Effects of prolonged oral reading on F0, SPL, subglottal pressure and amplitude characteristics of glottal flow waveforms

Effects of prolonged oral reading on F0, SPL, subglottal pressure and amplitude characteristics of glottal flow waveforms

Journalof Voice Vol. 13, No. 2, pp. 303-315 © 1999 SingularPublishingGroup,Inc. Effects of Prolonged Oral Reading on F0, SPL, Subglottal Pressure and...

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Journalof Voice Vol. 13, No. 2, pp. 303-315 © 1999 SingularPublishingGroup,Inc.

Effects of Prolonged Oral Reading on F0, SPL, Subglottal Pressure and Amplitude Characteristics of Glottal Flow Waveforms Erkki Vilkman, *Eija-Riitta Lauri, -~PaavoAlku, SEeva Sala, and **Marketta Sihvo Department of Otolaryngology and Phoniatrics, University of Oulu, Oulu; *Department of Otolaryngology and Phoniatrics, Helsinki University Central Hospital, Helsinki; ?Laboratory of Electronics and Information Technology, University of Turku, Turku; -~Department of Otolaryngology and Phoniatrics, Turku University Central Hospital, Turku; and **Department of Phoniatrics, Tampere University Hospital, Tampere, Finland.

Summary: The effects of prolonged (5 × 45 minute) reading (vocal loading) on fundamental frequency (F0), sound pressure level (SPL), subglottal (intraroral) pressure (p), and two glottal flow waveform parameters (AC amplitude of glottal flow, f, and negative peak amplitude of differentiated flow (d) of normal female and male subjects (N = 80) were studied. Two rest (morning and noon) and three loading (two in the morning and one in the afternoon) samples were recorded and analyzed. The glottal waveforms were obtained by inverse filtering of the acoustic pressure waveforms of speaking voice samples. The analyses were based on measurement and inverse filtering of the first stressed syllable of "paappa" words repeated 3 × 5 times for normal, as soft as possible, and as loud as possible phonation. In normal phonation the parameter values changed statistically significantly due to loading. In many cases the values obtained in the morning samples changed after the first loading session. This is interpreted as a vocal "warming-up effect." Especially in soft phonation p, d, and f were sensitive indicators of vocal loading. In both normal and soft phonation, the SPL, p, d, and f values tended to rise due to prolonged reading in the morning and afternoon samples, indicating increased effort (normal phonation) and a rise in the phonatory threshold (soft phonation). The lunch break vocal rest ("rest effect") considerably affected the parameter values in many cases. Key Words: Vocal loading--Fundamental frequency--Sound pressure level--Glottal flow waveform--Amplitude--Differentiated glottal flow.

The ergonomic and occupational health aspects of vocally demanding professions have been given inadequate attention. However, occupational voice problems do not lack significance from the point o f

view of either the patient or the society. Apparently, one of the main underlying problems is the shortage of knowledge concerning the environmental and ergonomic aspects of voice disorders as an occupational health problem.1 The main loading factor in voice/speech professions is the demand for prolonged use of the voice itself for the working task. The results of the effects of prolonged vocal use on voice function are quite scant

Accepted for publication December 15, 1997. Address correspondence and reprint requests to Professor Erkki Vilkman, Department of Otolaryngology and Phoniatrics, University of Oulu, FI-90220 Oulu, Finland.

