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Oral muzzle pressure effects in underwater communication
Journal of Phonetics (1977) 5, 265-271
Oral muzzle pressure effects in underwater communication Robert F. Coleman Division of Hearing and Speech S ciences, School of Medicine, Vanderbilt University, Nashville, Tennessee 37232, U.S.A.
and WilmaR. Krasik New England Medical Center Hospital Received 17th Septernber 1976
Abstract:
Most underwater communication systems require that a diver-talker speak into an oral muzzle arrangement, and with an air pressure higher than the surrounding water pressure. This investigation studied the effect of varying ambient pressure in the muzzle upon intelligibility of words. There was a systematic relationship between decreased intelligibility scores and increases in ambient pressures. An analysis of phoneme distortions showed that errors of " place" were most common for a group of listeners, and that nasal consonants tended to be heard as fricatives or plosives.
Introduction
Within the past decade there has been a greatly accelerated effort to investigate the problems of underwater communication, particularly for free divers wearing self-contained underwater breathing apparatus (SCUBA). The cumulative results of such research suggest that the communication process in inner space (underwater) is potentially far more complex than that encountered in outer space, such as in moon probes. A program designed to investigate the complex problems of underwater communication has been carried out by Hollien and his associates (Hollien & Tolhurst, 1970; Hollien, Coleman , Thompson & Hunter, 1970). One of the ongoing conclusions of these studies is that the intelligibility of divers talking through underwater communicators may be lim ited more by the talkers' input to the various systems, than in the electromechanical systems themselves. A major constraint placed upon a diver is the necessity for wearing an oral muzzle or mask to allow normal respiration and to provide a cavity into which he talks. In addition to adding another closely coupled acoustic cavity to the speech mechanism, it is necessary that the talker speak and exhale into a slightly raised pressure than that of the surrounding water, to prevent flooding of the oral muzzle. Leyden (1956) investigated breathing resistance in several configurations of underwater breathing systems and reported increases in pressure due to the systems themselves of from 0·1 to 17·0 cmH 2 0 . The role of varying air pressure in the production of human speech has been well recognized. Differences in air pressure within the vocal tract have been found to be asso-
266
R. F. Coleman and W. R. Krasik
ciated with vocal intensities (Van den Berg, 1956; Draper, Ladefoged & Whitteridge, 1960; Kunze, 1962; Arkebauer, 1964). Other aspects of speech associated with differences in air pressure include the rate of syllable production (Arkebauer, 1964), rate of air flow through the vocal tract (Flanagan, 1958; Kunze, 1962), different sounds or classes of sounds (Black, 1950; Arkebauer, 1964; Brown & McGlone, 1969a) and changes in pitch (Kunze, 1962). The literature concerning the relationship of air pressure variations and speech is sometimes contradictory, and is confounded by the fact that various investigators used different methodologies to arrive at their conclusions. However, general conclusions from these studies would indicate that the minimum transglottal pressure at which the larynx will oscillate is on the order of 1-3 cmH 2 0 . Nominal conversational speech is accompanied by pressures of 4-10 cmH 2 0 pressure, and maximum shouting may use a pressure of30 cmH 2 0. Consonants appear to be higher pressure components than vowels. Since divers, of necessity, must speak into a slightly raised atmospheric pressure due to the design of current SCUBA, it seems reasonable to assume that such elevated levels would have an effect on phonemes which rely for the most part on a pressure build-up in normal speech production. A further complication arises from the fact that such elevated pressures in the SCUBA system tend not to be constant, but rather vary according to the stiffness and release characteristics of the exhaust flutter valves on the systems. The purpose of this study was to examine the effect of varying ambient muzzle pressure on diver communication. A second purpose was to ascertain whether there is a differential effect of such varying pressure upon the intelligibility of discrete phonemes. Procedure
Selection ofspeakers Three male and two female speakers, who ranged in age from 23 to 33 years of age, with "normal" speech as defined by the experimenter were used in this study. One speaker was an experienced SCUBA diver, who had experience in underwater communication. Instrumentation A conventional SCUBA air tank and a three-stage regulator were modified to accommodate a Bioengionics Nautilus oral muzzle. The oral muzzle contained an EM-3 microphone, located approximately one inch from the speaker' s lips, to transduce speech to an Ampex 601 tape recorder. A "T" tube was tightly fitted with a rubber disc exhaust valve in its vertical outlet. The exhaust hose extending from the muzzle was positioned in one of the horizontal outlets; the third opening was sealed. Also installed in the muzzle was a probe tube which led to a "U" tube manometer and a Statham PC-97 differential pressure gage; the latter was attached to a Grass Model 7 polygraph. The pressure recording equipment was calibrated with the aid of the "U" tube water manometer to enable reading of pressure values directly from the graphic record during connected speech. The special "T" tube was rotated to permit the exhaust valve to lie on a vertical axis and was extended from an adjustable clamp located in a small water tank. Valve release pressure could be systematically varied by lowering and raising the "T" tube structure in the water. A SCUBA face mask with a purge valve was worn by all speakers. The equipment was located in a sound treated chamber. Word lists Word lists used for this experiment were lists A through E of the Griffiths' Rhyming Minimal Contrasts Articulation Test (Griffiths, 1967). These lists consist of 50 words each
