International Journal of Audiology 2015; Early Online: 1–7
Original Article
Impact of frequency compression on music perception Bruna S. S. Mussoi & Ruth A. Bentler
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Department of Communication Sciences and Disorders, University of Iowa, Iowa City, USA
Abstract Objective: To determine the effects of frequency compression on music perception, and the impact of previous music training and hearing status. It was hypothesized that lesser amounts of compression would be preferred, and that this pattern of preference would be more evident in the musically trained groups. Design: A paired-comparison paradigm was used. Subjects listened to pairs of musical passages as processed by a hearing aid with different frequency-compression settings. Subjects indicated their preferred passage and the strength of their preference. Study sample: Fifty-seven subjects divided in four groups, according to hearing status (normal hearing, mild-to-moderate hearing loss), and previous music experience (trained, not trained). Results: Subjects generally preferred the conditions with the lesser amount of compression. Listeners in the group with previous music training showed stronger preference for less compression than those without training, as did listeners with normal hearing when compared to subjects with hearing loss. Conclusions: Although less frequency compression was in general preferred, there was more variability in the comparisons involving the default settings for a 50-dB hearing loss (i.e. start frequency 4000 Hz, compression ratio 2.5:1) and no compression, suggesting that mild amounts of compression may not be detrimental to perceived sound quality.
Key Words: Hearing aids; frequency compression; music
or most persons with hearing impairment, the primary purpose of F hearing aids is to improve audibility of the speech signal. In particular, increasing access to high frequency sounds is often a goal when fitting hearing aids, since consonants tend to have high frequency content. Studies using low-pass filtered speech have shown that perception of consonants by normal-hearing children and adults improves with increasing provision of mid-to-high frequency cues (Hogan & Turner, 1998; Stelmachowicz et al, 2001). However, achieving the desired amount of high frequency output in hearing aids has not always been possible, due to (1) technical limitation of transducers, and (2) hearing aid feedback. At the same time, amplification in high frequency regions for listeners with moderately severe to profound sensorineural hearing loss may not always be desirable, as performance may decrease with increasing access to sounds above 4000 Hz (Ching et al, 1998; Hogan & Turner, 1998). This phenomenon has been explained by the existence of ʽdead regionsʼ in the cochlea, or places (usually at the cochlear base) with non-functioning inner hair cells/auditory neurons (Moore et al 2000). Several signal processing strategies aimed at maintaining a sufficiently wide input bandwidth have been proposed in the past few decades; these are referred to as frequency lowering schemes. Currently there are three such schemes available in hearing aids: (1) nonlinear frequency compression, a processing scheme that
compresses high frequency cues into the lower frequency spectrum; (2) frequency transposition, in which high frequency cues are extracted and added to the lower frequency spectrum; and (3) spectral cueing, an algorithm that maintains the original input spectrum while simultaneously introducing high frequency cues into the lower frequency spectrum. The present study was focused on the first scheme. In its current form, frequency compression leaves frequencies below a pre-determined cutoff unprocessed while shifting higher input frequencies above the cutoff into lower—though not overlapping—frequency regions. Since the shifted signal does not interact with lower frequencies, frequency compression has the potential advantages of avoiding spectral overlap and consequent masking of lower frequency content; however, it can introduce distortion by altering formant ratios, harmonic ratios, and spacing (McDermott, 2011). These are important not only for the perception of vowels (Peterson & Barney, 1952), but also of music (Dowling & Fujitani, 1971). Recent reports of the effectiveness of frequency compression schemes in clinical populations have been equivocal in their findings. In those studies, objective outcomes have mostly been determined by comparisons of phoneme perception in quiet (in VCV or CVC format), and by plural recognition and detection of /s/ sounds, with and without frequency compression.
Correspondence: Bruna S.S. Mussoi, Department of Communication Sciences and Disorders, University of Iowa, 250 Hawkins Drive, Iowa City, IA 52242, USA. E-mail: [email protected] (Received 10 October 2014; accepted 30 January 2015) ISSN 1499-2027 print/ISSN 1708-8186 online © 2015 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2015.1020972
2 B. S. S. Mussoi & R. A. Bentler
Abbreviations
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ANSI APHAB CVC HL-NT HL-T NH-NT NH-T SNR-50 VCV
American National Standards Institute Abbreviated profile of hearing aid benefit Consonant-vowel-consonant Hearing loss and no previous music training Hearing loss and previous music training Normal hearing and no previous music training Normal hearing and previous music training Signal-to-noise ratio for 50% correct recognition Vowel-consonant-vowel
Several studies have reported positive group findings in support of frequency compression over conventional processing in hearing aids. For example, Simpson et al (2005) reported on a body-worn experimental device with frequency compression that was tested on subjects with moderate-to-profound sensorineural hearing loss. After having practiced with the device for 4–6 weeks, group results showed better phoneme perception in quiet, although individually a significant benefit was obtained by only eight of their 17 subjects. Group improvements after some experience with frequency compression were also described by Glista et al (2009), as evidenced by lower detection thresholds for /s/ sounds, and improved consonant and plural perception. Individual data revealed substantial variability, however, with only one fourth of the adult subjects and two thirds of the child subjects showing significantly better consonant and/or plural perception. Wolfe et al (2010) also found a group advantage in sound field thresholds for the phonemes /s/ and /sh/, high-frequency warble tones and in plural recognition in children with moderate-to-severe hearing loss (individual results were not reported). No advantage was found for sentences in background noise. In a follow-up study these investigators observed an improvement of 1.5 dB SNR-50 (i.e. the signal-to-noise ratio that yields 50% correct speech recognition) after six months of experience with the frequency compression device as compared to early performance with conventional processing. However, no difference was observed when comparing initial and six-month scores with frequency compression (Wolfe et al, 2011). Lastly, Hopkins et al (2014) studied a large group of 46 listeners differing in length of experience with frequency compression and in the amount of high frequency hearing loss. Results showed a benefit with frequency compression when recognizing consonants (mostly due to improvements in the recognition of /s/), but not when listening to sentences in noise. There was substantial variability in individual benefit from frequency compression. Other investigations have not found such benefits with frequency compression. In a follow-up investigation to their 2005 study, Simpson et al (2006) failed to replicate the improved phoneme perception in listeners with steeply sloping normal-to-profound sensorineural hearing loss. Only one of five subjects showed benefit from frequency compression in background noise. Similarly, Gifford et al (2007) did not find differences in mean word and sentence recognition in quiet and in noise with and without frequency compression in hybrid cochlear implant candidates. However, some of their subjects tended to perform better without frequency compression (two of six subjects in quiet, four to five of six subjects in noise; significance not reported). These results are corroborated by those of Park et al (2012), who showed that although hearing aids improved speech perception in quiet and in noise over a contralateral cochlear implant alone, there was no difference between outcomes obtained with frequency compression and conventional hearing aids. Finally, one study found significant benefit of conventional processing over
frequency compression for spondees in noise when combined with a contralateral cochlear implant, both with group results and for 50% of the participants with an intra-subject analysis (Perreau et al, 2013). These authors also showed significantly better vowel perception with conventional processing for group data and for two of 17 subjects. Overall, studies seem to indicate that there may be perceptual benefits with frequency compression for some but not all subjects, and this benefit may be dependent on the specific measures utilized, as well as on the degree and configuration of hearing loss. A different approach to determining outcomes with frequency compression is to compare subjective benefit with or preference for a hearing aid with that processing enabled and disabled. For example, in one study, the abbreviated profile of hearing aid benefit - APHAB (Cox & Alexander, 1995) was used with that intent, and four out of six subjects reported more benefit with the conventional device, one subject with frequency compression, and another subject indicated similar benefit scores with the two devices (Simpson et al, 2006). In another investigation, APHAB results did not show significant differences in benefit with frequency compression as compared to the subjects’ own aids with conventional processing (Gifford et al, 2007). However, there were trends towards reports of greater aversiveness with frequency compression. Alternatively, listeners have been asked to directly compare hearing aid memories with and without frequency compression in their daily lives and report perceptions in a diary (Glista et al, 2009), with only two of thirteen adults preferring frequency compression (nine had no preference), and seven of eleven children preferring that scheme (three had no preference). In Perreau et al (2013), listeners rated conventional processing significantly higher than frequency compression in the sound quality questionnaire (Tyler et al, 2009) on a subscale that assessed speech understanding. Currently, there is increased focus on music perception with hearing aid amplification. In one survey 70% of hearing aid users indicated that listening to music is an important part of their lives, with half of the respondents stating that they listen to music every day (Leek et al, 2008); 78% indicated that they wear their hearing aids when listening to music. Given the possible negative effects of frequency lowering schemes on pitch, it might be expected that trained individuals would be more sensitive to alterations of that structure when processed by frequency compression. The goal of the present study was to determine the effects of frequency compression on music perception, and how those effects are impacted by (1) previous music training and (2) hearing status (normal hearing versus mild-to-moderate hearing loss). Music perception was assessed through subjective preference on a pairedcomparison paradigm using music passages with differing amounts of frequency compression. It was also of interest to examine what factors influenced preference. It was hypothesized that less compression would be preferred by listeners (especially in the comparisons involving the maximum compression settings), and that this pattern of preference would be more evident in the musically trained groups. Also, factors related to sound quality were expected to be prominent criteria for the subjective ratings of the music passages.
