MODULE 7

Auditory Cues for Space Perception

The last module was devoted to our visual perceptions of the world around us. This module will investigate the cues we receive through hearing. Most sounds affect our perception of our world; they give us cues to the direction and the distance of objects in our environment.!

As you read the text, keep the following questions in mind.

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AUDITORY SPACE PERCEPTION

Human beings are basically spatial animals; they experience space through hearing, touching, moving, and seeing. It is important, therefore, that we be able to perceive space. The different sensory experiences do not lead to different spaces, but to a single unified space; and generally, the sensory cues harmonize to a high degree.

Distance perception is based on amplitude which requires sensation from only one ear

Hearing provides us with cues about our world. Auditory perception of distance requires only one ear; it is monaural. Directional location of sound sources requires both ears; it is binaural.

The major stimulus characteristic of distance determined by a number of characteristics of the sound the amplitude of sound waves reaching the ear. Therefore, loud sounds seem to be heard as coming from a closer object and soft sounds as coming from a more distant one.

As a complex sound wave travels through the air, its complexity diminishes. For example, the sound of an airplane motor is far less complex when heard at a distance than when heard nearby. When it is close a variety of sounds are heard; at a distance only the low hum is audible.

Nearby sounds, in addition to being louder and more complex than distant ones, seem to fill more space or have more volume. A sound that has more volume is also heard as closer, particularly if it dominates other competing sounds.

Aside from these stimulus factors, set and meaning are also important. Individuals hear sounds at distances when they expect to, in accordance with their interpretations of the sound sources.

When previous knowledge and vision are eliminated, two ears are necessary for locating a sound's direction. Human beings are unable to localize sound in terms of auditory cues alone unless they stimulate the two ears differently.

The nature of binaural cues has been systematically investigated as shown in Figure 19. The subject is put inside a sound cage. His head is held motionless and he is required to judge the direction of sounds originating at various locations on the surface of the surrounding sphere.

Figure 19. A sound cage ( The subject's head is held motionless while the loudspeaker is moved to study sound localization). 124

Perception of sound location requires binaural cues, a sound creating different sensations in the two ears

When the sound is originated to the right or left of the median plane (the vertical plane passing through the middle of the head from front to back), the person judges the direction of the sound correctly. Analysis of the physical situation shows that the two ears are differently stimulated by a sound depending on its direction. If the sound originates on the right side, it reaches the right ear a split second sooner than it does the left ear. This slight time difference provides an essential cue to direction.

An intensity difference occurs whenever the sound source is closer to one ear than to the other. This is primarily caused by the sound "shadow" cast by the head; sound waves that have to pass around the head are somewhat disrupted and their intensity is slightly reduced.

A phase difference refers to the difference in phase of the cycle activating the two ears; the sound may be in different parts of its cycle when it strikes the nearer ear than when strikes the farther ear. This difference is important only when a tonal stimulus is involved; but it is especially helpful if the sound wave is of low frequency and, therefore, has a relatively long wavelength. Experiments have shown that some individuals locate sounds in terms of phase differences and others do not.

If the sound comes from anywhere in the median plane -- thus striking the two ears simultaneously and with equal intensityÄit is heard as being somewhere in the plane. However, the person cannot tell where in the plane, front, back, above, or below. This dislocation is referred to as a plane of confusion.

Similarly the person experiences a cone of confusion for sounds coming from one side of the median plane. The person can tell from which side the sound comes, but he cannot discriminate among the sounds originating at the many points on the surface of the cone. This confusion results because all points on the cone are the same distance farther from one ear than from the other; and a sound originating at any point on the cone will provide the same difference in stimuli at the two ears. (Figure 20)

Head movements produce different temporal patterns of stimulation at the two ears and enable people to discriminate among sounds that are otherwise indistinguishable as to direction.

Other factors play a decisive role in sound location. When a visual stimulus occurs simultaneously with an auditory stimulus, they tend to be located in the same place in space. This depends also, of course, upon the factors of learning and expectation (set), which play a major role in sound location as they do in all forms of perceptual organization. In addition, olfaction facilitates the location of odor-giving objects or animals, although this is more useful to animals than humans.

