Sound Waves
Review
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Do you remember anything about _______?
Transverse waves
Longitudinal waves
Mechanical waves
Electromagnetic waves
Discussion
• Discuss with your partner about differences of
transverse vs. longitudinal and mechanical vs.
electromagnetic waves.
Superposition
• When two waves exist at the same time in the
same space, the waves overlap. The
combination of two overlapping waves is
called superposition.
• The two waves interact to form an
interference pattern.
Constructive Interference
Destructive Interference
After interference
Standing Waves
Node
Antinode
Node and Antinode
• Node: a point in a standing wave that always
undergoes complete destructive interference
and therefore is stationary.
• Antinode: a point in a standing wave, halfway
between two nodes, at which the largest
amplitude occurs.
Wavelength of Standing Wave
L
(A)
(B)
(C)
A single loop corresponds to either a
crest or a trough alone, this standing
wave corresponds to one-half of a
wavelength. Thus, the wavelength in
this case is equal to twice the string
length (2L).
The wavelength of (B) standing wave:
_______
The wavelength of (C) standing wave:
_______
Sound Waves
As the prong swings to the
right, the air molecules in front
of the movement are forced
closer together.
Such a region of high molecular
density and high air pressure is
called a compression.
As the prong swings to the left,
the air molecules to the right
spread apart. The region of
lower density and pressure is
called a rarefaction.
Sound is a longitudinal wave
• Sound travels through the air at approximately 340 m/s.
• It travels through other media as well, often much faster than
that!
• Sound waves are started by vibration of some other material,
which starts the air moving.
Hearing Sounds
• We hear a sound as “high” or “low” depending on its frequency or
wavelength. Sounds with short wavelengths and high frequencies sound
high-pitched to our ears, and sounds with long wavelengths and low
frequencies sound low-pitched. The range of human hearing is from about
20 Hz to about 20,000 Hz.
• The amplitude of a sound's vibration is interpreted as its loudness. We
measure the loudness (also called sound intensity) on the decibel scale,
which is logarithmic.
Pure Sounds
• Sounds are longitudinal waves, but if we graph
them right, we can make them look like
transverse waves.
• When we graph the air motion involved in a
pure sound tone versus position, we get what
looks like a sine or cosine function.
• A tuning fork produces a relatively pure tone.
So does a human whistle.
Graphing a Sound Wave
Complex Sounds
• Because of the phenomena of “superposition”
and “interference” real world waveforms may
not appear to be pure sine or cosine
functions.
• That is because most real world sounds are
composed of multiple frequencies.
• The human voice and most musical
instruments produce complex sounds.
Beats
• As a result of interference, a fluctuation in the
loudness of the combined sounds is heard.
The periodic variation in the loudness of
sound is called beats.
Review Questions
t1
t2
t3
t4
t5
• At what times are two waves exactly out of
t t t
phase?
• At what times are the two waves exactly in
phase?
t t
1, 3, 5
2, 4
Characteristics of Sound Waves
• Frequency determines pitch.
• Pitch: how high or low sound is. Man’s vocal
sound ranges from 100 Hz to several hundred
Hz. Woman’s vocal sound ranges from 1000Hz
to several thousand Hz.
Let’s Practice.
• If you hear a higher pitch from a trumpet than
from a saxophone, how do the frequencies of
the sound waves from the trumpet compare
with those from the saxophone?
• The trumpet’s waves have a higher frequency
than the saxophone’s waves.
Characteristics of Sound Waves
• Speed of sound depends on the medium.
• Sound waves generally travel faster through
solids than through gases.
• Speed of sound in air (room temperature) 346
m/s; in water 1490 m/s; in copper 3560 m/s
The Doppler Effect
When a car is moving
to a listener, the pitch
of the car horn will be
higher as the car
approaches and will
be lower as the car
moves away.
This frequency shift is
known as the Doppler
effect, named for the
Austrian physicist
Christian Doppler.
The Doppler Effect
• http://www.animations.physics.unsw.edu.au/j
w/doppler.htm#source
• http://soundbible.com/579-Police-Siren.html
Let’s Practice
A moving bug is making disturbance on water. Which way is the bug moving?
A to B or B to A
Answer: A to B
Let’s practice.
(A)
(B)
1. Which wave has a stationary sound source?
Answer: A
2. Which way is the sound source in (B) moving?
Answer: to the right
Let’s practice.
• As a dolphin swims toward a fish, it sends out
sound waves to determine the direction the
fish is moving. If the frequency of the
reflected waves is increased, is the dolphin
catching up to the fish or falling behind?
• The dolphin is catching up to the fish.
Sound intensity and resonance
• Intensity is the rate of energy flow through a
given area.
• Intensity = Power/area
• Intensity of a spherical wave = power/4πr2
Practice
• What is the intensity of the sound waves
produced by a trumpet at a distance of 3.2m
when the power output of the trumpet is 0.20
W? Assume that the sound waves are
spherical.
• Intensity = 0.2 W /4π(3.2m)2=1.6 x 10-3 W/m2
Audibility depends on intensity and
frequency of sound
Relative intensity is measured in
decibels.
• Frequency of a sound wave determines its
pitch.
• Intensity of a wave determines its loudness, or
volume.
Ratios of Two dB Levels
Decibel(dB)
• Relative intensity: determined by relating the
intensity of a sound wave to the intensity at
the threshold of hearing.
• The volume doubles each time the decibel
level increases by 10.
• 10 dB (1.0 x 10-11 W/m2) is twice as loud as 0
dB (1.0 x 10-12 W/m2).
• 20 dB (1.0 x 10-10 W/m2 is twice as loud as 10
dB (1.0 x 10-11 W/m2).
Resonance
• Every object vibrates at its own special set of
frequencies, which together form its special
sound. That is called natural frequency.
• Natural frequency: one at which minimum energy
is required to produce forced vibrations.
• When the frequency of a forced vibration on an
object matches the object’s natural frequency, a
dramatic increase in amplitude occurs. This
phenomenon is called resonance.
Demonstration
Description
Draw a glass or plastic golf
tube out of a water bath
while holding an excited
tuning fork over one end.
Discussion
The first resonance occurs
when the length of the air
column is one-fourth of the
wavelength of the tuning
fork, because a node occurs
at the surface of the water
and an antinode occurs at
the top of the pipe.
• Resonance occurs at L = λ/4, and again at L =
3λ/4, and at L = 5λ/4.
• The speed of sound in the classroom can be
calculated using the formula for the speed of a
wave v = f λ.
Resonance
• Resonance is not restricted to wave motion.
• Pushing a friend on a swing is an example of
resonance.
• It occurs whenever successive impulses are
applied to a vibrating object in rhythm with its
natural frequency.
• The Tacoma Narrows Bridge disaster in 1940 is
attributed to wind-generated resonance.
Homework
• Conceptual Physics Page 403 #33 - #39
Homework
• Holt physics pages 507 – 509 Chapter 13
Review and Assess 1-37 (odd numbers only)