Perceptual Coherence : Hearing and Seeing

368 Perceptual Coherence
sounds, a third rubbing sounds, and so on (as in Gaver, 1993, above). Then
the responses of all the listeners to one sound were put in the appropriate
categories. If all listeners describe the sound as the same type of event,
then all responses will fall in one category, and the uncertainty will be zero.
If all of the descriptions are of different types of events and occur equally
in all the different categories, the uncertainty will be the maximum value
possible. Simply put, causal uncertainty is an indicant of how many different
events could have produced the sound. The measure of causal uncertainty
was highly correlated to the mean time it took to identify the sound,
and several acoustic features (e.g., presence of harmonics) also were correlated
to identification time. Thus, both the causal uncertainty and presence
of specific acoustic features influence identification.
Ballas (1993) used several methods to assess the effect of familiarity.
First, based on a written description (not the sound itself ), listeners rated
their familiarity with the object. Second, to measure the a priori probability
that the sound occurred in the natural environment, participants were asked
to report the first sound they heard when a timer randomly activated. On the
whole, there is a weak relationship between ecological frequency (method
2) and causal uncertainty. If a sound occurs frequently, we might expect listeners
to have more chances to discover the relevant acoustic cues. One
possible reason for the weak relationship is that most of the sounds that occurred
frequently were background sounds (air in heating ducts) that listeners
normally do not pay attention to.
Ballas (1993) concluded that identification is best conceptualized as
arising from information in different domains, rather than arising from a
single measure of some sort. The maximum prediction of the identification
time included four types of data: (1) temporal acoustic properties (similar
spectral bursts in noncontinuous sounds); (2) spectral acoustic properties
(average frequency of spectrum); (3) amplitude envelope (ratio of burst duration
to total duration); and (4) ecological (frequency of occurrence). This
is the same “no smoking gun” conclusion coming from experiments on
color constancy.
Source Constancy: Multiple Sounds From One Source
There is hardly any cross-referencing between the research on timbre as a
sound quality and the research on source identification, even though both
kinds of studies arrive at nearly the same temporal and spectral properties.
The research on timbre quality has emphasized human sounds (e.g., musical
instruments, speaking and singing voices) that can be produced at
widely different fundamental frequencies and amplitudes. The research on
source identification has concentrated on the identification of a single
sound (even if the sound was a set of discrete impacts, e.g., walking) that is