Materials and Methods Participants Participants were recruited via flyers and a mass e-mail sent to University of Iowa students, faculty, staff and retirees, both of which were approved by the Institutional Review Board. Fifty-seven subjects participated in this study, and were grouped as follows: 15 subjects with normal hearing and no previous music training
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(NH-NT group, M 30.3 years, range 21–40 years), 15 subjects with normal hearing and previous music training (NH-T group, M 28.3 years, range 20–45 years), 15 subjects with hearing loss and no previous music training (HL-NT group, M 57.9 years, range 28–72 years), and 12 subjects with hearing loss and previous music training (HL-T group, M 64.7 years, range 41–82 years). Normal hearing was defined by hearing thresholds equal to or better than 20 dB HL at the octave frequencies of 250–8000 Hz (re ANSI S3.6-2004). Subjects with hearing loss had symmetrical, mild-to-moderate sensorineural hearing loss that was in general gently sloping, with no more than 25 dB difference between 1000 Hz and the average of 2000, 3000, and 4000 Hz. Figure 1 shows mean thresholds for the two groups with hearing loss; symbols connected by solid lines represent thresholds for the HL-NT group, and by dashed lines for the HL-T group. As it is evident from Figure 1, thresholds were flatter across frequency for the group without musical training, despite careful attempts to recruit subjects with flat hearing loss. Seven subjects were excluded based on audiometric findings. Because of the novel signal processing scheme (frequency compression) which none of the subjects had experienced prior to this study, current hearing aid use was not considered relevant. Previous music training was ascertained through self-identification, and confirmed from responses to portions of the Iowa musical background and appreciation questionnaire (Gfeller et al, 2000). The first part of the questionnaire was used in this study, as it assesses a listener’s musical training or involvement. The first questions are focused on each participant’s number of years of lessons in singing, participation in music groups, and formal classes in music. Next, subjects were asked to rate themselves in a 1–5 scale ranging from no general knowledge/experience with music (a score of 1) to high experience in music (a score of 5). Collapsed across hearing condition, the two groups with and without previous music training differed significantly (p 0.05, t tests with Bonferroni correction for multiple tests) in years of music lessons in voice or an instrument (mean years for group without training 4.59, with training 13.13), years of participation in a music group (mean years for group without
Impact of frequency compression on music perception 3 training 5.72, with training 18.28), and in the number of classes in music appreciation or music theory (mean years for group without training 0.38, with training 9.87). In addition, the groups also differed on self-rating of general knowledge and experience with music. The group without previous training had a mean self-rating of 2.97, and the group with music training had a mean self-rating of 4.93. Therefore, the classification of musically ʽtrainedʼ and ʽuntrainedʼ participants based on responses to the Iowa musical background and appreciation questionnaire was deemed appropriate.
Stimuli A commercially available behind-the-ear hearing aid with frequency compression capability was used to produce the experimental stimuli. The hearing aid was programmed for a flat 50-dB hearing loss across the audiometric frequency range, and slight adjustments were made to ensure a flat 25-dB gain for average input sound levels, as verified in an Audioscan Verifit VF-1 analyser. Because we attempted to recruit subjects with mild-to-moderate gently sloping hearing loss, the hearing aids were not programmed per each subject’s hearing thresholds. The amplitude compression ratios across the 20 channels ranged between 1.4 and 1.6. The ANSI 3.22 specifications indicate a bandwidth of 100–6400 Hz, but this was altered by the frequency compression algorithm used in the study. A single, manual program was used for the recordings, in which hearing aid features such as noise reduction, wind management, and feedback management were turned off. The microphone was set to the omnidirectional mode. Three frequency compression conditions were investigated in the current study: (1) no frequency compression, (2) ʽdefaultʼ frequency compression (the manufacturer’s software default settings for a flat 50-dB hearing loss, with start frequency of 4000 Hz and compression ratio of 2.5:1), and (3) the maximum amount of frequency compression allowed by the software (start frequency of 1500 Hz, compression ratio of 4:1). Five tracks produced with different musical instruments were used to generate the experimental stimuli: Allemande, by Johan Sebastian Bach (guitar solo); Molto Allegro, by Mozart (classical music); Sonata for piano 1, by Mozart (piano solo); Parte prima, by Paganini (violin solo), and You light up my life, by LeAnn Rimes (female singer). Six excerpts lasting five seconds were extracted from different parts of each track, totaling 30 passages (5 tracks 6 excerpts). Each passage was presented at 70 dB SPL in a calibrated chamber where the hearing aid was coupled to a 2 cm3 coupler. Hearing aid output was recorded in each of the three frequency compression conditions (no compression, default, and maximum compression).
Procedures
Figure 1. Mean audiometric thresholds are shown; solid lines represent the HL-NT group (with hearing loss, no musical training) and dashed lines represent the HL-T group (with hearing loss and previous musical training).
A paired-comparison paradigm was used to assess each subject’s preference for the frequency compression conditions. Pairedcomparisons allow the listener to compare electroacoustic conditions in a simple task that does not place a heavy load on memory, allowing better comparisons of conditions that do not differ much (Amlani & Schafer, 2009). In the present study, recordings of the same passage with different frequency compression conditions were paired, yielding three comparison conditions: default maximum compression, default no compression, and maximum no compression. Experimental comparisons comprised 90 pairs of stimuli (30 passages the three comparison conditions outlined above). In addition, fifteen ʽcatch trialsʼ were included, where one passage from each instrument was compared to itself in the same frequency compression condition. For each comparison, the presentation order
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4 B. S. S. Mussoi & R. A. Bentler of frequency compression conditions within the pairs was counterbalanced. The presentation order of the 105 pairs was randomized across subjects using a custom script on E-prime v2.0. Subjects were tested in a single session, lasting 1–2 hours. After subjects provided consent, audiometric thresholds were obtained (or screened, for normal hearing subjects) and the relevant portions of the Iowa musical background questionnaire (Gfeller et al, 2000) were administered. For the paired-comparisons, subjects were seated in a double-walled sound-treated Industrial Acoustics Company (IAC) test booth, facing a touch-sensitive computer monitor (a mouse was also available). Stimuli were presented bilaterally through Sennheiser IE-8 earphones, at a presentation level deemed comfortable by each subject. As subjects listened to each passage, the corresponding half of the computer screen was highlighted; demonstrating which passage in the pair the subject was listening to (passage 1 or passage 2). Subjects were instructed to choose the passage in each pair that they preferred by selecting the corresponding half of the computer screen, based on any criteria that they considered important. After this 2-alternative forced choice task, subjects were asked to determine the strength of their preference for each pair, by selecting one of three options in a visual analog scale displayed on the computer monitor: ʽslightʼ, ʽmoderateʼ, and ʽstrongʼ. A new pair was presented 2-s after the preference selection, but subjects were allowed to respond to each pair at their own pace. A break was automatically offered after half of the pairs had been presented. A short practice session was completed before the experiment started, to familiarize subjects with the test setup. Six pairs of passages from a separate music track were processed similarly to the experimental stimuli and used for practice. No feedback was provided. All subjects demonstrated a clear understanding of the task after practice. Finally, after the paired-comparisons, subjects were asked to list the factors that were important to them when determining preference for the music passages.
meaning that there was a significant preference for one of the frequency compression conditions in a pair. As a result, 95% confidence intervals for the odds ratio (the ratio of the probability of a score being higher than 3, divided by its complement) were computed for each of the four groups (NH-NT, NH-T, HL-NT, and HL-T). A ratio equal to 1 indicates that the score was not significantly different than 3, or that there was no clear preference in this forced-choice task.
