Myoelastic-Aerodynamic Theory and Acoustics of Phonation - Tutorial

What is it about?

We review some historical and recent contributions to understanding the myoelastic-aerodynamic (MEAD) theory of voice production and the related acoustic phenomena below and above the vocal folds. At the time of the formulation of MEAD by van den Berg in late 1950s, it was assumed that vocal fold oscillate thanks to increased subglottal pressure (i.e. the pressure below the vocal folds) pushing the glottis to open and decreased subglottal pressure allowing the glottis to close. In vivo measurements of subglottal pressures during phonation invalidated these assumptions, however, and showed that at low fundamental frequencies subglottal pressure rather tends to reach a maximum value at the beginning of glottal closure and then exhibits damped oscillations. Therefore, the understanding of MEAD has changed through the years. Rather than subglottal pressure oscillations, a more efficient mechanism of transfer of aerodynamic energy to the vocal fold vibrations has been identified in the vertical phase differences (mucosal waves) making the glottal shape more convergent during glottis opening than during glottis closing. Along with other discoveries, these findings form the basis of our current understanding of MEAD.

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Photo by Jason Rosewell on Unsplash

Photo by Jason Rosewell on Unsplash

Why is it important?

Vocal fold oscillations are the basis for voice production. It is important to understand how come that the vocal folds oscillate. In popular texts, the explanations have often been incorrect. Myoelastic-aerodynamic (MEAD) theory explains that the vibratory characteristics of the vocal folds depend mainly on the elasticity of the vocal fold tissues (forming the ELASTIC component of MEAD). The elastic properties of the tissues have a major influence on the fundamental frequency of the vocal fold oscillations, very much like the frequency of string oscillations is governed by the elastic properties of the string. The elastic properties can be changed by laryngeal muscles (hence the term MYOELASTIC) making the vocal folds stiffer or slacker thus changing the fundamental frequency of the oscillations and making the pitch of the voice higher or lower. The energy for vibration is delivered to the vocal folds by air flowing from the lungs (forming the AERODYNAMIC component of MEAD). Part of the vibrational energy of the vocal folds dissipates within each cycle due to tissue viscosity and therefore energy needs to be added to the vocal folds throughout the vibratory cycles to make the vocal fold oscillations self-sustainable. This paper reduces the highly complex process of voice production to its essential elements so that the information is accessible also to nonscientists.

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