303

304

ERKKI VILKMAN ET AL

and far from conclusive. A rise in the fundamental frequency of phonation is a finding that has been made in many studies. However, this effect seems to have been dependent on the speech task used in the study. 2-7 In addition, a study on vocal fatigue, in which the output level and pitch of phonation were high, revealed a significant difference in the maintenance of voice between an untrained and a trained subject. In this case the changes indicating fatigue were mainly subjective complaints of tiredness and other subjective symptoms. The acoustic measures used correlated only with the subjective reports of the trained subject.8 From the methodological point of view, there are no established sensitive measures of vocal fatigue. As the glottal waveform represents the glottal source function and apparently reflects, to a considerable extent, the vibratory behavior of the glottis, inverse filtering might be a tool for describing loading changes in voice production. Inverse filtering, that is, cancellation of the effects of vocal tract resonances, is usually performed electronically, processing either the transoral airflow 9 or the acoustic signal. 1°,11 In an earlier study, the changes in period length were not seen to be directly related to prolonged voice use, but rather reflected effects of vocal warming-up and rest, respectively. The time-based parameters of glottal flow showed a gender difference, which was interpreted in such a way that females tended to change the type of phonation toward hyperfunction as a result of vocal loading more than males. 12,13 The negative peak amplitude of the first derivative of glottal flow (d) represents the main excitation of the vocal tract and is closely related to the sound pressure level (SPL) of vocal output. 14 Both are functions of subglottal pressure (p). The AC amplitude of glottal flow (f) has been found to correlate with SPL and also with the type of phonation. 14~19 These amplitude characteristics of the glottal flow waveform have not been used before for investigating vocal loading changes. The speech material of the present study was produced in accordance with the principles of speaking voice profile assessment (Sprechstimmprofil). 2° In this relatively widely used method, the subjects are asked to produce words or phrases using their normal loudness level and as soft and as loud a voice as possible. The extremes of the subjects' vocal capabilities are thereby determined. In this respect, the procedure Journal of Voice, Vol. 13, No. 2, 1999

differs from the earlier studies in which the loud and soft phonations were produced at a "comfortable"

level.14,17,19 The present study is a part of a project addressing the influences of prolonged reading and a number of ergonomic factors on voice. In the present investigation, the parameters F0, SPL, f, and d were analyzed. It was hoped that the experiment would help to shed light on the questions concerning the effects of vocal loading in general as well as on some methodological issues and gender differences.

SUBJECTS AND METHODS Subjects The subjects consisted of randomly chosen young university students. There were 40 females and 40 males with an average age of 22 years; age range was 18-45 years. They were all nonsmokers and had no known hearing defects or voice/speech disorders. A clinical examination (indirect laryngoscopy) of each subject was performed before the experiment was started. None of the subjects had attended voice therapy or formal voice training. Voice training has been reported to affect the development of vocal fatigue and recovery. 2,8 The subjects received written instructions concerning pretest sleep, medication, and alcohol use. Each subject participated only once in the experiment. The subjects practiced all the experimental tasks beforehand to avoid effects of learning during the test. The experimenter used standard instructions during the experiment. All of the subjects were completely naive to the exposure arrangements and hypotheses.

Laboratory facilities All data recordings were carded out in a sound-insulated room at the Institute of Occupational Health, Helsinki, Finland. The loading of the voice took place in a normal office room immediately adjacent to the recording room. The background noise level was <30 dB(A) in the recording room and <40 dB(A) in the loading rooms. The room temperature was 22°C _+ I°C.

Loading arrangements The voice of each subject was loaded by orally reading a novel of his or her own choice. There were two alternative output levels of reading; the subjects were

EFFECTS OF PROLONGED ORAL READING ON GLOTTAL FLOW WAVEFORMS

either sitting or standing while reading and relative ambient humidity was either low or high. The effects of these factors will be reported elsewhere. The schedule of individual loading tests can be seen in Table 1. The purpose was to simulate a (primary school) teacher's working day with five 45minute lessons.

