The Perceptual Process for Hearing
The first step in the perceptual process for hearing is to identify the environmental
Physical Aspects of Sound
This question is useful because it shows that we can use the word sound in two
o Physical definition: Sound is pressure changes in the air or other medium.
o Perceptual definition: Sound is the experience we have when we hear.
If a tree falls in the forest and no one is there to hear it, is there a sound? The
answer to the question is “yes” if we are using the physical definition, because the
falling tree causes pressure changes whether or not someone is there to hear them.
The answer to the question is “no” if we are using the perceptual definition,
because if no one is in the forest, there will be no experience.
Sound as Pressure Changes
When the diaphragm of the speaker moves out, it pushes the surrounding air
molecules together, a process called condensation, which causes a slight increase
in the density of molecules near the diaphragm. This increased density results in a
local increase in the air pressure above atmospheric pressure. When the speaker
diaphragm moves back in, air molecules spread out to fill in the increased space, a
process called rarefaction. The decreased density of air molecules caused by
rarefaction causes a slight decrease in air pressure.
This pattern of air pressure changes, which travels through air at 340 meters per
second (and through water at 1,500 meters per second), is called a sound wave
However, although air pressure changes move outward from the speaker, the air
molecules at each location move back and forth but stay in about the same place.
What is transmitted is the pattern of increases and decreases in pressure that
eventually reach the listener’s ear.
A pure tone occurs when changes in air pressure occur in a pattern described by a
mathematical function called a sine wave
This vibration can be described by noting its frequency —the number of cycles
per second that the pressure changes repeat—and its amplitude —the size of the
Frequency, the number of cycles per second that the change in pressure repeats, is
measured in units called hertz (Hz) , in which 1 Hz is 1 cycle per second.
As we will see, humans can perceive frequencies ranging from about 20 Hz to
Sound Amplitude and the Decibel Scale
One way to specify a sound’s amplitude would be to indicate the difference in
pressure between the high and low peaks of the sound wave. The decibel (dB) converts this large range of sound pressures into a more
dB = 20 x log (p/po)
ratio of two pressures: p, the pressure of the sound we are considering; and p , toe
reference pressure, usually set at 20 micropascals, which is the pressure near
hearing threshold for a 1,000Hz tone.
Logarithms come to the rescue by converting numbers into exponents or powers.
The logarithm of a number is the exponent to which the base, which is 10 for
common logarithms, has to be raised to produce that number.
When specifying the sound pressure in decibels, the notation SPL , for sound
pressure level, is added to indicate that decibels were determined using the
standard pressure p of 20 micropascals. In referring to the decibels or sound
pressure of a sound stimulus, the term level or sound level is usually used.
Complex Tones and Frequency Spectra
This property of repetition means that this complex tone, like a pure tone, is a
From the time scale at the bottom of the figure, we see that the tone repeats four
times in 20 msec. Because 20 msec is 20/1,000 sec = 1/50 sec, this means that the
pattern for this tone repeats 200 times per second. That repetition rate is called the
fundamental frequency of the tone.
Complex tones are made up of a number of pure tone (sinewave) components
added together. Each of these components is called a harmonic of the tone. The
first harmonic , a pure tone with frequency equal to the fundamental frequency,
is usually called the fundamental of the tone. The fundamental of this tone,
shown in Figure 11.5b, has a frequency of 200 Hz, which matches the repetition
rate of the complex tone.
Higher harmonics are pure tones with frequencies that are wholenumber (2, 3,
4, etc.) multiples of the fundamental frequency. This means that the second
harmonic of our complex tone has a frequency of 200 × 2 = 400 Hz
Adding the fundamental and the higher harmonics results in the waveform of the
Another way to represent the harmonic components of a complex tone is by
frequency spectra. The horizontal axis is frequency, not time, as is the case for
the waveform plot on the left. The position of each line on the horizontal axis
indicates the frequency of one of the tone’s harmonics, and the height of the line
indicates the harmonic’s amplitude. Frequency spectra provide a way of
indicating a complex tone’s fundamental frequency and harmonics that add up to
the tone’s complex waveform.
Note that removing a harmonic changes the tone’s waveform, but that the rate of
repetition remains the same.
Perceptual Aspects of Sound
Thresholds and Loudness Thresholds (the smallest amount of sound energy that can just barely be detected)
and loudness (the perceived intensity of a sound that ranges from “just audible” to
Loudness and Level
Loudness is the perceptual quality most closely related to the level or amplitude
of an auditory stimulus, which is expressed in decibels.
The relationship between level in decibels (physical) and loudness (perceptual)
was determined using the magnitude estimation procedure (Graph: Intensity vs.
Thresholds Across the Frequency Range: The Audibility Curve
Some frequencies have low thresholds—it takes very little sound pressure change
to hear them—and other frequencies have high thresholds—large changes in
sound pressure are needed to make them heard. This is illustrated by the
audibility curve .
This audibility curve, which indicates the threshold for hearing versus frequency,
indicates that we can hear sounds between about 20 Hz and 20,000 Hz and that
we are most sensitive (the threshold for hearing is lowest) at frequencies between
2,000 and 4,000 Hz, which happens to be the range of frequencies that is most
important for understanding speech. (Graph: Frequency vs. dB(SPL))
The light green area above the audibility curve is called the auditory response
area because we can hear tones that fall within this area. At intensities below the
audibility curve, we can’t hear a tone.
