Jitter and shimmer are measures used to describe pathological voice quality. Jitter, also known as frequency perturbation, measures the minute involuntary variations in timing between cycles of vibration and compares them to the fundamental frequency. Normal voices have jitter values ranging from 0.2 to 1 percent (Ferrand, 2007). Jitter values above this range indicate that vocal folds are not vibrating as regularly as they should be (Ferrand, 2007). Higher jitter levels suggest interference with normal vocal fold vibration and mucosal wave (Ferrand, 2007).
On the other hand, shimmer, also referred to as amplitude perturbation, measures the variability in the intensity of adjacent vibratory cycles of the vocal folds.
According to research by Ferrand (2007), shimmer values below 0.5 dB are considered normal in the human voice. Similarly, Titze (1994) states that jitter and shimmer are indicators of internal noises in the human body. Pathological voices usually show higher levels of shimmer, similar to jitter. These increased levels of jitter and shimmer may indicate issues with neuromuscular control. Additionally, the presence of a nodule or cyst on the vocal folds can cause an increase in vocal fold mass and result in less periodic vibrations.
Measuring the cycle-to-cycle variability of vibration can help in detecting changes in neuromuscular function or changes in the layers of the vocal folds, as stated by Ferrand (2007). The steadiness of the muscles controlling vocal fold vibration directly impacts the steadiness and periodicity of the acoustic signal. This, in turn, affects the levels of jitter and shimmer. The vocal note generated by vocal fold vibrations is a complex combination of periodic sound waves (regular and repetitive) and aperiodic sound waves (irregular and non-repetitive).
The vocal signal may contain aperiodic waves, which are random noise caused by irregular or asymmetric closure of the vocal folds. This noise affects the clarity of the vocal note and excessive noise can be perceived as hoarseness. The Harmonics-to-Noise (HNR) ratio is a decibel-based measure that indicates the proportion of harmonics to noise in the voice (Ferrand, 2007). In other words, HNR quantifies the amount of additive noise relative to the harmonic sound (Awen & Frankel, 1994). A lower HNR value indicates a greater presence of noise in the voice.
Laryngeal problems can lead to inadequate vocal fold adduction, resulting in increased random noise in the vocal note. If an individual has difficulty vibrating the vocal folds due to growths, paralysis of one or both folds, or other laryngeal issues, more air escapes during vibration causing turbulent noise (Pabon & Plomp, 1988). The higher the amount of noise, the more noticeable hoarseness, breathiness, or roughness will be perceived and the lower the HNR figure will be. In simpler terms, a low HNR indicates high hoarseness while a high HNR indicates low hoarseness. The harmonic to noise ratio strongly correlates with our perception of voice quality (Ferrand, 2007).
References
The information in this text is sourced from the book “Speech Science: An Integrated Approach to Theory and Clinical Practice” by C. T. Ferrand (2007). It also includes content from an article titled “Instrumental Measurement of Voice” by G. Williamson (December 5, 2008) found on the website “Speech Therapy: Information and Resources” at http://www.speech-therapy-information-and-resources.com/instrumental-measurement-of-voice.html, which was accessed on October 14, 2012.
