Chinese speech intelligibility and its relationship with the speech transmission index for children in elementary school classrooms Jianxin Peng, Nanjie Yan, and Dan WangLMW

Citation: The Journal of the Acoustical Society of America 137, 85 (2015); doi: 10.1121/1.4904519 View online: http://dx.doi.org/10.1121/1.4904519 View Table of Contents: http://asa.scitation.org/toc/jas/137/1 Published by the Acoustical Society of America

Chinese speech intelligibility and its relationship with the speech transmission index for children in elementary school classrooms Jianxin Peng,a) Nanjie Yan, and Dan Wang School of Physics and Optoelectronics, South China University of Technology, Guangzhou, 510640, People’s Republic of China

(Received 31 December 2013; revised 28 June 2014; accepted 12 November 2014) The present study investigated Chinese speech intelligibility in 28 classrooms from nine different elementary schools in Guangzhou, China. The subjective Chinese speech intelligibility in the classrooms was evaluated with children in grades 2, 4, and 6 (7 to 12 years old). Acoustical measurements were also performed in these classrooms. Subjective Chinese speech intelligibility scores and objective speech intelligibility parameters, such as speech transmission index (STI), were obtained at each listening position for all tests. The relationship between subjective Chinese speech intelligibility scores and STI was revealed and analyzed. The effects of age on Chinese speech intelligibility scores were compared. Results indicate high correlations between subjective Chinese speech intelligibility scores and STI for grades 2, 4, and 6 children. Chinese speech intelligibility scores increase with increase of age under the same STI condition. The differences in scores among different age groups decrease as STI increases. To achieve 95% Chinese speech intelligibility scores, the STIs required for grades 2, 4, and 6 children are 0.75, 0.69, and 0.63, respectively. C 2015 Acoustical Society of America. [http://dx.doi.org/10.1121/1.4904519] V [LMW]

Pages: 85–93

I. INTRODUCTION

The acoustical environments in elementary school classrooms have an important role in developing the ability of learning and cognition of children. Evidence shows that poor room acoustics, such as excessive noise and reverberation, reduces the speech intelligibility in a classroom, interrupt the verbal communication between teachers and children, and has a negative influence on the memory of children.1 Moreover, the ability to recognize speech under the same room acoustics condition has a large difference between children and adults. A number of previous experiments have indicated that younger children have greater difficulty in recognizing speech in conditions with reverberation and noise compared with adults.2,3 Younger children require higher signal-to-noise ratios (SNRs) than that of adults to achieve the same speech intelligibility scores. Younger children require less noisy acoustical conditions.4 All criteria for classroom acoustical conditions should be based upon speech intelligibility. The room acoustical factors that affect speech intelligibility include background noise level and reverberation time (RT or T30).5 However, Bradley pointed out that SNR is the more critical factor.6 Some objective acoustical measures, such as useful-to-detrimental sound ratio and speech transmission index (STI) combine room acoustics and a SNR component into a single objective measure to evaluate speech intelligibility in classrooms.7–9 The STI was developed by Houtgast and Steeneken,10 which is based on the assumption that the degradation of speech intelligibility in rooms is related to the a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]

J. Acoust. Soc. Am. 137 (1), January 2015

reductions in the amplitude modulations of speech signals by both room acoustics and ambient noise. The STI method has been shown to be successful in evaluating of Chinese speech intelligibility in rooms.11,12 Many published studies have investigated the speech intelligibility in elementary school classrooms.6,13–16 Houtgast13 conducted a series of speech intelligibility tests in classrooms for teachers and children aged 8 to 15 years old under a variety of road traffic noise conditions with RT from 0.7 s to 1.5 s. The relationship between A-weighted signal-to-noise ratio [S/N(A)] and speech intelligibility scores for Dutch was reported. Bradley6 investigated speech intelligibility using the English Fairbank rhyme test in occupied classrooms with RT from 0.39 s to 1.20 s for children aged 12 to 13 years old through a small loudspeaker with its directivity similar to human’s mouth. The relationships between English speech intelligibility scores and S/N(A), useful-to-detrimental sound ratio were found, respectively. Bradley and Sato14,15 likewise conducted a series of speech intelligibility tests using the Word Identification by Picture Identification test for children in grades 1, 3, and 6 in real elementary school classrooms. The relationship between S/ N(A) and English speech intelligibility scores was also revealed. They found that 15 dB SNR is inadequate to obtain better speech intelligibility for the children in elementary schools. Moreover, they reported that 6-yr-old children required a 7 dBA higher SNR to achieve the same speech intelligibility scores as 11-yr-old children.14 Astolfi et al.16 explored Italian speech intelligibility using diagnostic rhyme test (DRT) in four elementary classrooms with different reverberation time for children aged from 7 to 10 years old. The relationship between STI and Italian speech intelligibility scores was also built. The results showed that children in

