Effect of ambient noise on the vocal output and the preferred listening level of conversational speech

EFFECT OF AMBIENT NOISE ON THE VOCAL OUTPUT AND THE PREFERRED LISTENING LEVEL OF CONVERSATI ONAL S PEECH

E. VAN HEUSDEN,R. PLoMr' and L. C. W. Poes

lnstitute Jor Perception TNO, Soesterberg (The Netherlands)

SUMMAR Y

The effect of ambient noise on vocal output and the preferred listening level of conversational speech was investigated under conditions typical of everyday speech communication. For a speaker-listener distance of i m, vocal output and the preferred listening level in quiet were both about 50 dB( A ). Deviations from this value were observed when the noise level exceeded a level of about 40 dB( A ). The regression lines for the data points above this level showed a 3 dB rise for a lO dB rise in noise level. The experiments further suggest that both speaker and listener (when the latter is able to control the playback level of recorded speech) try to compensate for the noise interference by raising the level of speech in order to keep the (subjective) loudness of speech in noise equal to the loudness of speech in quiet.

1.

INTRODUCTION

It is a well known fact that people tend to raise their voices when ambient noise is introduced. The minimum noise level required to obtain a speech level significantly higher than the speech level in quiet may be considered to be the minimum level at which ambient noise interferes with speech production. This noise level is an important measure: beyond this level subjects put more effort into speaking than is required at lower levels or in quiet. It seems reasonable to interpret this level as representing the minimum level at which ambient noise begins to interfere with relaxed speech communication. It should be realised that absence of interference as a preferable condition does not imply that absence of any ambient sound would be ideal: that is not the case. Lane and Tranel I published an extensive survey of the literature on the effect of ambient noise on speech level, sometimes referred to as the Lombard sign after the 31 Applied Acoustics (12) (1979)--© Applied Science Publishers Ltd, England, 1979 Printed in Great Britain

32

E. VAN HEUSDEN, R. PLOMP, L. C. W. POLS

first person who studied it (e.g. Lombard2). At ambient noise levels above 50 dB(A) a 10 dB increase of noise level appears to result in an increase of approximately 4 dB for conversational speech. The data available do not make it possible to decide at what noise level people begin to raise their voices. Hence the present authors conducted further experiments in which the speech level in quiet was also investigated. These experiments are reported in Section 2. The effect of ambient noise on speech communication is, however, not limited to the speaker. It is also of interest to know how the speech level preferred by the listener depends upon the noise level. If the actual speech level is significantly lower than the preferred level, this may represent a strain for the listener, also to be interpreted as an annoying effect of the noise on speech communication. This question will be considered in Section 3. This approach may be interpreted as an attempt to translate non-specific noise annoyance into specific measures such as vocal output and preferred listening level.

2.

EFFECT OF NOISE ON VOCAL OUTPUT

Experiment 1 The main experiment was carried out in a soundproof anechoic room. The experimenter and the test subject were seated at a distance of 1 m, facing each other. In order to have the experimental conditions as close as possible to everyday speech communication, the experimenter and the test subject conversed on various topics (job, hobbies, etc.). As we wanted to obtain a noise-free recording of conversational speech, the noise was presented to the subject by means of headphones. The headphones used (Sennheiser H D 414) were selected because they are provided with an acoustically porous layer (up to 3 kHz) between the membrane and the subject's ear. This means that the subjects heard their own voices and the voice of the experimenter without attenuation. The level of the noise produced by the headphones was calibrated by matching its loudness with the loudness of a diffuse sound field. The spectrum of the noise was equal to the long-term average speech spectrum (see Fig. 1). This noise was chosen because a single competing talker or the voice babble of many talkers may be considered to be the most frequent interfering sound. It appears that average traffic noise has about the same sound spectrum. The conversational speech of the test subject was picked up by a microphone attached to the headphone set and recorded on tape. Because of the many 'silent' intervals in conversational speech, a special method was used for computing the root-mean-square (rms) value of the speech signal. This method, adopted from Brady, 3 estimates the speech level by exclusively using speech wave samples above a certain threshold value. The basis of this threshold-independent speech-level measure is the nearly uniform distribution, between some arbitrary threshold and a

