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Rasch (1979) investigated synchronization in performed ensemble music. He observed that in polyphonic music, the synchrony dictated by the score is never realised in the actual performance. In fact, the standard deviation of the differences of onset time in trios (three instruments) was typically 30-50ms. Nevertheless, this does not generally lead to the percept of uneven playing. There are similarities in the conceptual motivation for his study with that of Morton, Marcus and Frankish (1976); Morton et al wished to define what is

Chapter 2 - Non Speech Stimuli

evenly timed in a perceptually isochronous sequence, if it is not the physical onset; Rasch wished to define what is synchronous in a polyphonic piece, if it is not the physical onsets of the instruments?

He studied the onset difference time of trios; that is the measured difference between physical onsets of instruments. The onset time was defined as when the intensity passed a threshold of 15-20 dB below maximum.

He compared the onset difference times of six pieces, each involving different instrument combinations.

Piece 1 : three recorders (treble and tenor)

Piece 2: three recorders (treble, tenor and descant) Piece 3&4: oboe, clarinet and bassoon

Piece 5&6: violin, viola and cello

The mean asynchronies (ie. standard deviations of onset different times) were 30ms and 31ms for the two recorder pieces, 37ms and 27ms for the wind instrument pieces, and 49ms and 37ms for the string pieces. Thus the range of asynchronies varied with the different trios.

When the mean physical onset differences were considered, it appeared that the tenor (low frequency range) was most different from the treble in the first piece, and from the descant in the second piece; they seem to vary thus with the frequency range of the instrument. In the wind pieces, the clarinet was most different from the oboe. In both the string pieces the viola was most different to the violin.

Rasch felt that possibly the observed differences were related to the rise times of the notes of the instruments. He stated that the recorders all have short rise

times " so the beginnings are clearly marked' (Rasch 1979, my italics). He stated that the wind instruments also have relatively short rise times. The string instruments have longer rise times - between 50 and 200ms. Rasch hypothesized that shorter, sharper rise times make better synchronization both necessary and possible.

It must be stressed that the emphasis of this paper is less about the perceptual beats of the musical notes, and the implication for performed and perceived synchronization, than about how longer rise times lead to a failure of performers and listeners to notice asynchronies.

This concept was refined and extended in a later paper. Vos and Rasch (1981) studied the Perceptual Onset (P.O.) of musical tones. This they defined within all acoustic stimuli, as "the moment in time at which the stimulus is first perceived" (1981, p323). This opposed to the physical onset, which is the time when the signal is first generated. The perceptual onset is delayed relative to the physical onset. Note again that this concept of perceptual onset is again similar in motivation to the P-centre hypothesis, which they explicitly mention.

Vos and Rasch (1981) attempted to apply a simple threshold model to the perceptual onset of musical tones. To test this they used a dynamic rhythm setting task, and their experimental method made similar assumptions to those of Morton et al; tone sequences were defined as perceptually isochronous if the time intervals between successive perceptual onsets are equal. In a method similar to Marcus (see Chapter 4) this enabled them to determine the threshold amplitude for perceptual onset.

In this way they measured the effect of rise time and intensity of the stimuli, and the effect on perceptual onset. An important difference between this method and that of Marcus, etc, is that to alter the A - B interval, the subjects

Chapter 2 - Non Speech Stimuli

in Vos and Rasch’s experiments had to vary the duration of the B stimulus. That is, to shorten the A - B interval, they increased the steady state portion of B.

Their results indicated that the P.O. of a signal was located at the point that the physical onset passed 17% of the maximum - that is, 15dB SPL below maximum intensity.

They next varied the physical intensity of the signals. They found that the time difference between physical and perceptual onsets increases with decreasing

tone intensity. This incfease was described as a shift up of the relative threshold which leads to the perceptual onset; they stressed that this shift in threshold was small compared to the variation in intensity. This led them to conclude that the threshold could simply be define relative to the maximum intensity of the signal, regardless of the physical intensity.

Their final experiment investigated the signal to noise ratio of the experimental presentation and its effect on the threshold. They found that the relative threshold for the perceptual onset of the tones decreased with increased level above the masked threshold. These results were congruent with the findings of the previous experiment - the louder the tones, the smaller the threshold.

They thus concluded that the P.O. of a tone was due to the intensity of a signal passing a simple threshold. The threshold was relative to the maximum intensity of the signal, 6-15dB SPL below the maximum intensity of the tone, depending on the sensation level of the tone. Figure 2.1 shows a diagram of this model of perceptual onset. Their results seemed to suggest adaptation to the constant stimulus level, and indeed the repetitious nature of all dynamic rhythm setting tasks are "optimal conditions for adaptation" (1981, p331). They found that the temporal integration model (Schütte 1977, 1978) predicted the