261
Underwater communication
with five alternate forms. The alternate words represented a closed set of responses for listeners and permitted analyses to be made of consonant confusions. Test procedure
Each speaker trained for the experiment by reading several practice word lists at the different pressures used in the study. After familiarizing himself with a particular word list, the speaker read each test word preceded by the phrase "Say the word ... "The lists were quasi-randomized with respect to order of the lists and oral muzzle pressures. Amplitude levels were monitored for each speaker by use of a VU meter. The output of speech at the muzzle microphone was recorded on an Ampex 601 tape recorder. A Sony TC-355 tape recorder was used to dub from the master tapes to prepare listening tapes. A 4-5 s pause was inserted between each stimulus phase. The resultant tapes were played to listeners through an Ampex 620 speaker. Listening task
A total of 20 listeners who were all university graduate students were used. Prior to listening to the lists, each participant was required to pass a screening discrimination test with a minimum score of 96 %. After passing the screening procedure, listeners were presented 30 lists in random order for words, pressures, and speakers. Each word list required approximately 5 min to complete, and 5-10 min rest was allowed after sets of 10 lists were heard. The entire task required approximately 3-3·5 h. The task was divided into two or more sessions for all listeners. The listeners were provided with one of five randomized response sheets for each of the 30 test lists, and were required to circle the word heard. The listeners were forced to guess if not certain of the proper response. Results and discussion Intelligibility scores for each condition, as well as specific phoneme distortions, were derived from the listener responses. Mean intelligibility scores for six reading conditions for each of five speakers are presented in Table I. Table I Mean intelligibility scores for six reading conditions for each of five speakers. (NM* =no oral muzzle was worn by the speakers. M =Male; F= Female)
Muzzle back pressure (cmH 2 0)
Speakers Ml M2 M3 Fl F2 Mean
NM*
0
97·1 98·3 94-4 97·0 98 ·8
89-4 76·1 75·7 78 ·6 70·0
97·1
78·0
2
4
6
85·6 63·6 71·9 79·9 E8·7
79·1 60·4 69·2 78 ·8 63 ·5
82·9 59·6 63-4 72-8 53·3
75 ·0 59·0
73·9
70·2
66-4
64·7
55·5
73·8 60·1
268
R. F. Coleman and W. R. Krasik
Effect ofora/muzzle ofspeech intelligibility The effect of the oral muzzle itself on communication was determined from comparison of mean intelligibility scores at 0 cmH 2 0 and the "no muzzle" condition for all five speakers. An average 20% decrease in intelligibility resulted from donning the oral muzzle. For individual talkers, the difference in scores obtained with and without the oral muzzle ranged from 7·7 to 28·8 %- Thus, as other investigators have found (Hollien, 1969; Hunter, 1968) the simple addition of an oral muzzle has a significant and deleterious effect upon speech transmission. The fact that no speaker scored less than 94 % in the "no muzzle" condition attests to the quality of the underwater microphone used in the system, as well as the integrity of the entire recording-playback-listeningdesign. Effect ofpressure variation on speech intelligibility A systematic trend may be noted for intelligibility scores to decrease with increases in muzzle release pressure. The average loss of intelligibility was approximately 2·3 % per increase of 1 cmH 2 0 pressure. From a pilot experiment performed in a swimming pool with a diver wearing SCUBA equipment, at a depth of0-9 ft, peak release pressures for an underwater communication system were on the order of 4 cmH 2 0 pressure. During sustained conversation, nominal release pressure was approximately 0·75 to 1·50 cmH 2 0. Thus, the pressures used in the present study were assumed to be representative of those encountered in typical SCUBA communication work. Leyden's early research (1956) found breathing resistance pressures as high as 17 cmH 2 0 in a closed SCUBA system, a level considerably greater than that used in the present study. Effect of speaker sex on speech intelligibility In order to ascertain the possible influence of speaker sex on speech intelligibility, mean intelligibility scores for six reading conditions for the three male speakers, two female speakers, and all five speakers combined were calculated. The results are displayed in Fig.1 . The trend of decreased intelligibility with increases in pressure was noted for both sexes. The largest difference of 6·1% in intelligibility scores of males and females wearing an oral 100
u
90
~
0
u
"' a ;;:
"0
80
0
c:., u w
70
60 nm* 0
2
I
I
4
6
Muzzle bock pressure (c mH 20 )
Figure I
Plot of mean intelligibility scores for six reading conditions for three male speakers, two female speakers, and all five speakers combined. (nm =no oral muzzle.) Female ( · · ··· ·);male(---); combined(--).
