Quarterly Progress and Status Report. Masking effects of one s own voice

Transcription

1 Dept. for Speech, Music and Hearing Quarterly Progress and Status Report Masking effects of one s own voice Gauffin, J. and Sundberg, J. journal: STL-QPSR volume: 15 number: 1 year: 1974 pages:

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3 STL-QPSR 1/1974 B. MASKING EFFECTS OF ONE'S OWN VOICE Abstract Measurements are reported in which a voice trained subject matches his masked threshold during phonation. This threshold is found to be fairly similar to the threshold masked by the purely airborn signal which reaches the subject' s ear during phonation in an anechoic room. By reducing the number of partials in the last mentioned masker spectrum it is found that most higher partials fall below the masked threshold and seem to account for the masking effect. The differences between spectra radiated frontally from the mouth and spectra l'eaching the ear are studied, I The masking of one' s own voice can be described by the masked threshold which is measured during phonation. This threshold is interesting from many points of view. It is related to the perception of the own voice and to the excitation of the basilar membrane due to the phonation. Therefore, the masked threshold may inform about the auditory feedback signal'which the speaker perceives. This signal is presumably important for the control of the voice. We may raise the question, however, whether or not measurements of masked thresholds can be successfully made in a phonating subject. What is the order of magnitude of the scatter we obtain if a subject tries to match the hearing threshold while he is phonating? And is a subject capable of keeping the phonation sufficiently constant over the period of time needed for the measurements? Thus, are such threshold measurements meaningful at all? The purpose of the present investigation was to find out the answer to such questions. Attempts have been made to determine the masked threshold during phonation of different vowel sounds. The threshold has also been determined for the sound reaching the speaker' s ear when he phonates. Finally, the differences in the spectrum radiated frontally from the mouth and the spectrum reaching the ear have been measured for different vowels. The experiments have been conducted on oee subject only. Background The masking effect of one's own voice can be assumed to depend on three important factors: the stapedius reflex, the bone conduction, and the sound transfer from the mouth to the ears.

4 The stapediu s reflex attenuates the sound transmitted through the middle ear by reducing the vibration amplitude of the stapedius. The reflex has been shown to be released even at very weak levels of phona- tion. It mainly affects frequencies lower than 2 khz. In a silent subject the stapedius reflex is released when the ear is exposed to sounds with an SPL of 90 to 95 db (~p/ller, 1972). The bone conduction transmits vibrations from the vocal tract walls and the larynx region to the ear. The bone conducted vibrations yield energy to the cochlea in various ways. Sound is generated in the meatus due to its vibrating walls. In addition the skull vibrations are transmitted to the ossicles. Also, the vibrations forced directly upon the cochlea result in an excitation of the basilar membranes. We may assume that the phase relationship between bone- and air-conducted sound during phonation is frequency dependent. Moreover, the stapedius reflex is assumed to affect bone conduction (Tonndorf, 1972). The sound transfer from the mouth to the ear is normally dependent upon the reverberation of the room. In an anechoic room the transfer would to a large extent be dependent on the size of the sound radiating lip opening, the dimensions of the head, and the wavelength of the sound transferred. As yet the details of this sound transfer are not sufficiently investigated. Against this background it seems clear that the masking effect of one's own voice is hard to predict. Therefore, even a set of purely empirical data may provide valuable information on the masking effect and on the perception of the own voice. Measurements All threshold measurements were made in an anechoic chamber. A microphone was mounted at the subject's ear 7 cm above his meatus. This microphone was used for the control of the phonation level and for measuring the probetone amplitudes. A sinusoidal probetone was presented through a loud-speaker suspended 40 cm in front of the subject (cf. Fig. III-B- I). Each 0. 5 sec the probetone alternated between two sound levels differing by 6 db. While the subject phonated, he adjusted the probetone amplitude so that he could perceive the stronger parts of the probetone only. The probetone was given at 17 frequencies in rising

5 . PROBETONE * G A I r GATE r\e Fig. 111-B- I. Equipment used for measuring masked thresholds during phonation and for a purely airborne masker. M microphone mounted at the subject's ear VU instrument indicating SPL Pm potentiometer for regulation of the SPL of the purely airborne masker which was provided by a tape loop on the tape recorder (TR) G sinewave generator providing the probetone potentiometer for the subject's regulation of the probetone Pp amplitude MX mixer

