Reliable Jitter and Shimmer Measurements in Voice Clinics: The Relevance of Vowel, Gender, Vocal Intensity, and Fundamental Frequency Effects in a Typical Clinical Task *Meike Brockmann, †Michael J. Drinnan, ‡Claudio Storck, and §Paul N. Carding, *Zurich, Switzerland, yNewcastle upon Tyne, United Kingdom, zBasel, Switzerland, xNewcastle upon Tyne, United Kingdom Summary: The aims of this study were to examine vowel and gender effects on jitter and shimmer in a typical clinical voice task while correcting for the confounding effects of voice sound pressure level (SPL) and fundamental frequency (F0). Furthermore the relative effect sizes of vowel, gender, voice SPL, and F0 were assessed, and recommendations for clinical measurements were derived. With this cross-sectional single cohort study, 57 healthy adults (28 women, 29 men) aged 20–40 years were investigated. Three phonations of /a/, /o/, and /i/ at ‘‘normal’’ voice loudness were analyzed using Praat (software). The effects of vowel, gender, voice SPL, and F0 on jitter and shimmer were assessed using descriptive and inferential (analysis of covariance) statistics. The effect sizes were determined with the eta-squared statistic. Vowels, gender, voice SPL, and F0, each had significant effects either on jitter or on shimmer, or both. Voice SPL was the most important factor, whereas vowel, gender, and F0 effects were comparatively small. Because men had systematically higher voice SPL, the gender effects on jitter and shimmer were smaller when correct- ing for SPL and F0. Surprisingly, in clinical assessments, voice SPL has the single biggest impact on jitter and shimmer. Vowel and gender effects were clinically important, whereas fundamental frequency had a relatively small influence. Phonations at a predefined voice SPL (80 dB minimum) and vowel (/a/) would enhance measurement reliability. Furthermore, gender-specific thresholds applying these guidelines should be established. However, the efficiency of these measures should be verified and tested with patients. Key Words: Vowel–Voice intensity–Gender–Fundamental frequency–Jitter–Shimmer–Voice assessments. INTRODUCTION Why are jitter and shimmer measured? Measurements of voice frequency (jitter) and amplitude (shim- Small irregularities in the acoustic wave are considered as nor- mer) perturbation commonly form part of a comprehensive mal variation associated with physiologic body function and voice examination. These measures are used to provide supple- voice production.10,11 However, voice perturbation levels mentary information alongside visual laryngeal examination have been shown to considerably increase according to laryn- details and auditory perceptual voice assessment.1,2 Both geal pathology12–14 and to partially discriminate between func- perturbation parameters are obtained by computer analysis of tional voice disorder types.15,16 Investigations combining a prolonged vowel phonations or speech samples and quantify number of acoustic parameters and visual laryngeal assessment unintentional irregularity in the acoustic waves generated by methods suggest that jitter and shimmer may be valuable pre- the larynx. Therefore, voice perturbation analysis has been dictors of voice pathology.16,17 Additionally, correct recogni- described as an easily applicable, indirect, noninvasive mea- tion of pathologic voices has been reported to be as high as surement of laryngeal vibratory function, which can quantify above 95% when perturbation parameters are used in combina- the regularity and hence the stability of vocal fold vibration.3,4 tion with a mathematical pattern recognition model.18 Further- These measures may help describe and quantify pathological as more, investigations in ‘‘normal’’ sounding voices such as in well as healthy vocal fold vibration characteristics. The param- patients with laryngopharyngeal reflux or after thyroid surgery eters jitter and shimmer are widely used in clinical and scientific imply that perturbation parameters might even track subtle settings for diagnostic and descriptive purposes1,5,6 as well as to voice alterations not easily detectable by perceptual or visual document and evaluate voice treatment outcome.7–9 assessment methods.19,20 However, applications of jitter and shimmer analysis have Accepted for publication July 1, 2009. Conflicts of interest: All authors declare that they have no real or potential conflicts of been thwarted by unsatisfactory measurement reliability, sensi- interest influencing the presented research. From the *Speech Pathology Section, Department of Phoniatry and Speech Pathology, tivity, and specificity. This has been particularly true in the Clinic for Otorhinolaryngology, Head and Neck Surgery, University Hospital Zurich, Zur- analysis of ‘‘hoarse’’ voices with potentially severely aperiodic ich, Switzerland; yRegional Medical Physics Department, Newcastle General Hospital, sound signal structure.21–23 Yet even the analysis of mildly dys- Newcastle upon Tyne, United Kingdom; zDepartment of Phoniatry, University Hospital Basel, Basel, Switzerland; and the xDepartment of Speech, Voice and Swallowing, Free- phonic voices (with presumably more periodic sound signals) man Hospital, Newcastle upon Tyne, United Kingdom. 