269
Underwater communication
muzzle appeared at 0 cmH 2 0. Since pressure was not a factor in this condition it may be hypothesized that such differences are due to the resonance effects of the oral muzzle itself. The two speakers who achieved the highest intelligibility scores on this task were a male and a female. The only apparent differences between these two speakers and the other three who made lower intelligibility scores were that the superior talkers were physically larger, spoke with a higher overall intraoral breath pressure, and had a considerably slower speech rate than the other speakers. Phoneme analysis ofunderwater speech intelligibility
Use of the Griffiths' Rhyming Minimal Contrasts Articulation Test permitted an error analysis of the closed set of responses from listeners. Chi-squared tests of significance were used with the hypothesis that the word read by the speaker would be the expected word Labial
Labiadental
Dental
Alveolar
Palatal
Velar
Glottal
p Plosive
g
b
J v ~{)
Fricati ve
h
3
1] Affricative
Figure 2
d:;
Semivowel
w
Nasal
m
n
0
Place-manner phoneme chart demonstrating substitutions for consonants spoken by talkers. Each substitution listed occurred a significant number of times in a closed set response list (X 2 significance= 0·01.)
heard by the listeners. A confidence level ofO·OOl with one degree of freedom was chosen to denote a significant difference between the expected (speaker) word and observed (marked) word. It was apparent that as muzzle back pressure increased, more consonants were significantly distorted. Thus, one consonant at the "no oral muzzle" condition, nine at 0, 14 at 1, 20 at 2, 23 at 4, and 22 at 6 cmH 2 0 were missed a significant number of times. The almost equal number of words significantly missed at the higher pressures suggests that speaking into any raised pressure was the degrading factor in the task, without regard to the magnitude of pressures used. A second X2 analysis was made with the hypothesis that there was no significant difference between the expected word and any one of the phonemes that could have been substituted for the expected sound. In the first x2 analysis, the significance of missing the expected word was tested; in the second set, the significance of the specific substitutions made for the expected word was evaluated.
270
R. F. Coleman and W. R. Krasik
Phoneme substitution errors which occurred a significant number of times for the intended consonant enabled construction of a confusion matrix. Figure 2 displays the phoneme substitutions on a place-manner chart. For example, it can be seen that ftf was substituted for /k/ and vice versa. Examination of Fig. 2 will show that, for this experiment, the most obvious trend was for listeners to make "place" errors, and that plosive phoneme confusions were most common. Numerous phoneme confusions were also noted for fricatives, but to a lesser extent than plosives. There appeared to be a general trend for lingua-alveolar and linguavelar plosives, i.e. ft/ and /k/, to shift forward. The forward shift was also in evidence for their voiced cognates, fd/ and fgf. There also was a trend for nasals to shift toward plosives. This would not be unexpected in view of the facts that nasals are low pressure phonemes, and that an extra cavity, a face mask, was also added to the speech mechanism. The mask possibly functioned as a closely coupled resonator, shifting nasal resonances downward. Fatigue effects ofincreased muzzle pressure This investigation has demonstrated a systematic trend for speech intelligibility to be affected by increased oral muzzle pressure. Of equal significance to an underwater diving task, however, is the physical fatigue associated with such activity. The talkers used in this study found it extremely difficult to speak into a muzzle pressure of 4 and 6 cmH 2 0. Although the problem was not of significant magnitude to influence the results of the experiment, it was apparent that none of the subjects, including the experienced SCUBA diver, would voluntarily communicate over a sustained period because of the