Recording samples The samples were recorded using a microphone (Brtiel and Kj~er 4176) and a digital instrumentation recorder (Teac RD101T). The acoustic signal was recorded on two channels. The input level of one channel was attenuated by 20 dB. The microphone was placed 40 cm from the subject's lips. The distance was constantly monitored. All the recordings were made by the same researcher (M.S.). The material recorded for the purposes of the present study consisted of three strings of five/pa:p:a/ words (Finnish for "grandpa") produced normally, as softly, and as loudly as possible in this order. The loud voice quality can be described as shouting. Yelling or screaming was not accepted. The rate of

TABLE 1. The Schedule o f the Experiment 8.00 8.45 9.00 9.45

Clinical examination Rest sample 1 Reading 45 min Loading sample 1

10.00

Pause 15 min

10.15

Reading 45 rain

11.00

Pause 15 min

11.15

Reading 45 min

12.00 12.15

Loading sample 2 Clinical examination Lunch

13.00

Rest sample 2

13.15

Reading 45 min

14.00

Pause 15 min

14.15

Reading 45 min

15.00 15.15

Loading sample 3 Clinical examination

305

production was approximately 1 word per second. Three words in the middle of each string were further stored on a computer disc (Macintosh IIci) to be used in the analysis. During the task, the subjects kept a plastic tube between their lips in the angle of the mouth for intraoral pressure recording. The open end of the tube was placed a few centimeters in the mouth above the tongue. The other end of the tube was connected to a pressure transducer (F-J Manophone, Copenhagen, Denmark). The intraoral pressure has been shown to give an estimate of the subglottal pressure. 9

Measurement of the samples Subglottal pressure was estimated from calibrated, plotted recordings. The intraoral pressure values for the /p/ occlusion were measured from the middle member of the three strings at following the long/a/ vowel of the first syllable. This point was used for the analyses because it has been found to give the most reliable estimate of subglottal pressure.17 The long vowel of the first stressed syllable of the test word/pa:p:a/ was inverse-filtered to estimate the glottal flow waveform. The inverse filtering was performed using the IAIF method (Iterative Adaptive Inverse Filtering) implemented on a computer (Macintosh IIci). The IAIF method basically computes the model of the vocal tract transfer function by first cancelling the average effect of glottal excitation on the speech spectrum. The estimate of the glottal contribution is pitch asynchronously computed during several pitch periods with an iterative procedure. The bandwidth of speech was 4 kHz, and the IAIF analysis was performed with a time window of 32 ms. The low-frequency distortion due to ambient noise was removed by filtering all speech signals with a high-pass filter. The cutoff frequency was 30 Hz. The method has been described in detail elsewhere .10,11 In the IAIF method the estimated glottal flow is computed from the acoustic speech pressure waveforms that have been recorded in a free field, that is, without a flow mask. 9 Absolute flow values including the DC flow cannot be obtained using the methods based on the acoustic speech pressure waveform. 9 However, the IAIF method takes advantage of a scaling technique which permits a comparison of glottal flow waveforms in an amplitude domain on an arbitrary scale. The scaling is based on adjusting the Journal ofVoice, Vol. 13, No. 2, 1999

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ERKKI VILKMAN ET AL

DC gain of the all-pole vocal tract model to unity for all speech signals. The AC amplitude of the f values and the negative peak amplitude of the d values were automatically determined using an interactive analysis environment (N~ips~i, 21). An example of the principles of analysis is given in Figure 1. The amplitude values were determined over eight consecutive glottal periods for each of the three middle members of the/pa:p:a/word strings. Thus, the final f and d values used in the statistical analyses represent the mean of 24 periods. The SPL of the first stressed syllable of the test word was analyzed using a computer program (Intelligent Speech Analyser, ISA, developed by Raimo Toivonen, M.Sc.). A 160-ms segment was analyzed for 3 repetitions at 3 loudness levels. In other words, the segment analyzed was not exactly the same as the segment used for f and d measurements (8 periods).

Statistical treatment The results obtained are represented by arithmetic means (X) and standard deviations (SD). Statistical tests of the data were carried out using analysis of variance of repeated measurements (ANOVA). The subjects themselves were used as their own controls, which means that time was a within-subject variable. 22 The analyses were separately run for the morning and afternoon samples, because the initial analyses showed considerable differences between these samples and it seemed illogical to treat the second rest sample as a loading sample. It should naturally represent recovery rather than loading.

