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IGCSE Physics
Sound
Sound
Aims:
•To describe how to measure the speed of sound in air
by a simple direct method.
•To use an oscilloscope to determine the frequency of a
sound wave and appreciate that the pitch of a sound
depends on the frequency of vibration,
•To appreciate that the loudness of a sound depends on
the amplitude of vibration,
Sound cannot travel though a vacuum
Sound is made up of the vibrations of
atoms or molecules passing through a
substance, and so without atoms or
molecules (i.e. in a vacuum) sound
cannot travel
When the pump is turned on, the bell sounds quieter and quieter as
the air is pumped out. Eventually, it can hardly be heard at all thought he gong can still be seen striking the bell. This is because
the sound from the bell cannot travel though a vacuum, but light can.
The bell does not become totally silent, since some of its vibrations
are transferred to the container - the foam rubber helps to reduce this
effect.
Speed of Sound
You need a quiet open space at least 100 m long
to perform this investigation.
START
STOP
00:0034
00
100 m
1. When you see the cymbals crash, press START.
2. When you hear the cymbals crash, press STOP.
 Record the results of your sound experiments in a table.
sound
distance
(m)
time
(s)
speed
(m/s)
1
100
0.34
294
2
3
4
How are these values used to estimate the speed of sound?
100
distance
=
= 294 m/s
speed =
time
0.34
The speed of sound in air is about…
340 m/s
 Use the results of the cymbals experiment
to calculate your average speed of sound.
How does your calculation for the average speed of sound
compare with the real speed?
What errors could have affected the results of your cymbals
experiment?
Do you think the speed of sound in water is the same
as it is in air?
Measuring speed with echos
START
150 m
STOP
Stand at least 100 m from a large, flat wall with a stop watch.
1. Use a starting pistol (or clapper board) to make a sound.
2. Measure the time taken between firing the pistol and
hearing the echo. How far does the sound travel?
The sound of the starting pistol takes 0.92 s
to travel a distance of 300 m.
How can you use this result to estimate the speed of sound?
distance
speed =
time
=
300
0.92
= 326 m/s
Repeat the experiment several times to obtain an average.
How does your calculation for the average speed of sound
compare with the real speed?
Draw a labelled diagram of the arrangement
of suitable apparatus for the experiment.
Speed of sound in different materials
The speed at which it travels though a
material will therefore depend on how close
together the molecules are, and so upon the
density of a material - the more tightly
packed the molecules, the more quickly
vibrations can be transferred from molecule
to molecule, i.e. the faster the sound can
travel though the material.
Speed of sound in different materials
The speed of sound is approximately:
340m/s in air - gas molecules are relatively far apart,
and the transfer of vibrations (sound) is relatively
slow
1400m/s in water - liquid molecules are much closer
together, and transfer of vibrations is much more
rapid
6000m/s in steel - solid molecules are even closer
together, and transfer of vibrations is even more
rapid
Speed of sound in different materials
vgas<vliquid<vsolid
Soft materials dampen sound;
hard materials reflect it (echoes and
reverberations).
Visualising sound
Describing a sound wave with
diagrams or equations makes it easier
to communicate our ideas.
The Loudspeaker
Paper cone moves
backwards and forwards
When the cone moves
forwards it compresses
the air next to the speaker
When the cone
moves backwards it
rarefies the air next
to the speaker
H
L
H
L
H
L
H
L
H
The compressions and rarefactions move
away from the speaker as sound waves
HIGH
LOW
HIGH
LOW
Higher frequency sounds (higher pitch)
mean that the speaker vibrates backwards
and forwards more often
Sound Waves
Sound Waves
Amplitude
Sound Waves
Wavelength
Sound Waves
Wavelength
Wavelength
Oscilloscope
Oneoscilloscope
button can change
the
An
can
widthanofelectrical
the wave on the
show
screen
so that
you can
signal
on its
screen.
measure the frequency even if
the wavelength is very large
or very small.
One button can change the
height of the wave on the
screen so that you can
measure the amplitude even if
it is very large or very small.
Looking at Sound Waves
Trace
Microphone
A low frequency sound
at a medium volume
Oscilloscope
What is the difference between the sound wave of
a quiet sound and a loud sound?
quiet sound
loud sound
The loud sound has taller waves.
