An Introduction to Laryngeal Biomechanics
Pp. 120-164 (45)
Charles Brown and Tobias Riede
Laryngeal sounds in most frogs, in reptiles and most mammals are produced
by the interaction between an airstream through the larynx and soft tissue vocal folds
positioned laterally in the larynx. This produces a sound characterized by a
fundamental frequency (F0), a spectrum of higher frequencies, amplitude and duration.
The vibrating vocal folds disturb the airstream so that acoustic waves are generated
which travel along the vocal tract from which a small portion of sound energy is
radiated from mouth or nostrils. Laryngeal muscles are used for posturing of vocal
folds, they adduct and abduct, or elongate and shorten them. Not only posturing and
length changes of vocal folds affect the acoustic properties of a voice, but their
morphology is also an important determinant of the vocal output. Vocal folds in frogs,
reptiles and mammals are composed of several layers of tissue. An epithelial layer
covers a lamina propria. The lamina propria itself can be composed of more than one
layer, and the number of layers varies by species. The thyroarytenoid muscle, the third
distinct structural part of a mammalian vocal fold, is located lateral to the lamina
propria. The cellular and acellular morphology of vocal folds determines their
viscoelastic properties, and therefore are critical in determining how the tissue responds
to changes in airflow, posturing, and tension. The effect of multiple aspects on sound
output, for example, (a) active movements facilitated by laryngeal muscles, (b) vocal
fold morphology, (c) vocal fold viscoelastic properties and (d) vibration characteristics,
can be studied in isolation, but the full picture of laryngeal biomechanics requires the
investigation of the whole organ in action. One approach which we discuss here is the
excised larynx experiment. Although this approach cannot reproduce natural vocal
behavior, it helps reveal important aspects of the vocal fold functional morphology.
The use of perfused in situ larynx preparations and the differentiated stimulation
of motor efferent fibers of intrinsic laryngeal muscles, has helped characterize the acoustic space available to the vocal organ. We also describe the production of
ultrasonic vocalization in rodents, which do not rely on tissue oscillation but on a
purely aerodynamic process. For ultrasonic vocalization, the vocal folds are used as a
dynamic obstruction of the airway to produce sound by a whistling mechanism.
Fundamental frequency, Hyaluronan, Myoelastic-aerodynamic
theory, Stress-strain relationship, Stress relaxation, Vocal production.
Department of Psychology, University of South Alabama, Mobile, AL 36688, USA.