Perceptual Coherence : Hearing and Seeing

obviously misplaced). The majority of subjects (60%) weighted each of the
three common dimensions equally. The remaining listeners either weighted
attack time most heavily or weighted spectral centroid and spectral flux
equally. Musical background did not differentiate the listeners.
The degree of specificity was not predictable from instrument class, and
the hybrid instruments did not differ from the simulated instruments. The
authors suggested that two types of sound qualities determine specificity:
(1) qualities that vary in degree, such as raspiness, inharmonicity, or hollowness;
and (2) qualities that are discrete, such as presence of a thud or
damped offset.
Subsequently, Marozeau, de Cheveigne, McAdams, and Winsberg
(2003) investigated whether the dimensional representation of sound quality
was equivalent at different notes. There were two major conditions. In
the first, listeners judged the difference between two instruments playing
the same note. Three different notes within one octave were used in separate
experiments. In the second, listeners judged the difference between
two instruments playing different notes, the maximum note difference being
slightly less than an octave. The similarity judgments were quite consistent
across conditions. First, the three perceptual dimensions derived in
all conditions were identical. The first was a measure of attack time
(termed impulsiveness here); the second was a measure of the spectral centroid;
and the third dimension was spectral spread. Thus, the first two dimensions
are identical to those found by McAdams et al. (1995). The third
dimension found by McAdams et al. correlated with spectral flux in contrast
to spectral spread, but we would expect some differences as a function
of the particular instruments used. Second, if both instruments played the
same note, the spatial arrangement of the instruments at the three different
notes tended to be similar. When the notes played by the two instruments
differed by 11 semitones, there were significant shifts for roughly half of
the instruments, as shown in figure 8.6.
The results of McAdams et al. (1995) and Marozeau et al. (2003) demonstrate
that the dissimilarity in the perceived sound quality between two instrumental
notes can be represented by the temporal and spectral acoustic
properties of the sounds. The research outcomes reviewed below suggest
that differences in attack time and the frequency of the spectral centroid
consistently covary with perceptual judgments, but that differences in other
acoustic measures such as spectral flux or spectral spread affect judgments
only in specific contexts determined by the individual sounds themselves.
Spectral Differences
The Perception of Quality: Auditory Timbre 355
Every scaling experiment has found that the predominant factor in judged
similarity is differences in the amplitudes of the component frequencies,