Intraspeaker variability in fundamental frequency stability: An age-related phenomenon?

Intraspeaker variability in fundamental frequency stability: An age-related phenomenon?

Sue Ellen Linville

Department of Speech Pathology and Audiology, Marquette University, Milwaukee, Wisconsin 53233

(Received 1 March 1987; accepted for publication 14 October 1987)

The purpose of this investigation was to gather information on the extent to which intraspeaker variability on measures of jitter (%) and fundamental frequency standard deviation (Fo s.d. ) is age related in women. Fifteen repeat productions of the vowels ?i/,/a/, and/u/from 22 young women ( 18-22 years) were analyzed for Fo s.d. and jitter. Findings for these young speakers were compared with those for elderly speakers tested previously (Linville and Korabic, 1987). Results indicate that the aging process brings about increases in the variability individual women demonstrate on measures of Fo stability when producing sustained vowels as steadily as possible. Further, young speakers differed markedly from elderly speakers in the pattern of frequency instability variations observed across the three vowels tested.

PACS numbers: 43.70.Fq, 43.70.Gr

INTRODUCTION

Interest in studying the fundamental frequency stability characteristics of voices at different stages of adult life has developed from the desire to define normal vocal changes with aging. Such normative information would be useful as a basis for comparison with disordered populations. Previous research into age-related changes in fundamental frequency (F o) stability has suggested that, on the average, elderly speakers demonstrate reduced stability of Fo in comparison with young speakers (Wilcox and Horii, 1980; Stoicheft, 1981; Linville and Fisher, 1985a,b). However, when re- quested to produce sustained vowels as steadily as possible, some elderly speakers are equally as steady as young speakers in fundamental frequency (Wilcox and Horii, 1980; Lin- ville and Fisher, 1985a,b). In other words, elderly speakers are more variable on frequency stability measures than young speakers. Further, when individual elderly women produce multiple trials of vowels as steadily as possible, intraspeaker variability in Fo stability has been found to be quite considerable in some cases, suggesting the need for a large number of trials from any given elderly woman before selecting her "steadiest" production (Linville and Korabic, 1987). However, it is possible that young women demonstrate greater consistency in frequency stability on repeat trials of sustained vowels than elderly women. The purpose of this investigation was to determine the extent to which intraspeaker variability on measures of jitter and fundamental frequency standard deviation (Fo s.d.) is age related in women.

I. METHOD

A. Speakers

Speakers were 22 young women ranging in age from 18 to 22 years (mean -- 20.32, s.d. --0.95). Speaker selection was restricted to individuals who were nonsmokers with no

professional singing or acting experience. Speakers had no history of voice disorders, or present complaint of voice dis-

orders. In addition, all speakers were judged to have normal vocal quality by a speech pathologist. All speakers were in good health on the day of testing with no history of neuro- logical or chronic respiratory disease.

B. Recording procedures

Recording procedures were the same as those reported previously for elderly speakers (Linville and Korabic, 1987). Speakers were recorded individually in a quiet room using a high-quality reel-to-reel audio tape recorder (Sony Model TC630). Prior testing of the tape recorder insured that frequency response was within manufacturer's specifi- cations.

Speakers were required to sustain the vowels/i/,/a/, and/u/, as steadily as possible, for a minimum of 3 s, at a comfortable pitch and intensity level. Twenty-five repeat trials of each vowel were tape recorded at a mouth-to-micro- phone distance of 6 in. Vowel order was randomized across speakers. Prior to recording each vowel group, the speaker practiced producing that vowel as steadily as possible by monitoring productions on the CRT of a Visi-Pitch (Kay Elemetrics--model 6087).

C. Acoustic analysis

The fundamental frequency stability analysis system and reliability and validity of the system have been reported previously (Korabic et al., 1986; Linville and Korabic, 1987). Briefly, the tape recorder output was input to a Visi- Pitch (Kay Elemetrics--model 6087), which extracted the cycle-to-cycle fundamental frequency of the voice sample. Extracted frequency information from the Visi-Pitch was sampled by a microprocessor (Apple IIE) and stored in memory at two consecutive memory locations as a two-byte word. The effective sampling rate of the microprocessor was approximately 113.25 kHz.

