Perceptual Coherence : Hearing and Seeing

184 Perceptual Coherence
differences between durations (i.e., frequencies) emerged. By four repetitions
per segment (AAAABBBBCCCCDDDD ...), the autocorrelation
equaled 0.80 due to the correlation of 1.0 within the four repetitions and 0.0
between the different noise segments. There still was a noisy component,
and the overall sequence sounded gritty and rough. At this point, the different
timbre associated with each noise segment became apparent, and the
sequence seemed to vary in quality at the same pitch as the segments
changed. However, the gain in tonal strength and the increase in the perceived
variation in timbre for four repetitions were much less for segment
durations below 0.5 ms (frequencies above 2000 Hz). By eight repetitions
per segment (the autocorrelation equaled 8/9), the noisy component disappeared,
except for the 0.125 ms durations (8000 Hz). For this segment duration,
at which the second harmonic at 16000 Hz was presumed not to
affect perception, the tonal strength did not grow at all up to 12 repetitions
and the segments did not appear to change quality. The pitch strength for
sequences of identical noises also reached its maximum at eight repetitions,
so that there was no difference between changing the noise segment every
eight repetitions and continuously repeating the same noise segment.
It is interesting to contrast these results with those from sequences of
alternating noise segments (e.g., AAAABBBBAAAABBBB ...). If we
start with the simplest case, ABABAB ..., listeners perceived the sequence
as being made up of the repeating unit AB, not of alternating A
and B segments. Thus the pitch was one half the pitch that would have resulted
from A or B. The entire sequence was heard as completely tonal,
and the tonal quality did not change. As the number of repetitions increased
(e.g., from AABBAABB to AAAABBBBAAAABBBB), the percept
changed as the perceived repeating unit shifted back to the
individual segments. The buzzy noise changed quality due to the different
noises and seemed to warble back and forth. This shift had two effects, as
illustrated in figure 4.14.
First, initially the AABBAABB construction was more tonal than the
AABBCCDD construction. The tonal component decreased in strength as
the number of repetitions increased due to a shift from perceiving the repeating
unit as [AB] or [AABB] to perceiving the individual segments [A]
in [AAAA] and [B] in [BBBB] as repeating. In contrast, for the AABBC-
CDD construction, the tonal component consistently increased as the number
of repetitions increased, so that by 8 to 12 repetitions there was no
difference in the tonal strength between the two kinds of sequences.
Second, I asked listeners to judge the warble in the sound, and that is
shown in figure 4.14B. For the longer-duration segments (e.g., 8 ms segments),
there was a strong sense of timbre alternation at even 2 or 4 repetitions,
and any difference between ABAB and ABCD sequences in the
strength of alternation disappeared after about 8 to 12 repetitions. For the