Perceptual Coherence : Hearing and Seeing

372 Perceptual Coherence
experiment, often are not clearly connected to the sound production process,
and much of the time do not seem intuitive to me. Maybe that should
be expected because of the complexity of the stimuli due to the large number
of vibration modes of the source and filter and the number of unique receptor
cells, and because listeners will make use of any acoustic property
that works in a context.
A recent approach to source identification, termed physically inspired
modeling, may provide a systematic way to investigate the perception of
simple rigid objects. Physically inspired modeling makes use of the vibration
wave equations for the motions of simple bars to synthesize the sounds
of those objects. The advantage of this approach is that it allows the experimenter
to vary the parameters of the vibration modes (e.g., the decay rates)
and thereby isolate the properties that listeners actually use to identify the
objects. For example, Lutfi (2001) synthesized the sound of freely vibrating
hollow and solid bars and required listeners to distinguish between the two
types. The first vibration mode of hollow bars decays more quickly, is
louder, and has a lower frequency than the first mode of solid bars. Some
listeners used decay and frequency while others used only frequency to
make their judgments. By comparing the wave equations to discrimination,
Lutfi was able to show that the limits of human sensitivity (i.e., internal
noise) can account for the errors made as the intrinsic acoustic relationships
are perturbed.
Moreover, I have equated color constancy with the ability to predict
sound quality at one pitch and loudness from the sound quality of the same
object at a different pitch and loudness. At least at a superficial level, that
seems to be a correct analogy. To achieve color constancy, the observer
must remove the effects of the change in illumination to capture the unchanging
surface reflectance. To achieve sound source constancy, the listener
must remove the effects of changes in pitch and loudness to capture
the source resonances. But the source filter resonances change at different
excitation frequencies so that the listener can be, in effect, faced with a
different sound object when the excitation frequencies are widely different.
Without sounds at intermediate frequencies to characterize the connecting
transformation, it would be impossible to judge whether two sounds
come from the same object. Perhaps the analogy holds within overlapping
frequency ranges of about one octave where the source filters are fairly
constant.