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grade 2 understood fewer words than children in the higher grades when the STI is lower. To achieve 95% Italian speech intelligibility scores, the required STI for grade 2 is 0.82, and for grades 3–5 is 0.77. Prodi et al.17 investigated the noise and age influence on the perception of speech based on a series of Italian speech intelligibility tests in seven elementary classrooms. The relationship between STI and Italian speech intelligibility scores was also obtained by linear regression for different types of noise. However, the correlation coefficient was not satisfactory. The studies mentioned above were based on Western language. The relationship between objective acoustical indices [such as S/N(A), STI] and speech intelligibility scores may not suitable for children in China because Chinese is a tonal language that is different from Western languages. Speech intelligibility in an enclosure is satisfactory for English, but it is not necessarily satisfactory for Chinese, or vice versa.18 Peng and co-workers11,12,19 investigated the effect of RT and SNR on Chinese speech intelligibility and the relationship between Chinese speech intelligibility scores and STI through auralization techniques for adults. The results indicated a high correlation between Chinese speech intelligibility scores and STI in rooms. However, the relationship between Chinese speech intelligibility scores and STI in elementary school classrooms through in situ measurement for young listeners has not been reported so far. In the current study, Chinese speech intelligibility in the 28 existing elementary school classrooms was assessed by children in grades 2, 4, and 6 (7 to 12 years old). The Chinese speech intelligibility test signals recorded in anechoic chamber are reproduced through a loudspeaker with its directivity similar to human’s mouth. Chinese speech intelligibility scores were obtained at each listening position under different SNR conditions in these classrooms. The aim is to compare the Chinese speech intelligibility scores among children with different ages and investigate the relationship between subjective Chinese speech intelligibility scores and STI for children in elementary school classrooms. II. EXPERIMENT METHOD A. Classrooms

In this study, 27 classrooms with no acoustical treatment and 1 classroom with an acoustical treatment from nine schools in Guangzhou were investigated. Twenty-five of the classrooms were conventional rectangular-shaped classrooms, and another three were hexagon-shaped classrooms. For classrooms without an acoustical treatment, classrooms with grades 2, 4, and 6 were selected in each school. The walls and ceilings were plastered in these classrooms. The sidewalls contained two to three glass windows, and the floors were covered with ceramic tiles. All classrooms had wooden desks and chairs. For the classroom with the acoustical treatment, the ceiling was covered with mineral-fiber acoustic tiles, and other surfaces were similar to those classrooms with no acoustical treatment. The dimensions of all the classrooms are shown in Table I. The volume range of all classrooms was from 159.3 m3 to 273.3 m3. 86

J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015

TABLE I. The dimensions of all the classrooms. School Grade A

B

C

D

E

F

G

H

I

a

2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 2, 4, 6a 2 4 6 2 4 6

Shaped Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Rectangular Hexagon Hexagon Hexagon

Length  Width/m2 Height/m Volume/m3 8.40  6.75 8.60  6.15 8.60  6.15 8.27  6.75 8.27  6.75 8.27  6.75 8.15  5.75 8.15  7.05 8.15  7.05 8.72  7.32 8.72  7.32 8.72  7.32 9.30  6.90 9.30  6.90 9.30  6.90 8.82  6.60 8.72  6.92 8.72  6.92 9.35  7.90 9.35  7.90 9.35  7.90 9.35  7.90 8.75  6.25 8.75  6.25 8.75  6.25 4.90 4.90 4.90

