Spectral cues which influence monaural localization in the horizontal plane

An extensive series of behavioral tests was carried out to determine what region, or regions, of the sound spectrum were critical for locating sounds monaurally in the horizontal plane. Seven subjects were requested to locate narrow bands of noise centered at different frequencies, combinations of these noise bands, low-pass, high-pass, and broadband noise. As observed in an earlier study, increasing bandwidth did not necessarily lead to improved localization performance until the band became broad, including, for example, all frequencies above 4.0 kHz. What seems to be happening is that listeners perceive narrow bands of noise originating from restricted places in the horizontal plane which may differ one from another depending on the frequency composition of the stimulus. In several instances, if two noise bands were presented simultaneously, the resulting stimulus was located with reasonable accuracy provided each component, when presented singly, was perceived as emanating from clearly separate azimuthal positions. If, however, two noise bands, which were perceived to originate from approximately the same azimuthal position when presented singly, were now presented simultaneously, the resulting stimulus still was perceived to originate from the same region of the horizontal plane. This, then, is a case where augmenting the spectral content of the stimulus does not bring about improved performance. We suggest that the expression of judgmental biases in the apparent location of a band of noise may prove useful for understanding why some stimuli of specified width and center frequency are localizable while others are not.