33

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peak, of the logarithm of the momentary values of the speech waveform (Davenport4). Brady's equivalent peak level (epl) is defined as that peak level of a log-uniformly distributed random variable that would have given the same rms value above threshold as was measured for the speech sample. This method enables us to express the level as a single number which is neither influenced by silent intervals (which frequently occur in conversation) nor by background noise up to a certain level. The log-uniform distribution of the speech waveform has been verified for spoken Dutch. To find the conversion factor between the epl and the more often used long-term A-weighted rms speech level, both these values were computed for texts read by twenty male subjects. The long-term dB(A) value appeared to be 14dB below the computed equivalent peak level. Ten normal-hearing male subjects aged between 21 and 29 were used as speakers. Five of them were exposed to the noise in the following order: no noise, 35, 45, 55, 65 dB(A), no noise. The other five subjects were presented with the noise in the reverse order. Each condition lasted about 4min and the interval between two conditions was about 1 min. The speech level was computed for speech excerpts of 0.5min in order to obtain some insight into the intra-individual variation of conversational speech. This variation appeared to be comparable to the interindividual variation. As the final speech level for each subject we adopted the median

34

E. VAN HEUSDEN, R. PLOMP, L. C. W. POLS

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Fig. 2. Vocal output converted to the level at a distance of l m in front of the subject as a function of ambient noise level measured in a soundproof anechoic room during conversation between test subject and experimenter. The test subject and the experimenter were seated at a distance of I m, facing each other. The spectrum of the noise was equal to the long-term average speech spectrum. The open points represent the medians of the individual levels of ten subjects and the vertical bars represent the quartiles. The hatched area covers 50 per cent of the individual data points.

value of the long-term dB(A) values during the 0.5-min intervals measured at a distance of 1 m in front of the subject. The results for the ten test subjects are plotted in Fig. 2. The open points represent the medians of the individual levels and the vertical bars represent the quartiles.

Experiment 2 In order to verify whether the experimental conditions of the first experiment were too artificial, a second experiment with more practical, less rigorous, conditions was carried out. In this case the experimenter and the test subject moved from one place to another with different noise conditions, including a trip by car. The six conditions are specified in Table 1. As the test subjects now did not wear headphones, a small microphone was attached to their spectacles (or safety spectacles for those who did not wear glasses). The conversational speech was recorded on a portable recorder carried by the test subject. These recordings also included the ambient noise. Again, the speech level was computed by means of Brady's procedure which is resistant to interfering noise up to a certain level. Eight normal-hearing male subjects aged between 25 and 35 were used as speakers. The six experimental conditions were run through in different orders for

EFFECT OF AMBIENT NOISE ON VOCAL OUTPUT AND LISTENING LEVEL

35

TABLE 1 3

M E A S U R I N G C O N D I T I O N S F O R T H E DATA P O I N T S IN F I G .

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these subjects. Each condition lasted about 5 min but the subject had some time to become accustomed to the new situation: only the last 1 min was used for the level computation. The medians and quartiles of the individual speech levels converted to the levels at a distance of 1 m in front of the subject are plotted in Fig. 3 as a function of noise level.

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Fig. 3. Vocal output (converted to the level at a distance of 1 m in front of the subject) as a function of the ambient noise level during conversation between test subject and experimenter under six different conditions as specified in Table 1. The distance between test subject and experimenter was 1 m. The open points represent the medians of the individual levels of eight subjects and the vertical bars represent the quartiles. The dashed line represents the vocal output shown in Fig. 2.

