Perceptual Coherence : Hearing and Seeing

362 Perceptual Coherence
does not seem to be the case for source identification. Listeners will judge
widely different sounds as coming from the same event (e.g., walking) or
coming from the same source (e.g., singing or piano notes). It may be that
there is an acoustical invariant that occurs only for every sound from one
source, but I doubt that. I prefer to think that listeners develop transformations
or trajectories that link the sounds at different frequencies, intensities,
and sound qualities that are produced by one source or type of action.
If this conceptualization is correct, then any invariants will not be found
at a single pitch and loudness. Moreover, it is unclear whether the sound
qualities found at one note using multidimensional scaling are useful for
source identification. Some temporal properties such as onset time would
be useful because they would characterize different classes of instruments
across the playing range. Other spectral properties such as the frequency
centroid or spectral spread would not be useful because they can change
dramatically across the playing range. Right now, the answer is unknown.
Single Events
Many studies have investigated the identification of objects, including
instruments, speakers, and environmental events. Performance varies dramatically
depending on the objects, the stimulus presentation conditions,
listeners’ prior knowledge about the possible objects, and so on. On the
whole, identification is reasonably good, although it is quite difficult to assess
the relative effects of any the above factors. Listeners will make use of
whatever acoustic properties make the sounds most discriminable in the experimental
context.
Gaver (1993) suggested a way of organizing environmental events in
terms of physical actions. The three major categories are: (1) vibrating objects
due to impacts, scraping, or other physical actions; (2) aerodynamic
sounds due to explosions or continuous excitation; and (3) liquid sounds
due to dripping or splashing. Although this hierarchy does not necessarily
separate events in terms of their acoustic properties, we would expect a
strong relation between the physical events and their acoustic properties.
There is not a perfect correspondence between this classification and the
acoustic properties, particularly with respect to rhythmic patterning, but
this hierarchy does have heuristic value.
The research on timbre as a sound quality suggested that the acoustic
properties could be organized into one class dealing with the temporal
properties, particularly onset time, and a second class dealing with the
spectral properties. Such a split will prove useful in this section, as long as
we expand our conception of the temporal properties to include rhythmic
patterning within individual sounds as well as between sounds and as long
as we expand our conception of the spectral properties to include noise.