SECTION 1
Preview Vocabulary
Visual Vocabulary Compress and
decompress a spring, explaining that the
coils of a spring move because of
changes in the application of pressure.
Explain that a sound wave behaves the
same way, traveling by compression and
decompression of the medium (e.g., air,
water, etc.) through which it travels.
Apply this demonstration to the
definitions of compression and
rarefaction.
! Teach
Objectives
Explain how sound waves are
produced.
Relate frequency to pitch.
Compare the speed of sound in
various media.
Relate plane waves to spherical
waves.
Sound Waves
Key Terms
compression
rarefaction
pitch
Doppler effect
The Production of Sound Waves
Recognize the Doppler effect,
and determine the direction of
a frequency shift when there is
relative motion between a
source and an observer.
compression the region of a longitudinal wave in which the density and
pressure are at a maximum
rarefaction the region of a longitudinal
wave in which the density and pressure
are at a minimum
TEACH FROM VISUALS
Whether a sound wave conveys the shrill whine of a jet engine or the
melodic whistling of a bird, it begins with a vibrating object. We will
explore how sound waves are produced by considering a vibrating tuning
fork, as shown in Figure 1.1(a).
The vibrating prong of a tuning fork, shown in Figure 1.1(b), sets the air
molecules near it in motion. As the prong swings to the right, as in
Figure 1.1(c), the air molecules in front of the movement are forced closer
together. (This situation is exaggerated in the figure for clarity.) Such
a region of high molecular density and high air pressure is called a
compression. As the prong moves to the left, as in Figure 1.1(d), the
molecules to the right spread apart, and the density and air pressure in
this region become lower than normal. This region of lower density and
pressure is called a rarefaction.
As the tuning fork continues to vibrate, a series of compressions and
rarefactions forms and spreads away from each prong. These compressions and rarefactions spread out in all directions, like ripple waves on a
pond. When the tuning fork vibrates with simple harmonic motion, the
air molecules also vibrate back and forth with simple harmonic motion.
FIGURE 1.1 Point out that the vibrations
of the prongs cause the air molecules to
move back and forth.
Ask How is the position of the air
molecules related to the motion of the
prongs over time?
Answer: The air molecules follow the
prongs’ motion. There is an increase of
air pressure when a prong “pushes,”
leaving fewer molecules and lower
pressure behind. When the prong
returns, the pressure pattern shifts back.
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FIGURE 1.1
Compressions and Rarefactions
(a) The sound from a tuning fork is
produced by (b) the vibrations of each of
its prongs. (c) When a prong swings to the
right, there is a region of high density and
pressure. (d) When the prong swings back
to the left, a region of lower density and
pressure exists.
(c) Compression
(a)
(b)
(d) Rarefaction
©Richard Megna/Fundamental Photographs, New York
! Plan and Prepare
SECTION 1
404
Chapter 12
Differentiated
Instruction
BELOW LEVEL
As noted in the caption for Figure 1.1, the
sound from a tuning fork is produced by the
vibrations of each of its prongs. Students
should already know that sound is an indicator
of vibrating molecules. Explain that both the
object itself and the air must be vibrating in
order to produce sound. To illustrate these
vibrations, use a tuning fork if one is available.
Since it is impossible to see air molecules
moving, dip the tuning fork in water so
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404 Chapter 12
students can see both the tuning fork and the
water vibrating. Emphasize that it is the
disturbance that travels, not the molecules of
the medium. They vibrate in place.
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FIGURE 1.2
Teaching Tip
Representing Sound Waves (a) As this tuning fork vibrates,
(b) a series of compressions and rarefactions moves away from each
prong. (c) The crests of this sine wave correspond to compressions,
and the troughs correspond to rarefactions.
PH99E-C13-001-003a,bA
(a)
Figure 1.2 uses a sine curve to represent
the compressions and rarefactions
of a longitudinal wave produced by
a vibrating object. Compressions
correspond to crests, and rarefactions
correspond to troughs. Sometimes a
sine curve is used to represent
displacement rather than pressure
and density. For any given longitudinal
wave, the sine curve representing
pressure and the sine curve representing
displacement are 90° out of phase.
