Estimation of viscoelastic shear properties of vocal-fold tissues based on time-temperature superposition

Empirical data on the viscoelastic shear properties of human vocal-fold mucosa (cover) were recently reported at relatively low frequency (0.01 - 15 Hz). For the data to become relevant to voice production, attempts have been made to parametrize and extrapolate the data to higher frequencies using constitutive modeling [Chan and Titze, J. Acoust. Soc. Am. 107, 565-580 (2000)]. This study investigated the feasibility of an alternative approach for data extrapolation, namely the principle of time-temperature superposition (TTS). TTS is a hybrid theoretical-empirical approach widely used by rheologists to estimate the viscoelastic properties of polymeric systems at time or frequency scales not readily accessible experimentally. It is based on the observation that for many polymers, the molecular configurational changes that occur in a given time scale at a low temperature correspond to those that occur in a shorter time scale at a higher temperature. Using a rotational rheometer, the elastic shear modulus (G′) and viscous shear modulus (G″) of vocal-fold cover (superficial layer of lamina propria) tissue samples were measured at 0.01-15 Hz at relatively low temperatures (5°-37 °C). Data were empirically shifted according to TTS, yielding composite "master curves" for predicting the magnitude of the shear moduli at higher frequencies at 37 °C. Results showed that TTS may be a feasible approach for estimating the viscoelastic shear properties of vocal-fold tissues at frequencies of phonation (on the order of 100-1000 Hz).