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

Masking patterns for a 2-kHz sinusoidal masker at 45, 65 or 85 dB SPL were measured for three normal-hearing subjects, using a 3-AFC method with feedback (condition 1). The patterns showed distinct irregularities, particularly 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 dB 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, particularly for signal frequencies adjacent to the masker frequency. In condition 4, the lowpass noise was presented simultaneously with the MDI tones. Thresholds were well predicted as a combination of the effects of the lowpass noise and the MDI tones. In condition 5, a pair of low-frequency MDI tones was added to the masker. The thresholds had the same overall pattern as in condition 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 frequency separations up to at least 300 Hz and by the detection of combination tones for separations between 300 and 1000 Hz.