Aperiodicity, a characteristic of noise added to the vocal fold output, is responsible for distortion, raspiness, breathiness, jitter and shimmer.
‘Periodicity’ describes sounds that repeat exactly over and over again. In reality, these don’t exist; so, normally, sounds that we label as ‘periodic’ are actually ‘quasi-periodic’ (i.e., almost periodic; they repeat in a noticeably similar way over and over). Aperiodicities are, therefore, sounds that do not repeat. They’re effectively noise. However, this doesn’t mean that a supraglottic (i.e., occuring above the vocal fold level) periodic vibratory behavior isn’t aperiodic. If that vibratory behavior is out of sync with the vocal fold motion, that is also considered an aperiodicity. So, in general, aperiodicities are sounds that are out of sync with vocal fold motion.
Despite the usually quasi-periodic output of the vocal fold vibratory motion, we often come across aperiodicity in phonation.
In a spectrogram, aperiodicity is identified as:
(a) bands of noise in between the harmonics produced by the vocal folds in the cases of distortion, raspiness and breathiness, or
(b) irregular fluctuations in pitch (jitter) or intensity (shimmer) in all or part of the harmonic series
Aperiodicity is a perfectly safe technique. However, it can be a sign of vocal deterioration if it’s happening unintentionally, but even then those aperiodicities may be non-issues, especially if they’re not intense, consistent or widely undulating (wide jitter/shimmer).
Aperiodicities may be learned or unlearned behaviorally by any normal voice with training.
Distortion occurs when the aperiodic noise (which can be of glottic or supraglottic origin; i.e., it can be produced by the true vocal folds or by vibrating bodies above it) picks up enough intensity to significantly compete with the quasi-periodic sound output originating from the vocal folds.
Raspiness is the result of very audible aperiodicities with formants that hover around specific pitches but do not compete with harmonics in intensity.
It includes certain types of glottic and supraglottic distortion, which may be interpreted as raspiness, and, more importantly, also includes aspirate rasp (which happens when the false vocal folds approximate so that air slightly vibrates their edges, producing a pitch).
Breathiness occurs when a significant volume of air leaks through the glottis without being vibrated to a pitch, resulting in white noise.
Jitter & Shimmer
Jitter is characterized by an abnormal pitch flux, whereas shimmer is characterized by an abnormal amplitude flux.
Although these can be signs of involuntary destabilization or difficulty (as happens with age), jitter and shimmer can still be done purposefully for artistic choice.
Artistic freedom allows jitter and shimmer to be performed purposefully, for effect; however, these can be signs of unvoluntary destabilization or instability, as usually happens with age and during physical exercise. The direct causes involve loss of stabilizer muscle tone and the occurrence of tremors in muscles involving the Power and/or Source of the voice.
American Speech-Language-Hearing Association. Voice Disorders: Dysphonia. (n.d.). Retrieved from https://www.asha.org/PRPSpecificTopic.aspx?folderid=8589942600§ion=Signs_and_Symptoms
Jotz, G. P., Cervantes, O., Abrahão, M., Settanni, F. A. P., & Angelis, E. C. D. (2002). Noise-to-Harmonics Ratio as an Acoustic Measure of Voice Disorders in Boys. Journal of Voice, 16(1), 28–31. doi: 10.1016/s0892-1997(02)00068-1
Kreiman, J., & Gerratt, B. R. (2005). Perception of aperiodicity in pathological voice. The Journal of the Acoustical Society of America, 117(4), 2201–2211. doi: 10.1121/1.1858351