ArticlesHarmonics-to-Noise Ratio: An Index of Vocal Aging☆
Introduction
Given the importance of distinguishing between vocal changes that occur with normal aging and those that are associated with disease, finding a robust acoustic measure that reliably signals vocal performance in older adults remains an ongoing challenge. Fundamental frequency (F0) has been shown to change with aging, increasing with advancing age in elderly men, and decreasing in elderly women.1, 2, 3, 4 By itself, however, F0 does not yield a sufficiently detailed picture of vocal fold vibratory patterns to differentiate between normal and disordered vocal function. Other acoustic measures such as jitter have yielded inconclusive results in comparisons of younger and older adults' vocal function. For instance, Linville and Fisher5 reported higher mean jitter values in elderly women than in younger women. However, jitter levels did not contribute to the overall estimation of the speaker's age. Wilcox and Horii6 found significant jitter differences between young and elderly men, while Ringel and Chodzko-Zajko7 indicated that jitter values were more related to physiological health than to chronological age. Linville8 found considerable overlap in jitter levels among groups of young, middle-aged, and elderly women. Elderly women, on average, did not display more jitter than younger women.
Another acoustic measure that may be a more sensitive index of vocal function is the harmonics-to-noise ratio (HNR). HNR quantifies the relative amount of additive noise in the voice signal.9 Additive noise arises from turbulent airflow generated at the glottis during phonation.10 Inadequate closure of the vocal folds allows excessive airflow through the glottis, giving rise to turbulence. The resulting friction noise is reflected in a higher noise level in the spectrum.11 Noise in the signal may also result from aperiodic vocal fold vibration. The ratio thus reflects the dominance of harmonic (periodic) over noise (aperiodic) levels in the voice, and is quantified in terms of dB. Yumoto, Sasaki, and Okamura12 reported that adult speakers with normal voice quality obtained HNRs of 7.4 dB and above when producing isolated vowels; other research suggests that values between 11–13 dB are normal for adults.13, 14 Perceptually, HNR reflects voice quality. HNR seems to be one of the parameters that can be used to relate physiological aspects of voice production to a perceptual impression of the voice, because the degree of spectral noise is related to the quality of the vocal output.11 Indeed, it has been reported that HNR is a significant predictor of voice samples rated perceptually as rough.15, 16, 17
A limited number of studies have investigated HNR in elderly speakers. One of the few studies focusing on spectral characteristics in the voices of older adults found that HNR decreased in subjects aged 60 to 99 years old, reflecting a growing voice instability with normal aging.18 Ramig et al19 reported that older speakers in poor physical condition showed more spectral noise than older speakers in good phsyical condition or than younger speakers. However, rather than quantifying the degree of noise, Ramig et al used spectrograms to visually determine the level of spectral noise. Mueller1 made the assumption that because hoarseness is prevalent in the voices of elderly speakers, a lower HNR would be predicted. However, this assumption needs to be supported and quantified in order to distinguish between vocal changes that are within normal limits and those that may be due to pathology.
It is important to establish a normative acoustic database for laryngeal function in older adults. HNR might be a more sensitive index of vocal function in older speakers than is jitter, as the ratio reflects both cycle-to-cycle variability in frequency and amplitude, as well as additive noise generated at the glottis.20 As such, it could be a valuable adjunct in quantifying and differentiating between possible deterioration in voice function in adults who are aging normally and in those with voice problems.
The first purpose of this study, therefore, was to compare HNRs in three age groups of women: younger, middle-aged, and older adults. The second purpose was to compare HNR and jitter measures in terms of sensitivity to potential differences in vocal function in different age groups. The third purpose was to determine F0 differences among the three speaker groups.
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Subjects
Forty-two adult women with normal voices served as subjects, with 14 speakers in each of three groups: young adults, middle-aged adults, and elderly adults. Young adults ranged in age from 21 to 34 with an average age of 25 years. Young adults were recruited from the student population at Hofstra University in Hempstead, NY. Middle-aged adults ranged in age from 40 to 63, with an average age of 50 years. The middle-aged speakers were recruited from faculty members at Hofstra University, as well
Results
For more than 98% of the samples, neither HNR nor jitter differed by more than 0.02 dB and 0.02%, respectively.
Values for HNR, jitter, and F0 were averaged over the five vowel prolongations for each individual. Individual means were then averaged for each of the three groups of speakers. Table 1, Table 2, Table 3 display individual and group means and standard deviations for HNR, jitter, and F0.
In terms of HNR, young and middle-aged subjects obtained mean values of 7.82 dB and 7.86 dB,
Discussion
The major focus of this study was to determine whether HNR values changed with age. The second purpose was to determine whether HNR values and jitter values yielded similar results in three groups of women; the third purpose was to compare F0 values between the three groups.
The HNR values in the present study are comparable to some values reported in the literature, but lower than others. Bertino et al21 and Yumoto, Gould, and Baer22 reported values of 7.23, and 7.3 dB, respectively, for adult
Acknowledgements
This work was supported in part by a Hofstra College of Liberal Arts and Sciences Faculty Research and Development Grant. The author wishes to thank Jillian Resch and Michael Sayers for their outstanding assistance in collecting the data.
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Portions of this work were presented at the American Speech-Language-Hearing Association Annual Convention, San Francisco, CA, November 1999.