Sound is produced by vibrating sources and travels as longitudinal waves through a medium, consisting of alternating compressions and rarefactions. The speed of sound differs in various media, being fastest in solids and slowest in gases. Sound waves can be reflected, producing echoes that allow measurement of distances using the time between an initial sound and its echo. Ultrasound has frequencies above human hearing and is used in applications like quality control and prenatal scanning. Loudness depends on amplitude while pitch depends on frequency, with higher amplitudes and frequencies producing louder and higher-pitched sounds.

1. Sound

2. 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

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

4. 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.

5. 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.

6. How does a sound wave travel?
From Chp 12: General Wave Properties
 Sound waves are longitudinal waves  direction of
vibration of the medium’s molecules is parallel to the
direction the wave travels.
direction of wave motion
direction of particle vibration

7. How does a sound wave travel?
 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.

8. How does a sound wave travel?
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 ,

9. Graph of sound waves
The amplitude A of
a sound wave is the
maximum pressure
change.
1
The wavelength λ of
the sound wave is
the distance
between the centres
of two consecutive
compressions or
rarefactions.
2
1
1
22
Pressure – Distance Graph

10. Graph of sound waves
When it is at max.
distance away from
its original position, it
has max
displacement.
1
Zero displacement
when at original
position.
2
Displacement – Distance Graph
1
2

11. Graph of sound waves
Pressure
Highest pressure when
lowest displacement
1
Zero pressure when
highest displacement
2
1 2

12. Graph of sound waves
Displacement – Time Graph
 Follow one particle as per Chp 12 General Wave Properties

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

14. 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

15. 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

16. 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.

17. 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

18. 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
Approximate speed
of sound / m s-1
330 1500 5000
Very far apart
Closely packed
but further than
solid
Very closely
packed

19. How does a sound wave travel?
Summary
 Sound waves require a medium to travel.
 Sound travels fastest in solid, slowest in gas.

20. Learning Objectives
d) describe a direct method for the determination of the speed of
sound in air and make the necessary calculation

21. Measuring speed of sound in air
Procedure
1. Person A and B are positioned at a known distance apart.
2. Person A fires the starting pistol and when Person B sees the
flash of the starting pistol, he starts the stopwatch.
3. Person B stops the stopwatch when he hears the sound of the
pistol.
4. The time interval, t, is recorded.
5. How is the speed calculated?
𝐒𝐩𝐞𝐞𝐝 𝐨𝐟
𝐬𝐨𝐮𝐧𝐝
=
𝒅
𝒕

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

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

24. 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.

25. Forming Echoes
Echoes obey the laws of reflection.
The angle of incidence i is equal to the angle of reflection r.

26. Using Echoes
Echoes can be used to
 measure large distances (like the depth of the sea);
 detect the location of objects (i.e. echolocation).
Echolocation used to
detect the location of fish

27. Using Echoes
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=

28. 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 =

29. 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

30. 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

31. Learning Objectives
f) define ultrasound and describe one use of ultrasound, e.g. quality
control and pre-natal scanning

32. Audible sounds
 The human ear is capable of detecting sounds in a
certain range of frequencies.
 This range of frequencies is called the range of
audibility.
 For humans, this range is from 20 Hz (lower limit) to
20 000 Hz (upper limit).

33.  Humans cannot hear low frequency sounds (infrasound) and high
frequency sounds (ultrasound).
 Ultrasound is sound with frequencies above the upper limit of the
human range of audibility (>20 000 Hz)
Spectrum of sound frequencies

34. Uses of Ultrasound
Quality Control
 Ultrasound can be used to check for cracks and
cavities in concrete slabs and metal pipes.
Transmitter emits ultrasound
Ultrasound passes through the
concrete and is received by a sensor.
The presence and location of defects
are identified by comparing the
ultrasound emitted with the ultrasound
received.
1
2
3

35. Uses of Ultrasound
Pre-natal scanning
 Ultrasound can be to obtain images of structures in the
body.
 It is used, instead of X-rays, to examine the
development of foetuses because it does not harm the
foetus.

36. Learning Objectives
g) relate loudness of a sound wave to its amplitude and pitch to its
frequency

37. 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.

38. 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

39. Pitch
 Pitch is related to the frequency of a sound wave.
 The higher the frequency, the higher the pitch.
 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

40. Questions?