A brief tone of steady intensity may be heard as growing louder. Increasing loudness signals an approaching sound source, so this illusion may reflect the importance of detecting potential collision, analogous to ``looming'' in the visual modality. In previous studies, diotic stimuli, simulating a sound source, within a plane defined by zero degrees azimuth, have been used. However, collision can arise from any direction. Hence, the illusion should extend to monotic stimuli, which simulate a sound source defined by +/-90 degrees azimuth; the sound source would be approaching from the side. In this article, the illusion in monotic stimuli with 4 carrier frequencies between 0.5 kHz and 2 kHz is described. Stimuli each lasted 1.5 s, during which time intensity was increasing, decreasing, or remaining steady; the initial intensity was 40 dB SPL (sound pressure level relative to 0.0002 dynes/cm(2)). Three listeners made forced binary responses of ``growing louder'' or ``growing softer'' to stimuli. The magnitude of illusion was computed from the value of changing intensity that elicited equal numbers of each type of response; for each listener, those values were negative. Measures of sensitivity were also obtained but did not vary systematically. The illusion was a little greater than that found with diotic stimuli, possibly because of the reduced overall loudness of monotic stimulation. Also, the illusion increased at higher frequencies; this result might be explained in terms of the frequency dependency of attenuation of a sound source's intensity by the listener's head.
|Journal||Journal of General Psychology|
|Publication status||Published - Oct 1997|