Results Paired-comparisons of frequency compression conditions The purpose of this study was to investigate the effects of frequency compression on music perception in non-musicians and musicians with normal hearing and with hearing loss. Figure 2 shows the preference results for each of the four groups as a function of the frequency compression setting. The strength of preference is depicted by the different shading within each bar. Note that there are three compression settings being compared; each of them is paired to each of the other two an equal number of times. Therefore, each compression setting is present in two thirds or 66.6% of the pairs compared; thus if a compression setting is always preferred it will have a preference score of 66.6%. Also, preference scores for each group across the compression settings compared add up to 100% (e.g. the added percent of preference for no compression, default, and maximum compression in the NH-NT group totals 100%). Results of the paired-comparisons, shown in Table 1, revealed that in all pairs of comparisons the condition with less frequency compression was significantly preferred by all groups (i.e. no 95% confidence intervals contained the odds of 1). The only exception was that the group with hearing loss and no previous music training did not have a clear preference between default and no compression (95% CI 0.92, 1.37). Results from the fifteen ʽcatch trialsʼ in which two identical pairs of passages were compared indicated no significant preference for either the first or second passage presented.
Statistical analyses There were three pairs of frequency compression conditions compared in this study: (1) default maximum compression, (2) default no compression, and (3) maximum no compression. Because of the nature of the two-alternative forced choice task used in this study, it was of interest to incorporate the strength of preference into each subject’s response, instead of simply comparing the percentage of preference for each condition between the groups. Therefore, responses to each pair compared were transformed into data on an ordinal scale. A comparison score of 1, 2, or 3 was assigned if the second of the compression conditions in the pair was strongly, moderately, or slightly preferred, respectively, and a comparison score of 4, 5, or 6 was assigned if the first compression condition in the pair was slightly, moderately, or strongly preferred, respectively. Therefore, the extreme values on this 1–6 ordinal scale indicate a stronger preference for one of the two compression conditions compared. The following analyses were carried out for each of the three pairs of frequency compression settings compared. The cumulative multinomial logit model (SAS Proc GenMod, SAS Institute Inc.) was used to model the effects of the predictor variables hearing status, previous music training and music instrument (as a control) on the cumulative probability of obtaining each of the ordinal comparison scores. The significance of the parameters representing those predictor variables in the logistic regression equations was tested with chi-square tests, against the null hypothesis that the parameters were zero. Also of interest was the follow-up analysis revealing the probability of the response to a pair being significantly different from the neutral position (i.e. an odds ratio of 1) in either direction,
Figure 2. Total percent of preference for each compression setting across all pairs compared is depicted in the panels (left, no compression; middle, default compression; right, maximum compression). Within each panel, total preference differences can be visualized; strength of preference is shown in the stacked bars. Note that each compression setting is part of two thirds of the pairs compared; therefore, if a given compression condition is always preferred it will have a 66.6% preference score. Also, the percentages for each group across the three compression conditions add to 100% preference.
Impact of frequency compression on music perception 5
Table 1. For each group in each pair of compression conditions compared, the odds ratio and 95% confidence intervals are shown. An odds ratio of 1 means no preference for either of the conditions compared in a pair. A ratio larger than 1 means a preference for the first condition compared in a pair, while a ratio smaller than 1 means a preference for the second condition compared in a pair. Significant differences are indicated by an asterisk. Default Maximum compression Group
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NH-T NH-NT HL-T HL-NT
Odds
95% CI
48.2 15.9 11.9 3.9
(26.1, 89.1)* (10.9, 23.3)* (6.1, 23.4)* (1.9, 8.2)*
Default compression No compression Odds 0.18 0.32 0.65 1.12
95% CI (0.12, (0.22, (0.53, (0.92,
0.28)* 0.45)* 0.80)* 1.37)
Maximum No compression Odds 0.01 0.05 0.08 0.28
95% CI (0.008, (0.03, (0.04, (0.14,
0.02)* 0.08)* 0.16)* 0.56)*
The effect of music passage or instrument was also analyzed. Results suggest that passage had a significant effect in all comparisons. Follow-up tests revealed that default compression was significantly more preferred than maximum compression for the female singer than for piano [c2 (1) 9.67; p .002], and that no compression was significantly more preferred than default compression for the female singer than for guitar or piano [c2 (1) 12.93; p .0003]. No follow-up tests were significant for instrument effects in the comparison between maximum compression and no compression once corrections for multiple tests were applied. The instrument effects were consistent across groups.
Effects of hearing status To investigate the effects of hearing status on frequency compression preference, groups were collapsed across previous music training, yielding a group of normal-hearing subjects, and another of subjects with hearing loss. Results revealed that the effects of hearing status were significant in all paired-comparisons, in that the group with normal hearing had a significantly stronger preference for less compression than the group with hearing loss: stronger preference for default over maximum compression [c2(1) 8.17; p .004], for no compression over default compression [c2(1) 10.87; p .001], and for no compression over maximum compression [c2(1) 9.90; p .002]. These effects are shown in Figure 3(a), where the percentage of responses to each comparison score (ranging from 1 to 6, represented in the figure by their corresponding strength of preference) is plotted for each of the three paired-comparisons. Open and filled circles depict responses from the groups with normal hearing and hearing loss, respectively.
Effects of previous music training To test the effects of previous music training on preference for the different frequency compression conditions, groups were collapsed across hearing status yielding a group of subjects without music training and a group with music training. Results showed that the effects of previous music training were significant for all comparisons, where the groups with music training had a stronger preference for less compression than the groups without training; that is, they showed stronger preference for default over maximum compression [c2(1) 5.94; p .02], for no compression over default compression [c2(1) 7.51; p .006], and for no compression over maximum compression [c2(1) 5.77; p .02]. Music training effects are displayed in Figure 3(b), with open and filled triangles representing responses from the groups without training and with training, respectively. Interestingly, similar trends can be observed in the effects of hearing status and previous music training (Figure 3, panels a and b), in
Figure 3. Panels depict the percent of trials each frequency compression condition was preferred in a pair, per level of strength of preference. Each panel denotes a pair of conditions compared. Extreme values in each panel reflect a stronger preference for one of the conditions in the pair (indicated at the top of the panels). (a) Effects of hearing status on preference for a passage in each condition paired (top graph; open symbols refer to collapsed groups with normal hearing, filled symbols to groups with hearing loss). (b) Effects of previous music training on preference for a passage in each condition paired (bottom graph; open symbols refer to collapsed groups without training, filled symbols to collapsed groups with training).
6 B. S. S. Mussoi & R. A. Bentler that there is a clear stronger preference for less compression in the groups with normal hearing and groups with training whenever maximum frequency compression was one of the conditions compared.
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Criteria for preference The factors that were listed by subjects as important for their preference judgments were initially grouped according to the dimensions of sound quality proposed by Gabrielsson and Sjögren (1979): ʽclearness/distinctnessʼ, ʽsharpness/hardness-softnessʼ, ʽbrightnessdarknessʼ, ʽfullness-thinnessʼ, ʽdisturbing soundsʼ (i.e. presence of distortion), and ʽloudnessʼ. The two proposed dimensions of ʽfeeling of spaceʼ and ʽnearnessʼ were not cited often by the subjects and were therefore grouped with other responses in the category ʽotherʼ. In addition, the category ʽtinninessʼ appeared frequently and was considered to be relevant, given the sound processing studied. Figure 4 shows the number of subjects in each group that reported using each criterion. Note that there were no limits to the number of criteria a subject could list. Due to the open-ended nature of the questionnaire, results were highly variable and were therefore not statistically analyzed. However, the most salient features of frequency compression that influenced the subjects’ perception of music passages can be seen on Figure 4. As expected, those were mostly related to clearness and distortion factors that are a result of frequency compression.