6 STL-QPSR 1/ order between 100 and 4000 Hz. After the probetone amplitude adjust- ment had been completed at a given frequency the phonation was inter- rupted and the probetone amplitude was measured with the ear micro- phone. Thereafter the subject started the phonation again and the next probetone frequency was tested. This procedure was repeated several times so that three or four values were obtained for each probetone fre- quency. Thus, each point in a threshold curve represents the average of at least three values. The complete set of readings (3 collected within 40 min. 17) could be The ability of the subject to control voice production is decisive for the reliability of the results. This ability can be assumed to be higher in a trained than in an untrained voice. Therefore, a trained singer was used as subject. Nevertheless, a certain amount of spectral variability must be present also in vowels produced by trained voices. This varia- bility may be of two kinds: long-time variations and short-time varia- tions of the amplitudes of the spectral components. The long-time variability was estimated by comparing spectra of the same vowel produced in the beginning and at the end of a session. The stronger partials below 1 khz differed by.f. I db only, whereas differences smaller than 3 db were observed near the higher formants. The short- time variability (within 2 sec) of the partial amplitudes was found to be smaller than f 2 db for the more prominent components. The actual masker signal raising the threshold during phonation is unknown. On the other hand, the acoustic signal reaching the subject's ear during the phonation can be determined. This signal was picked up by the microphone at the subject' s ear and recorded on tape. A loop of this tape was presented through the loud-speaker as masker signal in a subsequent session. In this way the masking of the purely airborne sound reaching the subject' s ear during phonation could be measured. This sound was possible to reproduce with an accuracy of 4 db. The measurements were completely analogous to the earlier measurements. Thus, the masker signal was interrupted when the probetone amplitude adjustment had been completed, and at least three threshold values were collected for each of the 17 probetone frequencies. The three settings of the probetone amplitude made at the same fre- quency in the same session differed with more than 4 db only excep- tionally. In the measurements with a purely airborne masker the spread

7 STL-QPSR 1/1974 was slightly smaller, on the average. Occasionally, day- to-day-variations of the masked threshold occurred. They were observed to be within 3 db, approximately. This amount of spread is rather small and we may conclude that measurements of the masked threshold during phonation yield reasonably reliable results. However, in comparing masked thresholds obtained with different masker Signals it should be remembered that differences smaller than 3 db may be due to the limited accuracy in the measurements. Masked thresholds The masked thresholds for an [a)-vowel phonated at two levels and for an [ i ] are shown in Fig. 111-B-2a-c. The fundamental frequency was 110 Hz in all vowels. In the graphs in Fig. 111-B-2 are also shown the purely airborne masker spectrum recorded at the subject' s ear and the masked threshold pertaining to that masker. The two types of thresholds are generally grossly parallel and differ with less than 5 db as a rule. Two exceptions to this are found. One is the weaker [a 1 in the low frequency region and the other is found between 1 and 2 khz in the stronger [ a ]. In both these cases, the threshold measured during phonation is the lowest. If the bone conduction has the effect of suppressing a strong partial in the masker spectrum, the masked threshold can be expected to drop in the frequency region above this partial. This is a possible explanation to the threshold differences in these two cases. Only in the case of the purely airborne masker the relations between the masker spectrum and the masked threshold can be studied. It is well known that the masked threshold for a sinewave masker reaches a maximum at the masker frequency lying about 20 db belov the SPL of the masker. The threshold slopes steeply towards lower frequencies and slower towards higher frequencies. In our case the relationships between the masker spectrum and the threshold is more complicated, as we may expect. The threshold lies 10 to 15 db below the stronger low frequency components. Above a strong partial followed by considerably weaker partials the threshold falls at a rate of 13 to 20 d~/octave. As a consequence of this, the threshold lies higher than the spectrum envelope in the "valleys" between formants, and only one or two partials near a higher formant surpasses the level of the threshold. I I

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9 STL-QPSR 1/ Which partials determine the masking effects, then? It seems rea- sonable to assume that masker components lying below the threshold do not contribute to the masking effects. This assumption is supported by the results shown in Fig. 111-B- 3a and b. Lowpass-filtering the weaker [ a ] at 1. 1 khz did not affect the threshold significantly, as seen in Fig. 111-B-3a. In one version of the [ i ] only five partials clearly surpassed the masked threshold. These five partials were synthesized and presented as a masker to the subject. The results showed that the threshold was not affected significantly by this reduction of all partials lying below the threshold as is shown in Fig. 111-B-3b. Therefore it seems as if the masking effect of a vowel spectrum may be determined by a rather small number of partials. Comparing the timbre of the 1 complete and the reduced masker spectra yielded support for the as- sumption. that these few partials determine the timbre perceived as well. Transmission mouth- ear Our results suggest that the masking effect of one' s own voice is to a large extent dependent on quite few partials. This result is of course partly due to the fact that the masker spectrum was recorded at the sub- ject's ear in an anechoic room. How does this spectrum relate to the spectrum radiated frontally from the mouth? The answer to this question was obtained by measuring these two spectra and comparing them. One microphone was placed in front of the mouth and the other at the ear, as before. The distance from the mouth opening to both these micro- phones was 16 cm. The microphone signals were simultaneously re- corded on each of the two channels of a tape recorder. In this way, the same set of voice pulses could be identified and analyzed in the two recorded signals. The analysis was performed by an FFT computer program. Two pairs of spectra of each of the vowels [ u, a, i ] were compared. The differences in the partial amplitudes are shown in Fig. 111-B-4a, b, and c. It is seen from the figure that the differences vary considerably with frequency and vowel. Note also that the two values observed for the same partial generally agree within a few db. The frequent and abrupt changes in the curves are probably due to in- terferences between sound radiated from the lip opening and sound radiated from the cheeks and the neck. The frequency dependence shows that the signal reaching a speaker's ear in an anechoic chamber differs widely from the signal radiated frontally. I

10 b FREQU E NCY (~Hz) I I I I I f I I I c I FREQU E NCY (k~z) I I I I I I 1 1 Fig. 111-B-3. Airborne masker spectra and masked thresholds for an [a] and an [i ] (upper and lower graphs, respectively). Circles and solid line: mask4 threshold for the complete masker spectrum: triangles and dashed line: masked threshold for a reduced masker spectrum consisting only of the partials indicated by heavy lines. The dashed curve at the bottom of the graphs shows the threshold measured in silence.

