Factors affecting psychophysical tuning curves for normally hearing subjects.

These experiments were conducted to clarify the influence of beats and combination products on psychophysical tuning curves (PTCs) for normally hearing subjects. PTCs for 1- and 4-kHz sinusoidal signals were determined using as maskers a sinusoidal tone and 80-, 160-, and 320-Hz wide bands of noise. PTCs obtained using the sinusoidal masker showed distinct irregularities, particularly for masker frequencies close to the signal frequency. The PTCs determined for the noise maskers were more regular. The broader the masker, the more regular were the shapes of the PTCs. To reduce the detectability of beats produced by the interaction of the signal and masker, a pair of low-frequency tones, called "Modulation detection interference (MDI) tones", was used to introduce beats at the same rate. The MDI tones reduced the threshold level of the sinusoidal masker by up to 20 dB for frequencies within 300 Hz of the signal frequency; a similar but smaller effect was found when an 80-Hz wide masker was used. Adding a lowpass filtered (LF) noise to the sinusoidal or narrowband noise masker did not affect the low-frequency sides of the PTCs, suggesting no influence of combination products. The LF noise did affect the high-frequency sides of the PTCs, but this can be attributed to it reducing off-frequency listening. To achieve a PTC whose shape around the tip is minimally affected by beats, we propose using a noise masker with a bandwidth approximately equal to the bandwidth of the auditory filter for which the PTC is measured.