Perceptual Coherence : Hearing and Seeing

exists at several levels: (a) describing the differences between sounds that
are composed of a broad band of frequencies and do not have a clear pitch,
such as sawing or scraping; (b) describing the differences between sounds
at one frequency and intensity, that is, the ANSI (1973) definition; (c) describing
and identifying sources across different frequencies and intensities,
such as a clarinet as opposed to a saxophone; (d) describing and
identifying source categories across different sources, frequency, and intensity,
for instance, sopranos as opposed to mezzo-sopranos, woodwind
instruments as opposed to brass instruments. For (c) and (d), timbre must
be reconceptualized as a sound transformation that allows us to predict
an object’s sound at different frequencies and thereby allows us to track
objects in the environment. We might characterize (a) and (b) as a proximal
description of sound quality, and (c) and (d) as a distal description of
objects.
I am not ready to abandon timbre as an empty concept even though the
term is used in so many ways, like the term appearance, that it means
whatever we wish. Moreover, because sound production results in so many
acoustical properties, there may not be any fixed set of acoustic cues that
allow us to identify the same object in real environments with overlapping,
competing sounds of other objects. Perhaps the only possibility is to list the
possible acoustic variables and transformations, and then identify the subset
used at different times (this is the same conclusion we reached for the
cues to color).
In spite of this perceptual and conceptual inexactness, we still have the
basic perceptual issues. We presume that the goals of the auditory and visual
systems are identical: Namely, to construct a coherent representation
of objects in the external world. These objects will have inherent properties,
and perceptual systems should be tuned to pick up those properties.
At this point, we cannot distinguish between innate and learned mechanisms,
or the degree to which recognition is a purely inferential process,
but that does not matter.
Sound Production: The Source-Filter Model
The Perception of Quality: Auditory Timbre 335
Let us start by comparing the source-filter model for seeing to that for
hearing. For seeing (color), the source was the energy of the illuminant at
different frequencies; the filter was the reflectance of the surface at those
frequencies; and we multiplied the illumination by the reflectance to predict
the light reaching the eye. The assumption was that the light at all frequencies
reached the eye at the same time. For hearing (timbre), the model
is more complex. First, we need energy to excite the source. What makes
this complicated is that different ways of exciting the source (e.g., hitting,