The illusion of increasing loudness in brief steady tones: variation with carrier frequency

A brief tone of steady intensity is heard as growing louder; to be heard as steady, intensity must be decreasing. The present report concerns the influence of carrier frequency on this illusion. 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/cm2). Carrier frequencies were between 0.0625 and 8.0 kHz. Four listeners made forced binary responses of "growing louder" or "growing softer" to stimuli. Values of changing intensity that elicited equal numbers of each type of response were computed. As expected, these values were negative. The illusion was most pronounced for the lowest and highest frequencies. In contrast to the results of a recent study, the findings were that sensitivity to changing intensity did not vary systematically with the size of the illusion. The illusion might arise because many sounds slowly decay in intensity and because of the importance of detecting approaching sound sources, analogous to "looming" in the visual modality. Overall loudness of the sound source should assist in altering the listener; the increase in the illusion at the extremities of the frequency range might compensate for the reduced overall loudness at such frequencies.