Sampling information for 100 consecutive cycles follow- ing the initial 1 s of phonation was retrieved from memory

741 J. Acoust. Soc. Am. 83 (2), February 1988 0001-4966/88/020741-05500.80 ¸ 1988 Acoustical Society of America 741

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TABLE I. Mean, standard deviation (s.d.), and range ofjitter values ( % ) in young women on the 15 steadiest productions of/i/,/a/, and/u/.

/i/ /a/ /u/

Speaker Mean s.d. Range Mean s.d. Range Mean s.d. Range

1 1.14 0.19 0.88-1.53 0.35 0.04 0.27-0.42 1.11 0.13 0.88-1.28 2 1.42 0.27 0.87-1.78 0.53 0.08 0.35-0.62 1.39 0.49 0.53-2.04 3 1.39 0.40 0.79-1.88 0.69 0.27 0.34-1.16 1.33 0.14 1.13-1.51 4 0.75 0.09 0.60-0.90 0.38 0.05 •0.31-0.45 0.63 0.07 0.47-0.75 5 0.61 0.09 0.•. •. 0.71 0.42 0.07 0.33-0.51 0.87 0.10 0.67-1.00 6 1.37 0.15 1.07-1.59 0.61 0.05 0.53-0.66 0.61 0.08 0.47-0.76 7 0.77 0.06 0.65-0.85 0.34 0.04 0.25-0.38 0.81 0.20 0.37-1.13 8 0.56 0.12 0.36-0.73 0.42 0.12 0.26-0.61 0.49 0.10 0.31-0.63 9 0.66 0.09 0.50-0.78 0.51 0.10 0.27-0.61 0.80 0.12 0.59-1.02

10 1.10 0.11 0.97-1.32 1.79 0.27 1.36-2.30 0.82 0.09 0.57-0.92 11 0.66 0.07 0.55-0.75 0.65 0.08 0.45-0.74 0.51 0.05 0.43-0.57 12 1.25 0.14 0.99-1.43 0.67 0.09 0.51-0.87 1.53 0.14 1.25-1.72 13 1.42 0.14 1.20-1.63 0.38 0.05 0.26-0.45 0.87 0.13 0.71-1.10 14 0.43 0.06 0.35-0.53 0.29 0.04 0.21-0.33 0.41 0.07 0.31-0.54 15 0.75 0.05 0.67-0.82 0.31 0.05 0.23-0.38 0.74 0.15 0.55-0.98 16 1.20 0.22 0.78-1.52 1.23 0.17 0.90-1.46 1.13 0.09 1.02-1.32 17 0.70 0.17 0.49-1.00 0.48 0.07 0.32-0.57 0.47 0.05 0.39-0.52 18 1.01 0.16 0.71-1.27 0.39 0.14 0.23-0.65 0.73 0.07 0.61-0.86 19 0.75 0.10 0.57-0.92 0.27 0.05 0.15-0.36 0.48 0.09 0.37-0.63 20 2.25 0.37 1.66-2.74 0.68 0.07 0.57-0.78 1.87 0.33 1.37-2.41 21 0.26 0.06 0.14-0.34 0.37 0.06 0.23-0.45 0.23 0.05 0.15-0.29 22 0.92 0.17 0.65-1.13 0.31 0.05 0.22-0.38 0.69 0.09 0.55-0.81

GRP 0.97 0.47 0.14-2.74 0.55 0.36 0.15-2.30 0.84 0.43 0.15-2.41

and period and the Fo was computed for each cycle. The absolute difference in duration between adjacent cycles (jitter) was then determined along with mean period, mean jitter, mean Fo, percent jitter (mean jitter divided by mean period times 100), and Fo s.d.

II. RESULTS

Jitter values (%) for each speaker appear in Table I. For each individual, a mean value for her 15 steadiest productions of the vowel is given, along with the standard deviation and range for the 15 productions. As a group, speakers demonstrated the highest jitter when producing /i/ (M- 0.97%), an intermediate level when producing/u/ (M -- 0.84%), and the least jitter when producing /a/ (M -- 0.55% ). Student t tests indicated that differences among all three vowels were significant (p < 0.01 ). In addition, significant heterogeneity of variance was present among the three vowels (Fmax -- 1.31, p <0.01), with/i/ showing the greatest variability in jitter (s.d.- 0.47), followed by/u/ (s.d. ---- 0.43) and/a/ (s.d. -- 0.36). These findings contrast with those of elderly women (M--76 years; Linville and Korabic, 1987) for whom /a/ (M- 2.21%) showed significantly higher levels of jitter than the other two vowels (/i/ M--0.80%; /u/ M ---- 1.21%) and, with the exception of a single speaker, the greatest variability (/a/ s.d.- 1.52; /i/ s.d.----0.57; /u/ s.d.- 0.99). The differing pattern of jitter levels in young and elderly speakers across the three vowels is illustrated in Fig. 1. Data from Wilcox and Horii (1980) for young and elderly men are plotted in Fig. 1 for comparative purposes. For the women, this figure points out the reversal in jitter levels across vowels that occurred with aging. That is, elder-

