Speech reception with low-frequency speech energy Ruth
D. Rosenthal
Bernard M. Baruch College,City Universityof New York, 17 Lexington Avenue, New York, New York I0010
James K. Lanõ Brooklyn College,City Universityof New York, Bedford Avenue and Avenue H, Brooklyn, New York 11210
Harry Levitt City University of New York Graduate School 33 West 42nd Street, New York, New York 10036 (Received 9 August 1972; revised 11 April 1974)
The effectsof low-frequencyspeechbands on consonantreceptionwere investigated.A recordingof the FairbanksRhyme Test was filtered such that four passbandswere produced;a high band of 1100-2200 Hz and three low bandsof 55-110, 110-220, and 220-440 Hz, respectively.Articulation scoreswere determined for the high band only and for the high band in combination with each of the low bands for both monotic and dichotic listening conditions.Additional data for the low-bandonly conditionswere also obtained.The relative gain of the low band was varied in stepsof 10 dB from 0 to 40 dB. Results indicated that, comparedwith scoresfor the high band only, presentation of any one of the low bandssignificantlyincreasedarticulation scoresand that this increasewas greatestfor the 220-440-Hz band in combinationwith the high band. An analysisof the data in terms of articulatory featuresshowedthe greatestimprovementin the receptionof voicing, a substantialimprovementin the receptionof manner of articulation,and a moderateimprovementin the receptionof place of articulation.Of the various consonantssampledin the rhyme test, the nasalsbenefitedmost from the additional low-frequencyinformation. There was also a substantial improvementin the receptionof plosivesand the lateral, a moderateimprovementin the receptionof voicelessfricatives,and no measureableimprovementin the receptionof the glides.The relevanceof thesefindingsfor the designof speechreceptionaids for the hearing irnpairedis discussed. Subject Classification:70.30, 70.55.
INTRODUCTION
Relatively little attention has been paid to the speech information contained in the low frequencies of the speech signal. The reasons are partly historical. In the early days of telephony, practica] difficulties were encountered in the amplification and transmission of low-frequency stimuli and it was soon discovered that, for practical purposes, speech frequencies below about 300 Hz could be omitted
without
serious
reduction
in
intelligibility. 1 One consequence of these early observations is that the lower cutoff frequency for most speech carrier telephone systems has been set at either 250
Hz or 300 Hz. •' Similarly, numericalproceduresdesigned to predict the effect of frequency filtering or additive noise on speech intelligibility typically neglect
frequencycomponents belowabout200 Hz. a,4 A similar view of the relative unimportance of low frequencies for speech intelligibility in the context of hearing aid design
was providedby the Harvard Report.5 The primary aim of this pioneering study was, to determine appropriate design objectives for the construction of hearing aids. A major recommendation was that the frequency response of a hearing aid have a lower cutoff frequency not higher than 400 Hz or lower than 200 Hz, the preferred value being 300 Hz, and that below 200 Hz the frequency response shouldfall off at a rate of at least 10 dB per octave, if not higher. As a result, the majority of hearing aids now produced do not amplify signals below about 300 Hz.
There has, in recent years, been a re-evaluation of
higher intelligibility ttian the frequency response of conventional training aids. Ling's intelligibility tests, however, relied heavily on the prosodic characteristics of speech, much of the information for which is contained in the low frequencies of the speech signal. There is still some question as to whether the low frequencies are of value for the reception of consonants and other sounds which have the bulk of their acoustic energy in the high frequencies. One aim of this study is to examine the ex-
tent to which cues for consonant reception are contained in the low frequencies. Although these low-frequency cues may be of minor importance for speech reception
by normal-hearing listeners, they are (or could be) of substantial importance to the hearing-impaired listener, given a suitable hearing aid and appropriate auditory training. The large majority of hearing-impaired individuals have some residual hearing in the low-frequency region. The aim of the present experiment was to determine the relative contribution to consonant reception of auditory cues contained in the low frequencies of the speech signal. A split-band technique was used in which the gain in intelligibility was measured when the low-fre-
quencyband (LB) was addedto a high-frequencyband (HB). This approachwas used becauseprevious investigations in which only the low-frequency band was pre-
sentedin isolation7'8or in whichlow frequencieswere amplified with respectto highfrequencies,5 underesti-
this position. Lingohas shownthat amplicationwith
mate the information contained in the low frequencies. When low frequencies alone are presented, there is no
flat frequency response down to about 100 Hz results in
allowance
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J. Acoust.Soc. Am., Vol. 57, No. 4, April 1975
for the mutual
enhancement
of information
Copyright¸ 1975 by the AcousticalSocietyof America
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Rosenthal, Lang, and Levitt: Speechreception with low-frequencyenergy
from both low- and high-frequency bands; when presented together in monotic or dichotic listening there
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(iv) Left-right differences. Two of the four subjects
is the danger of mutual masking between different frequency components in the speech signal. Evidence in support of this view is provided by Huizing and
received the HB to the left ear; the other two received the HB to the right ear. Although the possibility of ear dominance was a factor of secondary interest, it was included in the experimental design since it did not re-
Taselaar,øwho showedthat whena low-frequencyband
quire additional
(LB) of 140-280 Hz, which by itself was unintelligible at any level,
was presented in combination with a high-
frequency speech band (HB) of 1128-2256 Hz, the level necessary to yield a 50% articulation score decreased by 8 dB when the bands were presented monotically, and by 22 dB when the bands were presented dichotically.