Discussion The main finding of the present study is that, consistent with our hypothesis, subjects preferred the music passages with lesser amounts of frequency compression. Interestingly, there was more variability in preference when the paired-comparisons involved a mild amount of compression (the ʽdefaultʼ condition) versus no compression, which suggests that as long as frequency compression is not extreme, musical quality may still be acceptable. This was true especially for subjects with hearing loss and no formal music training (arguably the most representative of the general population of hearing aid users), who did not show a significant preference for either condition in that comparison. Also, as expected, musically-trained individuals had a greater preference for conditions with less frequency compression compared to non-trained individuals. This finding supports the notion that decisions relative to feature settings such as frequency compression must
Figure 4. Factors listed by subjects in each group as being relevant to their preference for a music passage in the pairs compared are depicted. Subjects were given an open-ended form where they could list as many items as they wished.
be individualized for both lifestyle and for hearing deficit, insomuch as that information is provided. A stronger preference for less compression was also evident in subjects with normal hearing, as compared to those with hearing impairment. However, it is noteworthy that for the hearing-impaired group, listeners with training and without training preferred the maximum frequency compression in 7% and 16% of the comparisons, respectively (see Figure 2). From the criteria listed by subjects when determining their preference for the processed music passages, subjects with hearing loss and no previous training likely disliked the lesser compression conditions because of their high-pitched quality. It is also possible that preference for more frequency compression in the groups with hearing loss is related to a larger benefit from the provision of high frequency information without the high-pitched sound quality. Alternatively, for some musically trained individuals who had more sloping losses, the compressed stimuli might have been the only ones to provide audible high frequency information. Parsa et al (2013) also reported higher subjective quality ratings for frequency compressed speech and music by subjects with hearing loss as compared with their normal-hearing counterparts. In general the hearing-impaired subjects rated the frequency compression conditions more similarly than did the subjects with normal hearing, suggesting that differences in frequency compression settings may not be as apparent to them. Regarding instrument or passage effects, lesser amounts of frequency compression were preferred for the female singer passages across groups. It is possible that those effects are related to the fact that the female singer passages were the only ones with speech content; that is, musicians and non-musicians alike listen to speech on a regular basis, and thus both groups may have noticed the changes in sound quality in those passages. Finally, due to the open-ended questionnaire used to gather written subjective preference information, it was somewhat difficult to determine groupings in order to analyze the responses. Still, the data allow for examination of the general trends. Besides the expected changes in clearness of sounds and introduction of distortion by frequency compression, it was interesting to note that distortion factors seemed more salient to normal-hearing listeners, and that tinniness was more important for listeners with hearing loss and no music training. One of the limitations of the current study is that the fitting of the hearing aids was not individualized. It was assumed that in setting the hearing aid gain to flat 25 dB and adjusting the overall level for individual participants, the testing conditions would be more similar across all groups. Also, providing constant gain across the frequency range was assumed to preserve the sound quality of music by maintaining balance between the amplitudes of the fundamental frequency and its higher frequency harmonics (Chasin & Russo, 2004). However, as it can be seen from the mean hearing thresholds for the groups with hearing loss, in Figure 1, gain at higher frequencies may not have been optimal for some musically-trained subjects. In addition, except for turning off all special features of the hearing aid and maintaining constant gain across frequency, the hearing aid was not programmed as per the manufacturer’s recommendation for music. It has been suggested that some hearing aid settings can be optimized when listening to music, such as increasing input limiting levels to accommodate the larger dynamic range of music, lowering amplitude compression ratios and increasing compression kneepoints (Chasin & Russo, 2004). Another potential limitation of this study was the lack of time to adjust to the frequency-compressed sounds. However, recent investigations in which subjects underwent more extensive training with frequency compression still did not fully support acclimatization to
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frequency compression (e.g. Simpson et al, 2006; Glista et al, 2009; Wolfe et al, 2011; Perreau et al, 2013). A large cross-sectional study of hearing aid users with frequency compression experience ranging from 1 to 121 months found no correlation between frequency compression benefit (in speech perception) and duration of use (Hopkins et al, 2014). In fact, when looking at acclimatization to frequency compression for individual subjects, the effects have been variable (Glista et al, 2012). The present results can be taken at least as reflecting listeners’ initial perception of frequency-compressed music stimuli. Although initial perception may change over time, it is likely an important factor for new hearing aid users when deciding whether to continue device (or processing) use. One other potential confound is that subjects in the groups with hearing loss were older than those in the normal-hearing groups. As a result, it is possible that age effects may also have played a role in the observed effects of hearing status. For example, working memory, which tends to decline with age, has been shown to influence understanding of speech when distorted by frequency compression and the presence of noise (Arehart et al, 2013). Specifically, they found that listeners with higher working memory had better speech perception in the presence of those distortions than listeners with lower working memory. Thus, although speech perception was not tested in the current study, it might be expected that older adults (especially those with lower cognitive functioning) would prefer the conditions with less frequency compression. However, our results suggest the opposite, that subjects with normal hearing (who were younger) had a stronger preference for less compression than subjects with hearing loss (who were older). Finally, as described in the methods section, the musically-trained groups consisted of individuals with several years of practice with musical instruments and participation in bands, who also took several classes in music theory or appreciation. Most of those participants were graduate or undergraduate students in music, or active/ retired music faculty. Thus, it is possible that our subjects had more music experience than the general population of those who consider themselves musically knowledgeable, which could have made the effects of previous music training more salient.
Conclusion The present findings suggest that mild amounts of frequency compression are not detrimental to perceived sound quality, at least for sounds that are heavily based on harmonic structure, such as music. This was shown to be true even for subjects that are highly experienced musicians.
Acknowledgements The authors are grateful to Yu-Hsiang Wu for his assistance with equipment setup and helpful suggestions for study design. The authors would also like to thank Xuyang Zhang for his assistance with statistical analyses. Portions of this study were presented at the American Auditory Society Scientific and Technical Meeting, Scottsdale, Arizona, March 4–6, 2010, and at the American Academy of Audiology Convention, Chicago, Illinois, April 6–9, 2011. Declaration of interest: The authors report no conflicts of interest.
References Amlani A.M. & Schafer E.C. 2009. Application of paired-comparison methods to hearing aids. Trends Amplif, 13, 241–59.