3.15 3.25 3.25 3.45 3.77 3.77 3.40 3.40 3.40 3.50 3.50 3.50 2.90 3.45 3.45 3.40 3.40 4.05 3.70 3.70 3.70 3.15 3.70 3.70 3.70 3.40 3.40 3.40

178.6 171.9 171.9 192.6 210.5 210.5 159.3 195.4 195.4 223.4 223.4 223.4 186.1 221.4 221.4 197.9 205.2 244.4 273.3 273.3 273.3 232.7 202.3 202.3 202.3 212.1 212.1 212.1

Classroom with special acoustical treatment.

For each test, four listening position were arranged in each classroom. Figure 1 shows the schematic drawing of the listening positions in the classroom. In Fig. 1, other desks and chairs were not shown in the classroom. Chinese speech intelligibility tests were carried out with the grades 2, 4, and 6 children. The children were seated at four listening positions evenly distributed in the tested area of the classrooms where they studied, except for in the classroom with the acoustical treatment. In the acoustical treatment classroom, the grades 2, 4, and 6 children in the school participated in the test in groups separately. B. Speech intelligibility tests

Chinese speech intelligibility scores are strongly dependent on the type of speech intelligibility test word lists.20 In the current study, Chinese speech intelligibility scores were obtained using Chinese rhyme test word lists as specified by GB 4959–8521 and SJ2467–8422 in China. The test is similar to the modified rhyme test of English23 and ten word lists which contain 25 five-word rows of similar-sounding Chinese monosyllabic words were used. Five words in each row which were randomly arranged and differed only in the initial consonant sound (hao, sao, gao, zao, kao). The test words in the carrier phrase are “The x row reads y,” where x and y are replaced by the number of row and the pronunciation of the corresponding word. All word lists were recorded at the rate of 4.0 words per second and 65 dB sound pressure Peng et al.: Chinese speech intelligibility for children

FIG. 1. Schematic drawing of the listening positions in each classroom of different shaped classroom. Four listening positions (L1–L4) were arranged in side the classroom.

level (SPL) using an random-field microphone at a distance of 0.5 m from the speaker in an anechoic chamber. The recorded signals were edited by Cooledit Pro software and the mute signal was added between the stimuli of carrier sentence so that subjects could select the object word that they heard. Subjects had to identify which of the five words was presented to them. The Chinese speech intelligibility test signals recorded in an anechoic chamber were reproduced by a JBLLSR6325P loudspeaker with its directivity similar to human’s mouth. The loudspeaker was located at the center of the platform where the teacher would frequently stand and oriented toward the seating area (Fig. 1). It was set 1.5 m above the floor and 0.5 m from the blackboard on the front wall. The speech level at 1 m directly in front of the loudspeaker was set at 65 dBA by adjusting the volume of the loudspeaker when the subjects seated around the listening positions. Since the time-averaged frequency spectrum is different for male and female speakers, two speech-shaped noises were selected for the tests based on the average speech spectrum of the male and female speakers. The SNR was equal for all selected frequency bands because of speech-shaped noise with a frequency spectrum equivalent to the time-averaged speech spectrum of the test signal without pause. During the test, speech-shaped noises were likewise reproduced by the same loudspeaker with speech signals at a given SNR. The reproduction modes of test signals included five different SNRs (i.e., 5 dBA, 0 dBA, 5 dBA, 10 dBA, 15 dBA) and no speech-shaped noise case. Chinese speech intelligibility tests were carried out at four listening positions in each classroom with three to five different reproduction modes of the test signals. Therefore, there was no word list repeated for each grade of children in each classroom. C. Subjects

