CHAPTER 5
Audition
WILLIAM A. YOST
121HEARING AS SOUND SOURCE DETERMINATION 121
SOURCES OF SOUND: THE PHYSICS OF THE COMPLEX
SOUND WAVE 122
Simple Vibrations 122
Complex Vibrations 122
Sound Propagation 123
The Decibel 124
Reflections, Standing Waves, Reverberation, and
Sound Shadows 124
AUDITORY ANATOMY AND PHYSIOLOGY 125
The Peripheral Auditory System: Transduction
and Coding 125
Auditory Nerve 128
Central Auditory Nervous System 130
DETECTION 131
Thresholds of Hearing 131
Masking 132
Temporal Modulation Transfer Functions 134
DISCRIMINATION 134
SOUND LOCALIZATION 134
Sound Localization in Three-Dimensional Space 134
Models of Sound Localization 135
Lateralization Versus Localization 135
The Effects of Precedence 136
SOUND SOURCE SEGREGATION 136
Spectral Separation 136
Temporal Separation 136
Spatial Separation 137
Pitch and Timbre: Harmonicity and Temporal
Regularity 138
Spectral Profile 139
Common Onsets and Offsets 140
Common Modulation 140
Models or Theories of Sound Source Segregation 141
AN OVERVIEW OF THE FUTURE STUDY
OF AUDITION 142
REFERENCES 144HEARING AS SOUND SOURCE DETERMINATION
Hearing allows an organism to use sound to detect, discrimi-
nate, and segregate objects in its surrounding world (de
Cheveigne, 2001). A simple nervous system could allow a
primitive animal to detect the presence of the sound produced
by prey on one side of the animal and to use a motor system,
like a fin, on the opposite side of the animal to propel it to-
ward the prey. Such a simple auditory detector would not be
adaptive if the sound were from a predator. In this case, the
system needs to be able to discriminate prey from predator
and to activate a different response system (i.e., a fin on the
same side of the body) to escape the predator. If the world
consisted of either prey or predator, but not both, this primi-
tive animal might survive. In the real world, however, prey
and predator commingle. In the real world, the auditory sys-
tem requires greater complexity in order to segregate prey
from predator and then to make an appropriate neural deci-
sion to activate the proper response.
Sounds in the world do not travel from their sources to an
animal along independent paths; rather, they are mixed into
one complex sound wave before reaching the ears of an animal.
As we will learn, the peripheral auditory system codes the
spectral-temporal attributes of this complex sound wave. The
rest of the auditory nervous system must interpret this code in
order to reveal information about the sources of the complex
sound wave in order that detection, discrimination, and espe-
cially segregation can occur (Yost, 1992a). As Bregman (1990)
describes, the complex sound wave produces an auditory scene
in which the images of this scene are the sound producing
sources. Auditory scene analysis is based on perceptual mech-
anisms that process the spectral-temporal neural code laid
down by the inner ear and auditory nerve.
Hearing therefore involves sound, neural structures that
code for sound, and perceptual mechanisms that process this
neural code. Then this information is integrated with that
from other sensory systems and experiences to form a com-
plete auditory system. This chapter begins with a discussion