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.