Chapter 16: Sound
Section 1:
The Nature of
Sound
Section 2: Properties of
Sound
Section 1:
Sound
The Nature of
Sound & Longitudinal Waves
Remember, sound begins with
a vibration.
How sound travels:
Sound is a disturbance that
travels through a medium as a
longitudinal wave.
Sound is a
disturbance
that travels
through a
medium
as a
longitudinal
wave.
How Sounds Are Made
As the drum
vibrates back &
forth, it creates
compressions &
rarefactions in the
air.
How Sounds Are Made
When a person
speaks or sings,
the vocal cords
vibrate. The
vibrations produce
longitudinal sound
waves in the air.
Sounds in Solids & Liquids
Example:
When you knock on the door,
the particles of the door vibrate.
The vibration creates sound
waves that travel through the
door.
When the sound waves reach
the other side of the door, they
make sound waves in the air on
When sound waves
enter a room
through an open
door, they diffract
(spread out).
Sound can only travel in there is
a medium to transmit the
compressions & rarefactions.
In outer space, there are no
molecules to compress or
rarefy.
The energy of the original
vibrations has nothing
through which to travel.
How Sound Bends
When sound waves hit a barrier with
a small hole in it, some of the waves
pass through the hole (Similar to the
diffraction shown in a harbor).
When sound waves go through a
doorway, they spread out.
Because of diffraction, you can hear
sounds from around corners.
Waves passing a corner spread out
as they pass.
The Speed of Sound
Speed of sound depends
upon the elasticity, density,
and temperature of the
medium.
Speed of sound at room
temp. is about 342 m/s.
Speed of
sound depends
on the medium
through which
it is traveling!
Elasticity = the ability of a
material to bounce back after
being disturbed.
If a medium is very elastic, its
particles easily go back to their
original positions.
Sound travels more quickly in
mediums that have a high degree
of elasticity because when the
particles are compressed, they
quickly spread out again.
Which one of
these would
sound travel
faster in?
Some substances are more
elastic than others.
Sponges & rubber
bands
.
are more elastic than
modeling clay.
Solid materials are usually
more elastic than liquids or
gases, so compressions &
rarefactions travel very well
in solids.
Liquids aren’t very elastic
Gases are generally very
inelastic & are the poorest
Density
Speed of sound depends on how
close together the particle of the
substance are.
In materials of the same state of
matter – solid, liquid, or gas- sound
travels more slowly in denser
mediums.
The denser the medium, the more
mass it has in a given volume. The
particles of a dense material do not
move as quickly as those of a less
dense material.
Temperature
In a given medium, sound
travels more slowly at lower
temperatures and faster at
higher temperatures.
At 20 ºC, speed of sound in
air = 340m/s.
At 0 ºC, speed of sound in air
=
330 m/s.
At higher altitudes the air is colder,
Moving Faster than Sound
Oct. 14, 1947,
Chuck Yeager
became the
first person to
fly faster than
the speed of
sound. He
flew at a high
altitude in
order to do
this. Why?
Moving Faster than Sound
Oct. 15, 1997,
Andy Green
became the
first person to
drive a land
vehicle faster
than the speed
of sound.
Why did he
choose to
drive in the
desert?
Section 2:
Properties of
Sound
Intensity
When a sound wave carriers a large
amount of energy, the molecules of the
medium move a greater distance as the
waves pass by, and the sound wave
has a greater amplitude.
The intensity of a sound wave is the
amount of energy the wave carries per
second through a unit area.
Measured in watts per square meter
(W/m2)
Loudness
Sound waves of higher amplitude
have a greater intensity because
they carry more energy per second
through a given area.
Though intensity & loudness aren’t the
same thing, the greater the intensity of a
sound wave, the louder it is.
Loudness describes what you actually hear.
Measured in decibels (dB).
Sounds louder than 100dB can cause
damage to your ears.
A loudspeaker
gives out sound
by vibrating
cones of
material. The
greater the
amplitude of
vibration, the
greater the
volume, or
loudness, of the
sound.
Frequency
Most people can hear sounds with
frequencies between 20 Hz &
20,000 Hz.
Sounds with frequencies above the normal
human range of hearing are called
ultrasound.
Sounds with frequencies below the normal
human range of hearing are called
infrasound.
Pitch
The pitch of a sound is a
description of how high or low
the sound seems to a person.
The pitch of a sound that you
hear depends on the frequency
of the sound wave.
The key farthest to the left on a piano is
attached to the longest string. This key
plays the note with the lowest pitch.
Resonance
All objects vibrate naturally.
Frequency of the object depends
on the shape & type of the object.
If the frequency of sound waves
exactly matches the natural frequency
of an object, the sound waves can add
to the object’s vibrations.
Resonance occurs when the frequency
of the sound waves & the natural
frequency of the object are the same.
Some musical
instruments can produce
notes w/ vibrations that
match the natural
frequency of a crystal
glass. If the note is
sustained, the amplitude
of vibration can cause
the glass to shatter.
The Doppler Effect
Doppler Effect = the apparent
change in frequency as a wave
source moves in relation to the
listener.
If the waves are sound waves,
the change in frequency is
heard as a change in pitch.
The Doppler Demonstration:
Doppler put a musical band on
an open flatcar of a train.
He stood on the ground nearby. As
the train approached him, the notes
the musicians played seemed to be a
higher pitch.
As the train passed, the notes
seemed to drop in pitch.
Doppler repeated the experiment, but
this time he stood on the train and had
the musicians play while they were
seated on the ground.
Doppler heard the same changes in
pitch as the train he rode approached
and passed the band
The effect was the same regardless of
who was moving, the band or Doppler.
Changing Pitch
As a sound source moves
toward the listener, the waves
reach the listener with a higher
frequency. The pitch appears to
increase because of the Doppler
effect.
As the police car speeds by, the pitch of the
siren seems to change. Ahead of the car, the
sound waves are piling up, so the pitch is
higher. Behind the car the waves spread out,
so the pitch is lower.
When a plane flies faster than the speed
of sound, waves pile up to form the
shock wave known as the sound
barrier.