Perceived Roughness of Two Simultaneous Pure Tones

Previous studies have shown that the sensation of roughness produced by two simultaneous pure tones primarily depends on the beat rate of the tones, that is the difference in frequency between the beating tones. In the present study, a map of the dependence of roughness on the beat rate and the centre frequency of a pair of pure tones was determined by the method of absolute magnitude estimation. The results show that the general course of the function relating roughness to the beat rate is in agreement with published studies: roughness reaches a maximum at a certain beat rate and gradually decreases below and above that rate. The beat rate at which maximum roughness is obtained increases from 24 Hz for a centre frequency of 63 Hz to 84 Hz for a 4000-Hz centre frequency. In contrast with the conclusions of previous studies, the frequency interval between two tones that yields maximum roughness at various centre frequencies is not a constant fraction of the bandwidth of the auditory filter, nor is the frequency interval at which roughness disappears equal to the auditory filter bandwidth.