physical effort involved. Leyden's (1956) study of resistance components in an underwater breathing apparatus reported respiration pressures at depth (132 ft) of 17 cmH 2 0 pressure for a particular type of exhaust valve. Such an escalation of pressure, even allowing for the superior physical condition of most professional SCUBA divers, would undoubtedly result in considerable fatigue and serious degradation of speech communication. It may be hypothesized from the experience of the subjects in this study, that differential pressures of no more than 2- 4 cmH 2 0 should be allowed for acceptable speech communication. Conclusions
In summary, it appears that the following conclusions are warranted from the results of this investigation. ( 1) There was a systematic decrease in speech intelligibility with increases in oral muzzle release pressures. (2) The major effect on speech degradation was due to the effects of donning an oral muzzle and face mask. (3) There appeared to be no significant differences attributed to speakers' sex in this study. (4) Plosive consonants appeared to be distorted most often with a moderate trend for lingua-velar and lingualveolar consonants to be shifted forward in place. (5) Nasal consonants were shifted perceptually from nasal manner of production to fricative and plosive manner, while maintaining the same approximate place of articulation. References Arkebauer, H. J., Hixon, T. J. & Hardy, J. C. (1967). Peak intraoral air pressures during speech. Journal of Speech and Hearing Research 10, 196-208.
Underwater communication
271
Black, J. W. (1950). The pressure component in the production of consonants. Journal of Speech and Hearing Disorders 15, 207-20. Brown, W. S. & McGlone, R. E. (1969). Relation of intraoral air pressure to oral cavity size. Folia Phoniatrica 21, 321-31. Draper, M., Ladefoged, P. & Whitteridge, D. (1960). Expiratory pressure and air flow during speech. British Medical Journal18, 1837-43. Flanagan, J. L. (1958). Some properties of the glottal sound source. Journal of Speech and Hearing Research 1, 99-116. Griffiths, J.D. (1967). Rhyming minimal contrasts: a simplified diagnostic test. Journal of the Acoustical Society of America 42, 236-41. Hollien, H ., Coleman, R. F., Thompson, C. & Hunter, K. H. (1970). Evaluation of diver communication systems under controlled conditions. Undersea Technology Handbook , Arlington, Viriginia, Compass Publications, A-81 to A-87. Hollien, H . & Tolhurst, G. (1969). A research program in diver communication. Naval Research Reviews, December 1969. Hunter, E. K. (1968). Problems of diver communication. Institute of Electrical and Electronics Engineers, Transactions on Audio and Electroacoustics, A U-16, 118-20. Kunze, L. H . (1962). An investigation of the changes in sub-glottal air pressure and rate of flow accompanying change in fundamental frequency, intensity, vowels, and voice registers in adult male speakers. Doctoral dissertation, University of Iowa. Leyden, C. J. (1956). Breathing resistance in SCUBA components. U.S. Navy Experimental Diving Unit Research Report 4-57, Project WS185-005, 1-23. Van den Berg, J. W. (1956). Direct and indirect determination of the mean subglottic pressure. Folia Phoniatrica 8, 1-24.
Oral muzzle pressure effects in underwater communication Robert F. Coleman Division of Hearing and Speech S ciences, School of Medicine, Vanderbilt University, Nashville, Tennessee 37232, U.S.A.
and WilmaR. Krasik New England Medical Center Hospital Received 17th Septernber 1976
Abstract:
Most underwater communication systems require that a diver-talker speak into an oral muzzle arrangement, and with an air pressure higher than the surrounding water pressure. This investigation studied the effect of varying ambient pressure in the muzzle upon intelligibility of words. There was a systematic relationship between decreased intelligibility scores and increases in ambient pressures. An analysis of phoneme distortions showed that errors of " place" were most common for a group of listeners, and that nasal consonants tended to be heard as fricatives or plosives.
