Masking in simulated free‐field

R. W. Gatehouse [Proc. 12th I.C.A. 2, B2‐7 (1986); J. Acoust. Soc. Am. Suppl. 1 82, S108 (1987)] reported a free‐field masking effect that appears anomalous on the surface: A pure‐tone probe positioned at a nonzero azimuth is maximally masked by a broadband noise when the azimuthal position of the masker does not coincide with that of the probe. In the present experiments, similar results were obtained with stimuli presented in a simulated free‐field using head‐related transfer functions measured in an artificial head [J. Blauert and P. Laws, Acustica 29, 273–277 (1973)], with a 3‐kHz probe located at a 45° left azimuth and a white‐noise masker positioned at either 45°, 30°, or 20° left azimuths. Maximum masking was observed at the 30° noise location. This nonmonotonic masking effect may not be due to anything but the head‐induced diffraction patterns characteristic to the 3‐kHz range: Measurement of the noise power (at the output of the transfer functions inside the 1/3‐oct band surrounding the probe showed that, as the noise was displaced from the 45° left to the 30° left position, the decrease of the noise level in the left ear was less important than the increase in the right ear. Adding this variable‐level noise to the fixed signal should thus result in monaural masked thresholds that are not appreciably different in the left (i.e., the near) ear but that monotonically increase with decreasing left azimuths in the right (i.e., the distant) ear. These predictions were, indeed, confirmed by results of masking experiments using the same stimuli presented monaurally on the two ears. [Work supported by Department of Veterans Affairs.]