36

E. VAN H E U S D E N , R. P L O M P , L. C. W . POLS

Discussion Figures 2 and 3 indicate that the subjects differed considerably in their voice levels at equal noise levels. An analysis of variance of the data underlying Fig. 2 revealed that there is no significant interaction between subjects and noise levels. This illustrates that the spread is due to differences in the overall speech levels of the subjects rather than to differences in the shapes of the individual curves. Figure 3 indicates a gradual transition from noise levels which have no effect on the vocal output to noise levels which certainly do. In order to be able to define a minimum noise level at which ambient noise interferes with speech production, the data points of Fig. 2 were approximated by two straight lines. The horizontal line is the average of the lower two data points, the sloping line is the least-squares fit of the three data points at higher noise levels. This line rises 2.9 dB for a 10 dB shift in noise level. The slope is substantially smaller than the 4dB increase found by other investigators for conversational speech. This discrepancy can be explained by the fact that those authors included much higher noise levels than our upper limit of 65 dB(A). Up to this level, the data points of Korn 5 have the same slope as our data. From the diagram we may conclude that the noise level at which the speech level begins to increase is about 40 dB(A). The median speech level in quiet of 49.1 dB(A) is in agreement with the average level of 49.5 dB(B) observed by Gardner. 6 The data points in Fig. 3, representing speech levels in more practical situations, appear to be located slightly higher than those in Fig. 2. The difference of 1.5 dB at low noise levels has to be regarded as reflecting a systematic difference between the two groups of subjects since it was also present when the groups were compared in an anechoic room. The 4dB difference at higher noise levels cannot be explained fully by this group effect. Differences in reverberation and noise spectrum may have caused the deviations from the measured values of Experiment 1. It is possible that expressing the ambient noise level in dB(A) was not adequate for the noise in the car (strong low frequency components) and in the central-heating room (continuous background plus intermittent noise). Furthermore, in Experiment 1 so-called conversational (relaxed) speech was involved, whereas in the highest level condition in Experiment 2 the character of the speech changed because, in this condition, unconstrained conversation was not possible. Data obtained in experiments performed under conditions where the distance between subject and experimenter was 4 m fit very well those in Fig. 3 after subtracting 4 dB. Gardner 6 found a similar difference of 4dB in vocal output between a distance of 1 m and one of 4m. The absence of data points between 40 and 60dB(A) does not, in this case, allow computation of a level beyond which the speech level was affected by the noise. 3.

EFFECT OF NOISE O N P R E F E R R E D L I S T E N I N G LEVEL

Experiment 1 As was said in the Introduction, it is of interest to know not only how the voice

EFFECT OF AMBIENT NOISE ON VOCAL OUTPUT AND LISTENING LEVEL

37

output, but also how the speech level preferred by the listener, depends upon the level of ambient noise. Rice et al. 7 performed experiments in which subjects were asked to adjust the level of simulated traffic noise to a level at which the noise started to interfere with the ability to relax and enjoy listening to speech presented at a constant sound level. Richards 8 used white noise at 55 dB SPL and higher levels. It appeared to be desirable to perform new experiments with the same interfering noise as that mentioned in the previous section. The experiment was carried out in a reverberation room of 65 m 3 of which the reverberation time was reduced to 0.5 sec by means of absorbing material. The test subject was seated in front of a loudspeaker located at a distance of 1 m. Speech recordings of four speakers reading aloud different excerpts from a popular text on audition were presented by means of this loudspeaker. Noise with the average spectrum of speech was reproduced by six loudspeakers arranged randomly in the room in order to create a diffuse sound field. Eight normal-hearing subjects aged between 19 and 37 were used as listeners. The ambient noise conditions were 40, 50, 60, 70dB(A) and no noise, presented in a random order. The experiment was divided into two parts: in one part subjects were asked to adjust the level of presented speech as if they were listening to a radio (preferred level) and, in the other part, to adjust the level to the minimally required level for understanding (required level). For the preferred level no further explanations about the criterion to be used were given. We used the B6k6sy up-down adjustment method to force the subject to make any decisions in a short period of time. This method was performed in the following way: the speech level changed automatically every 0.5 sec in 1 dB steps. The direction of these steps, up or down, was controlled by the subject who was instructed to push a button (1 dB steps up) when he wanted an increasing level and to release the button (1 dB steps down) when he wanted a decreasing level. The average level over an interval of 25 sec following the first six transitions was regarded as the adjusted level. The average of two adjusted levels for each speaker was accepted as the final preferred level. The subjects made preferred-level adjustments for two speakers and required-level adjustments for the remaining two speakers. Hence, for each speaker, four preferred and four required levels were obtained. Again, the speech levels were measured by means of the Brady method (see Section 2). The results for the eight subjects are plotted in Fig. 4. The open points represent the medians and the vertical bars the quartiles. Experiment 2

