Physiological measurements and mathematical models for interaural pathways

Abstract

The middle-ear cavities of birds, reptiles, and anurans are often connected through a tube, which can act as an interaural pathway between both eardrums. Such an arrangement of acoustically effective structures is described as a pressure-difference receiver. It is generally assumed that pressure-difference receivers enhance the skills for directional hearing in animals. Sound pressures and phases were measured with probe-microfones in the interaural pathway from Zebra Finches during azimuthal rotation. The results show pressure differences between right and left middle ear, which are dependent on the azimuth and the frequency of the applied sound. A mathematical model was constructed, which gives the resulting pressure and phase of two overlaying waves that travel different distances. Assuming one wave traveling through the pathway and a second around a sphere, the resulting wave from dependency on azimuth and frequency was calculated. The pattern shows the same characteristic pressure differences as the physiological measurements. The same procedures were performed with a sphere as a hardware model for a head with an interaural pathway. The results show again the expected characteristics. The combination of physiological measurements, mathematical modeling, and testing this model with hardware models gives a meaningful explanation for the acoustical properties of middle ears as a basis for psychological acoustics.