(b)
(c)
Misconception Alert!
Sound waves are longitudinal.
Point out that some individuals may be
able to hear sounds slightly below 20 Hz
or above 20 000 Hz because the range
of frequencies defined as audible is
based on the ability of the average
human ear.
In sound waves, the vibrations of air molecules are parallel to the direction of wave motion. Thus, sound waves are longitudinal. The simplest
longitudinal wave produced by a vibrating object can be represented by
a sine curve. In Figure 1.2, the crests correspond to compressions (regions
of higher pressure), and the troughs correspond to rarefactions (regions
of lower pressure). Thus, the sine curve represents the changes in air
pressure due to the propagation of the sound waves. Note that Figure 1.2
shows an idealized case. This example disregards energy losses that
would decrease the wave amplitude.
(tl) ©Richard Megna/Fundamental Photographs, New York
Characteristics of Sound Waves
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As discussed earlier, frequency is defined as the number of cycles per unit
of time. Sound waves that the average human ear can hear, called audible
sound waves, have frequencies between 20 and 20 000 Hz. (An individual’s
hearing depends on a variety of factors, including age and experiences
with loud noises.) Sound waves with frequencies less than 20 Hz are called
infrasonic waves, and those above 20 000 Hz are called ultrasonic waves.
It may seem confusing to use the term sound waves for infrasonic or
ultrasonic waves because humans cannot hear these sounds, but these
waves consist of the same types of vibrations as the sounds that we can
hear. The range of audible sound waves depends on the ability of the
average human ear to detect their vibrations. Dogs can hear ultrasonic
waves that humans cannot.
Did YOU Know?
Elephants use infrasonic sound waves
to communicate with one another.
Their large ears enable them to detect
these low-frequency sound waves,
which have relatively long wavelengths.
Elephants can effectively communicate
in this way, even when they are
separated by many kilometers.
Sound
405
ENGLISH LEARNERS
Use of the word loudness in the description
of pitch may trouble English learners.
They may not understand the distinction
between loud sounds and high sounds. To
prevent confusion, be sure to isolate pitch as
a function of frequency and loudness as a
function of amplitude.
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Sound 405
Frequency determines pitch.
pitch a measure of how high or low a
sound is perceived to be, depending on
the frequency of the sound wave
Demonstration
SOUND WAVES IN A SOLID
Purpose Show sound waves traveling
through a solid.
Speed of sound depends on the medium.
Sound waves can travel through solids, liquids, and gases. Because waves
consist of particle vibrations, the speed of a wave depends on how quickly
one particle can transfer its motion to another particle. For example, solid
particles respond more rapidly to a disturbance than gas particles do
because the molecules of a solid are closer together than those of a gas
are. As a result, sound waves generally travel faster through solids than
through gases. Figure 1.3 shows the speed of sound waves in various
media.
Materials coat hanger, two strings
Procedure Open the coat hanger, and
tie a string at each end. Ask a volunteer
to hold the ends of the strings tautly
next to his or her ears while you hit the
coat hanger with a pen. Ask the volunteer to describe the sounds he or she
heard. sounds like bells Have other
students in the class hold the strings to
their ears as you continue to hit the
coat hanger. Explain that the vibrations
of the coat hanger traveled through the
strings and produced the ringing sounds
that students observed.
The frequency of an audible sound wave determines how high or low
we perceive the sound to be, which is known as pitch. As the frequency of
a sound wave increases, the pitch rises. The frequency of a wave is an
objective quantity that can be measured, while pitch refers to how
different frequencies are perceived by the human ear. Pitch depends not
only on frequency but also on other factors, such as background noise
and loudness.
Ultrasound Images
U
ltrasonic waves can be used to produce images
of objects inside the body. Such imaging is
possible because sound waves are partially
reflected when they reach a boundary between two
materials of different densities. The images produced by
ultrasonic waves are clearer and more detailed than
those that can be produced by lower-frequency sound
waves because the short wavelengths of ultrasonic
waves are easily reflected off small objects. Audible and
infrasonic sound waves are not as effective because
their longer wavelengths pass around small objects.