In order to obtain some insight into the degree to which the preferred speech level depends upon the understandability of the speech material, t h e following experiment was performed. For Dutch listeners, English has to be expected to be less redundant than Dutch. Therefore, we made a .speech recording of a bilingual speaker who read an excerpt from an English book about the history of science and

38

E. VAN HEUSDEN, R. PLOMP, L. C. W. POLS

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Fig. 4. Preferred (upper curve) and required (lower curve) listening levels as a function of ambient noise level. The test subject was seated in a room of 65 m 3 with a reverberation time of 0.5 sec. Spoken texts were reproduced by a loudspeaker located in front of the subject at a distance of I m. The open points represent the medians of the individual adjusted levels of eight subjects, the vertical bars represent the quartiles. The hatched area covers 50 per cent of the individual data points. The regression lines for 50 dB(A) and higher noise levels rise 3.1 dB and 6.4 dB per 10 dB, respectively, for the preferred and required listening levels.

its translation into Dutch. All test conditions were the same as in the previous experiment. Ten normal-hearing Dutch male subjects who had a fair knowledge of English were used as listeners. The preferred level adjustments were made for ambient noise conditions of 40 and 50 dB(A) and no noise, presented in a random order. Every adjustment was made twice.

EFFECT OF A M B I E N T NOISE O N V O C A L O U T P U T A N D L I S T E N I N G LEVEL

39

The results showed that Dutch subjects preferred a 3 to 5 dB higher speech level for the English than for the Dutch speech excerpts. No dependence of this difference on noise level could be observed. Discussion

Figure 4 indicates that the preferred-level adjustments differed considerably among the subjects. An analysis of variance of the data revealed, as in Section 2, that there is no significant interaction between subjects and noise levels. So, again, the spread is due to differences in the levels preferred by the subjects rather than to differences in the shapes of the individual curves. The data points belonging to the preferred level were approximated by two straight lines in order to define the minimum noise level at which ambient noise interferes with the preferred level. The horizontal line is the average of the lower two data points, the sloping line is the leastsquares fit of the three data points at higher noise levels. This line rises by 3.1 dB for a 10dB shift in noise level. The slope is smaller than the 5 dB increase between 55 and 65dB SPL ambient white noise found by Richards 8 in his Most Comfortable Loudness (MCL) measurements. From the present data we may conclude that the noise level at which the preferred speech level begins to increase is about 35 dB(A). Rice et al. v found, in terms ofL~ 0, a level of about 6 dB below the used speech level of 54 dB(A) at which simulated traffic noise just starts to interfere with the ability to relax and enjoy listening. The preferred level in quiet of 49.2 dB(A) is in agreement with the vocal outputs reported in Section 2. The curve for the preferred-level adjustments is very similar to the curves for the speech level produced (Figs. 2 and 3). This means that for a speaker-listener distance of I m a speaker produces a speech level equal to the preferred level for a listener. The least-squares fit of the four data points (40 dB(A) and higher) of the minimally required level for understanding shows a 6.4dB increase for a 10dB shift in noise level. As a result, the preferred level and the required level converge for increasing ambient noise level. The absence of data points between 40 dB(A) and no noise and the large spread of the levels adjusted in the no noise condition do not make it possible to define a level beyond which the required level is interfered with by noise but it is evident that the level is below the 35 dB(A) noise level. It must be emphasised that the adjusted values for the highest noise levels impaired understanding because, from other experiments, we know that for these signal-to-noise ratios the sentence intelligibility is very low. The experiment on the influence of understandability upon level showed, for the Dutch subjects, a significantly higher preferred level for the English than for the Dutch text. Although this experiment was performed for only one speaker, the conclusion seems justified that listeners prefer a higher speech level when the redundancy of the text is reduced.

