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Chapter 9

PSYB51H3 Chapter 9: Chapter 9

Course Code
Matthias Niemeier

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Chapter 9: Hearing: Physiology and Psychoacoustics
It is often said that humans are visual animals, but hearing is very important as well
Our ears are always open, and we can hear in the dark
It is easy to take hearing for granted
Deafness deprives you of the most fundamental of human abilities: communication through speech
At night your sense of vision, which helps compensate for hearing loss when it’s light out, is
compromised by darkness
The Function of Hearing
Many fundamental principles apply to vision, hearing, and all the other senses
However, each sense developed at different periods in our evolutionary history and in response to
different environmental challenges
What Is Sound?
Sounds are created when objects vibrate
The vibrations of objects (the sound source) cause molecules in the object’s surrounding medium (for
humans, usually the Earth’s atmosphere) to vibrate as well, and this vibration in turn causes pressure
changes in the medium
These pressure changes are best described as waves, and the pattern of displacement will move
outward from the source until something gets in the way
Although the patterns of sound waves do not change as they spread out, the initial amount of pressure
change is dispersed over a larger and larger area as the wave moves away, so the wave becomes less
prominent as it moves farther from its source
Depending on the medium, sound waves travel at a particular speed—the denser the substances, the
faster the sound waves move through them
For example, the speed of sound through air (depending on the humidity level) is about 340 metres
per second, and the speed of sound through water is about 1500 metres per second
Why is there a lag time between seeing lightning and hearing thunder?
Because light waves move through air almost a million times faster than sound waves do
Basic Qualities of Sound Waves: Frequency and Amplitude
The magnitude of the pressure change in a sound wave—the difference between the highest pressure
area and the lowest pressure area—is called the amplitude, or intensity, of the wave
In other words, amplitude/intensity: the magnitude of displacement (increase or decrease) of a
sound pressure wave. Amplitude is perceived as loudness
For light waves, we usually describe the pattern of fluctuations by measuring the distance between
peaks in the waves—i.e., the “wavelength”
Although sounds have wavelengths, we typically describe their patterns by noting how quickly the
pressure fluctuates; this rate of fluctuation is known as the frequency of the wave
In other words, frequency: for sound, the number of times per second that a pattern of pressure
change repeats. Frequency is perceived as pitch
Sound wave frequencies are measured in hertz (Hz), where 1 Hz equals 1 cycle per second
For example, the pressure in a 500-Hz wave goes from its highest point down to its lowest point
and back up to its highest point 500 times per second
The amplitude and wavelength of light waves corresponds to brightness and colour, respectively, just as
the amplitude and frequency of sound waves are highly correlated with loudness and pitch,
Loudness: the psychological aspect of sound related to perceived intensity (amplitude). The more
intense a sound wave is, the louder it will sound
Pitch: the psychological aspect of sound related mainly to perceived frequency. Low-frequency
sounds correspond to low pitches (e.g., low notes played by a tuba), and high-frequency sounds
correspond to high pitches (e.g., the high notes from a piccolo)

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Human hearing uses a limited range of the frequencies present in environmental sounds. If you are
relatively young and you’ve been careful about your exposure to loud sounds, you may be able to
detect sounds that vary from about 20 to 20,000 Hz
Elephants hear vibrations at even lower frequencies, whereas dogs can be called with whistles that
emit sounds at frequencies too high for humans to hear. The sonar systems used by some bats use
sound frequencies above 60,000 Hz
Humans hear across a wide range of sound intensities. The ratio between the faintest sound humans
can detect and the loudest sounds that do not cause serious damage to the human ear is more than 1
to 1,000,000
To describe differences in amplitude across such a broad range, sound levels are measured on a
logarithm scale using units called decibels (dB)
Decibels define the difference between two sounds in terms of the ratio between sound pressures.
Each 10:1 sound pressure ratio is equal to 20 dB, so a 100:1 ratio is equal to 40 dB
The equation for defining decibels is dB = 20 log (p / p0)
The variable p corresponds to the pressure (intensity) of the sound being described
The constant term p0 is a reference pressure and is typically defined to be 0.0002 dyne/cm2, and
levels are defined as dB SPL (sound pressure level). This level (0.0002 dyne/cm2) is close to the
minimum pressure that can be detected at frequencies for which hearing is most sensitive, and
decibel values greater than zero describe the ratio between a sound measured and 0.0002 dyne/
The range of human hearing extends from 0 to over 120 dB SPL, and this decibel range corresponds to
a ratio of greater than 1,000,000:1
If the pressure of the sound that you’re measuring (p) is equal to 0.0002 dyne/cm2, then dB = 20 log (1).
Because the log of 1 is zero, a sound pressure that low would be equal to 0 dB SPL
Sounds with amplitudes even smaller than p0 have negative decibel levels
An important thing to remember about logarithm scales such as decibels is that relatively small decibel
changes can correspond to large physical changes
For example, an increase of 6 dB corresponds to a doubling of the amount of pressure
Figure 9.4 shows the decibel levels of some common sound sources
Sine Waves and Complex Sounds
All sounds can be described as a combination of sine waves
Sine wave or pure tone: a waveform for which variation as a function of time is a sine function
Complex sounds are best described in a spectrum (plural spectra) that displays how much energy, or
amplitude, is present at multiple frequencies, as shown in Figure 9.5
Spectrum: a representation of the relative energy (intensity) present at each frequency
Sounds with harmonic spectra (Figure 9.6) are typically caused by a simple vibrating source, such as
the string of a guitar. Each frequency component in such a sound is called “harmonic.” The first
harmonic, called the fundamental frequency, is the lowest-frequency component of the sound. All the
other harmonics have frequencies that are integer multiples of the fundamental
Definition of harmonic spectrum: the spectrum of a complex sound in which energy is at integer
multiples of the fundamental frequency
Fundamental frequency: the lowest-frequency component of a complex periodic sound
The shape of the spectrum is one of the most important qualities that distinguish different sounds
The properties of sound sources determine the spectral shape of sounds; thus, these shapes can help
us identify sound sources
For example, (Figure 9.6) spectra from three musical instruments. Each instrument is producing a
tone with the same fundamental frequency (262 Hz—note C4, or middle C), and the same
harmonics (524 Hz, 786 Hz, 1048 Hz, and so on). However, the shapes of the spectra (the pattern
of amplitudes for each harmonic) vary
Timbre is a term used to describe the quality of a sound that depends, in part, on the relative
energy levels of harmonic components