Why It Matters
ULTRASOUND IMAGES
Students may be familiar with X rays as a
means of examining organs within the
body. Point out that ultrasound waves
are much safer than X rays, which are
high-energy electromagnetic radiation.
Ultrasound waves are used to observe
fetuses because X rays can produce birth
defects and because ultrasound
provides more detail for soft tissue.
In order for ultrasonic waves to “see” an object inside
the body, the wavelength of the waves used must be
about the same size as or smaller than the object. A
typical frequency used in an ultrasonic device is about
10 MHz. The speed of an ultrasonic wave in human tissue
is about 1500 m/s, so the wavelength of 10 MHz waves is
λ = v/f = 0.15 mm. A 10 MHz ultrasonic device will not
detect objects smaller than this size.
Physicians commonly use ultrasonic waves to observe
fetuses. In this process, a crystal emits ultrasonic pulses.
The same crystal acts as a receiver and detects the
reflected sound waves. These reflected sound waves are
converted to an electrical signal, which forms an image
on a fluorescent screen. By repeating this process for
different portions of the mother’s abdomen, a physician
can obtain a complete picture of the fetus, as shown
above. These images allow doctors to detect some types
of fetal abnormalities.
©Dr. Najeeb Layyous/Photo Researchers, Inc.
! Teach continued
406
Chapter 12
Problem
Solving
TAKE IT FURTHER
Humans can hear at frequencies of
20 Hz ≤ 20,000 Hz. Have students determine
the ranges of wavelengths that a sound wave
in each of the following media would need to
be in order to be audible to humans:
Water between 0.0745 m and 74.5 m
Sea water between 0.0765 m and 76.5 m
Air at 25°C between 0.0173 m and 17.3 m
They should use the speed equation: v = f λ.
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The speed of sound also depends on the temperature of the medium.
As temperature rises, the particles of a gas collide more frequently. Thus,
in a gas, the disturbance can spread faster at higher temperatures than at
lower temperatures. In liquids and solids, the particles are close enough
together that the difference due to temperature changes is less
noticeable.
FIGURE 1.3
Teaching Tip
SPEED OF SOUND IN
VARIOUS MEDIA
Medium
v (m/s)
Gases
air (0°C)
331
Sound waves propagate in three dimensions.
air (25°C)
346
Sound waves actually travel away from a vibrating source in all three
dimensions. When a musician plays a saxophone in the middle of a room,
the resulting sound can be heard throughout the room because the sound
waves spread out in all directions. The wave fronts of sound waves
spreading in three dimensions are approximately spherical. To simplify,
we shall assume that the wave fronts are exactly spherical unless stated
otherwise.
air (100°C)
366
Spherical waves can be represented graphically in two dimensions
with a series of circles surrounding the source, as shown in Figure 1.4.
The circles represent the centers of compressions, called wave fronts.
Because we are considering a three-dimensional phenomenon in two
dimensions, each circle represents a spherical area.
Because each wave front locates the center of a compression, the
distance between adjacent wave fronts is equal to one wavelength, λ.
The radial lines perpendicular to the wave fronts are called rays.
FIGURE 1.4
helium (0°C)
972
hydrogen (0°C)
1290
oxygen (0°C)
317
Liquids at 25°C
methyl alcohol
1140
sea water
1530
water
1490
Solids
aluminum
5100
copper
3560
iron
5130
lead
1320
vulcanized rubber
Spherical Waves In this
representation of a spherical
wave, the wave fronts represent
compressions, and the rays show
the direction of wave motion. Each
wave front corresponds to a crest of
the sine curve. In turn, the sine curve
corresponds to a single ray.
54
Wave front
Source
Ray
Sine curve
Conceptual Challenge
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(br) ©Peter Arnold, Inc.
Music from a Trumpet Suppose you hear
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music being played from a trumpet that is
across the room from you. Compressions
and rarefactions from the sound wave reach
your ear, and you interpret these vibrations as
sound. Were the air particles that are vibrating near your ear carried across the room by
the sound wave? How do you know?
Emphasize that the medium through
which a disturbance travels is vibrating in
place. For example, when a sound wave
travels through the air, the air vibrates.
