Level effects in psychophysical two-tone suppression.

Measurements of psychophysical two-tone suppression in a number of subjects are described. Levels of the stimulus components (suppressee, L1, and suppressor, L2) were the primary experimental variables. In all experiments the pulsation threshold was used with the probe frequency fr fixed at the suppressee frequency f1. In an initial experiment f1 was fixed at 1 kHz. The suppressor frequency f2 ranged from 0.2 to 1.4 kHz. At appropriate levels all subjects showed significant suppression. Suppression was found to decrease to zero as f2 approached f1. The amount of suppression depended on both L1 and L2 in a way not accounted for by any of the current theories of two-tone suppression. At higher overall levels suppression became increasingly prominent. The amount of two-tone suppression in a given stimulus condition depended strongly on the subject. The maximum amount of suppression measured was about 35 dB. In a second experiment it was verified that suppression follows the same pattern at other frequencies f1 (0.5, 2, and 4 kHz). Data for equal f2/f1 ratios were quite similar. The two-tone suppression effect decreased in a noisy environment. Within a 20-dB range of signal-to-noise ratios the effect of noise changed from negligible to the virtually complete elimination of two-tone suppression.