Resolution of pairs of simple and complex sounds in simulated auditory space

Abstract

A simulated acoustic replica of a sound source moving along a circle with a 4‐m radius in the frontal horizontal half‐plane was generated by obtaining, in 5‐deg azimuthal steps, external ear transfer functions for both ears of an artificial head placed in an anechoic room. When an arbitrary sound is passed through the digital filter defined by the left and right transfer functions corresponding to a given angle, the sound will acquire a subjective azimuth comparable to which the transfer function was measured [J. Blauert and P. Laws, Acustica 29, 273–277 (1973)]. This technique was used to measure resolution thresholds for pairs of amplitude‐modulated as well as pure sinusoids. With the listener required to identify the relative location of the two sounds in a two‐alternative forced‐choice paradigm, in one set of experiments the two sounds were fixed and the minimum audible angular separation was obtained, whereas, in another set, the angular separation was fixed and the minimum modulation‐ or carrier‐frequency separation was measured. Generally, both kinds of resolution are better around 0 azimuth than at the sides: In the “auditory fovea,” simultaneous resolution for pure tones and AM sounds (a 3‐ or 6‐kHz carrier sinusoidally modulated in the 150‐ to 400‐Hz range) is almost as good as the jnd obtained in the usual sequential presentation. The poorest spatial resolution was obtained for a pair of 6‐kHz carriers modulated by a 100‐ and a 400‐Hz sinusoid and the best for pairs of AM tones with different carriers. [Work supported by the Veterans Administration and by a NATO Travel Fellowship.]