Asynchronous Preparation of Tonally Fused Intervals in Polyphonic Music

An analysis of a sample of polyphonic keyboard works by J.S. Bach shows that synchronous note onsets are avoided for those harmonic intervals that most promote tonal fusion (such as unison, fifths and octaves). This pattern is consistent with perceptual research showing an interaction between onset synchrony and tonal fusion in the formation of auditory streams (e.g., Vos, 1995). The results provide further support for the notion that polyphonic music is organized so as to facilitate the perceptual independence of the concurrent parts. IN the perception of auditory scenes, a number of factors are known to influence the formation of auditory streams (Bregman, 1990; McAdams & Bregman, 1979; van Noorden, 1975). Both time domain and frequency domain factors contribute to the perception of either separate or integrated sound images. In the case of concurrently sounding tones, two important factors are tonal fusion and onset synchrony. Tonal fusion is the tendency for two or more harmonically related tones to fuse and form a single auditory image. Onset synchrony is the tendency for two or more tones having coincident onsets to evoke a single auditory image. Onset asynchronies as small as 10 ms have been shown to facilitate the segregation of two tones so they form independent sounds (Rasch, 1978). However, Vos (1995) has noted that the experimental paradigms used in such experiments employ cyclic (repeated) stimuli that make it easier to segregate the component sounds. Consequently, such experimental measures ought to be regarded as ideal threshold values rather than values typical for source separation in common listening situations. Using an experimental procedure that better approximates the uncertainties attending naturally-occurring sounds, Vos found that for non-cyclic stimuli, asynchronies of even 20 ms are insufficient to evoke segregation of the component tones. Larger amounts of asynchrony are necessary: Vos has suggested that for significant effects on perceptual separation to occur, asynchronies greater than 50 ms may be needed (Vos, 1995; p. 414). Beyond 50 ms, larger asynchronies may well continue to facilitate the segregation of auditory sources. It is plausible that the ceiling effect for source segregation due to onset asynchrony may not appear until the inter-onset interval is 100 ms or longer. This latter value is more typical of the asynchronies implied in musical scores where notated asynchronies between two parts are rarely less than a sixteenth or thirty-second note in duration. At 96 beats per minute, for example, two notes whose onsets differ by a sixteenth-note duration would have a nominal asynchrony of 156 ms. In the case of polyphonic music, musical samples have already been shown to be consistent with the pursuit of asynchronous onsets between concurrent parts. Rasch (1981) carried out an analysis of vocal works by Praetorius, and showed that onset asynchrony between the notated parts increases as the number of voices is increased. Experiments by Huron (1989a) and Parncutt (1989) have shown that listeners' abilities to track auditory streams become degraded as the number of concurrent voices increases. Hence, the observed increase in asynchrony might be expected to compensate for the increased difficulty of parsing auditory scenes that contain many parts. Even in the case of two-part polyphonic music, Huron (1989b, 1993) showed that, compared with a meter-controlled distribution, Bach's 15 two-part Inventions exhibit a systematic tendency to avoid synchronous onsets between the parts. In the case of tonal fusion, evidence also indicates that polyphonic music is organized so as to avoid tonally-fused intervals. Using data on tonal fusion collected by Stumpf (1890) and by DeWitt and Crowder (1987), Huron (1991a) showed that, compared with a pitch-proximity-controlled distribution of