Temperature also provides spatial information, but this sensitivity aids only relatively simple acts, such as locating the direction of a draft or a warm gust.

Figure 20 (Illustration of cone of confusion) No matter et what point a sound originates on a conical surface like the one illustrated, it is always the same distance farther from one ear than from the other. Accordingly, though the hearer can tell from which side the sound comes, he cannot discriminate among the many possible locations. (After Boring, Langfeld, and Weld, 1948)

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MODULE 7
PROGRESS CHECK 1

Now test yourself without looking back.

1. A person who is deaf in one ear receives:
a. distance cues.
b. directional cues.
c. (neither)

2. Where is a sound when it is in the plane of________________________

3. When does an intensity difference between the ears occur?________________________________________

4. Which of the following factors may be helpful in locating the direction of a sound?
a. Smell
b. Sight
c. Expectation and learning
d. Monaural cues

ANSWER KEY PAGE 143

3 OR MORE CORRECT PAGE 130 FEWER THAN 3 CORRECT PAGE 127


MODULE 7
EXERCISES

Auditory Space Perception
Auditory Space Perception
PurposeHearingCue
distancemonaural (one ear)relative volume
directionbinaural (two ears)phase difference

Which of the following cues could a person deaf in one ear perceive?
a. Relative complexity
b. Time difference
c. Phase difference
d Relative loudness

_______________________________________3

A cannon at close range seems to fill the entire space but at a distance is just one of many sounds. This is an example of a_________________________________ (distance/directional) cue.

____________________________________________ 1

The specific cue in the question above is relative_______________________________________7

In order to perceive direction from a sound, the sound must stimulate both ears differently. Suppose a sound reaches the right ear a split second before the left ear. In which direction would the origin of the sound be?___________________________________________________6

The specific cue in the previous question is_____________________________4

Because the path is longer to the left ear, the sound wave that reaches the left ear is at a different point in its cycle than the wave that reaches the right ear. Which cue is that?_________________________________________2

The path also causes the intensity of the sound waves to be some- what reduced when they reach the farther ear. This cue is_________________________________________________________8

Time, phase, and intensity differences are binaural cues to the_____________________________________ of the origin of the sound.

___________________________________________5

ANSWERS

1 distance
2 phase difference
3 a, d
4 time difference
5 direction
6 on the right
7 volume
8 intensity difference

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An imaginary cone can be determined on either side of the plane of - confusion. A sound originating anywhere on the conical surface will be the same distance farther from one ear than from the other. The directional cues, therefore, will be able to locate the sound as being:
a. on the plane of confusion.
b. within the cone of confusion.
c. from one side or the other.
d. none of these(
___________________________________________4

The origin of a sound is at equal distance from both ears if it is in the median plane Thus sounds coming from directly behind, in front of, or over a person can be confusing. These sounds originate in the:
a. cone of confusion.
b. plane of confusion.
c. (neither)

___________________________________________3
When a sound is in a plane or cone of confusion, other factors
are necessary to locate the sound. Which of these do you think
might be useful?
a. Visual cues
b. Olfactory cues
c. Taste cues
d. Head movements

_______________________________________4
NOW TAKE PROGRESS CHECK 2

ANSWERS
2 a,b,d
3 b
4 b,c > 128


MODULE 7
PROGRESS CHECK 2

1. Directional auditory cues require:
a. hearing in one ear only.
b. hearing in both ears.
c. binaural hearing.
d. monaural hearing.

2. If the origins of two sounds are the same distance away from one ear as from the other, the origins are
on the same_________________________________________.

3. Which of the following are distance cues?
a. Relative loudness
b. Phase difference
c. Intensity difference
d. Relative complexity

4. Which of the following are directional auditory cues for space perception?
a. Relative loudness
b. Phase difference
c. Intensity difference

ANSWER KEY PAGE 143

3 OR MORE CORRECT ~ PAGE 130
FEWER THAN 3 CORRECT INSTRUCTOR CONFERENCE

UNIT 6 Table of Contents

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