The air molecules do not move along
with the sound.
Answers
Conceptual Challenge
1. no; As with all waves, the disturbance
travels, not the material itself.
Individual air molecules do not move
across the room with the sound wave.
Instead, each molecule vibrates in
place, and the vibrations are transferred from one particle to the next.
2. Because light travels so much faster
than sound, the speed of light can be
considered to be effectively infinite
in this case, and the flash is considered to have occurred at the same
instant we see it. Thus, the time it
takes the sound wave to reach the
listener multiplied by the speed of
sound in air gives the approximate
distance between the observer and
the lightning bolt.
Lightning and Thunder Light waves travel
nearly 1 million times faster than sound
waves in air. With this in mind, explain how
the distance to a lightning bolt can be determined by counting the seconds between the
flash and the sound of the thunder.
Sound
REALITY CHECK
Make the answers to the Conceptual Challenge
more concrete by encouraging students to use
numbers and equations to calculate examples
that quantify their qualitative answers.
407
Multiply the speed of sound in air (331 m/s at
0°C) by the time it took for the sound
to reach
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6:41:41 AM
the listener.
331 m/s × 10 s = 3310 m
Therefore, the lightning is 3310 meters away.
For example, assume a student sees lightning
and counts slowly to 10 (equal to approximately
10 seconds) before hearing the thunder. How far
away is the lightning?
Sound
407
Rays indicate the direction of the wave motion. The sine curve used in
our previous representation of sound waves, also shown in Figure 1.4,
corresponds to a single ray. Because crests of the sine curve represent
compressions, each wave front crossed by this ray corresponds to a crest
of the sine curve.
FIGURE 1.5
! Teach continued
Spherical Waves Spherical wave
fronts that are a great distance from the
source can be approximated with parallel
planes known as plane waves.
Consider a small portion of a spherical wave front that is many
wavelengths away from the source, as shown in Figure 1.5. In this case,
the rays are nearly parallel lines, and the wave fronts are nearly parallel
planes. Thus, at distances from the source that are great relative to the
wavelength, we can approximate spherical wave fronts with parallel
planes. Such waves are called plane waves. Any small portion of a spherical wave that is far from the source can be considered a plane wave. Plane
waves can be treated as one-dimensional waves all traveling in the same
direction, as in the chapter “Vibrations and Waves.”
Demonstration
Rays
THE DOPPLER EFFECT
Purpose Show that the observed
frequency of sound waves depends on
the relative motion between the source
of the sound waves and the observer.
W
HRW • Holt Physics
PH99PE-C13-001-00 -A
Materials battery-operated highvolume oscillator (available at most local
electronics stores), appropriate batteries
for the oscillator, foam ball large enough
to hold the oscillator and batteries
Procedure Carefully cut into the foam
ball and remove enough material so that
the oscillator and the batteries will
snugly fit inside the ball. Connect the
batteries to the oscillator, and place the
oscillator and batteries securely inside
the ball. Allow the students to toss the
ball about the classroom. Have the
students note the differences in the
observed frequency of the sound of the
oscillator when the ball is traveling
toward them, when the ball is traveling
away from them, and when the ball is
at rest.
o ts
The Doppler Effect
If you stand on the street while an ambulance speeds by with its siren on,
you will notice the pitch of the siren change. The pitch will be higher as
the ambulance approaches and will be lower as it moves away. As you
read earlier in this section, the pitch of a sound depends on its frequency.
But in this case, the siren is not changing its frequency. How can we
account for this change in pitch?
Relative motion creates a change in frequency.
If a siren sounds in a parked ambulance, an observer standing on the
street hears the same frequency that the driver hears, as you would
expect. When an ambulance is moving, as shown in Figure 1.6, there is
relative motion between the moving ambulance and a stationary
observer. This relative motion affects the way the wave fronts of the sound
FIGURE 1.6
waves produced by the siren are
perceived by an observer. (For simplicDoppler Effect As this ambulance moves to the left, Observer A hears the siren
ity’s sake, assume that the sound waves
at a higher frequency than the driver does, while Observer B hears a lower frequency.
produced by the siren are spherical.)
