Perceptual Coherence : Hearing and Seeing

the oddball note to be the one most different in pitch, exactly the same result
found for the singers. In contrast, if the two instruments were clarinet
or English horn and trumpet (i.e., different instrument classes), then listeners
were able to identify the oddball note.
From these results, it seems that source constancy is built up by developing
a sense of how the sounds from one source will change across pitch.
For difficult discriminations such as between singing voices or woodwind
instruments, listeners need a fine-grain auditory image of the timbre at each
pitch to detect the oddball note, and that requires many notes to be able to
derive the necessary detailed trajectory of each source. We can imagine that
given only two notes, the listener will accept a wide range of sounds as
possibly coming from the same source. As the number of given notes increases,
the added information about the timbre transformations allows the
listener to restrict the range of possible sounds. For easier discriminations,
such as between a woodwind and brass instrument that sound widely different,
listeners do not need a detailed trajectory, so that a broadly tuned
one based on just a few notes will suffice.
Poulim-Charronnat, Bigand, Madurell, Vieillard, and McAdams (2004)
have investigated a different issue. Suppose we present a musical passage
using one sound source (e.g., a piano) and then present the same or a different
passage using a second source (e.g., a clarinet). Will listeners be able to
generalize across the two timbres and discriminate between the two passages?
On the surface, it seems that it should be an easy kind of constancy,
particularly for Western classical music, which mainly revolves around harmonic
structure. Yet Poulim-Charronnat et al. found that the discrimination
task was surprisingly difficult if one source was a solo piano and the second
source was an orchestra. Even skilled musicians were unable to do better
than roughly 65–70% correct when the timbre changed for passages from a
Liszt symphony. The same listeners were able to achieve an accuracy of
90% if the timbre did not change. The poor performance when the timbre
changed could be due to the large difference in the sources and resulting
timbre. It is easy to predict that performance will vary as a function of the
difference in the source timbres, and in fact this provides a converging operation
on the results from the multidimensional scaling experiments.
Summary
The Perception of Quality: Auditory Timbre 371
Of all the chapters, this is the one I feel most uncomfortable about. There
does not seem to be a consistent thread running through all of these results
except for the fact that listeners can and do identify objects and properties
of objects by their sound quality alone. The acoustic measures that correlate
with the perceptual judgments tend to change from experiment to