Impedance Matching in Sound Production and Hearing: a Comparative Study

Abstract

Sound production involves stages of impedance matching between the higher-density body of the animal and lower-density air. Hearing involves further impedance matching, from air to the higher-density sensory cells. Initial stages of sound production may include a frequency multiplier that converts slow muscle contractions into higher frequency mechanical vibrations. The frequency multiplier may also determine the sound frequency. Larger sound sources allow better impedance amtching with the air so sound radiation often exploits acoustic transformers to increase the effective size of the sound source. In many cases the conversion efficiency of muscle power to sound power is high, giving a large effective range for the signal. Sound detection often uses acoustic transformers to concentrate the sound onto relatively dense vibrating sturctures that are coupled to the sensory cells, providing the inverse of the impedance matching that occurs in sound production. These transformers may be associated with directional mechanisms and may drive arrays of receptors that allow frequency analysis. Sound production and hearing are special cases of the general phenomenon of impedance matching that occurs throughout biomechanics.