40

E. VAN HEUSDEN, R. PLOMP, L. C. W. POLS 4.

RELATION BETWEEN VOCAL OUTPUT AND PREFERRED LISTENING LEVEL

Experiment The similarity of the curves for vocal output and preferred listening level as a function of ambient noise level raised the question of whether both phenomena are based upon a single underlying mechanism. Loudness and intelligibility are obvious candidates; the latter, however, is less probable in view of the fact that a constant signal-to-noise ratio is a condition for equal intelligibility. Therefore we designed an experiment on the relationship between loudness of speech and ambient noise level. The test conditions were the same as in the experiments of Section 3. A speech recording was made of one speaker reading an excerpt from a popular text on audition. The speaker read the text with minimum pauses between sentences to facilitate the loudness adjustments. Four normal-hearing subjects who had experience in psychophysical experiments took part in this experiment. The experiment was divided into two sessions: in both sessions we asked the subject to match loudness of recorded speech with the loudness of the same speech presented against a background of noise, but in one of these sessions the speech tape was played back in the reverse direction to make sure that loudness was the only cue used by the subject. Every cycle of speech and noise presentations was as follows: 3.0 sec quiet and 3'0 sec noise while the speech (0.75 sec) was presented in the middle of the quiet and of the noise periods (see Fig. 5). The earlier start of the noise was to facilitate the subjects focusing their attention exclusively on the loudness of the speech; if noise and speech had started at the same time the subjects could easily have been misled by the loudness differences at the transitions from speech alone to speech plus noise. As with the experiments described in Section 3, we used the B6kesy up down tracking method for the level adjustments. The average of the adjusted speech level over 10 cycles after the first six transitions was regarded as the matched value. The average of five of these adjustments was accepted as the final level.

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EFFECT OF AMBIENT NOISE ON VOCAL OUTPUT AND LISTENING LEVEL

41

The ambient noise levels were 40, 50, 60 and 70 dB(A), with speech levels in quiet equal to 40, 45, 50, 55 and 60dB(A). Again, speech levels were measured with Brady's method. The order of presentation was randomised. No significant differences were found between the results obtained from the loudness matching experiments in which normal speech and speech played back in reverse were used. Therefore the data for these conditions were combined. In Fig. 6 the results for the four subjects are plotted. The open points represent the medians. The quartiles were, on the average, comparable with the values represented in the previous figures.

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Fig. 6. Loudness adjustments of speech in noise matched to speech in quiet as a function of the ambient noise level. Parameter is the level of speech in quiet. Experimental conditions as in Fig. 4. The rise per 10dB of the regression lines through the data points is indicated for each condition.

Discussion

The inter-individual variation between the loudness matching adjustments of speech in quiet and speech in noise appeared to be rather large for the four subjects. The combination of high noise levels and low speech levels in quiet were especially difficult conditions. As in Sections 2 and 3 an analysis of variance of the data revealed that the spread is due to differences in the adjusted levels of the subjects

42

E. VAN HEUSDEN, R. PLOMP, L. C. W. POLS

rather than differences in the slopes of the individual curves. For each speech level in quiet the corresponding data points were approximated by a regression line in order to find the minimum noise level at which noise interferes with the loudness of speech. The horizontal lines represent, by definition, the level of speech in quiet matched to speech in quiet. In Section 2 we found a vocal output in quiet of about 50 dB(A). The curve in Fig. 6 belonging to 50 dB(A) speech level in quiet shows an increase of 3-8 dB for every 10 dB shift in noise level. From this curve we furthermore conclude that the noise level at which the loudness of speech begins to decrease is 35-40 dB(A). This is in agreement with the findings of Pollack 9 who described a loudness matching experiment of speech in quiet and speech masked by white noise. He found that noise levels of 35 dB SPL and lower had no effect on the loudness of speech of about 50dB(A). The spread of the individual data points does not allow us to draw any conclusion from the differences between the intersections of the sloping and the horizontal lines of Fig. 6.