1.4

,,)

1.2

KEY

• ELDERLY WOMEN • YOUNG WOMEN

ß ' .'El ELDERLY MEN ß " ./• YOUNG MEN

I I I I ß u

FIG. 1. Mean jitter (%) values on the 15 steadiest productions of/i/,/a/, and/u/for young women (present investigation ) and elderly women ( Lin- ville and Korabic, 1987). Data for young and elderly men (Wilcox and Horii, 1980) obtained from one sample of a sustained vowel are plotted also for comparative purposes.

742 J. Acoust. Soc. Am., Vol. 83, No. 2, February 1988 Sue Ellen Linville: Intraspeaker variability 742


ly women demonstrated considerably more jitter on/a/ than the other two vowels, while young women showed the least jitter on/a/. To a lesser degree, male speakers (Wilcox and Horii, 1980) appear to show a similiar pattern. Elderly males demonstrated higher jitter levels on/a/(M = 0.84) than the other two vowels (/i/M = 0.76;/u/M = 0.58), while young males showed low levels of' jitter on both/a/ (M = 0.53) and/u/(M = 0.51 ), with higher jitter levels on /i/(M = 0.61 ). However, it is important to note that Wil- cox and Horii (1980) did not test the significance of' observed mean differences across vowels for each of' the two age groups, making comparisons of' the aging pattern of' jitter levels across vowels in males and females difficult.

A two-factor split plot analysis of'variance (Kirk, 1968) was per•orrned comparing the jitter levels observed for young women in this study with those of' elderly women (Linville and Korabic, 1987), as a function of' vowel type. This analysis produced a significant group by vowel interaction (F= 281.17, p<0.01), and subsequent analyses revealed that differences between the two age groups were significant for all three vowels (/i/F = 13.91, p < 0.01; /a/ F= 438.18, p c0.01; /u/ F= 33.03, p •0.01). In other words, elderly women demonstrated significantly higher jitter levels than young women on/a/and/u/, but young women demonstrated significantly higher levels than elderly women on/i/.

Examination of the data in Table I reveals that individ-

ual young women varied somewhat in the range of jitter values each demonstrated. Eleven women (50%) demonstrat-

ed fairly low levels of variability (range of 0.5% jitter or less) on their 15 steadiest productions of all three of the vowels. The remaining 11 women (50%) demonstrated ranges that

ß

exceeded 0.5% jitter on at least one vowel tested, with four women ( 18% ) displaying ranges in excess of 0.5% on two vowels. In comparison, elderly women (Linville and Kora- bic, 1987) demonstrated considerably higher levels of intraspeaker variability. Of 18 old women, only one (6%) demonstrated ranges of 0.5% jitter or less on all three vowels. Of the remaining 17 women, five (28%) demonstrated ranges in excess of 0.5% on all three vowels, seven (39%) on two vowels, and five (28%) on one vowel.

The F o s.d. values [in semitones (ST) ] are presented in Table II. The Fo s.d. was highly correlated with jitter (0.87), so findings for these two measures tended to parallel one another. The Fo s.d. values were highest for/i/(M = 0.06), lower for /u/ (M=0.05), and the lowest for /a/ (M = 0.04). The Student t-test analysis indicated that differences among all three vowels were significant (p • 0.01 ). Heterogeneous variances were evident (Fmax = 1.23, p•0.01), with /i/ showing the greatest variability (s.d. = 0.079), followed by /a/ (s.d. =0.077) and /u/ (s.d. = 0.064). Once again, these findings contrast with those of elderly speakers (Linville and Korabic, 1987), for whom F o s.d. values were significantly higher for /a/ (M = 0.35) than the other two vowels (/u/M = 0.23;/i/ M = 0.18), and in whom variability on/a/ (s.d. = 0.23) was significantly greater than the other vowels (/i/ s.d. = 0.07;/u/s.d. = 0.13). The differing patterns of F o s.d. levels across vowels as a function of speaker age are shown in Fig. 2. As was the case with jitter, elderly speakers demonstrated considerably higher Fo s.d. levels on/a/than the other vowels, while young speakers showed the opposite pattern.

