Intensity Changes at the Ear as a Function of the Azimuth of a Tone Source: A Comparative Study

Abstract

Sound intensity of a tone (4, 10, and 20 kHz) was measured at the external auditory meatus as a function of the azimuth (at every 15°) of the sound source. The observations were made under free-field conditions, and man, squirrel monkey, rat, and bat were used as subjects. At 4 kHz, rat and man showed consistent variation of intensity as a function of azimuth. Only irregular variations were obtained in monkey and bat. Interaural-intensity differences (IIDS) at 4 kHz were estimated to be sufficient to support localization of a tone at a minimum angle of 2.1° (rat) and 1.125° (man) from the midline. At 10 kHz, rat, bat, and monkey showed consistent variations of intensity with azimuth of at least 15 dB. IIDs at 10 kHz were estimated to be sufficient to support the localization of a tone at a minimum angle of 1.125° (monkey), 2.0° (bat), and 1.5° (rat) from the midline. At 20 kHz, consistent intensity variations of at least 14 dB were obtained in rat, bat, and monkey. IIDs were estimated as sufficient to support localization at a minimum angle of 1.5° (monkey), 2.25° (bat), and 3.6° (rat).