Intonation and the perceptual separation of simultaneous voices

Abstract: The present paper examines the role of speech pitch in the perceptual separation of simultaneous speech messages, when both messages are spoken by the same speaker and there are no differences in directional hearing. In a first experiment, employing resynthesized speech with completely monotonous pitch, it is shown that intelligibility of the target message can be manipulated by introducing an artificial constant difference in pitch between target speech and interfering speech. Within certain limits intelligibility increases with increasing difference in pitch. In a second experiment natural speech is employed for both target and interfering messages. The interfering speech is always spoken with normal intonation, whereas the target messages are either spoken with normal intonation or deliberately spoken in a monotone. For both intonation conditions the messages are either spoken within the same pitch range as the interfering speech (SAME PITCH), or within a considerably higher pitch range (DIFFERENT PITCH). For the messages spoken with normal intonation the SAME PITCH condition is considerably less intelligible than the DIFFERENT PITCH condition. For the monotonously spoken messages the results are less clear. Here the effect of a difference in pitch range is probably confounded with the effects of other properties of speech which result from a monotonous pronunciation. The main results of these experiments can be related to the phenomenon of “perceptual fusion”, occurring whenever two simultaneous sounds have identical pitches, and to “perceptual tracking”: whenever the pitches of target and interfering speech cross each other, the listener runs the risk of inadvertently switching his attention from the target speech to the interfering speech.