RESULTS The average F 0, SPL, p, d, and f values for females and males are given in Tables 2 and 3, respectively. The ANOVA results for statistically significant gender differences in the parameter levels, effects of loading and gender-dependent loading effects are given in Tables 4, 5, and 6, respectively.

Fundamental frequency (F0) The average F 0 values were always highest for loud phonation and lowest for soft phonation for females. Among males the first rest sample showed one exception to this general trend (F 0 in normal phonation was the lowest). The afternoon values (Rest 2, Loading 3) were higher than the morning samples for Journal of Voice, Vol. 13, No. 2, 1999

both genders at all the three loudness levels. During the 1-day experiment, the greatest change in many cases was observed after the lunch break (not immediately after vocal loading). In normal phonation, the F 0 values significantly statistically increased in the morning samples. The F 0 rise took place mostly after the first loading session.

Sound pressure level (SPL) SPL values at all the three loudness levels of phonation were significantly higher for males than for females. Loading affected the SPL values of normal phonation in both the morning and the afternoon samples. Both females and males showed a systematic rise in SPL. After the lunch break the values slightly dropped and rose again due to continued loading. In the morning samples the soft phonation SPL also tended to rise significantly, and this trend was systematic among female subjects.

Subglottal pressure (p) The p values were always lowest for soft phonation and highest for loud phonation. The p levels of loud phonation were higher for males than for females. Normal and soft phonations showed no gender difference. In the morning and afternoon samples the p values significantly statistically increased for soft and normal phonation due to loading. In the morning samples the p rise took place after the first loading session. In male normal phonation the values did not drop due to the lunch break rest. In the morning samples of soft phonation there was a significant interaction between loading and gender; the male values increased more.

Negative peak amplitude of differentiated flow (d) The d values were lowest in soft phonation and highest in loud phonation. The values were significandy lower for females than males. The d values of soft and normal phonation significandy statistically increased during loading for both females and males in the morning samples and in the afternoon samples of soft phonation. The female values in normal and soft phonation generally logically and systematically reflected both loading and rest. The values rose due to loading and dropped due to rest. The trends of males were not as systematic; for

EFFECTS OF PROLONGED ORAL READING ON GLOTTAL FLOW WAVEFORMS

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Journal o f Voice, Vol. 13, No. 2, 1999

308

ERKKI VILKMAN ET AL T A B L E 2. Average values (X +_ SD) of the

T A B L E 3. Average values ('X + SD) of

variables in the five recordings for females at three loudness levels (n -- 40)?

the variables in the five recordings for males at three loudness levels (n = 40)?

Loud

Normal

Soft

Loud

Normal

Soft

205.9 69.5 0.48 38.4 27.2

± 24.0 ± 4.4 ± 0.16 _+ 16.7 ± 9.8

203.6 58.4 0.25 13.0 13.2

+ 22.4 _+ 4.2 _+ 0.12 _+ 6.1 + 6.1

Rest 1 F0 SPL p d f

228.9 9619 2.54 388.4 155.8

_+ 54.7 _+ 7.8 ± 0.96 ± 193.8 .+ 89.2

100.9 72.1 0.47 66.1 71.2

_+ 14.2 _+ 4.5 _+ 0.15 ± 20.9 -+ 21.8

105.1 62.2 0.23 23.5 42.5

± 17.6 ± 4.2 ± 0.08 -4- 11.8 + 18.2

Loading 1 F0 335.0 ± 48.9 SPL 92.9 ± 4.3 p 1.82 + 0.76 d 98.9 ± 88.1 f 35.4 ± 29.5