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Timbre: the psychological sensation by which a listener can judge that two sounds with the same
loudness and pitch are dissimilar. Timbre quality is conveyed by harmonics and other high
Basic Structure of the Mammalian Auditory System
Web Activity 9.2: Structure of the Auditory System is good for this section
Outer Ear
Sounds are first collected from the environment by the pinna (plural pinnae), the curly, funnel-like outer
part of the ear on the side of the head that we typically call an ear
Only mammals have pinnae, and the particular shapes of the pinnae play an important role in our ability
to localize sound sources
Sound waves are funneled by the pinna into and through the ear canal, which extends about 25 mm
into the head
The length and shape of the ear canal enhance sound frequencies between about 2000 and 6000 Hz,
but the main purpose of the canal is to protect the structure at its end, the tympanic membrane
(eardrum), from damage
Ear canal: the canal that conducts sound vibrations from the pinna to the tympanic membrane and
prevents damage to the tympanic membrane
Tympanic membrane: the eardrum; a thin sheet of skin at the end of the outer ear canal. The
tympanic membrane vibrates in response to sound (the pressure changes of sound waves)
The eardrum in most cases when damaged can heal itself; however there are cases in which the
eardrum is damaged beyond repair
Middle Ear
Together, the pinna and ear canal make up a division of the auditory system called the outer ear
The tympanic membrane is the border between the outer ear and middle ear, an air-filled chamber
which consists of the middle bones called the ossicles, that amplify sound waves
The first ossicle, the malleus, is connected to and receives vibration from the tympanic membrane and
is attached to the second ossicle, the incus, which connects the malleus to the third ossicle, the stapes
The stapes transmits the vibrations of sound waves to the oval window, which is the flexible opening to
the cochlea
The oval window forms the border between the middle ear and the inner ear
The ossicles are the smallest bones in the human body, and they amplify sound in two ways
First, the joints between the bones are hinged in a way that make them work like levers: a modest
amount of energy on one side of the fulcrum (joint) becomes larger on the other. This lever action
increases the amount of pressure change by about a third
Second, the ossicles increase the energy transmitted to the inner ear by concentrating energy from
a larger to a smaller surface area: the tympanic membrane, which moves the malleus, is about 18
times as large as the oval window. Therefore, pressure on the oval window is magnified 18 times
relative to the pressure on the tympanic membrane
Amplification provided by the ossicles is essential to our ability to hear faint sounds because the inner
ear is made up of fluid-filled chambers
The ossicles play an important role for loud sounds as well. The middle ear has two muscles: the
tensor tympani (attached to the malleus) and the stapedius (attached to the stapes). These two
muscles are the smallest muscles in the human body. They function to tense when sounds are very
loud, restricting the movement of the ossicles and thus muffling pressure changes that might be large
enough to cause damage
Acoustic reflex: a reflex that protects the ear from intense sounds, via contraction of the stapedius and
tensor tympani muscles
Unfortunately, this acoustic reflex follows the onset of loud sounds by about one-fifth of a second, so it
cannot protect against abrupt loud sounds, such as the firing of a gun
The muscles of the middle ear can be tensed during swallowing, talking, and general body movement,
helping to keep the auditory system from being overwhelmed by sounds generated by our own bodies
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