Impact of frequency compression on music perception 7 Arehart K.H., Souza P., Baca R. & Kates J.M. 2013. Working memory, age, and hearing loss: Susceptibility to hearing aid distortion. Ear Hear, 34, 251–260. Chasin M. & Russo F.A. 2004. Hearing aids and music. Trends Amplif, 8, 35–47. Ching T.Y.C., Dillon H. & Byrne D. 1998. Speech recognition of hearingimpaired listeners: Predictions from audibility and the limited role of high-frequency amplification. J Acoust Soc Am, 103, 1128–1140. Cox R.M. & Alexander G.C. 1995. The abbreviated profile of hearing aid benefit. Ear Hear, 16, 176–186. Dowling W.J. & Fujitani D.S. 1971. Contour, interval and pitch recognition in memory for melodies. J Acoust Soc Am, 49, 524–531. Gabrielsson A. & Sjögren H. 1979. Perceived sound quality of soundreproducing systems. J Acoust Soc Am, 65, 1019–1033. Gfeller K., Christ A., Knutson J.F., Witt S., Murray K.T. et al. 2000. Musical backgrounds, listening habits, and aesthetic enjoyment of adult cochlear implant recipients. J Am Acad Audiol, 11, 390–406. Gifford R.H., Dorman M.F., Spahr A.J. & McKarns S.A. 2007. Effect of digital frequency compression (DFC) on speech recognition in candidates for combined electric and acoustic stimulation (EAS). J Speech Lang Hear Res, 50, 1194–1202. Glista D., Scollie S., Bagatto M., Seewald R., Parsa V. et al. 2009. Evaluation of nonlinear frequency compression: Clinical outcomes. Int J Audiol, 48, 632–644. Glista D., Scollie S. & Sulkers J. 2012. Perceptual acclimatization post nonlinear frequency compression hearing aid fitting in older children. J Sp Lang Hear Res, 55, 1765–1787. Hogan C.A. & Turner C.W. 1998. High-frequency audibility: Benefits for hearing-impaired listeners. J Acoust Soc Am, 104, 432–441. Hopkins K., Khanom M., Dickinson A.M. & Munro K.J. 2014. Benefit from non-linear frequency compression hearing aids in a clinical setting: The effects of duration of experience and severity of high-frequency hearing loss. Int J Audiol, 53, 219–228. Leek M.R., Molis M.R., Kubli L.R. & Tufts J.B. 2008. Enjoyment of music by elderly hearing-impaired listeners. J Am Acad Audiol, 19, 519–526. McDermott H.J. 2011. A technical comparison of digital frequency-lowering algorithms available in two current hearing aids. PLOS ONE, 6, 1–6. Moore B.C.J., Huss M., Vickers B.R., Glasberg B.R. & Alcantara J.I. 2000. A test for the diagnosis of dead regions in the cochlea. Br J Audiol, 34, 205–224. Park L.R., Teagle H.F.B., Buss E., Roush P.A. & Buchman C.A. 2012. Effects of frequency compression hearing aids for unilaterally implanted children with acoustically amplified residual hearing in the nonimplanted ear. Ear Hear, 33, e1–e12. Parsa V., Scollie S., Glista D. & Seelisch A. 2013. Nonlinear frequency compression: Effects on sound quality ratings of speech and music. Trends Amplif, 17, 54–68. Perreau A.E., Bentler R.A. & Tyler R.S. 2013. The contribution of a frequencycompression hearing aid to contralateral cochlear implant performance. J Am Acad Audiol, 24, 105–120. Peterson G.E. & Barney H.L. 1952. Control methods used in a study of the vowels. J Acoust Soc Am, 24, 175–184. Simpson A., Hersbach A.A. & McDermott H.J. 2005. Improvements in speech perception with an experimental nonlinear frequency compression hearing device. Int J Audiol, 44, 281–292. Simpson A., Hersbach A.A. & McDermott H.J. 2006. Frequencycompression outcomes in listeners with steeply sloping audiograms. Int J Audiol, 45, 619–629. Stelmachowicz P.G., Pittman A.L., Hoover B.M. & Lewis D.E. 2001. Effect of stimulus bandwidth on the perception of /s/ in normal- and hearingimpaired children and adults. J Acoust Soc Am, 110, 2183–2190. Tyler R., Perreau A. & Ji H. 2009. Validation of the spatial hearing questionnaire. Ear Hear, 30, 466–475. Wolfe J., John A., Schafer E., Nyfeller M., Boretzki M. et al. 2010. Evaluation of nonlinear frequency compression for school-age children with moderate to moderately severe hearing loss. J Am Acad Audiol, 21, 618–628. Wolfe J., John A., Schafer E., Nyfeller M., Boretzki M. et al. 2011. Long-term effects of non-linear frequency compression for children with moderate hearing loss. Int J Audiol, 50, 396–404.
Original Article
Impact of frequency compression on music perception Bruna S. S. Mussoi & Ruth A. Bentler
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Department of Communication Sciences and Disorders, University of Iowa, Iowa City, USA
Abstract Objective: To determine the effects of frequency compression on music perception, and the impact of previous music training and hearing status. It was hypothesized that lesser amounts of compression would be preferred, and that this pattern of preference would be more evident in the musically trained groups. Design: A paired-comparison paradigm was used. Subjects listened to pairs of musical passages as processed by a hearing aid with different frequency-compression settings. Subjects indicated their preferred passage and the strength of their preference. Study sample: Fifty-seven subjects divided in four groups, according to hearing status (normal hearing, mild-to-moderate hearing loss), and previous music experience (trained, not trained). Results: Subjects generally preferred the conditions with the lesser amount of compression. Listeners in the group with previous music training showed stronger preference for less compression than those without training, as did listeners with normal hearing when compared to subjects with hearing loss. Conclusions: Although less frequency compression was in general preferred, there was more variability in the comparisons involving the default settings for a 50-dB hearing loss (i.e. start frequency 4000 Hz, compression ratio 2.5:1) and no compression, suggesting that mild amounts of compression may not be detrimental to perceived sound quality.
Key Words: Hearing aids; frequency compression; music
or most persons with hearing impairment, the primary purpose of F hearing aids is to improve audibility of the speech signal. In particular, increasing access to high frequency sounds is often a goal when fitting hearing aids, since consonants tend to have high frequency content. Studies using low-pass filtered speech have shown that perception of consonants by normal-hearing children and adults improves with increasing provision of mid-to-high frequency cues (Hogan & Turner, 1998; Stelmachowicz et al, 2001). However, achieving the desired amount of high frequency output in hearing aids has not always been possible, due to (1) technical limitation of transducers, and (2) hearing aid feedback. At the same time, amplification in high frequency regions for listeners with moderately severe to profound sensorineural hearing loss may not always be desirable, as performance may decrease with increasing access to sounds above 4000 Hz (Ching et al, 1998; Hogan & Turner, 1998). This phenomenon has been explained by the existence of ʽdead regionsʼ in the cochlea, or places (usually at the cochlear base) with non-functioning inner hair cells/auditory neurons (Moore et al 2000). Several signal processing strategies aimed at maintaining a sufficiently wide input bandwidth have been proposed in the past few decades; these are referred to as frequency lowering schemes. Currently there are three such schemes available in hearing aids: (1) nonlinear frequency compression, a processing scheme that
compresses high frequency cues into the lower frequency spectrum; (2) frequency transposition, in which high frequency cues are extracted and added to the lower frequency spectrum; and (3) spectral cueing, an algorithm that maintains the original input spectrum while simultaneously introducing high frequency cues into the lower frequency spectrum. The present study was focused on the first scheme. In its current form, frequency compression leaves frequencies below a pre-determined cutoff unprocessed while shifting higher input frequencies above the cutoff into lower—though not overlapping—frequency regions. Since the shifted signal does not interact with lower frequencies, frequency compression has the potential advantages of avoiding spectral overlap and consequent masking of lower frequency content; however, it can introduce distortion by altering formant ratios, harmonic ratios, and spacing (McDermott, 2011). These are important not only for the perception of vowels (Peterson & Barney, 1952), but also of music (Dowling & Fujitani, 1971). Recent reports of the effectiveness of frequency compression schemes in clinical populations have been equivocal in their findings. In those studies, objective outcomes have mostly been determined by comparisons of phoneme perception in quiet (in VCV or CVC format), and by plural recognition and detection of /s/ sounds, with and without frequency compression.
