Eric Macaulay is a graduate student in the Department of Physics and Astronomy at Michigan State University.
Listeners localized high-frequency sine tones from a loudspeaker array spanning azimuths from 0◦ (straight ahead) to 90◦ (extreme right) in an anechoic room. Probe microphones were placed in the listeners’ ear canals to measure the sound pressure waveforms. Listeners verbally responded with their perceived source angle. The only cue for localizing tones at 1500 Hz and higher is the interaural level difference (ILD) between the sound pressure levels in the listener’s ears. Sound waves diffract around the head and create an acoustical bright spot on the opposite side of the sound source. This is analogous to the Arago (or Fresnel or Poisson) bright spot in optics. Because of the bright spot, the ILD is a non-monotonic function of azimuth. At a frequency of 1500 Hz the wavelength is comparable to the diameter of the human head, so the bright spot is fairly broad and the ILD has a maximum value at about 50◦ azimuth. Therefore the ILD was a misleading cue to listeners when the azimuth was greater than 50◦. Further experiments in which the amplitudes of the tones were modulated showed that some listeners can benefit from the interaural time differences (ITD) in the envelope.
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