The changes in sound pressure level (SPL) at the two ears produced by azimuthal displacements of a free-field sound source in the interaural horizontal plane in frontal space were measured in anesthetized cats and the resultant interaural intensity differences (IIDs) were derived by subtraction. The mean curves at different frequencies indicate that azimuthal displacements from the median sagittal plane are generally associated with increases in SPL at the near (directly irradiated) ear and decreases at the far (shadowed) ear. Detailed measurements at 1° and 2° azimuthal steps in some animals revealed that at frequencies above 6-8 kHz there were marked variations in tympanic SPL at the far ear at lateral azimuths; these variations are apparently produced by interference between sets of 'creeping' surface waves propagated around the head in each direction. Near-ear changes were generally non-monotonic, becoming smaller or, in some cases, negative at large azimuthal displacements. At some frequencies above 8-10 kHz, functions for individual animals exhibited sharp decreases in level at the near ear (and consequently negative IIDs) at small azimuthal displacements. The general characteristics of the near-ear changes are in accord with previous evidence on the directionality of pinna amplification effects, although the sharp notches at some frequencies and azimuths presumably reflect more complex resonance and interference phenomena. Differences in the IID-azimuth relationship at different frequencies correlate well with data on the acuity of localization of high-frequency tones by cats.
- Interaural intensity difference
- Sound pressure transformation