207.5 70.9 0.52 42.8 27.7

+ 26.0 ± 4.2 _+ 0.15 ± 14.7 ± 9.8

200.7 59.7 0.25 14.7 14.6

± ± ± ± ±

24.2 4.9 0.07 6.5 6.0

Loading 1 F0 SPL p d f

229.2 96.8 2.75 381.8 151.6

± ± ± ± ±

59.0 7.3 1.65 167.6 74.2

107.0 74.8 0.57 82.6 80.2

± 16.4 ± 4.6 _+ 0.19 ± 23.5 ± 31.0

103.5 63.2 0.28 27.3 50.1

± 15.2 ± 3.5 _+_0.09 ± 12.1 ± 20.5

Loading 2 F0 340.3 ± 53.2 SPL 92.8 + 4.3 p 1.75 ± 0.79 d 101.0 ± 97.2 f 37.1 + 33.2

208.4 71.9 0.51 46.1 29.7

± 24.5 _+ 4.4 ± 0.17 _+ 16.5 -+ 9.8

202.5 60.0 0.26 16.1 16.5

_+ 23.0 _+ 4.3 ± 0.08 _+ 9.1 + 9.7

Loading 2 F0 SPL p d f

230.1 96.6 2.48 367.6 150.0

+_ 51.5 -+ 6.5 + 1.11 _+ 150.0 _+ 69.1

106.2 75.0 0.55 82.3 81.3

,+ 14.9 -+ 3.7 + 0.18 _+ 22.8 -+ 31.9

105.0 62.4 0.27 25.3 47.1

± 16.8 + 3.6 + 0.08 _+ 10.2 _+ 18.5

Rest 2 F0 SPL p d f

212.6 71.2 0.50 44.9 29.3

± ± ± ± ±

26.0 4.4 0.21 18.9 12.2

205.6 59.8 0.24 14.0 13.8

± 22.7 ± 6.2 _+ 0.09 _+ 7.9 ± 7.8

Rest 2 Fo SPL p d f

237.5 96.4 2.71 383.9 157.0

_+ 52.7 ± 6.3 + 1.10 -+_ 167.5 ± 71.5

108.8 74.0 0.57 81.1 88.2

± 14.3 ± 3.6 _+ 0.18 + 23.5 _+ 31.2

107.1 63.3 0.26 27.0 48.2

± 16.6 ± 3.7 +_ 0.08 _+ 13.1 _+ 19.8

213.7 72.0 0.53 49.3 30.9

± 26.9 ± 4.3 ± 0.23 _+ 19.4 _+ 11.8

204.8 60.7 0.25 17.4 16.9

± ± ± ± +

Loading 3 F0 SPL p d f

231.7 ± 52.6 109.6 ± 15.4 105.5 ± 17.5 95.4 + 5.4 74.9 _+ 3.4 63.5 +_ 3.9 2.68 ± 1.19 0.59 -+ 0.20 0.29 ± 0.08 407.8 ± 186.1 78.2 ± 19.7 28.6 ± 12.4 164.4 + 75.6 79.6 + 28.4 51.3 ± 19.8

Rest 1 Fo SPL p d f

331.1 92.7 1.77 105.5 39.9

346.9 92.5 1.72 94.4 34.7

+ ± ± + -+

± ± ± ± ±

52.5 4.4 0.75 102.7 36.5

51.0 4.1 0.77 92.8 33.2

Loading 3 F0 346.5 ± 53.1 SPL 93.5 ± 4.3 p 1.82 ± 0.78 d 102.7 ± 101.3 f 34.8 ± 33.2

21 4.8 0.09 9.1 8.1

F 0 = fundamental frequency (Hz), SPL = sound pressure level (dB), p = subglottal (intraoral) pressure (kPa), d = peak differentiated flow amplitude (data points × 10-3), f = AC flow amplitude (data points × 10-3).

morning

samples

showed

the greatest change

after

In the morning samples the f values rose in normal and soft phonation.

the first loading session. In the afternoon loud phonation samples the values significantly increased.

*F0 = fundamental frequency (Hz), SPL = sound pressure level (dB), p = subglottal (intraoral) pressure (kPa), d = peak differentiated flow amplitude (data points × 10 3), f = AC flow amplitude (data points × 10 3).