24 Address correspondence and reprint requests to Meike Brockmann, Head of Speech Pa- has been shown to have at best moderate test-retest reliability. thology Section, Department of Phoniatry and Speech Pathology, Clinic for Otorhinolaryn- Furthermore, pathology-specific norm values have not been gology, Head and Neck Surgery, University Hospital Zurich, Frauenklinikstrasse 24, 8091 3,4 Zurich, Switzerland. E-mail: [email protected] established to date. As a result, voice perturbation measure- Journal of Voice, Vol. 25, No. 1, pp. 44-53 ments have been cautiously interpreted. Measurement reliability 0892-1997/$36.00 Ó 2011 The Voice Foundation requires considerable improvement to enhance the potential of doi:10.1016/j.jvoice.2009.07.002 jitter and shimmer analysis in the voice clinic setting. Meike Brockmann, et al Vowel Influence on Jitter and Shimmer 45 Recording and measurement technique effects of a number of influencing factors such as (1) an unequal re- The limited measurement reliability might partially be ex- cording equipment between studies not complying to current plained by a number of technical confounding factors such as guidelines and/or (2) the use of both male and female subjects microphone type and placement, recording distance, analysis in the same experiments29,30 and (3) measurements at different method, and background noise. Also, measurement reliability voice intensities and fundamental frequencies5,26 and/or (4) an decreases with increasing voice irregularity because the method unequal recording distance (Table 1). However, all these factors depends on exact sound pressure level (SPL) and F0 recogni- have to be considered when determining the relative influence tion.5,17,25,26 To minimize confounding effects, guidelines of different vowels on jitter and shimmer measurements. defining an appropriate acoustic assessment equipment and From a theoretical perspective, various hypotheses have been setup and also suitable voice types have been published.5 proposed to explain or reject the possibility of vowel effects on However, from a clinical perspective, also the recording in- voice intensity and fundamental frequency perturbation. Ac- structions and interindividual differences in performing the cording to the source-filter theory, jitter and shimmer should given voice tasks might account for the high measurement var- not be affected by vowel articulation influencing the overtone iability. In clinical assessments, patients are usually instructed to spectrum through alterations of the vocal tract shape (filter) sustain the vowels /a/, /u/, /o/, or /i/ ‘‘atcomfortable loudness and because this does not alter the signal of the sound source pitch.’’1 This voice task is used to avoid potentially confounding (larynx).31 This is supported by electroglottographic (EGG) intensity and pitch change effects on acoustic measurements as measurements showing no distinct vowel effects.29 However, well as a possible influence of articulatory movements.1,5,27 the physical linkage hypothesis proposes an interrelation be- However, it is unclear whether these instructions are sufficiently tween vocal tract and laryngeal movements. Investigations in rigorous to produce maximum reliability in clinical practice. the position of the hyoid-larynx complex32 or the head and tongue33 showed associations with changes in fundamental Vowel influence frequency and also the amount of jitter and shimmer. Vowel effects in measurements of jitter and shimmer have been investigated by a number of researchers and are summarized in Gender differences Table 1. The results range from the description of distinct vowel Based on the results in Table 1, it might be assumed that phys- differences28 to no vowel effects at all.29 Highest jitter has been iologic gender differences were the strongest confounding fac- found in /u/, /i/, or /a/ and lowest shimmer in /i/ or /u/ (Table 1). tor in current investigations regarding vowel effects. However, These contradictory results clearly show that the influence of the evidence regarding gender differences also is contradictory vowel articulation has not been fully understood to date. in the published literature hindering a clear attribution. From a methodological standpoint, the reported results appear Comparing two similar designed studies examining