Introduction
Within the past decade there has been a greatly accelerated effort to investigate the problems of underwater communication, particularly for free divers wearing self-contained underwater breathing apparatus (SCUBA). The cumulative results of such research suggest that the communication process in inner space (underwater) is potentially far more complex than that encountered in outer space, such as in moon probes. A program designed to investigate the complex problems of underwater communication has been carried out by Hollien and his associates (Hollien & Tolhurst, 1970; Hollien, Coleman , Thompson & Hunter, 1970). One of the ongoing conclusions of these studies is that the intelligibility of divers talking through underwater communicators may be lim ited more by the talkers' input to the various systems, than in the electromechanical systems themselves. A major constraint placed upon a diver is the necessity for wearing an oral muzzle or mask to allow normal respiration and to provide a cavity into which he talks. In addition to adding another closely coupled acoustic cavity to the speech mechanism, it is necessary that the talker speak and exhale into a slightly raised pressure than that of the surrounding water, to prevent flooding of the oral muzzle. Leyden (1956) investigated breathing resistance in several configurations of underwater breathing systems and reported increases in pressure due to the systems themselves of from 0·1 to 17·0 cmH 2 0 . The role of varying air pressure in the production of human speech has been well recognized. Differences in air pressure within the vocal tract have been found to be asso-
266
R. F. Coleman and W. R. Krasik
ciated with vocal intensities (Van den Berg, 1956; Draper, Ladefoged & Whitteridge, 1960; Kunze, 1962; Arkebauer, 1964). Other aspects of speech associated with differences in air pressure include the rate of syllable production (Arkebauer, 1964), rate of air flow through the vocal tract (Flanagan, 1958; Kunze, 1962), different sounds or classes of sounds (Black, 1950; Arkebauer, 1964; Brown & McGlone, 1969a) and changes in pitch (Kunze, 1962). The literature concerning the relationship of air pressure variations and speech is sometimes contradictory, and is confounded by the fact that various investigators used different methodologies to arrive at their conclusions. However, general conclusions from these studies would indicate that the minimum transglottal pressure at which the larynx will oscillate is on the order of 1-3 cmH 2 0 . Nominal conversational speech is accompanied by pressures of 4-10 cmH 2 0 pressure, and maximum shouting may use a pressure of30 cmH 2 0. Consonants appear to be higher pressure components than vowels. Since divers, of necessity, must speak into a slightly raised atmospheric pressure due to the design of current SCUBA, it seems reasonable to assume that such elevated levels would have an effect on phonemes which rely for the most part on a pressure build-up in normal speech production. A further complication arises from the fact that such elevated pressures in the SCUBA system tend not to be constant, but rather vary according to the stiffness and release characteristics of the exhaust flutter valves on the systems. The purpose of this study was to examine the effect of varying ambient muzzle pressure on diver communication. A second purpose was to ascertain whether there is a differential effect of such varying pressure upon the intelligibility of discrete phonemes. Procedure
Selection ofspeakers Three male and two female speakers, who ranged in age from 23 to 33 years of age, with "normal" speech as defined by the experimenter were used in this study. One speaker was an experienced SCUBA diver, who had experience in underwater communication. Instrumentation A conventional SCUBA air tank and a three-stage regulator were modified to accommodate a Bioengionics Nautilus oral muzzle. The oral muzzle contained an EM-3 microphone, located approximately one inch from the speaker' s lips, to transduce speech to an Ampex 601 tape recorder. A "T" tube was tightly fitted with a rubber disc exhaust valve in its vertical outlet. The exhaust hose extending from the muzzle was positioned in one of the horizontal outlets; the third opening was sealed. Also installed in the muzzle was a probe tube which led to a "U" tube manometer and a Statham PC-97 differential pressure gage; the latter was attached to a Grass Model 7 polygraph. The pressure recording equipment was calibrated with the aid of the "U" tube water manometer to enable reading of pressure values directly from the graphic record during connected speech. The special "T" tube was rotated to permit the exhaust valve to lie on a vertical axis and was extended from an adjustable clamp located in a small water tank. Valve release pressure could be systematically varied by lowering and raising the "T" tube structure in the water. A SCUBA face mask with a purge valve was worn by all speakers. The equipment was located in a sound treated chamber. Word lists Word lists used for this experiment were lists A through E of the Griffiths' Rhyming Minimal Contrasts Articulation Test (Griffiths, 1967). These lists consist of 50 words each