The

last sample

of males

showed the highest value of the day in loud phona-

In the afternoon samples the f

values in soft phonation

increased for both females

and males. In the afternoon samples there was a gender differ-

tion despite the relatively stable SPL value.

ence in normal

AC amplitude of glottal flow (f)

males and rose slightly for females. In loud phona-

The f values always grew from soft to loud phona-

phonation.

The values dropped

for

tion the female f values were lowest in the afternoon

tion. They were higher for males than for females at

samples, whereas the highest male value was mea-

all l o u d n e s s l e v e l s o f p h o n a t i o n .

s u r e d i n t h e l a s t s a m p l e o f t h e test.

Journal of Voice, Vol. 13, No. 2, 1999

E F F E C T S O F P R O L O N G E D ORAL R E A D I N G O N GLOTI"AL F L O W WAVEFORMS

309

T A B L E 4. F and p values of statistically significant

T A B L E 5. F and p values of statistically significant

gender differences (female and male n = 40) in the parameter values for the different loudness levels?

effects of loading (time) on the parameter values for different loudness levels?

AM

PM

F

p

F

AM p

Loud

F0 SPL p d f

< < < < <

0.000 0.001 0.000 0.000 0.000

106.12 7.20 18,39 86.74 94.65

< 0.000 < 0.01 < 0.000 < 0.000 < 0.000

576.47 18.02 111.79 158.74

< < < <

0.000 0.000 0.000 0.000

536.40 13.37 75.02 130.75

< 0.000 < 0.001 < 0.000 < 0.000

427.71 21.69 46.78 202.23

< < < <

0.000 0.000 0.000 0.000

329.59 10.09 34.94 144.44

< 0.000 < 0.01 < 0.000 < 0.000

Soft

F0 SPL d f

F

p

F

p

Loud

104.58 11.18 15.83 91.72 85.24

Normal

F0 SPL d f

PM

*ANOVA, between subjects, separately for morning (df = 2, 128) and afternoon (dr = 1, 64) samples. F 0 -- fundamental frequency, SPL = sound pressure level, p = subglottal (intraoral) pressure, d = negative peak amplitude of the differentiated flow, f = AC flow amplitude.

d

4.22

< 0.05

4.22 10.95

< 0.05 < 0.01

9.82 5.62 4.11

< 0.01 < 0.05 < 0.05

Normal

Fo SPL p d f

4.42 15.08 18.38 14.34 3.25

< < < < <

0.05 0.0001 0.000/ 0.0001 0.05

3.28 17.89 3.66 3.75

< < < <

0.05 0.0001 0.05 0.05

Soft

SPL p d f

*ANOVA, within subjects, separately for morning (df = 2, 128) and afternoon (df = 1, 64) samples. F0 = fundamental frequency, SPL = sound pressure level, p = subglottal (intraoral) pressure, d = negative peak amplitude of the differentiated flow, f = AC flow amplitude.

T A B L E 6. F and p values of statistically significant

interaction between gender and loading (time) on the parameter values for different loudness levels? AM