A total of 480 children in grades 2, 4, and 6 participated in the tests in 28 classrooms. The number of children who participated in the tests in each grade is 160. The gender of all children is not taken into account, but the difference in the number of boys and girls was nearly negligible. The J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015

grade 2 children were 7 to 8 years old; the grade 4 children were 9 to 10 years old; and the grade 6 children were 11 to 12 years old. All children were native Mandarin speakers, and no medical reports of their hearing were reported from them and their parents. They represented the typical general listening audiences. As shown in Fig. 1, four listening positions were arranged in each classroom, and four subjects were seated around each listening position. Therefore, a total of 16 subjects participated in the test in each classroom. For each testing condition, two test word lists (one with a male speaker, the other a female speaker) were used. All subjects received a few minutes of instruction prior to the test and were told that they should not communicate with each other while completing the word tests. The subjects were asked to mark the words they heard. The four subjects’ Chinese intelligibility scores at each listening position across all eight lists (4 children  2 talkers ¼ 8 lists) were calibrated according ISO/TR 4870,24 and the averaged speech intelligibility score was obtained for each test condition. D. Acoustical measurements

The room impulse responses were measured by using a sine sweep signal generated from Cool Edit Pro with Aurora acoustical plug-in25 at the four listening positions with subjects in classroom after subjective Chinese speech intelligibility test. The sine sweep signal was radiated into the classroom from the same loudspeaker and source location as the Chinese speech intelligibility tests. The room’s acoustical parameters, such as early decay time (EDT), T30, earlyto-late sound energy ratio (C50), and STI without noise, were calculated from the room impulse response using Dirac 4.0. Table II shows the descriptive statistics of acoustical parameters in 28 classrooms. EDT(500–1000 Hz) is the average EDT value from the 500 Hz to 1000 Hz octave band, and similar subscripts apply for other parameters. At the meantime, the background noise level was measured by B&K 2250 sound analyzer for each listening position. Table III shows the background noise level of four listening positions in 28 classrooms. Because of the effect of background noise in the classrooms during the test, the SNR at each listening position for Peng et al.: Chinese speech intelligibility for children

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TABLE II. Descriptive statistics of acoustical parameters in 28 classrooms. Grade 2

4

6

Parameter

Mean

SD

Min

Max

EDT(5001000 Hz) /s T30(5001000 Hz) /s C50(5004000 Hz) /dB STI /EDT(5001000 Hz) /s T30(5001000 Hz) /s C50(5004000 Hz) /dB STI /EDT(5001000 Hz) /s T30(5001000 Hz) /s C50(5004000 Hz) /dB STI /-

0.85 0.89 2.59 0.66 0.92 0.93 2.54 0.66 0.97 0.99 1.89 0.64

0.29 0.26 2.26 0.06 0.29 0.26 2.66 0.07 0.31 0.27 2.37 0.07

0.37 0.45 0.58 0.57 0.36 0.43 0.80 0.56 0.37 0.45 1.79 0.53

1.44 1.34 8.55 0.79 1.44 1.34 10.37 0.86 1.44 1.37 8.55 0.79

each test condition was altered and was not equal to the SNR at the reproduction of the test signals. To determine speech and noise levels and to calculate the STI values combined with room impulse response for each test condition, speech and noise levels were also recorded at four listening positions during the speech intelligibility test in each classroom. To avoid the influence of the background noise level, the octave band speech levels without pauses were calibrated by using speech-shape noise with a sound pressure level of 80 TABLE III. The BNL in four listening positions in 28 classrooms under occupied condition. BNL /dBA School

Grade

L1

L2

L3

L4

A

2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 2 4 6 2, 4, 6a 2 4 6 2 4 6

47.9 48.5 49.0 52.2 51.7 52.3 51.1 43.8 50.9 46.9 46.9 46.9 52.0 49.4 46.9 51.6 43.4 45.5 44.4 44.8 45.8 44.6 43.5 42.2 44.0 48.8 50.1 51.3

47.8 48.8 49.4 50.9 51.8 50.6 51.1 43.6 47.8 47.6 47.6 47.6 52.2 49.1 44.6 50.9 41.2 46.3 44.9 43.1 44.0 44.3 42.9 43.1 43.6 47.1 48.5 51.7

47.8 49.4 51.0 51.1 50.6 50.6 51.7 43.4 47.3 48.2 46.6 46.6 52.3 48.3 46.2 49.4 42.3 45.0 45.0 44.4 45.7 44.8 42.7 42.3 43.0 47.7 47.7 51.0

48.5 49.4 51.1 51.7 52.3 50.4 50.7 44.9 49.4 47.8 45.7 45.7 52.5 48.8 47.9 51.8 44.1 45.7 45.2 45.4 45.5 45.9 42.0 42.7 42.1 48.1 47.9 50.7

B

C

D

E

F

G

H

I

a

Classroom with special acoustical treatment.