Correspondence: Bruna S.S. Mussoi, Department of Communication Sciences and Disorders, University of Iowa, 250 Hawkins Drive, Iowa City, IA 52242, USA. E-mail: [email protected] (Received 10 October 2014; accepted 30 January 2015) ISSN 1499-2027 print/ISSN 1708-8186 online © 2015 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2015.1020972
2 B. S. S. Mussoi & R. A. Bentler
Abbreviations
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ANSI APHAB CVC HL-NT HL-T NH-NT NH-T SNR-50 VCV
American National Standards Institute Abbreviated profile of hearing aid benefit Consonant-vowel-consonant Hearing loss and no previous music training Hearing loss and previous music training Normal hearing and no previous music training Normal hearing and previous music training Signal-to-noise ratio for 50% correct recognition Vowel-consonant-vowel
Several studies have reported positive group findings in support of frequency compression over conventional processing in hearing aids. For example, Simpson et al (2005) reported on a body-worn experimental device with frequency compression that was tested on subjects with moderate-to-profound sensorineural hearing loss. After having practiced with the device for 4–6 weeks, group results showed better phoneme perception in quiet, although individually a significant benefit was obtained by only eight of their 17 subjects. Group improvements after some experience with frequency compression were also described by Glista et al (2009), as evidenced by lower detection thresholds for /s/ sounds, and improved consonant and plural perception. Individual data revealed substantial variability, however, with only one fourth of the adult subjects and two thirds of the child subjects showing significantly better consonant and/or plural perception. Wolfe et al (2010) also found a group advantage in sound field thresholds for the phonemes /s/ and /sh/, high-frequency warble tones and in plural recognition in children with moderate-to-severe hearing loss (individual results were not reported). No advantage was found for sentences in background noise. In a follow-up study these investigators observed an improvement of 1.5 dB SNR-50 (i.e. the signal-to-noise ratio that yields 50% correct speech recognition) after six months of experience with the frequency compression device as compared to early performance with conventional processing. However, no difference was observed when comparing initial and six-month scores with frequency compression (Wolfe et al, 2011). Lastly, Hopkins et al (2014) studied a large group of 46 listeners differing in length of experience with frequency compression and in the amount of high frequency hearing loss. Results showed a benefit with frequency compression when recognizing consonants (mostly due to improvements in the recognition of /s/), but not when listening to sentences in noise. There was substantial variability in individual benefit from frequency compression. Other investigations have not found such benefits with frequency compression. In a follow-up investigation to their 2005 study, Simpson et al (2006) failed to replicate the improved phoneme perception in listeners with steeply sloping normal-to-profound sensorineural hearing loss. Only one of five subjects showed benefit from frequency compression in background noise. Similarly, Gifford et al (2007) did not find differences in mean word and sentence recognition in quiet and in noise with and without frequency compression in hybrid cochlear implant candidates. However, some of their subjects tended to perform better without frequency compression (two of six subjects in quiet, four to five of six subjects in noise; significance not reported). These results are corroborated by those of Park et al (2012), who showed that although hearing aids improved speech perception in quiet and in noise over a contralateral cochlear implant alone, there was no difference between outcomes obtained with frequency compression and conventional hearing aids. Finally, one study found significant benefit of conventional processing over
frequency compression for spondees in noise when combined with a contralateral cochlear implant, both with group results and for 50% of the participants with an intra-subject analysis (Perreau et al, 2013). These authors also showed significantly better vowel perception with conventional processing for group data and for two of 17 subjects. Overall, studies seem to indicate that there may be perceptual benefits with frequency compression for some but not all subjects, and this benefit may be dependent on the specific measures utilized, as well as on the degree and configuration of hearing loss. A different approach to determining outcomes with frequency compression is to compare subjective benefit with or preference for a hearing aid with that processing enabled and disabled. For example, in one study, the abbreviated profile of hearing aid benefit - APHAB (Cox & Alexander, 1995) was used with that intent, and four out of six subjects reported more benefit with the conventional device, one subject with frequency compression, and another subject indicated similar benefit scores with the two devices (Simpson et al, 2006). In another investigation, APHAB results did not show significant differences in benefit with frequency compression as compared to the subjects’ own aids with conventional processing (Gifford et al, 2007). However, there were trends towards reports of greater aversiveness with frequency compression. Alternatively, listeners have been asked to directly compare hearing aid memories with and without frequency compression in their daily lives and report perceptions in a diary (Glista et al, 2009), with only two of thirteen adults preferring frequency compression (nine had no preference), and seven of eleven children preferring that scheme (three had no preference). In Perreau et al (2013), listeners rated conventional processing significantly higher than frequency compression in the sound quality questionnaire (Tyler et al, 2009) on a subscale that assessed speech understanding. Currently, there is increased focus on music perception with hearing aid amplification. In one survey 70% of hearing aid users indicated that listening to music is an important part of their lives, with half of the respondents stating that they listen to music every day (Leek et al, 2008); 78% indicated that they wear their hearing aids when listening to music. Given the possible negative effects of frequency lowering schemes on pitch, it might be expected that trained individuals would be more sensitive to alterations of that structure when processed by frequency compression. The goal of the present study was to determine the effects of frequency compression on music perception, and how those effects are impacted by (1) previous music training and (2) hearing status (normal hearing versus mild-to-moderate hearing loss). Music perception was assessed through subjective preference on a pairedcomparison paradigm using music passages with differing amounts of frequency compression. It was also of interest to examine what factors influenced preference. It was hypothesized that less compression would be preferred by listeners (especially in the comparisons involving the maximum compression settings), and that this pattern of preference would be more evident in the musically trained groups. Also, factors related to sound quality were expected to be prominent criteria for the subjective ratings of the music passages.
Materials and Methods Participants Participants were recruited via flyers and a mass e-mail sent to University of Iowa students, faculty, staff and retirees, both of which were approved by the Institutional Review Board. Fifty-seven subjects participated in this study, and were grouped as follows: 15 subjects with normal hearing and no previous music training
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(NH-NT group, M 30.3 years, range 21–40 years), 15 subjects with normal hearing and previous music training (NH-T group, M 28.3 years, range 20–45 years), 15 subjects with hearing loss and no previous music training (HL-NT group, M 57.9 years, range 28–72 years), and 12 subjects with hearing loss and previous music training (HL-T group, M 64.7 years, range 41–82 years). Normal hearing was defined by hearing thresholds equal to or better than 20 dB HL at the octave frequencies of 250–8000 Hz (re ANSI S3.6-2004). Subjects with hearing loss had symmetrical, mild-to-moderate sensorineural hearing loss that was in general gently sloping, with no more than 25 dB difference between 1000 Hz and the average of 2000, 3000, and 4000 Hz. Figure 1 shows mean thresholds for the two groups with hearing loss; symbols connected by solid lines represent thresholds for the HL-NT group, and by dashed lines for the HL-T group. As it is evident from Figure 1, thresholds were flatter across frequency for the group without musical training, despite careful attempts to recruit subjects with flat hearing loss. Seven subjects were excluded based on audiometric findings. Because of the novel signal processing scheme (frequency compression) which none of the subjects had experienced prior to this study, current hearing aid use was not considered relevant. Previous music training was ascertained through self-identification, and confirmed from responses to portions of the Iowa musical background and appreciation questionnaire (Gfeller et al, 2000). The first part of the questionnaire was used in this study, as it assesses a listener’s musical training or involvement. The first questions are focused on each participant’s number of years of lessons in singing, participation in music groups, and formal classes in music. Next, subjects were asked to rate themselves in a 1–5 scale ranging from no general knowledge/experience with music (a score of 1) to high experience in music (a score of 5). Collapsed across hearing condition, the two groups with and without previous music training differed significantly (p 0.05, t tests with Bonferroni correction for multiple tests) in years of music lessons in voice or an instrument (mean years for group without training 4.59, with training 13.13), years of participation in a music group (mean years for group without
Impact of frequency compression on music perception 3 training 5.72, with training 18.28), and in the number of classes in music appreciation or music theory (mean years for group without training 0.38, with training 9.87). In addition, the groups also differed on self-rating of general knowledge and experience with music. The group without previous training had a mean self-rating of 2.97, and the group with music training had a mean self-rating of 4.93. Therefore, the classification of musically ʽtrainedʼ and ʽuntrainedʼ participants based on responses to the Iowa musical background and appreciation questionnaire was deemed appropriate.
Stimuli A commercially available behind-the-ear hearing aid with frequency compression capability was used to produce the experimental stimuli. The hearing aid was programmed for a flat 50-dB hearing loss across the audiometric frequency range, and slight adjustments were made to ensure a flat 25-dB gain for average input sound levels, as verified in an Audioscan Verifit VF-1 analyser. Because we attempted to recruit subjects with mild-to-moderate gently sloping hearing loss, the hearing aids were not programmed per each subject’s hearing thresholds. The amplitude compression ratios across the 20 channels ranged between 1.4 and 1.6. The ANSI 3.22 specifications indicate a bandwidth of 100–6400 Hz, but this was altered by the frequency compression algorithm used in the study. A single, manual program was used for the recordings, in which hearing aid features such as noise reduction, wind management, and feedback management were turned off. The microphone was set to the omnidirectional mode. Three frequency compression conditions were investigated in the current study: (1) no frequency compression, (2) ʽdefaultʼ frequency compression (the manufacturer’s software default settings for a flat 50-dB hearing loss, with start frequency of 4000 Hz and compression ratio of 2.5:1), and (3) the maximum amount of frequency compression allowed by the software (start frequency of 1500 Hz, compression ratio of 4:1). Five tracks produced with different musical instruments were used to generate the experimental stimuli: Allemande, by Johan Sebastian Bach (guitar solo); Molto Allegro, by Mozart (classical music); Sonata for piano 1, by Mozart (piano solo); Parte prima, by Paganini (violin solo), and You light up my life, by LeAnn Rimes (female singer). Six excerpts lasting five seconds were extracted from different parts of each track, totaling 30 passages (5 tracks 6 excerpts). Each passage was presented at 70 dB SPL in a calibrated chamber where the hearing aid was coupled to a 2 cm3 coupler. Hearing aid output was recorded in each of the three frequency compression conditions (no compression, default, and maximum compression).
Procedures
Figure 1. Mean audiometric thresholds are shown; solid lines represent the HL-NT group (with hearing loss, no musical training) and dashed lines represent the HL-T group (with hearing loss and previous musical training).
