Section 7.6 Questions
Understanding Concepts
1. A woman makes a sound and, 3.5 s later, the echo returns from a
nearby wall. How far is the woman from the wall, assuming that
the speed of sound is 350 m/s?
2. A man drops a stone into a mineshaft 180 m deep. If the temperature is 20.0°C, how much time will elapse between the moment
when the stone is dropped and the moment when the sound of
the stone hitting the bottom of the mineshaft is heard?
3. A ship is travelling in a fog parallel to a dangerous, cliff-lined
shore. The boat whistle is sounded and its echo is heard clearly
11.0 s later. If the air temperature is 10.0°C, how far is the ship
from the cliff?
Making Connections
4. Since the human ear is not very sensitive to low frequencies,
such as heartbeats, the doctor usually uses a stethoscope
(Figure 11). Research and explain how the construction of the
stethoscope intensifies the sounds coming out of the chest.
Figure 11
A stethoscope
5. A submarine’s sonar receives strong echoes from ships and other
submarines, but weak echoes from thermal currents in the ocean.
Why?
6. Describe how a bat uses sound energy to locate its next meal.
7. Ultrasound has many applications in the medical field, but there
are other uses. Research and report on industrial and other
applications of ultrasound.
Reflecting
8. Ask family members or friends if they have had any ultrasound
medical examinations performed on them. Describe the definition
of ultrasound to them and write down the types of procedures
they have had.
source of sound
room
Figure 1
Sound waves are diffracted as they pass
through a doorway from one room into the
next.
258 Chapter 7
7.7
Diffraction and Refraction
of Sound Waves
Have you noticed that you can always hear the sounds of a classroom through an
open door, even though the other students may be out of sight and separated by
a wall? You can also easily hear sounds travelling through an open window and
even around the corner of a building. The “sound around the corner” effect is so
familiar to us that we do not give it a second thought. Sound waves can travel
around corners because of diffraction. Diffraction describes the ability of waves
to move around an obstacle or to spread out after going through a small opening
(Figure 1).
7.7
As was discussed in Chapter 6, waves with relatively long wavelengths diffract more than those with short wavelengths. The diffraction of a wave depends
on both the wavelength and the size of the opening. Lower frequency sound
waves have relatively long wavelengths compared with the size of the openings
they commonly encounter and, thus, are more likely to diffract. Higher frequency sounds have shorter wavelengths and diffract less. For example, when
sound travels out from a loudspeaker, the bass notes (lower frequency) are diffracted more and travel easily into the next room, while the treble notes (higher
frequency) do not. Test this out on your home stereo!
You know that the speed at which a sound travels through air can be affected
by the temperature of the air; sound waves travel faster in warm air than they do
in cold air. What happens when sound waves travel from air at one temperature
to air at a different temperature? When sound waves move at an angle from air at
one temperature to air at a different temperature, they are refracted—that is, they
change their direction (Figure 2).
Suppose there is a layer of cold air over a layer of warm air. As shown in
Figure 3, sound waves tend to be refracted upward from an observer, which
decreases the intensity of the sound heard by the observer. However, at night, the
cooler air near the surface of the Earth tends to refract sound waves toward the
surface, so they travel a greater horizontal distance. This is particularly true over
flat ground or on water since there is also less absorption of the sound. Watch
what you say outdoors at night—you may be surprised at how far your voice
carries!
source
ground
normal
normal
warm air
cold air
Figure 2
Sound waves are refracted in the same
manner as light rays.
observer
day
cooler air
warmer air
source
ground
observer
night
warmer air
cooler air
source
ground
observer
Figure 3
The sound wave produced by a person’s voice
travels faster in warm air and “gains” on the
part of the wave in the cooler layer. The
wave is refracted away from the surface on a
warm day and toward the surface when a
cooler layer of air is near the surface.
Properties of Sound Waves 259
Practice
Understanding Concepts
1. Define diffraction of sound waves.
2. Define refraction of sound waves.
3. Why are lower frequency sound waves more likely to diffract than
higher frequency sound waves?
SUMMARY
Diffraction and Refraction of
Sound Waves
• Sound waves can be diffracted and refracted.
• Diffraction is greater when the sound wavelengths are larger.
Section 7.7 Questions
Understanding Concepts
1. Which sound waves from a home entertainment centre will be
easier to hear in the next room through an open doorway, those
from a woofer (low frequency) or a tweeter (high frequency)? Why?
2. For a person with equal sensitivity in both ears, would the direction from which a sound is coming be more easily identified if the
sound has a high pitch or a low pitch? Explain your reasoning.
Applying Inquiry Skills
3. Figure 4 shows noise barriers that are present on many highways.
Propose a design that would improve on these barriers to make
them more effective. Draw an example of your barrier and explain
why it is more effective, using the concepts of diffraction and
reflection.
Making Connections
4. (a) Explain how the noise barriers in Figure 4 affect highway
noises of differing frequencies.
(b) Why is it not feasible to install the barriers shown in Figure 4
on all urban roads?
(c) What else can be done to limit the spread of road noise?
Reflecting
5. “People who choose to purchase or rent a home near a noisy
highway should not expect the government to erect sound barriers.” Take a position for or against this statement and prepare
arguments in support of your position.
Figure 4
Noise barriers
7.8
The Interference of Sound Waves
It is quite common for two or more sound waves to travel through a medium at
the same time. When two or more sound waves act on the same air molecules at
the same time, interference occurs. In Chapter 6, you examined the interference of
transverse water waves and found that water waves can interfere constructively or
destructively. Do sound waves exhibit many of the properties of wave interference?
260 Chapter 7