88

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dBA at 1 m directly in front of the loudspeaker and measured by B&K 2250 sound analyzer. Test signal will be adjusted to 65 dBA at 1 m. The octave-band noise (combined with speech-shaped noise and ambient noise in classroom) levels were determined from the recording during the tests. The modulation transfer function (MTF) and STI can be calculated from the room impulse responses, speech and noise level for each test condition by using Aurora acoustical plug-in24 according to the standard IEC 60268–16.26

III. RESULTS A. Regression model

A third-order polynomial equation was selected to simply represent the nonlinear variation of speech intelligibility scores as a function of objective acoustical measures such as S/N(A) and useful-to-detrimental sound ratio U80 by Bradley.8,15 However, Fletcher27 for the Articulation Index and later Lochner and Burger28 revealed that intelligibility variations were described by an “S” form. Recently, Astolfi et al.16 indicated that a logarithmic model was the best fitting model in relation to the polynomials form. In the present study, three different regression models for the relationship between the Chinese speech intelligibility scores and STI were compared. The model functions of the third-order polynomial, logarithmic, and “S” form are given in Eqs. (1), (2), and (3), SI ¼ d þ cSTI  bSTI2 þ aSTI3 ;

(1)

SI ¼ a  blnðSTI þ cÞ;

(2)

SI ¼ 100ð1  10STI=a Þb ;

(3)

where a, b, c, and d are the regression parameters. Figure 2 shows the regression results of different models for the relationship between Chinese speech intelligibility scores and STI for the grade 4 children. The regression parameters of the three different regression models are shown in Table IV.

FIG. 2. (Color online) Regression results of different models for the relationship between Chinese speech intelligibility scores and STI for grade 4 children. Peng et al.: Chinese speech intelligibility for children

TABLE IV. Regression parameters of the three different regression models for grade 4 children.

Third-order polynomial Logarithmic “S” form

a

b

c

d

R

SD

R2

369.0

679.0

491.3

36.6

0.936

5.14

0.876

119.2 0.402

38.8 2.637

0.08

0.936 0.933

5.13 5.20

0.876 0.871

Table IV shows that the three regression models for Chinese speech intelligibility versus STI almost have almost the same standard deviations (SDs) and coefficient of determination R2. The R2 indicates that the variation in the dependent variable can be explained and accounted for by the independent variables in this regression analysis. However, the adjusted R2 considers the existence of the additional independent variables of regression and adjusts this R2 value to a more accurate view of the explanatory power of regression.29 The increase in R2 caused by regression parameters has nothing to do with the fitting quality. The adjusted R2 can be defined as29 2 R ¼ 1 

n1 ð 1  R2 Þ; nk1

(4)

where n is the number of independent variables in observed data and k is the number of regression parameters of the regression equation. The adjusted R2 has eliminated the effects of the number of regression parameters, and the fitting quality increases with the increase in adjusted R2. Aside from the adjusted R2, two other criteria Schwarz Criterion (SC)30 and Akaike Information Criterion (AIC)31 were used to compare the goodness-of-fit of regression equation with different regression parameter numbers. The equations are given by the follows: "X # e2i kþ1 þ lnðnÞ; (5) SC ¼ ln n n "X # e2i 2ðk þ 1Þ þ ; (6) AIC ¼ ln n n