A two-factor split plot ANOVA comparing young and

TABLE II. Mean, standard deviation (s.d.), and range of fundamental frequency standard deviation values (in semitones) in young women on the 15 steadiest productions of/i/,/a/, and/u/.

/i/ /a/ /u/

Speaker Mean s.d. Range Mean s.d. Range Mean s.d. Range

1 0.21 0.05 0.14-0.31 0.10 0.02 0.07-0.14 0.19 0.03 0.16-0.27 2 0.22 0.06 0.14-0.39 0.11 0.05 0.06-0.24 0.19 0.06 0.09-0.28 3 0.21 0.04 0.15-0.25 0.11 0.02 0.07-0.16 0.20 0.02 0.16-0.22 4 0.12 0.02 0.07-0.14 0.07 0.02 0.06-0.11 0.11 0.02 0.09-0.16 5 0.12 0.02 0.09-0.16 0.09 0.01 0.06-0.10 0.14 0.01 0.11-0.16 6 0.18 0.03 0.10-0.21 0.12 0.03 0.09-0.20 0.11 0.02 0.08-0.14 7 0.13 0.01 0.11-0.15 0.09 0.02 0.06-0.13 0.13 0.02 0.09-0.17 8 0.10 0.01 0.07-0.11 0.09 0.01 0.07-0.11 0.09 0.02 0.06-0.12 9 0.13 0.03 0.09-0.19 0.12 0.04 0.07-0.21 0.14 0.04 0.10-0.27

10 0.17 0.02 0.15-0.23 0.25 0.03 0.19-0.33 0.13 0.01 0.10-0.15 11 0.12 0.02 0.10-0.15 0.12 0.02 0.10-0.15 0.11 0.02 0.08-0.16 12 0.18 0.03 0.14-0.24 0.11 0.02 0.08-0.17 0.21 0.02 0.17-0.23 13 0.21 0.03 0.17-0.28 0.09 0.02 0.07-0.13 0.13 0.02 0.10-0.16 14 0.11 0.02 0.07-0.17 0.09 0.02 0.06-0.16 0.12 0.03 0.08-0.18 15 0.14 0.02 0.11-0.18 0.08 0.01 0.06-0.10 0.14 0.05 0.09-0.28 16 0.16 0.04 0.07-0.26 0.17 0.02 0.14-0.20 0.16 0.02 0.15-0.22 17 0.12 0.02 0.09-0.15 0.12 0.03 0.08-0.18 0.10 0.02 0.07-0.15 18 0.15 0.02 0.12-0.19 0.09 0.02 0.05-0.15 0.12 0.01 0.10-0.13 19 0.13 0.02 0.10-0.17 0.09 0.03 0.05-0.14 0.10 0.03 0.07-0.16 20 0.32 0.05 0.22-0.40 0.14 0.02 0.11-0.19 0.28 0.04 0.21-0.35 21 0.07 0.02 0.04-0.13 0.09 0.02 0.06-0.13 0.09 0.04 0.04-0.18 22 0.14 0.03 0.10-0.19 0.10 0.03 0.06-0.21 0.12 0.02 0.09-0.14

GRP 0.16 0.06 0.04-0.40 0.11 0.04 0.05-0.33 0.14 0.05 0.04-0.35



KEY

--[• ELDERLY WOMEN • YOUNG WOMEN

I I I [ I u

FIG. 2. Mean fundamental frequency standard deviation (F o s.d.) values (in semitones) on the 15 steadiest productions of/i/,/a/, and/u/for young women (present investigation) and elderly women (Linville and Korabic, 1987).

elderly speakers on Fo s.d. as a function of vowel type produced a significant group by vowel interaction (F = 162.51, p < 0.01 ). Differences between the two age groups were significant for all three vowels (/i/F= 13.94, p <0.01; /a/ F= 552.75, p <0.01;/u/F= 108.17, p <0.01), although, in contrast to jitter, elderly speakers demonstrated significantly higher Fo s.d. levels than young speakers on all three vowels.