Franklin•ø'• similarly foundimprovementsin intelligibility when a low-frequency band (LB) of 240-480 Hz was added to a high-frequency band (HB) of 1020-2040
measurements.
In addition to the above measurements, reference data were also obtained for the HB-only condition and some additional data, with different subjects, for the LB-only condition. The use of different subjects for the LB-only condition was the result of an oversight in planning, the original subjects being unavailable at the time the tests were performed.
The test material consisted of the Fairbanks Rhyme
Test.•s Recordings ofthefive listsweremadebya
Hz, and this improvement was substantially larger when the two bands were presented dichotically. In contrast,
male speaker using a Sony Condensor microphone (mod-
Linden •' andPalva•3foundan improvementin intelligi-
el C-37A) at a distance of 12 in. from the speaker's
bility when a low-frequency band was added to a highfrequency band, but that there was no significant difference between the monotic and dichotic modes of pre-
"the next word is .... "An interval of 5 sec separated each utterance.
sentation.
corder, the test words gave a peak reading between -3
The
relative
levels
used
in these
various
studies differed appreciably, a factor which may have contributed significantly to the diversity of the findings.
For this reason the present study was designed to
cover a broad range of relative levels, for •eachof three separate low-frequency bands, and for both monotic and dichotic listening. As a first step, the study was limited to normal-hearing listeners in order to obtain normative data on the nature and accessibility of the low-frequency cues. A follow up study with hearing14 impaired listeners is now in progress.
lips.
Each word was preceded by the carrier phrase Monitored
on the VU meter
and + 2 VU.
The recordings were made at 7.5 ips on a Magnecorder model 1028 tape recorder, then filtered through a General Radio Multifilter (model 1925)and re-recorded on an Ampex AG-500 tape recorder to produce the test tapes. The upper track of each tape contained the highfrequency band, 1100-2200 Hz. The lower track contained one of the three low-frequency bands, 55-110 I-Iz, 110-220 Hz, or 220-440 I-Iz, respectively. These bands were made up by combining sets of three stan-
dard 1/3-octave-bandfilters.
I. METHOD
A four-factor experimental design was used on each of four normal-hearing subjects. The four factors were:
(i) Low-frequency band. Three low-frequency bands (LB) were considered, 55-110 Hz, 110-220 Hz, and 220-440 Hz, respectively. (ii) Level of low-frequency band. The low-frequency bands were presented at five levels: 0 dB, 10 dB, 20 dB, 30 dB, and 40 dB re normal speech level (NSL), respectively. By "normal speech level" is meant the level in the frequency band of interest prior to any filtering or frequency-selective amplification. It is used as a reference to indicate changes in relative level that may be introduced between portions of the speech spectrum. For .example, X dB NSL for a given frequency band indicates that the speech signal has been
of the re-
Figure 1 showsthe fre-
quency response of the system. The skirts of the filters fell off at the rate of approximately 72 dB per oc-
tave. As a result of combiningthe 1/3-octave-band filters, slight ripples on the order of + 1 dB occurred in the passband. During the experiment,
the test tapes were played
back on a Magnecorder tape reproducer (model 1028). Channel I carried the high-frequency speech band, Channel II the low-frequency band. Channel II was played through a I