Introduction to Acoustics

sentences (see Fig. 13.20). They varied the ITDs ofthe
two sentences in the range 0 to ±181 µs. For example,
one sentence might lead in the left ear by 45 µs, while
the other sentence would lead in the right ear by 45 µs
(as in Fig. 13.20). The sentences were based on natural
speech but were processed so that each was spoken on
a monotone, i. e., with constant F0. TheF0 difference
between the two sentences was varied from 0 to 4 semitones.
Subjects were instructed to attend to one particular
sentence. At a certain point, the two sentences contained
two different target words aligned in starting time and
duration (“dog” and “bird”). The F0sandtheITDsofthe
two target words were varied independently from those
of the two sentences. Subjects had to indicate which of
the two target words they heard in the attended sentence.
They reported the target word that had the same ITD as
the attended sentence much more often than the target
word with the opposite ITD. In other words, the target
word with the same ITD as the attended sentence was
grouped with that sentence. This was true even when the
target word had the same ITD as the attended sentence
but a different F0. Thus, subjects grouped words across
time according to their perceived location, independent
of F0 differences. Darwin and Hukin [13.198] concluded
that listeners who try to track a particular sound source
over time direct attention to auditory objects at a particular
subjective location. The auditory objects themselves
may be formed using cues other than ITD, for example,
onset and offset asynchrony and harmonicity.
It should be noted that, for discrete sequences of
musical tones, the auditory system does not necessarily
form streams according to perceived location, especially
when that cue competes with other cues. This is illustrated
by an effect, called the scale illusion, reported
by Deutsch [13.217]. She presented two sequences of
tones via headphones, one sequence to each ear. The nth
tone in the left ear was synchronous with the nth tone
in the right ear. The sequences were created by repetitive
presentation of the C major scale in both ascending
and descending form, such that when a component of
the ascending scale was in one ear, a component of
the descending scale was in the other, and vice versa.
However, the tones from each scale alternated between
ears. Within each ear there were often large jumps in
frequency between successive tones. Most subjects perceived
the sounds as two streams, organized by the
frequency proximity of successive tones. One stream
(which was often heard towards one ear) was heard as
a musical scale that started high, descended and then increased
again, while the other stream (which was usually
heard towards the opposite ear) was heard as a scale that
Psychoacoustics 13.8 Auditory Scene Analysis 491
started low, ascended, and then decreased again. Thus,
the true location of the tones had little influence on the
formation of the perceptual streams.
Another example come from the opening bars of the
last movement of Tchaikovsky’s sixth symphony. This
contains interleaved notes played by the first and second
violins, who according to 19th century custom sat on
opposite sides of the stage. These notes are perceived
as a single stream, despite the difference in location,
presumably because of the frequency proximity between
successive notes.
A number of composers have exploited the fact that
stream segregation occurs for tones that are widely separated
in frequency. By playing a sequence of tones
in which alternate notes are chosen from separate frequency
ranges, an instrument such as the flute, which is
only capable of playing one note at a time, can appear
to be playing two themes at once. Many fine examples
of this are available in the works of Bach, Telemann and
Vivaldi.
Judgment of Temporal Order
It is difficult to judge the temporal order of sounds
that are perceived in different streams. An example
of this comes from the work of Broadbent and Ladefoged
[13.218]. They reported that extraneous sounds in
sentences were grossly mislocated. For example, a click
might be reported as occurring a word or two away
from its actual position. Surprisingly poor performance
was also reported by Warren et al. [13.219] for judgments
of the temporal order of three or four unrelated
items, such as a hiss, a tone, and a buzz. Most subjects
could not identify the order when each successive item
lasted as long as 200 ms. Naive subjects required that
each item last at least 700 ms to identify the order of
four sounds presented in an uninterrupted repeated sequence.
These durations are well above those which are
normally considered necessary for temporal resolution.
The poor order discrimination described by Warren
et al. is probably a result of stream segregation.
The sounds they used do not represent a coherent class.
They have different temporal and spectral characteristics,
and, as for tones widely differing in frequency, they
do not form a single perceptual stream. Items in different
streams appear to float about with respect to each other
in subjective time. Thus, temporal order judgments are
difficult. It should be emphasized that the relatively poor
performance reported by Warren et al. [13.219] is found
only in tasks requiring absolute identification of the order
of sounds and not in tasks which simply require
the discrimination of different sequences. Also, with
Part D 13.8