As seen with jitter, individual young women also varied in the range of Fo s.d. values each demonstrated (Table II). Eleven women (50%) demonstrated fairly low levels of variability (range of 0.1 ST or less) on their 15 steadiest productions of all three vowels. The remaining 11 women (50%) demonstrated ranges that exceeded 0.1 ST on at least one vowel, with four women ( 18% ) displaying the ranges in excess of 0.1 ST on two vowels, and 1 (5%) on all three vowels. In comparison, elderly women (Linville and Kora- bic, 1987) displayed considerably higher levels of intraspeaker variability. None of the 18 elderly women demonstrated Fo s.d. ranges of 0.1 ST or less on all three vowels. In fact, 15 elderly women (83%) displayed ranges exceeding 0.1 ST on all three vowels, 1 (6%) on two vowels, and 2 ( 11% ) on one vowel.

Both jitter ( % ) (r = -- 0.12) and F o s.d. (r = -- 0.08 ) showed small but significant correlations with mean Fo in young speakers (jitter p <0.01; Fo s.d., p <0.05). In other words, there was a slight tendency for frequency instability to increase as mean Fo decreased, despite the fact that both frequency stability measures are adjusted to take into ac- count the mean Fo level. Interestingly, this correlation exist- ed despite the fact that/a/showed the least frequency instability while being produced by speakers at a significantly lower mean Fo level (M=237.15 Hz) than /i/ (M = 249.23 Hz) and/u/ (M = 250.96 Hz). A similiar, although higher, correlation (jitter r- --0.36; Fo s.d.,

250

225

200

KEY

• ELDERLY WOMEN • YOUNG WOMEN

I I I I n u

FIG. 3. Average mean Fo (Hz) values on the 15 steadiest productions of sustained/i/,/a/, and/u/for young women (present investigation) and elderly women (Linville and Korabic, 1987).

r = --0.30) was reported previously in elderly speakers (Linville and Korabic, 1987). However, since elderly speakers also showed the greatest frequency instability on the low- frequency vowel/a/, the correlation was easier to explain. Findings with young speakers suggest that jitter and Fo s.d. levels in sustained vowels tend to increase somewhat as mean

Fo is decreased regardless of the inherent frequency level of the vowel being produced.

A two-factor split plot analysis of variance was per- formed comparing mean Fo levels on the three vowels for young speakers and elderly speakers analyzed previously (Linville and Korabic, 1987). This analysis produced a significant group by vowel interaction (F = 29.48,p <0.01 ), as illustrated in Fig. 3. Subsequent analyses revealed that differences between the two age groups were significant for all three vowels (/i/ F= 182.29, p<0.01; /a/ F= 281.80, p <0.01;/u/F= 183.87, p <0.01 ), although the groups differed on/a/to a greater degree than the other two vowels. In other words, elderly speakers tended to produce all of their vowels at lower frequency levels than young speakers, although the aging effect was particularly strong for the vowel /a/.

III. DISCUSSION

Findings from this study suggest that the aging process brings about increases in the variability individual women demonstrate on measures of Fo stability when producing sustained vowels "as steadily as possible." Young speakers not only tended to display lower levels of jitter and Fo s.d. than elderly speakers (Linville and Korabic, 1987), but also demonstrated lower levels of intraspeaker variability on these measures.

There is some suggestion from this investigation that Fo s.d. may be a better discriminator of vocal aging in women than jitter. Findings supporting this conclusion include: ( 1 ) Elderly speakers demonstrated significantly higher Fo s.d.



values than young speakers on all three vowels tested while demonstrating higher jitter values on only two vowels; and (2) elderly speakers demonstrated high levels of intraspeaker variability more consistently on Fo s.d. than jitter.