261
Underwater communication
with five alternate forms. The alternate words represented a closed set of responses for listeners and permitted analyses to be made of consonant confusions. Test procedure
Each speaker trained for the experiment by reading several practice word lists at the different pressures used in the study. After familiarizing himself with a particular word list, the speaker read each test word preceded by the phrase "Say the word ... "The lists were quasi-randomized with respect to order of the lists and oral muzzle pressures. Amplitude levels were monitored for each speaker by use of a VU meter. The output of speech at the muzzle microphone was recorded on an Ampex 601 tape recorder. A Sony TC-355 tape recorder was used to dub from the master tapes to prepare listening tapes. A 4-5 s pause was inserted between each stimulus phase. The resultant tapes were played to listeners through an Ampex 620 speaker. Listening task
A total of 20 listeners who were all university graduate students were used. Prior to listening to the lists, each participant was required to pass a screening discrimination test with a minimum score of 96 %. After passing the screening procedure, listeners were presented 30 lists in random order for words, pressures, and speakers. Each word list required approximately 5 min to complete, and 5-10 min rest was allowed after sets of 10 lists were heard. The entire task required approximately 3-3·5 h. The task was divided into two or more sessions for all listeners. The listeners were provided with one of five randomized response sheets for each of the 30 test lists, and were required to circle the word heard. The listeners were forced to guess if not certain of the proper response. Results and discussion Intelligibility scores for each condition, as well as specific phoneme distortions, were derived from the listener responses. Mean intelligibility scores for six reading conditions for each of five speakers are presented in Table I. Table I Mean intelligibility scores for six reading conditions for each of five speakers. (NM* =no oral muzzle was worn by the speakers. M =Male; F= Female)
Muzzle back pressure (cmH 2 0)
Speakers Ml M2 M3 Fl F2 Mean
NM*
0
97·1 98·3 94-4 97·0 98 ·8
89-4 76·1 75·7 78 ·6 70·0
97·1
78·0
2
4
6
85·6 63·6 71·9 79·9 E8·7
79·1 60·4 69·2 78 ·8 63 ·5
82·9 59·6 63-4 72-8 53·3
75 ·0 59·0
73·9
70·2
66-4
64·7
55·5
73·8 60·1
268
R. F. Coleman and W. R. Krasik
Effect ofora/muzzle ofspeech intelligibility The effect of the oral muzzle itself on communication was determined from comparison of mean intelligibility scores at 0 cmH 2 0 and the "no muzzle" condition for all five speakers. An average 20% decrease in intelligibility resulted from donning the oral muzzle. For individual talkers, the difference in scores obtained with and without the oral muzzle ranged from 7·7 to 28·8 %- Thus, as other investigators have found (Hollien, 1969; Hunter, 1968) the simple addition of an oral muzzle has a significant and deleterious effect upon speech transmission. The fact that no speaker scored less than 94 % in the "no muzzle" condition attests to the quality of the underwater microphone used in the system, as well as the integrity of the entire recording-playback-listeningdesign. Effect ofpressure variation on speech intelligibility A systematic trend may be noted for intelligibility scores to decrease with increases in muzzle release pressure. The average loss of intelligibility was approximately 2·3 % per increase of 1 cmH 2 0 pressure. From a pilot experiment performed in a swimming pool with a diver wearing SCUBA equipment, at a depth of0-9 ft, peak release pressures for an underwater communication system were on the order of 4 cmH 2 0 pressure. During sustained conversation, nominal release pressure was approximately 0·75 to 1·50 cmH 2 0. Thus, the pressures used in the present study were assumed to be representative of those encountered in typical SCUBA communication work. Leyden's early research (1956) found breathing resistance pressures as high as 17 cmH 2 0 in a closed SCUBA system, a level considerably greater than that used in the present study. Effect of speaker sex on speech intelligibility In order to ascertain the possible influence of speaker sex on speech intelligibility, mean intelligibility scores for six reading conditions for the three male speakers, two female speakers, and all five speakers combined were calculated. The results are displayed in Fig.1 . The trend of decreased intelligibility with increases in pressure was noted for both sexes. The largest difference of 6·1% in intelligibility scores of males and females wearing an oral 100
u
90
~
0
u
"' a ;;:
"0
80
0
c:., u w
70
60 nm* 0
2
I
I
4
6
Muzzle bock pressure (c mH 20 )
Figure I
Plot of mean intelligibility scores for six reading conditions for three male speakers, two female speakers, and all five speakers combined. (nm =no oral muzzle.) Female ( · · ··· ·);male(---); combined(--).
