Measurement of the Linear and Nonlinear Mechanical Properties of the Oscine Syrinx: Implications for Function

We have measured the vibrational modes of the sound producing membrane in the syrinx of zebra finches and canaries. Excised syringes were driven with a frequency-swept acoustic pressure wave through the trachea and the resulting vibrations measured using a laser interferometer. The sound pressure behind the medial membranes was measured with a calibrated probe microphone placed through the contralateral bronchus. The frequency-dependent membrane compliance was measured at 10-20 different positions, giving a detailed picture of the linear vibrational modes of the two membrane components, the medial labium (ML) and the medial tympaniform membrane (MTM). Nonlinear properties of the membrane was determined by measuring the linear response at several superimposed static pressures. In zebra finches, membrane compliance is dominated by the lowest vibrational mode, a narrrow mechanical resonance at roughly 700 Hz that extends over the entire membrane. Several higher frequency modes were also observed. The frequencies of the vibrational modes are determined by the mechanical properties of the heavier ML, rather than the thinner MTM. We thus propose that mechanical resonances of the ML are critical in determining the oscillatory frequency of the syrinx. We also suggest that the MTM may serve to produce a wave-like motion of the membranes, increasing the efficiency of sound production.