logarithmic and “S” form have smaller values of SC and the AIC of the logarithmic form have the smallest values. However, the fitness indices for the three models are all fall within the goodness-of-fit range and only vary in less than 2%. All of the three models for the relationship between STI and Chinese speech intelligibility scores are capable to fit the test data in the present study. In contrast, Fig. 2 shows the slight differences in the fitting curve of Chinese speech intelligibility score versus STI from three different models. The speech intelligibility scores from the third-order polynomial fitting curve are almost the same as the speech intelligibility scores from the logarithmic fitting curve. The speech intelligibility scores from the thirdorder polynomial and logarithmic fitting curves are slightly higher than the speech intelligibility scores from the “S” form fitting curve when STI is greater than 0.53. However, the speech intelligibility scores from the third-order polynomial fitting curve may not monotonically increase with an increasing STI. The speech intelligibility scores from the third-order polynomial and logarithmic fitting curve may be more than 100% when the STI is greater than a certain value. These two cases would not occur when the speech intelligibility scores were obtained from the “S” form fitting. From the above-mentioned analysis, the “S” form model is used for the relationship between Chinese speech intelligibility scores and STI in this study. B. Relationship between the Chinese speech intelligibility scores and STI

The Chinese speech intelligibility scores obtained from grades 2, 4, and 6 are plotted against the corresponding STI from different listening positions and test conditions in Fig. 3. The lines shown on the figure are the results of an “S” form fitting model to the data. The regression parameters, correlation coefficients and standard deviations are shown in Table VI. The results indicated high correlations between Chinese speech intelligibility scores and STI for grades 2, 4, and 6 children. The STI can explain 80.0%, 87.1%, and 83.2% of the variance of Chinese speech intelligibility scores in real elementary classrooms for grades 2, 4, and 6 children, respectively.

where ei is the difference value between the observed and regression values. The goodness-of-fit of regression equation 2 increases with the decrease in SC and AIC values. The R , SC, and AIC of different kinds of regression models between Chinese speech intelligibility scores and STI for grade 4 children are presented in Table V. Table V shows that the 2 2 R , SC and AIC of three models are different. The R of the logarithmic form have the largest value. Both the 2 TABLE V. The R , SC, and AIC of different kinds of regression models between speech intelligibility and STI for grade 4 children.

2 R SC AIC

Third-order polynomial

Logarithmic

“S” form

0.874 3.41 3.31

0.876 3.38 3.30

0.870 3.38 3.32

J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015

FIG. 3. (Color online) Relationships between Chinese speech intelligibility scores and STI for grade 2, 4, and 6 children and best-fit “S” form. Peng et al.: Chinese speech intelligibility for children

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TABLE VI. The regression parameters, correlation coefficients and standard deviations of “S” fitting model.

Grade 2 Grade 4 Grade 6

a

b

R

SD

R2

0.425 0.402 0.385

2.916 2.637 2.171

0.894 0.933 0.912

6.6 5.20 4.67

0.800 0.871 0.832

IV. DISCUSSION A. Comparison with different age groups

To compare the difference in speech intelligibility scores under the same STI condition for different age groups, Fig. 4 shows the best-fit curves between Chinese speech intelligibility scores and STI for different age groups. Peng11 used the auralization technique and employed the same test word lists as the present study to obtain the speech intelligibility scores. Peng investigated the relationship between Chinese speech intelligibility scores and STI for adults (without using the “S” form-fitting curve). This relationship is plotted in Fig. 4 using the “S” form-fitting curve. The Chinese speech intelligibility scores are increasing with the increase of STI for both children and adults. The Chinese speech intelligibility scores increase as the age increases under the same STI condition. However, the differences in Chinese speech intelligibility scores among different age groups decrease as the STI increases. This finding indicates that children have more difficulty in understanding speech in poor classroom acoustical conditions compared with adults. The listening ability of the four age groups can be compared by considering the required STI value for 95% Chinese speech intelligibility scores indicated by best-fit curves. To achieve 95% Chinese speech intelligibility scores, the STIs required for grades 2, 4, and 6 children, and adults are 0.75, 0.69, 0.63, and 0.56, respectively. In this study, the test word differs only in the initial consonant in each word list. In a reverberation-plus-noise listening condition, the phoneme identification abilities of children may not mature until the late teenage years.32 The ability to identify the manner of articulation for initial consonants requires listeners to be sensitive to the rapid spectrum

FIG. 4. (Color online) Comparison of the best-fit curves for different age groups. 90

