Biophysical Aspects of Directional Hearing in the Tammar Wallaby, Macropus Eugenii

The biophysical properties of the external ear of the Tammar wallaby, Macropus eugenii (Desmarest), have been investigated using probe microphones implanted in the ear canal. An acoustic axis of the pinna exists above 2kHz which is located close to the horizonal plane for natural ear positions, whereas azimuthal location of the acoustic axis is determined by pinna orientation on the head. The maximum on-axis acoustic pressure gain of the external ear reaches 25-30 dB for frequencies near 5 kHz. This results from pressure transformation by the horn-like pinna combined with resonance of the auditory meatus. The directionality of the pinna is similar to the sound diffraction properties of a circular aperture with an average radius based on the circumference of the pinna face. These properties determine the acceptance angle of the main lobe containing the acoustic axis and the spatial location of nulls. Large binaural intensity differences, exceeding 30dB, can be produced by the interaction of peaks and nulls between monaural directivity patterns, depending on the relative position of each pinna.