A paired-comparison paradigm was used to assess each subject’s preference for the frequency compression conditions. Pairedcomparisons allow the listener to compare electroacoustic conditions in a simple task that does not place a heavy load on memory, allowing better comparisons of conditions that do not differ much (Amlani & Schafer, 2009). In the present study, recordings of the same passage with different frequency compression conditions were paired, yielding three comparison conditions: default maximum compression, default no compression, and maximum no compression. Experimental comparisons comprised 90 pairs of stimuli (30 passages the three comparison conditions outlined above). In addition, fifteen ʽcatch trialsʼ were included, where one passage from each instrument was compared to itself in the same frequency compression condition. For each comparison, the presentation order
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4 B. S. S. Mussoi & R. A. Bentler of frequency compression conditions within the pairs was counterbalanced. The presentation order of the 105 pairs was randomized across subjects using a custom script on E-prime v2.0. Subjects were tested in a single session, lasting 1–2 hours. After subjects provided consent, audiometric thresholds were obtained (or screened, for normal hearing subjects) and the relevant portions of the Iowa musical background questionnaire (Gfeller et al, 2000) were administered. For the paired-comparisons, subjects were seated in a double-walled sound-treated Industrial Acoustics Company (IAC) test booth, facing a touch-sensitive computer monitor (a mouse was also available). Stimuli were presented bilaterally through Sennheiser IE-8 earphones, at a presentation level deemed comfortable by each subject. As subjects listened to each passage, the corresponding half of the computer screen was highlighted; demonstrating which passage in the pair the subject was listening to (passage 1 or passage 2). Subjects were instructed to choose the passage in each pair that they preferred by selecting the corresponding half of the computer screen, based on any criteria that they considered important. After this 2-alternative forced choice task, subjects were asked to determine the strength of their preference for each pair, by selecting one of three options in a visual analog scale displayed on the computer monitor: ʽslightʼ, ʽmoderateʼ, and ʽstrongʼ. A new pair was presented 2-s after the preference selection, but subjects were allowed to respond to each pair at their own pace. A break was automatically offered after half of the pairs had been presented. A short practice session was completed before the experiment started, to familiarize subjects with the test setup. Six pairs of passages from a separate music track were processed similarly to the experimental stimuli and used for practice. No feedback was provided. All subjects demonstrated a clear understanding of the task after practice. Finally, after the paired-comparisons, subjects were asked to list the factors that were important to them when determining preference for the music passages.
meaning that there was a significant preference for one of the frequency compression conditions in a pair. As a result, 95% confidence intervals for the odds ratio (the ratio of the probability of a score being higher than 3, divided by its complement) were computed for each of the four groups (NH-NT, NH-T, HL-NT, and HL-T). A ratio equal to 1 indicates that the score was not significantly different than 3, or that there was no clear preference in this forced-choice task.
Results Paired-comparisons of frequency compression conditions The purpose of this study was to investigate the effects of frequency compression on music perception in non-musicians and musicians with normal hearing and with hearing loss. Figure 2 shows the preference results for each of the four groups as a function of the frequency compression setting. The strength of preference is depicted by the different shading within each bar. Note that there are three compression settings being compared; each of them is paired to each of the other two an equal number of times. Therefore, each compression setting is present in two thirds or 66.6% of the pairs compared; thus if a compression setting is always preferred it will have a preference score of 66.6%. Also, preference scores for each group across the compression settings compared add up to 100% (e.g. the added percent of preference for no compression, default, and maximum compression in the NH-NT group totals 100%). Results of the paired-comparisons, shown in Table 1, revealed that in all pairs of comparisons the condition with less frequency compression was significantly preferred by all groups (i.e. no 95% confidence intervals contained the odds of 1). The only exception was that the group with hearing loss and no previous music training did not have a clear preference between default and no compression (95% CI 0.92, 1.37). Results from the fifteen ʽcatch trialsʼ in which two identical pairs of passages were compared indicated no significant preference for either the first or second passage presented.
Statistical analyses There were three pairs of frequency compression conditions compared in this study: (1) default maximum compression, (2) default no compression, and (3) maximum no compression. Because of the nature of the two-alternative forced choice task used in this study, it was of interest to incorporate the strength of preference into each subject’s response, instead of simply comparing the percentage of preference for each condition between the groups. Therefore, responses to each pair compared were transformed into data on an ordinal scale. A comparison score of 1, 2, or 3 was assigned if the second of the compression conditions in the pair was strongly, moderately, or slightly preferred, respectively, and a comparison score of 4, 5, or 6 was assigned if the first compression condition in the pair was slightly, moderately, or strongly preferred, respectively. Therefore, the extreme values on this 1–6 ordinal scale indicate a stronger preference for one of the two compression conditions compared. The following analyses were carried out for each of the three pairs of frequency compression settings compared. The cumulative multinomial logit model (SAS Proc GenMod, SAS Institute Inc.) was used to model the effects of the predictor variables hearing status, previous music training and music instrument (as a control) on the cumulative probability of obtaining each of the ordinal comparison scores. The significance of the parameters representing those predictor variables in the logistic regression equations was tested with chi-square tests, against the null hypothesis that the parameters were zero. Also of interest was the follow-up analysis revealing the probability of the response to a pair being significantly different from the neutral position (i.e. an odds ratio of 1) in either direction,
Figure 2. Total percent of preference for each compression setting across all pairs compared is depicted in the panels (left, no compression; middle, default compression; right, maximum compression). Within each panel, total preference differences can be visualized; strength of preference is shown in the stacked bars. Note that each compression setting is part of two thirds of the pairs compared; therefore, if a given compression condition is always preferred it will have a 66.6% preference score. Also, the percentages for each group across the three compression conditions add to 100% preference.
Impact of frequency compression on music perception 5
Table 1. For each group in each pair of compression conditions compared, the odds ratio and 95% confidence intervals are shown. An odds ratio of 1 means no preference for either of the conditions compared in a pair. A ratio larger than 1 means a preference for the first condition compared in a pair, while a ratio smaller than 1 means a preference for the second condition compared in a pair. Significant differences are indicated by an asterisk. Default Maximum compression Group
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NH-T NH-NT HL-T HL-NT
Odds
95% CI
48.2 15.9 11.9 3.9
(26.1, 89.1)* (10.9, 23.3)* (6.1, 23.4)* (1.9, 8.2)*
Default compression No compression Odds 0.18 0.32 0.65 1.12
95% CI (0.12, (0.22, (0.53, (0.92,
0.28)* 0.45)* 0.80)* 1.37)
Maximum No compression Odds 0.01 0.05 0.08 0.28
95% CI (0.008, (0.03, (0.04, (0.14,
0.02)* 0.08)* 0.16)* 0.56)*
The effect of music passage or instrument was also analyzed. Results suggest that passage had a significant effect in all comparisons. Follow-up tests revealed that default compression was significantly more preferred than maximum compression for the female singer than for piano [c2 (1) 9.67; p .002], and that no compression was significantly more preferred than default compression for the female singer than for guitar or piano [c2 (1) 12.93; p .0003]. No follow-up tests were significant for instrument effects in the comparison between maximum compression and no compression once corrections for multiple tests were applied. The instrument effects were consistent across groups.
Effects of hearing status To investigate the effects of hearing status on frequency compression preference, groups were collapsed across previous music training, yielding a group of normal-hearing subjects, and another of subjects with hearing loss. Results revealed that the effects of hearing status were significant in all paired-comparisons, in that the group with normal hearing had a significantly stronger preference for less compression than the group with hearing loss: stronger preference for default over maximum compression [c2(1) 8.17; p .004], for no compression over default compression [c2(1) 10.87; p .001], and for no compression over maximum compression [c2(1) 9.90; p .002]. These effects are shown in Figure 3(a), where the percentage of responses to each comparison score (ranging from 1 to 6, represented in the figure by their corresponding strength of preference) is plotted for each of the three paired-comparisons. Open and filled circles depict responses from the groups with normal hearing and hearing loss, respectively.
