Critical Bands and Two-Tone Suppression in Band-limiting Experiments

Abstract

Band-limiting experiments were performed at seven noise spectrum levels (N0=-20, -10, 0, 10, 20, 30, and 40dB SPL) for forward, simultaneous, and backward masking. Masked threshold for a 2kHz sinusoidal signal (duration 15ms and rise-fall time 5ms) was examined as a function of the bandwidths of the bandpass masker (duration 600ms and rise-fall time 10ms) in which its center was fixed at 2kHz, and its upper cut-off frequency was fixed at 2.15kHz, and lower cut-off frequency was fixed at 1.85kHz.The results were as follows:1) Masked threshold for a 2kHz sinusoidal signal was increased for the masker bands greater than three critical bands at delay time of 8ms in the simultaneous masking. This overshoot effect reached its full magnitude (about 10dB), if the masked threshold at a long delay time (300ms) was elevated at least 15dB above their threshold in quiet environment.2) Masked threshold at the delay time of 300ms in the simultanous masking showed that the gradients for subcritical bandwidths were significant at 3dB/oct and they were gradually transferred to the broad-band threshold for N0 greater than 0dB SPL.But in the forward and backward masking it showed that the gradients for subcritical bandwidths were less than 3dB/oct.3) Critical bands estimated in the simultaneous masking were well discribed by a level-dependent critical band with upward spread. The effective masker bandwidths produced by the forward and backward masking might be narrower than the critical band measured by the simultaneous masking.4) With the forward masking, threshold was exponentially decreased at the supercritical bandwidths. As the masker bandwidth was further increased by ten times of the critical bandwidth and masker level was increased, the masked threshold became more decreased. This suppression was increased by about 15dB for N0=40dB.5) With the backward masking the most striking features of these data were that a large decrease in the masked threshold was noted for the supercritical masker bandwidths, and the lower frequency bands showed the greater suppression than the higher frequency bands. This suppression pattern for the forward and backward masking consisted with intensity difference of 30dB.