Observer
A
Observer
B
Although the frequency of the siren
remains constant, the wave fronts reach
an observer in front of the ambulance
(Observer A) more often than they
would if the ambulance were stationary. The reason is that the source of the
sound waves is moving toward the
observer. The speed of sound in the air
does not change, because the speed
depends only on the temperature of the
air. Thus, the product of wavelength
and frequency remains constant.
Because the wavelength is less, the
frequency heard by Observer A is
greater than the source frequency.
408
Chapter 12
Differentiated
Instruction
BELOW LEVEL
Some students may think that the observed
frequency rises as the source of sound
approaches an observer and decreases as the
source moves away. Stress the fact that the
observed frequency is higher or lower and
that it changes only when the source passes
the observer. This concept is illustrated in
Figure 1.6, which shows that the distance
between wave fronts is constant for each
observer.
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5/18/2011 6:41:42 AM
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For the same reason, the wave fronts reach an observer behind the
ambulance (Observer B) less often than they would if the ambulance
were stationary. As a result, the frequency heard by Observer B is less than
the source frequency. This frequency shift is known as the Doppler effect.
The Doppler effect is named for the Austrian physicist Christian Doppler
(1803–1853), who first described it.
Doppler effect an observed change in
frequency when there is relative motion
between the source of waves and an
observer
We have considered a moving source with respect to a stationary
observer, but the Doppler effect also occurs when the observer is moving
with respect to a stationary source or when both are moving at different
velocities. In other words, the Doppler effect occurs whenever there is
relative motion between the source of waves and an observer. (If the
observer is moving instead of the source, the wavelength in air does not
change, but the frequency at which waves arrive at the ear is altered by
the motion of the ear relative to the medium.) Although the Doppler
effect is most commonly experienced with sound waves, it is a phenomenon common to all waves, including electromagnetic waves, such as
visible light.
Assess and Reteach !
Assess Use the Formative Assessment
on this page to evaluate student
mastery of the section.
Reteach For students who need
additional instruction, download the
Section Study Guide.
Response to Intervention To reassess
students’ mastery, use the Section Quiz,
available to print or to take directly
online at HMDScience.com.
SECTION 1 FORMATIVE ASSESSMENT
Reviewing Main Ideas
1. What is the relationship between frequency and pitch?
2. Dolphin echolocation is similar to ultrasound. Reflected sound waves
allow a dolphin to form an image of the object that reflected the waves.
Dolphins can produce sound waves with frequencies ranging from
0.25 kHz to 220 kHz, but only those at the upper end of this spectrum are
used in echolocation. Explain why high-frequency waves work better
than low-frequency waves.
3. Sound pulses emitted by a dolphin travel through 20°C ocean water at a
rate of 1450 m/s. In 20°C air, these pulses would travel 342.9 m/s. How
can you account for this difference in speed?
Interpreting Graphics
4. Could a portion of the innermost wave front shown in Figure 1.7
be approximated by a plane wave? Why or why not?
5. Figure 1.8 is a diagram of the Doppler effect in a ripple tank.
In which direction is the source of these ripple waves moving?
6. If the source of the waves in Figure 1.8 is stationary, which way must the
ripple tank be moving?
Figure 1.7
HRW • Holt Physics
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Critical Thinking
7. As a dolphin swims toward a fish, the dolphin sends out sound waves
to determine the direction the fish is moving. If the frequency of the
reflected waves is higher than that of the emitted waves, is the dolphin
catching up to the fish or falling behind?
Answers to Section Assessment
1. A greater frequency is perceived as a
higher pitch.
2. Higher frequencies function well in
echolocation because their relatively short
wavelengths are able to detect smaller
objects. (Longer wavelengths would
disperse around small objects.)
3. Sound waves travel faster through water
than through air because the molecules of
water are closer together and, as a result,
can spread vibrations more quickly.
Figure 1.8
HRW • Holt Physics
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Sound
409
4. no; The wave front must be far from the
source (relative to the wavelength)
to be
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6:41:43 AM
approximated by plane waves.
5. to the right
6. to the left
7. The dolphin is catching up to the fish.
Sound
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