Effects of previous music training To test the effects of previous music training on preference for the different frequency compression conditions, groups were collapsed across hearing status yielding a group of subjects without music training and a group with music training. Results showed that the effects of previous music training were significant for all comparisons, where the groups with music training had a stronger preference for less compression than the groups without training; that is, they showed stronger preference for default over maximum compression [c2(1) 5.94; p .02], for no compression over default compression [c2(1) 7.51; p .006], and for no compression over maximum compression [c2(1) 5.77; p .02]. Music training effects are displayed in Figure 3(b), with open and filled triangles representing responses from the groups without training and with training, respectively. Interestingly, similar trends can be observed in the effects of hearing status and previous music training (Figure 3, panels a and b), in
Figure 3. Panels depict the percent of trials each frequency compression condition was preferred in a pair, per level of strength of preference. Each panel denotes a pair of conditions compared. Extreme values in each panel reflect a stronger preference for one of the conditions in the pair (indicated at the top of the panels). (a) Effects of hearing status on preference for a passage in each condition paired (top graph; open symbols refer to collapsed groups with normal hearing, filled symbols to groups with hearing loss). (b) Effects of previous music training on preference for a passage in each condition paired (bottom graph; open symbols refer to collapsed groups without training, filled symbols to collapsed groups with training).
6 B. S. S. Mussoi & R. A. Bentler that there is a clear stronger preference for less compression in the groups with normal hearing and groups with training whenever maximum frequency compression was one of the conditions compared.
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Criteria for preference The factors that were listed by subjects as important for their preference judgments were initially grouped according to the dimensions of sound quality proposed by Gabrielsson and Sjögren (1979): ʽclearness/distinctnessʼ, ʽsharpness/hardness-softnessʼ, ʽbrightnessdarknessʼ, ʽfullness-thinnessʼ, ʽdisturbing soundsʼ (i.e. presence of distortion), and ʽloudnessʼ. The two proposed dimensions of ʽfeeling of spaceʼ and ʽnearnessʼ were not cited often by the subjects and were therefore grouped with other responses in the category ʽotherʼ. In addition, the category ʽtinninessʼ appeared frequently and was considered to be relevant, given the sound processing studied. Figure 4 shows the number of subjects in each group that reported using each criterion. Note that there were no limits to the number of criteria a subject could list. Due to the open-ended nature of the questionnaire, results were highly variable and were therefore not statistically analyzed. However, the most salient features of frequency compression that influenced the subjects’ perception of music passages can be seen on Figure 4. As expected, those were mostly related to clearness and distortion factors that are a result of frequency compression.
Discussion The main finding of the present study is that, consistent with our hypothesis, subjects preferred the music passages with lesser amounts of frequency compression. Interestingly, there was more variability in preference when the paired-comparisons involved a mild amount of compression (the ʽdefaultʼ condition) versus no compression, which suggests that as long as frequency compression is not extreme, musical quality may still be acceptable. This was true especially for subjects with hearing loss and no formal music training (arguably the most representative of the general population of hearing aid users), who did not show a significant preference for either condition in that comparison. Also, as expected, musically-trained individuals had a greater preference for conditions with less frequency compression compared to non-trained individuals. This finding supports the notion that decisions relative to feature settings such as frequency compression must
Figure 4. Factors listed by subjects in each group as being relevant to their preference for a music passage in the pairs compared are depicted. Subjects were given an open-ended form where they could list as many items as they wished.
be individualized for both lifestyle and for hearing deficit, insomuch as that information is provided. A stronger preference for less compression was also evident in subjects with normal hearing, as compared to those with hearing impairment. However, it is noteworthy that for the hearing-impaired group, listeners with training and without training preferred the maximum frequency compression in 7% and 16% of the comparisons, respectively (see Figure 2). From the criteria listed by subjects when determining their preference for the processed music passages, subjects with hearing loss and no previous training likely disliked the lesser compression conditions because of their high-pitched quality. It is also possible that preference for more frequency compression in the groups with hearing loss is related to a larger benefit from the provision of high frequency information without the high-pitched sound quality. Alternatively, for some musically trained individuals who had more sloping losses, the compressed stimuli might have been the only ones to provide audible high frequency information. Parsa et al (2013) also reported higher subjective quality ratings for frequency compressed speech and music by subjects with hearing loss as compared with their normal-hearing counterparts. In general the hearing-impaired subjects rated the frequency compression conditions more similarly than did the subjects with normal hearing, suggesting that differences in frequency compression settings may not be as apparent to them. Regarding instrument or passage effects, lesser amounts of frequency compression were preferred for the female singer passages across groups. It is possible that those effects are related to the fact that the female singer passages were the only ones with speech content; that is, musicians and non-musicians alike listen to speech on a regular basis, and thus both groups may have noticed the changes in sound quality in those passages. Finally, due to the open-ended questionnaire used to gather written subjective preference information, it was somewhat difficult to determine groupings in order to analyze the responses. Still, the data allow for examination of the general trends. Besides the expected changes in clearness of sounds and introduction of distortion by frequency compression, it was interesting to note that distortion factors seemed more salient to normal-hearing listeners, and that tinniness was more important for listeners with hearing loss and no music training. One of the limitations of the current study is that the fitting of the hearing aids was not individualized. It was assumed that in setting the hearing aid gain to flat 25 dB and adjusting the overall level for individual participants, the testing conditions would be more similar across all groups. Also, providing constant gain across the frequency range was assumed to preserve the sound quality of music by maintaining balance between the amplitudes of the fundamental frequency and its higher frequency harmonics (Chasin & Russo, 2004). However, as it can be seen from the mean hearing thresholds for the groups with hearing loss, in Figure 1, gain at higher frequencies may not have been optimal for some musically-trained subjects. In addition, except for turning off all special features of the hearing aid and maintaining constant gain across frequency, the hearing aid was not programmed as per the manufacturer’s recommendation for music. It has been suggested that some hearing aid settings can be optimized when listening to music, such as increasing input limiting levels to accommodate the larger dynamic range of music, lowering amplitude compression ratios and increasing compression kneepoints (Chasin & Russo, 2004). Another potential limitation of this study was the lack of time to adjust to the frequency-compressed sounds. However, recent investigations in which subjects underwent more extensive training with frequency compression still did not fully support acclimatization to
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frequency compression (e.g. Simpson et al, 2006; Glista et al, 2009; Wolfe et al, 2011; Perreau et al, 2013). A large cross-sectional study of hearing aid users with frequency compression experience ranging from 1 to 121 months found no correlation between frequency compression benefit (in speech perception) and duration of use (Hopkins et al, 2014). In fact, when looking at acclimatization to frequency compression for individual subjects, the effects have been variable (Glista et al, 2012). The present results can be taken at least as reflecting listeners’ initial perception of frequency-compressed music stimuli. Although initial perception may change over time, it is likely an important factor for new hearing aid users when deciding whether to continue device (or processing) use. One other potential confound is that subjects in the groups with hearing loss were older than those in the normal-hearing groups. As a result, it is possible that age effects may also have played a role in the observed effects of hearing status. For example, working memory, which tends to decline with age, has been shown to influence understanding of speech when distorted by frequency compression and the presence of noise (Arehart et al, 2013). Specifically, they found that listeners with higher working memory had better speech perception in the presence of those distortions than listeners with lower working memory. Thus, although speech perception was not tested in the current study, it might be expected that older adults (especially those with lower cognitive functioning) would prefer the conditions with less frequency compression. However, our results suggest the opposite, that subjects with normal hearing (who were younger) had a stronger preference for less compression than subjects with hearing loss (who were older). Finally, as described in the methods section, the musically-trained groups consisted of individuals with several years of practice with musical instruments and participation in bands, who also took several classes in music theory or appreciation. Most of those participants were graduate or undergraduate students in music, or active/ retired music faculty. Thus, it is possible that our subjects had more music experience than the general population of those who consider themselves musically knowledgeable, which could have made the effects of previous music training more salient.
Conclusion The present findings suggest that mild amounts of frequency compression are not detrimental to perceived sound quality, at least for sounds that are heavily based on harmonic structure, such as music. This was shown to be true even for subjects that are highly experienced musicians.
Acknowledgements The authors are grateful to Yu-Hsiang Wu for his assistance with equipment setup and helpful suggestions for study design. The authors would also like to thank Xuyang Zhang for his assistance with statistical analyses. Portions of this study were presented at the American Auditory Society Scientific and Technical Meeting, Scottsdale, Arizona, March 4–6, 2010, and at the American Academy of Audiology Convention, Chicago, Illinois, April 6–9, 2011. Declaration of interest: The authors report no conflicts of interest.
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