The relative role of beats and combination tones in determining the shapesof masking patterns at 2 kHz: I. Normal-hearing listeners

Masking patterns for a 2-kHz sinusoidal masker at 45, 65 or 85 dB SPK were measured for three normal-hearing subjects, using a 3-AFC method with feedb ack (condition 1). The patterns showed distinct irregularities, particularl y at the highest masker level. In condition 2, a lowpass noise was added to mask combination tones. The noise increased thresholds mainly for the 85 d B masker, for signal frequencies of 2.3-3.0 kHz. In condition 3, a pair of high-frequency tones ('modulation detection interference (MDI) tones') was used to introduce beats at the same rate as produced by the interaction of the masker and signal. Thresholds were higher than for condition 1, particu larly for signal frequencies adjacent to the masker frequency. In condition 4, the lowpass noise was presented simultaneously with the MDI tones. Thre sholds were well predicted as a combination of the effects of the lowpass n oise and the MDI tones. In condition 5, a pair of low-frequency MDI tones w as added to the masker. The thresholds had the same overall pattern as in c ondition 4. We conclude that the shapes of masking patterns measured using a 2-kHz masker are influenced by the detection of beats for masker-signal f requency separations up to at least 300 Hz and by the detection of combinat ion tones for separations between 300 and 1000 Hz. (C) 2000 Elsevier Scienc e B.V. All rights reserved.