NA Sound

Learning Objectives
a) describe the production of sound by vibrating sources
b) describe the longitudinal nature of sound waves in terms of the
processes of compression and rarefaction

What is sound?
 Sound is a form of energy that is transferred from one
point to another as a longitudinal wave.

How is sound produced?
 Let’s take a look at how the following produce sounds!
 Guitar: https://www.youtube.com/watch?v=wGKYYcDNIsM
 Drum: https://www.youtube.com/watch?v=osFBNLA7woY
 Tuning Fork: https://www.youtube.com/watch?v=VCERs0v1OoI
 Voice
 Sound is produced by vibrating sources placed in a
medium.

How does a sound wave travel?
Tuning Fork
 If a vibrating tuning fork is dipped into a glass of water, the
water in the glass will splash out.
 This is because the water is displaced by the vibrating tuning
fork.
 The vibrating tuning fork displaces the
molecules of the medium around it, which
in turn displaces their neighbouring molecules.

From Chp 11: General Wave Properties
 Sound waves are longitudinal waves  direction of
vibration of the medium’s molecules is parallel to the
direction the wave travels.

 All longitudinal waves travel as a series of compressions (C)
and rarefactions (R).
 Compressions are regions where the medium’s density is
higher than the surrounding density.
 Rarefactions are regions where the medium’s density is
lower than the surrounding density.

Layers of air are in
undisturbed positions.
When the prongs push
outwards, a region of
compression is produced.
1
2
When the prongs move
inwards, a region of
rarefaction is produced.
3
The prongs continue to
vibrate (move inward and
outward) and a series of
compressions and
rarefactions is set up.
54 ,

Summary
 Sound waves require a medium to travel.
 When sound waves travel, a series of compressions and
rarefactions is set up in the medium.

Learning Objectives
c) explain that a medium is required in order to transmit sound waves
and the speed of sound differs in air, liquids and solids

Can sound travel in vacuum?
A bell jar experiment
The electric bell is switched
on and the air in the jar is
slowly pumped out to create
a vacuum.
What do you think will be
observed as the air is pumped
out?
https://www.youtube.com/watch?v
=hIOqX4uJtYY

Can sound travel in vacuum?
A bell jar experiment
• When the electric bell is on
and the vacuum pump is off,
you will hear the bell.
• When all the air is pumped
out, the bell jar becomes a
vacuum.
• The sound of the bell
becomes softer and softer
until you can no longer hear it.

Medium of Transmission
 Sound cannot travel in vacuum.
 Sound needs a medium to travel from one point to another.
 Travels at different speed in different media.
Medium Air Water Iron
Approximate speed of sound
/ m s-1
330 1500 5000
Speed of sound
in gas
< Speed of sound
in liquid
< Speed of sound
in solid

Medium of Transmission
 Why does sound travel fastest in solid?
Recall: Kinetic Model of Matter
 The closer the particles are packed, the faster the sound
wave is able to be transferred.
Medium Air Water Iron
Particle
arrangement
Very far apart Closely packed
but further than
solid
Very closely
packed
Approximate speed
of sound / m s-1
330 1500 5000

Summary
 Sound waves require a medium to travel.
 Sound travels fastest in solid, slowest in gas.

Learning Objectives
d) describe how the reflection of sound may produce an echo, and
how this may be used for measuring distances

Echoes
Why do you hear echoes when you shout in an
empty room?

Forming Echoes
An echo is formed when a sound is reflected off a hard, flat surface
such as a large wall.
An echo is the
repetition of a sound
due to the reflection of
sound.

Using Echoes
Echoes can be used to find distances.
In the sea
 The ship sends out a pulse of sound
(a signal).
 By noting the time taken for the
sound to be reflected back to the
ship, and with knowledge of the
speed of sound in the sea, we can
calculate the depth of the sea.
2 × depth of sea
time taken
Speed of
sound in sea=

Using Echoes
Against a wall
 A man stands at a distance, d, from a large brick wall
 Every time he claps, he hears an echo after time t s
 It is the time taken for the sound to travel from the man to the wall
and back, over a distance of 2d
 If the speed of sound travelling through air, v (340m/s), is known,
the distance of the reflecting surface from the source can be
calculated using
2d
t
Speed of
sound =

Using Echoes
Practice Qn 1
A boy standing 68 m from a high wall claps his hands and hears an
echo 0.4 s later. Find the speed of the sound in air.
Ans:
Total distance travelled by the sound
= 2 × 68
Speed of sound =
𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐭𝐫𝐚𝐯𝐞𝐥𝐥𝐞𝐝
𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧
=
𝟐×𝟔𝟖
𝟎.𝟒
= 340 m/s

Using Echoes
Practice Qn 2
A person standing at a distance, d, from a tall cliff claps his hands and
hears an echo 0.6 s later. Assuming the speed of the sound in air is
330 m/s, calculate the distance between the person and the cliff.
Ans:
Total distance travelled by the sound = 2d
Speed of sound =
𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐭𝐫𝐚𝐯𝐞𝐥𝐥𝐞𝐝
𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧
330 =
𝟐𝒅
𝟎.𝟔
d =
𝟑𝟑𝟎×𝟎.𝟔
𝟐
= 99 m

Learning Objectives
e) relate loudness of a sound wave to its amplitude and pitch to its
frequency

Sound applet
Observe how the sound changes as the amplitude and frequency is
being changed.
 As the amplitude increases, the sound becomes louder.
 As the frequency increases, the pitch of the sound becomes
higher.

Loudness
 Loudness is related to the amplitude of a sound wave.
 The larger the amplitude, the louder the sound.
 Recall that the amplitude of a wave is the maximum displacement
of a point from its rest position.
Which sound is louder – Sound A or Sound B?
Sound A Sound B

Pitch
 Pitch is related to the frequency of a sound wave.
 The higher the frequency, the higher the pitch.
 Humans can only hear from 20 Hz to 20 000 Hz.
 Recall that the frequency of a wave is the number of complete
waves produced per second.
Which sound has the lower pitch – Sound A or Sound B?
Sound A Sound B

NA Sound

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