PM

DISCUSSION

F In t e r m s o f F o, S P L , a n d p v a l u e levels f o r n o r m a l p h o n a t i o n , the levels o f the p r e s e n t s t u d y w e r e in line w i t h the o t h e r studies. C o m p a r e d to the o t h e r studies u s i n g inverse filtering m e t h o d s on soft a n d l o u d p h o n a t i o n s a m p l e s , the S P L a n d p v a l u e s o f the p r e s e n t s t u d y w e r e l o w e r a n d higher, respectively, b e c a u s e the e x t r e m e s w e r e sought. 14,17,19 T h e soft p h o n a t i o n s a m p l e s o f the p r e s e n t s t u d y r e p r e s e n t e d the p h o n a t o r y t h r e s h o l d values. 18,23 T h e v a l u e s o b t a i n e d for the A C a m p l i t u d e o f glottal f l o w (f) a n d the n e g a t i v e p e a k a m p l i t u d e o f diff e r e n t i a t e d glottal f l o w (d) c a n n o t b e d i r e c t l y c o m p a r e d to studies in w h i c h the g l o t t a l f l o w e s t i m a t i o n h a s b e e n b a s e d on the r e c o r d i n g o f oral flow. 9 H o w ever, the a m p l i t u d e v a l u e s (arbitrary units) b a s e d on the i n v e r s e filtering o f the a c o u s t i c signal s h o w e d t r e n d s s i m i l a r to the studies b a s e d on f l o w m e a s u r e m e n t . T h e a m p l i t u d e v a l u e s i n c r e a s e d w i t h an in-

p

F

p

normal

p d f

3.88 3.68

< 0.05 < 0.05 6.04

< 0.05

soft

p

3.22

< 0.05

*ANOVA, within subjects, separately for morning (df = 2, 128) and afternoon (df-- 1, 64) samples, p = subglottal (intraoral) pressure, d = negative peak amplitude of the differentiated flow, f = AC flow amplitude.

c r e a s i n g l o u d n e s s level, w h i c h h a s also b e e n reporte d b y H o l m b e r g , H i l l m a n , a n d Perkell, t4 Herteggtrd, 17 a n d Sulter. 19 In a d d i t i o n , studies a n a l y z i n g the t y p e o f p h o n a t i o n a l o n g the b r e a t h y - p r e s s e d axis u s i n g a m p l i t u d e - b a s e d p a r a m e t e r s on an a r b i t r a r y scale

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have produced logical and reliable results.2425 In the present study, the d and f values could be measured automatically and they reflected clear loading effects. Compared to time-based parameters, these amplitude parameters appeared to be more sensitive in this respect. One of the reasons is that the determination of time instants of glottal opening as well as glottal closing are based on the experimenter's decision, which is often difficult due to gradual glottal opening and inaccuracies caused by formant ripple.12,13 The aim of the present study, as well as the whole project, was to explore loading changes in various parameters to improve the understanding of the background of occupational health and diseases of professional voice users. In the light of the relevant literature, 14,15A7-19,23,26 the interpretation of the parameter changes can be outlined as follows: (1) in normal phonation, an Fo, SPL, p rise, and d rise and an f drop would indicate increased physical laryngeal work in terms of an increased number of vibrations, adductory force, and tissue acceleration; (2) in loud phonation, an F o, SPL, p rise, and d rise would reflect the motivation and the remaining physical resources to produce as loud a voice as possible, in which context an f drop would indicate a change toward a pressed type of phonation; (3) in soft phonation, a SPL, p rise and d rise would imply a phonatory threshold shift, which might reflect an impairment of the viscoelastic characteristics of the vocal folds. The results of this 1-day test revealed phenomena related to the results and discussion of our earlier studies) 2,13 First, it is obvious that the difference between the first rest sample and the first loading sample should be considered as the time during which the "warming up" of the voice takes place. This interpretation is supported by a perceptual and questionnaire study. 27 Against this background, for instance, the F 0 changes do not directly reflect vocal loading. The main effect, an F 0 rise in normal phonation, was already observed after the first loading session and should apparently be interpreted as a warming-up change connected with the initial adaptation of the vocal apparatus to the loading. Second, an F 0 rise for all loudness levels was connected with the lunch break, which also supports the view that the F 0 rise reflects phenomena other than direct loading changes in voice production. In general, this illustrates the basic problem of studies on vocal strain: There are no ways of getting direct inforJournal of Voice, Vol. 13, No. 2, 1999