Interestingly, young speakers differed markedly from elderly speakers (Linville and Korabi½, 1987) in the pattern of frequency instability variations observed across the three vowels tested (Figs. 1 and 2). Young speakers demonstrated the lowest levels of jitter and Fo s.d. on the low vowel/a/, with the high vowels/i/and/u/showing higher levels of frequency instability. In contrast, elderly speakers showed the greatest frequency instability on the low vowel /a/. Further, jitter levels for young and elderly males on these three vowels (Wilcox and Horii, 1980) appear to form a pattern similiar to that seen in these female speakers (Fig. 1 ). While no firm conclusion is possible as to the cause of these apparent age-related differences, it is possible that this phenomenon is related to inherent differences in tongue position among the three vowels. It has been hypothesized that articulatory movements of the tongue affect intralaryngeal events such as tension of the aryepiglottic folds (Zenker and Zenker, 1960) or vertical tension of the vocal folds (Ohala, 1977). Honda ( 1983 ) hypothesized that more forward positions of the hyoid, as seen with the vowel/i/, produce an upward and forward pull on the apex of the arytenoid cartillages, which may lengthen the vocal folds, enhance medio- compression, or stabilize the arytenoid cartillages. Such ad- justments, if present for the high front vowel/i/, might make this vowel more resistant to aging effects on frequency stability than other tongue position vowels, especially the low back vowel/a/.

A second possible explanation for this finding lies with aging effects on inherent mean Fo differences among the vowels. Although aging appeared to result in lower mean Fo levels for all three vowels in these women, the lowering observed for/a/was particularly marked (Fig. 3 ). It also was noted that frequency instability tended to increase as mean Fo decreased. Perhaps the finding that/a/showed particularly high levels of frequency instability in elderly women, and relatively low levels in young women, is related, to some extent, with the mean F o level on which the vowel/a/was produced by elderly women.

Overall results of this study suggest that fewer repeat trials would be necessary from a young woman, in comparison with an elderly woman, to determine her steadiest production of a given vowel. Aging appears to increase intraspeaker variability on jitter and F o s.d. measures from sustained vowels produced "as steadily as possible."

Further, aging tends to increase the magnitude of frequency instability in sustained vowels, although frequency stability levels in both young and elderly women are highly vowel dependent. These results suggest the need for careful consi- deration of the age group of the speaker before choosing a vowel from which to obtain frequency stability measure- ments. It appears that/a/would require fewer trials to ade- quately sample frequency stability in young women's sustained vowels while/i/generally would require the fewest trials from elderly women. However, as discussed previously (Linville and Korabi½, 1987), an individual elderly woman might be expected to deviate substantially from the vowel pattern described as "typical" for elderly women. Therefore, investigators may wish to make multiple samples of frequency stability in vowels of varying tongue positions in elderly women before deciding on a vowel that produces the "steadiest" results for an individual speaker. An interesting ques- tion that emerges from this study is the extent to which male speakers show similiar age-related patterns of frequency instability. Research is currently underway in our laboratory addressing that issue.

ACKNOWLEDGMENTS

Thanks are expressed to Mary Beth Aiello for assistance with running subjects and analyzing data for this project. Thanks also to Fran Johannes and Gail Kempster for assistance with statistical analysis.

Honda, K. (1983). "Relationship between pitch control and vowel articu- lation," in Vocal Fold Physiology Contemporary Research and Clinical Issues, edited by D. Bless and J. Abbs (College-Hill, San Diego).

Kirk, R. (1968). Experimental Design: Procedures for the Behaoioral Sci- ences (Brooks-Cole, California).

Korabic, E., Kuos, S., and Linville, S. E. (1986). "Analysis of pitch pertu- bation (jitter) using a Visi-Pitch and a micorprocessor," J. Comput. Us- ers Speech Hear. 2, 86-93.

Linville, S. E., and Fisher, H. B. (1985a). "Acoustic characteristics of women's voices with advancing age," J. Gerontol. 40, 324-330.

Linville, S. E., and Fisher, H. B. (1985b). "Acoustic characteristics of per- ceived versus actual vocal age in controlled phonation by adult females," J. Acoust. Soc. Am. 78, 40-48.

Linville, S. E., and Korabic, E. (1987). "Fundamental frequency stability characteristics of elderly women's voices," J. Acoust. Soc. Am. 81, 1196- 1199.

Ohala, J. (1977). "Speculation on pitch regulation," Phonetica 34, 310- 312.

Stoicheft, M. (1981). "Speaking fundamental frequency characteristics of nonsmoking female adults," J. Speech Hear. Res. 24, 437-441.

Wilcox, K., and Horii, Y. (1980). "Age and changes in vocal jitter," J. Gerontol. 35, 194-198.

Zenker, W., and Zenker, A. (1960). "Ueber die regelung der stimmilippen- spannung durch von aussen eingreifende Mechanismen," Folia Phoniatr. 12, 1-26.



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Intraspeaker variability in fundamental frequency stability: An age-related phenomenon?