Phonatory characteristics of the excised human larynx in comparison to other species

Abstract

Objective: The purpose of this study was to determine the conditions needed to elicit phonation from excised human larynges and the resultant range of phonations produced; compare that with similar information previously obtained from canine, pig, sheep, and cow; and relate those findings to previously reported information about viscoelastic properties of the vocal fold tissue (ie, stress-strain curves and Young's modulus).

Methods: Six human larynges of the geriatric group (age range, 70-89) were mounted on the bench without supraglottic structures, and phonation was achieved with the flow of heated and humidified air through the tracheal tube. Using various sutures to mimic the function of the laryngeal muscles, the larynges were put through a series of sustained oscillations with adduction as a control parameter.

Results: The human larynges oscillated with an average frequency that was close to the canine larynges, but the oscillation behavior and wide frequency range were similar to those of pig larynges. The similarity of the wide vibration frequency ranges of human and pig larynges may be because of the nonlinear behavior of their elasticity, which is related to the high collagen content of the vocal folds. On the contrary, other species with limited frequency ranges showed almost linear stress-strain curves because of the higher elastin and lower collagen contents.

Conclusions: The physiological differences in the linearity and ranges of oscillation of excised larynges reported in this study and previous studies are reflective of the tissue composition and mechanics.

Figure 1

A mounted excised human larynx with control sutures and EGG electrodes.

Figure 2

Moving averaged values of subglottal pressure (Ps), flow rate (Fl), fundamental frequency (F0), pressure amplitude (Pa), and sound pressure level (SPL) for excised human larynx # 4 during pressure-flow increasing sweep.

Figure 3

Averaged values of phonation threshold pressure (PTP), subglottal pressure (Ps), sound pressure level (SPL), and fundamental frequency (F0) across 6 human excised larynges, with average values and standard deviation on top of each bar. Due to the equipment failure, the SPL is missing for the larynges 1–3.

Figure 4

Comparative phonatory characteristics of pig, sheep, cow, dog, and human larynges, including phonation threshold pressure (PTP), subglottal pressure (Ps), sound pressure level (SPL), and fundamental frequency (F0).

Figure 5

Phonatory characteristics of canine larynges including average values of the phonation threshold pressure (PTP), subglottal pressure (Ps), sound pressure level (SPL), and fundamental frequency (F0).

Figure 6

Pressure-frequency relations for typical samples of dog, pig, sheep, cow, and human excised larynges during increasing pressure-flow sweeps.

Figure 7

Average Young’s modulus comparison of vocal fold samples from different species. Each bar represents a species with its standard deviation as error bar.

Figure 8

Stress-strain loops of various vocal fold samples. A- dog vocal folds, B- cow vocal folds, C- pig vocal folds, and D- human vocal folds. E0 stands for low-strain Young’s modulus and EH for high-strain Young’s modulus (both in kPa).