The misleading acoustic bright spot

The brain localizes the source direction of a pure tone at low frequency by interaural phase difference (IPD), and at high frequency by interaural level difference (ILD), a logarithmic measure of the ratio of sound intensities at the two ears. (See PHYSICS TODAY, November 1999, page 24 .) Localization by IPD shuts off abruptly around 1 kHz, where phase ambiguity could cause a disastrous 180° mistake. But nature doesn’t protect us from all acoustic misinformation. At frequencies up to 4 kHz, wavelengths are still comparable to the size of the head, so diffraction around the head might be misleading. At much higher frequencies, where diffraction is negligible, the head casts a proper acoustic shadow and ILD is a reliable clue to how far the source is off to the left or right. A new paper by Eric Macaulay and coworkers in the Psychoacoustics Group at Michigan State University compares sound-localization attempts of test subjects at 1.5 kHz with wave-propagation calculations that predicted they should often be badly misled by a diffractive phenomenon analogous to Fresnel’s optical bright spot. And indeed they were. The acoustic bright spot is a diffractive enhancement in the middle of the shadow cast by the head. The MSU results show that the effect consistently misleads hearers by spoiling the monotonic growth of ILD with increasing departure of the source from the forward direction. The photo shows a tiny unobtrusive microphone being put in a subject’s ear in the group’s anechoic test room to measure ILDs and correlate them with his guesses about source location. (E. J. Macaulay, W. M. Hartmann, B. Rakerd, J. Acoust. Soc. Am., in press .) —Bertram Schwarzschild