mation on the subtle changes in the vibratory properties of the vocal folds or the metabolic or functional state of the laryngeal muscles. It is obvious that behavioral factors are involved to a notable extent, that is, the vocal outcome of the apparent mucosal changes depends on the subject's reaction to the change. Respiratory and laryngeal muscular compensation for the loading effects are most likely to be an essential part of the outcome. However, there is a total lack of knowledge as to which perceptual factors monitored by the speaker lead to corrections in the control system of voice production. One aspect underlying the pitch rise might be that the subjects tend to use this maneuver to avoid a breathy or rough voice quality. In both female and male normal phonation, the SPL slightly, but systematically, increased. In a 1hour laboratory test, SPL was also found to rise due to voice use. 2 In the present study, the lunch break caused a slight SPL drop, which can be considered logical as far as the effects of vocal loading are concerned. A similar trend was seen in the phonatory threshold values (soft phonation) for females; therefore, loading increased the values, while rest lowered them. From the point of view of the physiological interpretation of the effects of loading, it is interesting to note that females were able to produce the highest SPL value in the last loading sample of the day. This might be interpreted in such a way that there was no true laryngeal or respiratory muscle weakness due to fatigue. Because SPL is very closely correlated with subglottal pressure,17-19, 23 it is not astonishing that in normal and soft phonation the p values also rose due to loading. In soft phonation the effects of loading were systematic. Another factor closely related to SPL is d (c.f., 15,16,18). For females the observed trends in d in normal and soft phonations were identical to SPL; for males there were some deviations. The same was true of f changes. However, the male f values in normal phonation after the lunch break tended to be relatively high and continued straining of the voice made the values drop. This might imply a change towards increasingly "flow" type phonation due to the 1-hour voice rest.15,16,18 There is no doubt that d and f are not only related to SPL and p but also to the type of phonation, such as the vocal fold adduction.15A6, 24-26 In this light, gender differences are of great interest. It is well known that females have more functional and organ-

EFFECTS OF PROLONGED ORAL READING ON GLOTFAL FLOW WAVEFORMS

ic voice problems than males (see, e.g., Vilkman, 1 for a review). We found the gender-specific changes in d values as an indication of a change toward hyperfunction. A similar hypothesis was based on the earlier findings on loading changes in time-based parameters of glottal waveforms.12,13 This hypothesis calls for further testing. Besides, the major factor underlying the female majority in clinical materials is most likely the fact that, due to its higher frequency of vibrations, the vocal apparatus of females is under a greater strain in working situations and hence liable to fatigue and, eventually, damage.t From the clinical point of view, it seems that, when we make clinical voice recordings, we should give more attention to the patient's vocal behavior during the same-day sessions. This appears to be especially true when we record patients with mild dysphonia or functional voice disorders. Further, when short-term clinical vocal loading tests are performed, it has to be borne in mind that the results can be severely affected or even reversed, depending on the baseline level of the test. On the other hand, even short but wellcontrolled clinical loading tests may be useful when the issue that is tested is understood (i.e., warmingup, loading, or vocal rest effects). As a final practical conclusion, the voice changes due to vocal strain observed in the present series of untrained, healthy, young subjects can be interpreted as causing potential risks for vocal health in the long run. However, it has to be kept in mind that the changes were generally rather small. A tentative preventive voice health program based on these results would include learning to control the naturally occurring voice changes in vocally demanding situations. In short, students of vocally demanding professions should be taught to avoid increased pitch and loudness levels as well as excessive adductory force. These conclusions suggested by the results of the present study are apparently well in line with experiences based on clinical work, voice therapy, and principles of vocal training. A c k n o w l e d g m e n t s : The skillful technical assistance by Toomas Altosaar, M Sc Eng., Seppo Helle, Ph.D., and Vesa V~ilim~ki, Dr. Techn, is gratefully acknowledged. The authors also wish to thank Erkki Nykyri, Ph.D., Pertti Mutanen, Ph.D., and Petri Toivanen, M.A., for their expert help with the statistical analyses. The research was

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supported by the Finnish Foundation of Occupational Health (project 91010).

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