11 FREQUENCY (k~z) FREQUENCY (k~z) Fig Solid lines and dots: differences in partial amplitudes between vowel spectra simultaneously sounding at the car and 16 em in front of the mouth of a phonating subject. The dot-dashed line shows the corresponding values for a frequency sweep generated by a point source at the subject's mouth. The dashed line shows the average spectrum level differences in octave bands of 12 vowels recorded at the brim of the lips and at the ear (adapted after von B&k&sy).

12 STL-QPSR 1/ In Fig. 111-B-4 two other curves are shown for comparison. One shows the differences which wodd occur if the lip opening behaved as a poifit source. This curve was obtained as the differences in the responses to a ~ihekrave sweep generated by a point source (the STL-Ionophone) and recorded with the ~icro~hohes just mentioned (Fransson & Jansson, 197i). The Ionophone was a few mm in front of the subject's closed lips. The dip in the curve near 3 khz probably depends on interferences be- tween sound which travels directly to the ear and sound which reaches the ear from behind the head. We would expect that a vowel produced with a small lip opening would give values lying closer to the curve per- taining to the point source than would vowels produced with larger lip openings. This is also the case; the curve pertaining to the [ u] - vowel lies higher than the curves pertaining to the other two vowels. The other curve presented in the graphs in Fig. III-B-4 reproduces data given by von ~ & k &(1960). s ~ These data were recorded with one microphone at the brim of the lips and one near the ear. The values were normalized to a difference of 0 db at 0 Hz for the sake of compari- son. Thus, we would expect that B&~&sY's curve would provide a very gross average of our data. This can also be said to be the case except for a difference below 0.7 khz. Disregarding the numerous dips our curves may be described as follows. The differences between the spectrum reaching the speaker' s ear and the spectrum radiated frontally from the mouth are very small below 500 Hz. Partials near 1 khz are reduced with 8 db on the average and partials near 3 khz with 10 db. Discussion and conclusion The results appear to show that masked thresholds can be determined with an accuracy of 3 db in a phonating voice-trained subject. The threshold measured during phonation is not always identical with the threshold pertaining to the masker signal which reaches the ear during phonation. The differences are probably the combined effects of the bone conduction, the stapedius reflex and the sound transfer from the mouth to the ears. If we may generalize our observations the following can be said as regards the masked threshold in relation to a vowel masker spectrum. All strong low frequency components in the masker spectrum lie around

13 STL-QPSR 1/ db above the masked threshold, whereas nearly all higher partials lie below the threshold except those which fall close to formants. Thus, partials in a spectral "valley" between two formants do not seem to reach the level of the masked threshold. The results suggest that only a small number of partials accounts for the masking effect and probably also for the timbre perceived. This agrees with the findings of Kakusho et a1 (1968). In normal acoustic surroundings the reverberation will counteract the lowpass filter effect characterizing the sound transfer from the mouth to the ear in an anechoic room. However, the amplitudes of the higher partials reaching the ear will depend strongly of the acoustic sur- roundings. The results of a previous investigation yielded support for the assumption that a singer finds it easier to base the voice control on the vibration sensations in the head than on the auditory feedback signal (Sundberg, 1974). The results of the present investigation seem to provide an explanation to this. The auditory feedback signal must vary severely with the acoustic properties of the room in which the singer sings. This is not so for the head vibrations, which thus is likely to provide a more useful feedback signal. Acknowledgments This work was supported by the Bank of Sweden Tercentenary Fund. References von B&k&sy, G. : "Bone Conduction", Chapter 6 in Experim nts in Hearing, pp , New York Fransson, F. and Jansson, E. : "Properties of the STL Ionophone Transducer", STL-QPSR 2-3/1971, pp Kakusho, 0., Kato, K., and Kobayashi, T. : "Just Discriminable Change and Matching Range of Acoustic Parameters of Vowels", Acustica 20, pp (1968). - ~4ller, A. : "The Middle Ear", Chapter 4 in Foundations of Modern Auditory Theory I1 (ed. J. Tobias), pp , New York Sundberg, J. : "Articulatory Interpretation of the ' Singing Formant' ", J-Acoust. Soc.Am :4, p. &38 sqq (1974). Tonndorf, J. : "Bone Conduction1', Chapter 5 in Foundations of Modern Auditory Theory I1 (ed. J. Tobias), pp , New York 1972.