269
Underwater communication
muzzle appeared at 0 cmH 2 0. Since pressure was not a factor in this condition it may be hypothesized that such differences are due to the resonance effects of the oral muzzle itself. The two speakers who achieved the highest intelligibility scores on this task were a male and a female. The only apparent differences between these two speakers and the other three who made lower intelligibility scores were that the superior talkers were physically larger, spoke with a higher overall intraoral breath pressure, and had a considerably slower speech rate than the other speakers. Phoneme analysis ofunderwater speech intelligibility
Use of the Griffiths' Rhyming Minimal Contrasts Articulation Test permitted an error analysis of the closed set of responses from listeners. Chi-squared tests of significance were used with the hypothesis that the word read by the speaker would be the expected word Labial
Labiadental
Dental
Alveolar
Palatal
Velar
Glottal
p Plosive
g
b
J v ~{)
Fricati ve
h
3
1] Affricative
Figure 2
d:;
Semivowel
w
Nasal
m
n
0
Place-manner phoneme chart demonstrating substitutions for consonants spoken by talkers. Each substitution listed occurred a significant number of times in a closed set response list (X 2 significance= 0·01.)
heard by the listeners. A confidence level ofO·OOl with one degree of freedom was chosen to denote a significant difference between the expected (speaker) word and observed (marked) word. It was apparent that as muzzle back pressure increased, more consonants were significantly distorted. Thus, one consonant at the "no oral muzzle" condition, nine at 0, 14 at 1, 20 at 2, 23 at 4, and 22 at 6 cmH 2 0 were missed a significant number of times. The almost equal number of words significantly missed at the higher pressures suggests that speaking into any raised pressure was the degrading factor in the task, without regard to the magnitude of pressures used. A second X2 analysis was made with the hypothesis that there was no significant difference between the expected word and any one of the phonemes that could have been substituted for the expected sound. In the first x2 analysis, the significance of missing the expected word was tested; in the second set, the significance of the specific substitutions made for the expected word was evaluated.
270
R. F. Coleman and W. R. Krasik
Phoneme substitution errors which occurred a significant number of times for the intended consonant enabled construction of a confusion matrix. Figure 2 displays the phoneme substitutions on a place-manner chart. For example, it can be seen that ftf was substituted for /k/ and vice versa. Examination of Fig. 2 will show that, for this experiment, the most obvious trend was for listeners to make "place" errors, and that plosive phoneme confusions were most common. Numerous phoneme confusions were also noted for fricatives, but to a lesser extent than plosives. There appeared to be a general trend for lingua-alveolar and linguavelar plosives, i.e. ft/ and /k/, to shift forward. The forward shift was also in evidence for their voiced cognates, fd/ and fgf. There also was a trend for nasals to shift toward plosives. This would not be unexpected in view of the facts that nasals are low pressure phonemes, and that an extra cavity, a face mask, was also added to the speech mechanism. The mask possibly functioned as a closely coupled resonator, shifting nasal resonances downward. Fatigue effects ofincreased muzzle pressure This investigation has demonstrated a systematic trend for speech intelligibility to be affected by increased oral muzzle pressure. Of equal significance to an underwater diving task, however, is the physical fatigue associated with such activity. The talkers used in this study found it extremely difficult to speak into a muzzle pressure of 4 and 6 cmH 2 0. Although the problem was not of significant magnitude to influence the results of the experiment, it was apparent that none of the subjects, including the experienced SCUBA diver, would voluntarily communicate over a sustained period because of the physical effort involved. Leyden's (1956) study of resistance components in an underwater breathing apparatus reported respiration pressures at depth (132 ft) of 17 cmH 2 0 pressure for a particular type of exhaust valve. Such an escalation of pressure, even allowing for the superior physical condition of most professional SCUBA divers, would undoubtedly result in considerable fatigue and serious degradation of speech communication. It may be hypothesized from the experience of the subjects in this study, that differential pressures of no more than 2- 4 cmH 2 0 should be allowed for acceptable speech communication. Conclusions
In summary, it appears that the following conclusions are warranted from the results of this investigation. ( 1) There was a systematic decrease in speech intelligibility with increases in oral muzzle release pressures. (2) The major effect on speech degradation was due to the effects of donning an oral muzzle and face mask. (3) There appeared to be no significant differences attributed to speakers' sex in this study. (4) Plosive consonants appeared to be distorted most often with a moderate trend for lingua-velar and lingualveolar consonants to be shifted forward in place. (5) Nasal consonants were shifted perceptually from nasal manner of production to fricative and plosive manner, while maintaining the same approximate place of articulation. References Arkebauer, H. J., Hixon, T. J. & Hardy, J. C. (1967). Peak intraoral air pressures during speech. Journal of Speech and Hearing Research 10, 196-208.
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271
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