Vocal microtremor in normophonic and mildly dysphonic speakers

Jean Schoentgen Université Libre Bruxelles Brussels - Belgium

Vocal microtremor (definition) • Modulation of the phonatory frequency • Distinct from pathological vocal tremor • 1 - 15 Hz (Titze, 1995, Sataloff, 1997) • Two features : modulation level and modulation frequency

Vocal microtremor (examples)

Motivation ?

• Tremor data are scarce • Vocal jitter & microtremor are base-line phenomena • Measurement of vocal tremor frequency via the cycle length time series • Test predictions of a simulation model of jitter and tremor (Schoentgen, 2001)

Experiment I : Objectives ?

• Recording data (tremor level & frequency) • Differences between vowel timbres ?

• Differences between male & female speakers ?

• Differences between normophonic & mildly dysphonic speakers ?

Corpora • Sustained vowels [a], [i], [u] • 22 males, 16 females (normophonic) • 16 males, 28 females (dysphonic)  Voice type : monocycle periodic  Register : modal  No register or type breaks, or voice arrests  No cycle length outliers  No excessive additive noise or jitter  No pathological vocal tremor

Method (tremor frequency) 1. Estimation of the average cycle length 2. Upsampling (160 kHz) and low-pass filtering of the speech signal 3. Extraction of the vocal cycle length time series via peak picking 4. Removal of frequency drift or glissando 5. Calculation of the magnitude spectrum of the time series 6. Search for the statistically significant spectral peaks 7. Tremor frequency = weighted average of spectral peak positions

Examples of spectra

Method (tremor level) 1. Upsampling (160kHz) and low-pass filtering of the speech signal 2. Extraction of the vocal cycle length time series via peak picking 3. Removal of frequency drift or glissando 4. Smoothing of the time series to decrease jitter 5. Tremor level -> standard deviation of the smoothed cycle length perturbations (divided by average cycle length)

Results (1) • No statistically significant differences for the modulation frequency (Hz) and modulation level (%) between :  Male & female speakers  Normophonic & mildly dysphonic speakers  Vowel timbres

Results (2) Feature Modulation level Modulation frequency Inter-quartile interval 0.4 % - 1.3 % 2.2 - 4.5 Hz

Results (3) • Dissimilarities between modulation data reported by different studies are due to different cutoff frequencies below which spectral peaks are considered not to contribute to vocal microtremor

Experiment II : Objective • Compare the size of vocal cycle length perturbations owing to jitter and frequency tremor

Corpus & Method • 22 male and 16 female speakers sustained [a], [i] and [u].

o Upsampling (160kHz) and low-pass filtering of the speech signal o Extraction of the vocal cycle length time series via signal zero-crossings o Removal of frequency drift or glissando

Linear auto-regressive analysis of the cycle length time series (e.g. Schoentgen, 1995) • Present perturbation = weighted sum of past perturbations + de-correlated noise  De-correlated noise -> vocal jitter  Weighted sum -> vocal tremor (by default) • Calculate sample standard deviation for each (& divide by average cycle length)

Example

Results (1) : [a] (inter-quartile ranges) Relative raw perturbations Modulation level Relative vocal jitter male female 0.6 - 1.1% 0.6 - 1.4% 0.5 - 1.0% 0.6 - 1.2% 0.3 - 0.5% 0.3 - 0.4%

Results (2) • Vocal jitter (%) < vocal tremor (%) (statistically significant) • Moderate significant correlation between vocal jitter & tremor (in %) • No significant tremor differences between vowel timbres • No significant tremor differences between speaker genders

Conclusion • Vocal frequency (micro)tremor data can be obtained via the cycle length time series • This may be generalized to pathological tremor data, but an additional stage may be required which is the re-sampling of the cycle length time series at equal intervals