The upper boundary of the auditory response area is the curve marked “threshold
of feeling.” Tones with these high amplitudes are the ones we can “feel”; they can
become painful and can cause damage to the auditory system.
Elephants can hear stimuli below 20 Hz. Above the high end of the human range,
dogs can hear frequencies above 40,000 Hz, cats can hear above 50,000 Hz, and
the upper range for dolphins extends as high as 150,000 Hz.
Thus, each frequency has a threshold or “baseline”—the decibels at which it can
just barely be heard, as indicated by the audibility curve—and loudness increases
as we increase the level above this baseline.
Another way to understand the relationship between loudness and frequency is by
looking at the red equal loudness curves in Figure 11.8. These curves indicate
the sound levels that create the same perception of loudness at different
frequencies. An equal loudness curve is determined by presenting a standard pure
tone of one frequency and level and having a listener adjust the level of pure tones
with frequencies across the range of hearing to match the loudness of the
All frequencies between about 30 Hz and 5,000 Hz sound equally loud at this
Pitc , the perceptual quality we describe as “high” or “low,” can be defined as
the property of auditory sensation in terms of which sounds may be ordered on a
musical scale The physical property that is related to this low to high perceptual experience is
frequency, with the lowest note on the piano having a fundamental frequency of
27.5 Hz and the highest note 4,166 Hz
The perceptual experience of increasing pitch that accompanies increases in a
tone’s fundamental frequency is called tone height
Because of this similarity, we say that notes with the same letter have the same
tone chroma . Every time we pass the same letter on the keyboard, we have gone
up an interval called an octave . Tones separated by octaves have the same tone
Notes with the same chroma have fundamental frequencies that are wholenumber
multiples of one another. Thus, a male with a lowpitched voice and a female with
a highpitched voice can be regarded as singing “in unison,” even when their
voices are separated by an octave or more.
The constancy of pitch, even when the fundamental or other harmonics are
removed, is called the effect of the missing fundamental , and the pitch that we
perceive in tones that have harmonics removed is called periodicity pitch . The
term periodicity pitch indicates that pitch is determined by the period or repetition
rate of the sound waveform. Pitch, therefore, is determined not by the presence of
the fundamental frequency, but by information, such as the spacing of the
harmonics and the repetition rate of the waveform
Timbre is the quality that distinguishes between two tones that have the same
loudness, pitch, and duration, but still sound different.
When two tones have the same loudness, pitch, and duration, but sound different,
this difference is a difference in timbre.
When we describe one person’s voice as sounding “nasal” and another’s as being
“mellow,” we are referring to the timbres of their voices.
Graph: Frequency vs. Response
Timbre also depends on the time course of a tone’s attack (the buildup of sound
at the beginning of the tone) and of the tone’s decay (the decrease in sound at the
end of the tone).
Thus, timbre depends both on the tone’s steadystate harmonic structure and on
the time course of the attack and decay of the tone’s harmonics
From Pressure Changes to Electricity
The auditory system accomplishes three basic tasks during this journey. First, it
delivers the sound stimulus to the receptors. Second, it transduces this stimulus
from pressure changes into electrical signals. Third, it processes these electrical
signals so they can indicate qualities of the sound source such as pitch, loudness,
timbre, and location.
The Outer Ear
When we talk about ears in everyday conversation, we are usually referring to the
pinnae , the structures that stick out from the sides of the head.
Sound waves first pass through the outer ear , which consists of the pinna and the
auditory canal (Figure 11.11). The auditory canal is a tubelike structure, about 3 cm long in adults, that protects the delicate structures of the middle ear from the
hazards of the outside world. The auditory canal’s 3cm recess, along with its
wax, protects the delicate tympanic membrane , or eardrum , at the end of the
canal and helps keep this membrane and the structures in the middle ear at a
relatively constant temperature.
Resonance occurs in the auditory canal when sound waves that are reflected back
from the closed end of the auditory canal interact with sound waves that are
entering the canal.
This interaction reinforces some of the sound’s frequencies, with the frequency
that is reinforced the most being determined by the length of the canal. The
frequency reinforced the most is called the resonant frequency of the canal. Also
increases the sound pressure level of frequencies
The Middle Ear
The middle ear is a small cavity, about 2 cubic centimeters in volume, that
separates the outer and inner ears. This cavity contains the ossicles , the three
smallest bones in the body. The first of these bones, the malleus (also known as
the hammer), is set into vibration by the tympanic membrane, to which it is
attached, and transmits its vibrations to the incus (or anvil), which, in turn,
transmits its vibrations to the stapes (or stirrup). The stapes then transmits its
vibrations to the inner ear by pushing on the membrane covering the oval window
If vibrations had to pass directly from the air in the middle ear to the liquid in the
inner ear, less than 1 percent of the vibrations would be transmitted. The ossicles
help solve this problem in two ways:
o By concentrating the vibration of the large tympanic membrane onto the
much smaller stapes, which increases the pressure by a factor of about 20;
o By being hinged to create a lever action—an effect similar to what
happens when a fulcrum is placed under a board, so that pushing down on
the long end of the board makes it possible to lift a heavy weight on the
If ossicles are damaged it would be necessary to increase the sound level by a
factor of 10 to 50 to achieve the same hearing
The middle ear also contains the middleear muscles , the smallest skeletal
muscles in the body. These muscles are attached to the ossicles, and at very high
sound levels they contract to dampen the ossicles’ vibration. This reduces the