5.

CONCLUSIONS

From Figs. 2, 4 and 6 we may conclude that both the vocal output level and the preferred listening level as functions of the ambient noise level are rather similar to the level of speech in noise matched to speech in quiet with respect to loudness. The increase in vocal output (2.9 dB) per 10 dB increase in noise level is in agreement with the increase in preferred listening level (3.1 dB) and both values are not too different from the increase in the level of speech in noise matched, according to loudness, to speech in quiet at a level of 50 dB(A). The similarity holds also for the minimum noise levels beyond which noise begins to interfere: 40 dB(A) in Fig. 2, 35 dB(A) in Fig. 4 and 38 dB(A) in Fig. 5. Actually, there is a continuous changeover from a horizontal part to a sloping part of the curves. If we want to use a single number, a value of 40dB(A) seems to be justified. As the vocal output in quiet is about 50 dB(A), we may conclude that both speakers and listeners neglect the noise as long as the signal-to-noise ratio exceeds about 10dB. The experiments suggest that both the speaker and the listener (when the latter is able to control the playback level of recorded speech) try to compensate for the reduction in loudness of speech due to noise interference by raising the level of speech in order to keep the (subjective) loudness of speech in noise equal to the loudness of speech in quiet. We should realise however, that maintaining the same loudness does not mean that intelligibility is kept constant. In terms of signal-tonoise ratio, which is the decisive measure for intelligibility, the introduction of noise interferes more and more for increasing noise level.

EFFECT OF AMBIENT NOISE ON VOCAL OUTPUT AND LISTENING LEVEL

43

ACKNOWLEDGEMENTS T h e a u t h o r s wish to t h a n k It. W. S c h o o n d e r b e e k w h o p e r f o r m e d t h e e x p e r i m e n t s o n the v o c a l o u t p u t in p r a c t i c a l c o n d i t i o n s . W e also t h a n k D r . T. H o u t g a s t for his coo p e r a t i o n in p l a n n i n g the e x p e r i m e n t s a n d d i s c u s s i o n o f t h e results. T h e r e s e a r c h was s u p p o r t e d by the M i n i s t r y o f H e a l t h a n d E n v i r o n m e n t a l Protection of the Netherlands.

REFERENCES 1. H. LANEand B. TRANEL,The Lombard sign and the role of hearing in speech, Journal of Speech and Hearing Research, 14 (1971), p. 677. 2. E. LOMBARD,Le signe de r616vation de la voix, Annales des Maladies de I'Orielle et du Larynx, 37 (1911), p. 101. 3. P. T. BRADY, Equivalent peak level: A threshold-independent speech-level measure, Journal of the Acoustical Society of America, 44 (1966), p. 695. 4. W. B. DAVENPORT,JR., An experimental study of speech-wave probability distributions, Journal of the Acoustical Society of America, 24 (1952), p. 390. 5. T. S. KORN, Effect of psychological feedback on conversational noise reduction in rooms, Journal of the Acoustical Society of America, 26 (1954), p. 793. 6. M. B. GARDNER,Effect on noise, system gain, and assigned task on talking levels in loudspeaker communication, Journal of the Acoustical Society of America, 40 (1966), p. 955. 7. C. G. RICE, B. M. SULLIVAN,J. G. CHARLES,C. G. GORDONand J. A. JOHN, A laboratory study of nuisance due to traffic noise in a speech environment, Journal of Soundand Vibration, 37 (1974), p. 87. 8. A. M. RICHARDS,Most comfortable loudness for pure tones and speech in the presence of masking noise, Journal of Speech and Hearing Research, 18 (1975), p. 498. 9. I. P•••AcK• The e•ect •f white n•ise •n the l•udness •f speech •f assigned average leve•• J•urna• •f the Acoustical Society of America, 21 (1949), p. 255.