Jitter and shimmer are measures of the cycle-to-cycle variations of fundamental frequency and amplitude, respectively, which have been largely used for the description of pathological voice quality. Jitter is also known as frequency perturbation and refers to the minute involuntary variations in the timing variability between cycles of vibration. In essence, it is a measure of frequency variability in comparison to the client’s fundamental frequency. Research shows that jitter values in normal voices range from 0. 2 to 1 percent. Ferrand, 2007) Jitter values above this level indicate that the vocal folds are vibrating in a way that is not as periodic as it should be. (Ferrand, 2007). Higher jitter levels suggest that something is interfering with normal vocal fold vibration and the mucosal wave. (Ferrand, 2007) Whereas jitter is a measure of the percentage irregularity in the pitch of the vocal note, shimmer is a measure of the percentage irregularity in the amplitude of the vocal note. It is often referred to as amplitude perturbation. Shimmer, therefore, measures the variability in the intensity of adjacent vibratory cycles of the vocal folds.
Research estimates that shimmer values below 0. 5 dB are normal in the human voice. (Ferrand, 2007). Jitter and shimmer reflect the internal noises of the human body. (Titze, 1994) As with jitter, pathological voices will typically exhibit a higher percentage of shimmer. Higher jitter and shimmer levels can reflect problems in neuromuscular control. Increased levels of jitter and shimmer can also be caused by a nodule or cyst on the vocal folds which would increase the vocal fold mass and vibration would become less periodic.
Measuring the cycle-to-cycle variability of vibration can allow us to detect changes in neuromuscular function or changes in the layers of the vocal folds. (Ferrand, 2007) The more steady the muscles that control vocal fold vibration are, the more steady and periodic the acoustic signal will be and the lower the jitter and shimmer. The vocal note produced by the vibrations of the vocal folds is complex and made up of periodic (regular and repetitive) and aperiodic (irregular and non-repetitive) sound waves.
The aperiodic waves are random noise introduced into the vocal signal owing to irregular or asymmetric adduction (closing) of the vocal folds. Noise impairs the clarity of the vocal note and too much noise is perceived as hoarseness. The Harmonics-to-Noise (HNR) ratio is a measure of the proportion of harmonic sound to noise in the voice measured in decibels. (Ferrand, 2007). HNR quantifies the relative amount of additive noise (Awen & Frankel, 1994) The lower the HNR, the more noise in the voice.
Laryngeal pathology may lead to poor adduction of the vocal folds and, therefore, increase the amount of random noise in the vocal note. If a person has some kind of problem in vibrating the vocal folds, due to a growth, paralysis of one or both vocal folds, or other kind of laryngeal problem, a larger amount of air escapes during vibration, creating turbulent noise. (Pabon & Plomp, 1988) The greater the proportion of noise, the greater the perceived hoarseness, breathiness or roughness and the lower the HNR figure will be, i. e. low HNR indicates a high level of hoarseness, and a high HNR indicates a low level of hoarseness. There is a strong relationship between how we perceive voice quality and the harmonic to noise ratio (Ferrand, 2007).
Ferrand, C. T. (2007). In Speech science: An integrated approach to theory and clinical practice. Boston: Pearson/Allyn and Bacon. Williamson, G. (2008, December 5). Instrumental Measurement of Voice. Speech Therapy: Information and Resources. Retrieved October 14, 2012, from http://www. speech-therapy-information-and-resources. com/instrumental-measurement-of-voice. html