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Chapter 10

PSYB51H3 Chapter 10: Chapter 10

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Matthias Niemeier

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Chapter 10: Hearing in the Environment
Sound Localization
Auditory localization is quite different from determining the location of a visual object
If you could see an owl in front of you, you would know that it was to the left or the right of your fovea
because its image would appear on the right or left side of your retina
But the owl’s hoots enter your ears in exactly the same place (funnelled through the pinnae into the
middle and inner ear) regardless of where the owl is
Just as having two eyes turns out to be one of the keys to determining visual depth relations, having
two ears is crucial for determining auditory locations
For most positions in space, the sound source will be closer to one ear than the other. Thus, there are
two potential types of information for determining the source of a sound
First, even though sound travels fast, the pressure wave will not arrive at both ears at the same
time. Sound arrives sooner, albeit very slightly, at the ear closer to the source
Second, the intensity of a sound is greater at the ear closer to the source
These are our first two auditory localization cues
Interaural Time Difference
Interaural time difference (ITD): the difference in time between a sound arriving at one ear versus the
If the source is to the left, the sound will reach the left ear first. If it’s to the right, it will reach the right ear
Thus, we can tell whether a sound is coming from our right or left by determining which ear receives
the sound first
The term that is used to describe locations on an imaginary circle extended around us—front, back, left,
and right—in a horizontal plane is azimuth
Azimuth: the angle of a sound source on the horizontal plane relative to a point in the center of the
head between the ears. Azimuth is measured in degrees, with 0 degrees being straight ahead. The
angle increased clockwise toward the right, with 180 degrees being directly behind
Figure 10.4 shows interaural time differences, and it shows that ITDs are largest, about 640
microseconds when sounds come directly from the left or directly from the right, although this value
varies somewhat depending on the size of your head
A sound coming from directly in front of or directly behind the listener produces an ITD of 0; the
sound reaches both ears simultaneously
Studies have shown that listeners can detect interaural delays of as little as 10 microseconds
The medial superior olives (MSOs) are the first places in the auditory system where inputs from both
ears converge
Medial superior olive (MSO): a relay station in the brains stem where inputs from both ears
contribute to detection of the interaural time difference
Interaural Level Difference
The second cue to sound localization is called the interaural level difference, or ILD, in sound
ILD: the difference in level (intensity) between a sound arriving at one ear versus the other
Sounds are more intense at the ear closer to the sound source because the head partially blocks the
sound pressure wave from reaching the opposite ear
The properties of the ILD relevant for auditory localization are similar to those of the ITD:
Sounds are more intense at the ear that is closer to the sound source, and less intense at the ear
farther away from the source
The ILD is largest at 90 and –90 degrees, and it is nonexistent at 0 degrees (directly in front) and
180 degrees (directly behind)
Between these two extremes, the ILD generally correlates with the angle of the sound source, but
because of the irregular shape of the head, the correlation is not quite as precise as it is with ITDs
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