J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015

changes over time intervals of 10 to 30 ms.33 Under a low STI condition, the long reverberation and the high noise level mask those rapid spectrum changes; the task becomes especially difficult to understand for younger children. Masking by noise and reverberation reduces the amount of acoustical information available to listeners. The younger children may require more acoustic signal information than that of elder children or adults for the correct identification of the test words. Thus, elder children might be more adept at identifying the test words from partial or limited acoustic signal information than younger children.32,34 In contrast, children are less able to focus attention on task-relevant information and resist interference from irrelevant sounds than adults.3,35–37 Evidence shows the poorer selective attention in children with respect to the auditory domain. Evidence also demonstrates that children have a higher susceptibility to informational masking in auditory signal detection tasks38,39 and experience more intrusions from the distractor message in binaural listening tasks.40 Children are likewise less flexible in using perceptual strategies for speech identification, causing difficulty in taking advantage of the available auditory cues in unfavorable listening conditions.41 All of these factors may cause a difference in Chinese speech intelligibility scores between younger children and elder children (or adults) under the same STI condition. B. Comparison between different studies

Speech intelligibility scores for children were examined as a function of S/N(A) by Houtgast, Bradley, and Sato in elementary classrooms.6,13–15 The difference in the relationship between these two indices and speech intelligibility scores cannot be directly compared because S/N(A) and STI are two different objective acoustical indices used to evaluate speech intelligibility in a room. However, STI is regarded as the key independent variable in the present study in accordance with the study of Astolfi et al.16 Figure 5 shows the best-fit curve between Chinese speech intelligibility scores and STI for grades 2 and 4 in the present study. The results (i.e., curve between STI and Chinese speech intelligibility scores) obtained by Astolfi et al. are also plotted in Fig. 5. Moreover, Fig. 5 shows the results (i.e., curve between STI and Italian speech intelligibility scores) obtained by Astolfi et al. for grades 3, 4, and 5 as a group. The curve between STI and Italian speech intelligibility scores from the group (in the work of Astolfi et al.) is compared with the results from grade 4 in the present study. Astolfi et al. used a diagnostic rhyme test (DRT) to obtain the Italian speech intelligibility scores and different types of noise were added to the test signals to create different listening conditions at seven different listening positions in existing classrooms with RT from 0.37 to 1.54 s. The subjects were selected from children aged from 7 to 10 years old from grades 2, 3, 4, and 5 from Italy. The best-fit curve between STI and Italian speech intelligibility scores is described by a logarithmic curve.16 A comparison of the speech intelligibility curves for children with the curves from the study by Astolfi et al. has revealed some similarities, but also some differences including sample size, ages of the children, languages, test Peng et al.: Chinese speech intelligibility for children

FIG. 5. (Color online) Comparison of the regression curve between speech intelligibility scores and STI with Chinese and Italian for children.

methods, and word lists. It can be seen from Fig. 5 that all four curves indicate speech intelligibility scores increasing with the increase of STI value. When the STI values are greater than 0.53, there is a broadly consistent trend in speech intelligibility scores between Chinese and Italian for grade 2. However, when the STI values are lower than 0.53, Chinese speech intelligibility scores are lower than the Italian speech intelligibility scores. Similar results can be obtained for grade 4, and the point of convergence is 0.56. The similarities and differences in speech intelligibility scores between Chinese and Italian is related to subjects, languages, types of noise source, test methods, fitting methods, word lists, and test conditions. Figure 6 shows that the relationship between STI and SNR(A) at listening positions under all test conditions for grade 2 and 4 children in the present study. It can be seen from Fig. 6 that the STI values decreases with the decrease of SNR(A). A lower STI value means a case with a lower SNR(A) and/or a longer reverberation time at the listening position in the classroom. When SNR(A) is lower, the dynamic range of SPL in western languages is considerably

greater than that in Chinese.18 This may be an explanation of the fact that some Italian words may be understood by picking up only the high peaks under lower SNR(A) condition. Moreover, the interfered noise in this study is speech-shaped noise combined with ambient noise during the test, but in the study of Astolfi et al., the traffic, babble, fan-coil, and impact noise were used.16 The speech-shaped noise has more masking effect on speech intelligibility test signals than traffic, babble, fan-coil, and impact noise because speech-shaped noise has a similar frequency spectrum as speech intelligibility test signals. Furthermore, the speech test signals and speechshaped noise were from the same loudspeaker in this study. However, the speech test signals were from the source located at the platform in the classroom and noises source were located at the center of classroom or outside the classroom in the study by Astolfi et al.16 Spatial separation of target speech and interfered noise may contribute to the improvement in the speech intelligibility because spatial release from masking resulted in the improvement of SNR.42 In addition, the reverberation times measured from 28 classrooms (Table II) were almost higher than that of Italian classrooms in the study of Astolfi et al. Room reverberation blurs temporal and spectral cues and flattens formant transitions.43 Noise masks the weak consonants to a greater degree than the higher intensity vowels. The combined effects of reverberation and noise under the lower SNR(A) conditions are more detrimental to speech intelligibility.44 All these factors may contribute to the lower speech intelligibility scores obtained in the Chinese classroom and resulted in that the Chinese speech intelligibility scores are lower than Italian speech intelligibility scores for children when the STI values were lower. Chinese is a tonal language that is different from or nontonal languages in Western countries. The speech material and linguistic level of subjects are important factors. In the present study, Chinese rhyme test word lists and the native language listeners were used. Five words in each row were randomly arranged and differed only in the initial consonant sound in each word list. The subjects were asked to mark the word they thought was correct that they heard at a time. However, Astolfi et al.16 used DRT, but there are not all children being native Italian listener. The subjects heard one word at a time and marked which of the two words they thought was correct. Moreover, Astolfi et al.16 used a logarithmic function to fit the relationship between STI and speech intelligibility scores. In present study, the “S” form curve was used. In Fig. 2, it can be seen that the fitting value of the “S” form curve is lower than that of the logarithmic form with a high STI value. All these factors likewise influence the speech intelligibility scores of children for these two languages in classrooms. On the basis of the comparison of Chinese and Italian speech intelligibility for children, it seems necessary to further investigate the effect related to speech material (test word lists), linguistic level of subjects, the fitting methods as well as test conditions. V. CONCLUSION

FIG. 6. (Color online) Relationship between S/N(A) and STI for grade 2 and 4. J. Acoust. Soc. Am., Vol. 137, No. 1, January 2015

This study investigated the Chinese speech intelligibility of elementary school children from grades 2, 4, and 6 in real Peng et al.: Chinese speech intelligibility for children

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classrooms with noise and reverberation. The Chinese speech intelligibility tests were performed in 28 active elementary classrooms in nine different elementary schools in Guangzhou evenly divided among grades 2, 4, and 6 children (7 to 12 years old) and detailed acoustical measurements were also performed. Subjective Chinese speech intelligibility scores were obtained from rhyme tests, and the STIs were calculated under each listening position and test condition. Different fitting models of between subjective Chinese speech intelligibility scores and STI were explored. The effects of age on the speech intelligibility scores and the findings from different studies are also discussed. Among the three fitting models, the “S” model is the suitable model between Chinese speech intelligibility scores and STI for children in the study. High correlations are observed between subjective Chinese speech intelligibility scores and STI for grades 2, 4, and 6 children. Higher STI scores result in higher Chinese speech intelligibility scores for different age groups. The speech intelligibility scores increase as age increases under the same STI condition. To achieve 95% Chinese speech intelligibility scores, the STIs required for grades 2, 4, and 6 children and adults are 0.75, 0.69, 0.63, and 0.56, respectively. The differences in Chinese speech intelligibility scores among different age groups decrease as STI increases. The Chinese speech intelligibility scores are lower than the Italian speech intelligibility scores based on research done by Astolfi et al.16 when there is small STI value. This result is related to noise type, and test method and conditions. ACKNOWLEDGMENTS

The authors thank the students who participated in subjective evaluation of Chinese speech intelligibility. Special thanks are given to Dr. Siu-Kit Lau for helpful discussions and for his revisions of the manuscript. This work is supported by National Natural Science Foundation of China (Grant No. 10774048, 11374106), Science and Technology Planning Project of Guangdong Province, China (Grant No. 2011B061300066). 1

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