The louder the sound, the greater the amplitude.
What would the sound wave of a very loud sound look like?
What is the difference between the sound wave of
a low pitch sound and a high pitch sound?
low pitch sound
high pitch sound
The high pitch sound has a shorter wavelength,
so more waves are visible. It has higher frequency waves.
What would the sound wave of a very low sound look like?
Looking at Sound Waves
A higher frequency
sound at a lower volume
Looking at Sound Waves
A low frequency sound
at a low volume
Looking at Sound Waves
A high frequency sound
at a high volume
Which trace represents the loudest sound?
A
B
Sound A is the loudest.
Sound A has the largest amplitude, which means the
wave has more energy and so the sound is louder.
Which trace represents the sound with the highest pitch?
A
B
Sound B is the highest pitched.
Sound B has the shortest wavelength and the most
number of waves visible, so it has the highest frequency.
Match the description to the oscilloscope pattern:
A
B
C
D
1. Low-pitched sound, very loud
D
2. Loud, high-pitched sound
C
3. Medium sound with medium pitch
B
4. Quiet, high-pitched sound
A
Match the oscilloscope traces to the source of the sound.
A
B
C
D
1. A flute playing a single clear note
B
2. A dog whistle
C
3. A milk bottle dropped onto a floor
D
4. A rumble of thunder
A
Amplitude and Frequency
The human can hear sound between about 20Hz and
20,000Hz.
Ultrasonic waves
1.A signal generator can produce
electrical signals which can be
converted to sound waves by a speaker.
The sound waves can be greater than
the hearing range for humans
(ultrasonic waves).
2.Ultrasonic waves can be used in
industry for cleaning, quality control,
and for pre-natal scanning
Ultrasonic waves
Uses of sound
a. Acoustics – the study of sound.
Soft materials dampen sound;
hard materials reflect it (echoes and
reverberations).
b. SONAR – Sound Navigation and
Ranging (echolocation).
c. Ultrasound imaging
d. Kidney stones & gallstones.
Oscilloscopes in detail
Oscilloscopes can be complicated but we
need to know how to work out frequency
from the image on the screen.
Oscilloscope picture
Oscilloscope diagram
The height of the wave on the screen is the amplitude of the signal
and can be controlled by the ‘volts/div’ dial.
The width of the wave on the screen is the frequency of the signal
and can be controlled by the ‘time base’ or ‘time/div’ dial.
Reading the screen
•The height of the
screen shows the
amplitude or voltage
of the signal.
•The width of the
screen can be used to
determine the
frequency of the signal
if it is changing.
1 cm squares
•Each large square is
one centimetre wide.
•Each little division is
two millimetres apart.
•The middle
horizontal line
displays zero voltage.
d.c. signals 1
If the scale of the
oscilloscope is
set to 4V per
division.
This signal
would be -8
volts.
d.c. signals 2
•If the scale of
the oscilloscope
is set to 0.2V per
division.
This signal
would be + 0.4
volts.
d.c. signals 3
•If the scale of
the oscilloscope
is set to
1V/division.
This signal
would be -2.8
volts.
d.c. signals 4
•The scale has
been set to
1V/division.
•The signal is
still d.c.
It is changing from 1 V to 3 V but is always positive.
a.c. signals 1
•This is a
correct a.c.
signal.
•At 1 cm per
volt it has an
amplitude of 2
volts.
It contains both positive and negative voltages.
Timebase controls horizontal speed
Timebase in action 2
Changing the volts/division
Calculating frequency
To work out the frequency of the wave displayed
on the screen you need to do three things:
1. Measure a complete wave on the screen in cm,
2. Multiply this number by the ‘Timebase’
setting to get the period,
3. The frequency = 1  period.
Example calculation 1
The wavelength of a signal on an oscilloscope
screen is 3 cm long. The timebase setting is
0.1 s/div.
•Wavelength = 3 cm
•Period = 3 × 0.1 s = 0.3 s
•Frequency = 1  0.3 = 3.33 Hz
Example calculation 2
The wavelength of a signal on an oscilloscope
screen is 4 cm long. The timebase setting is
0.01 s/div.
•Wavelength = 4 cm
•Period = 4 × 0.01 s = 0.04 s
•Frequency = 1  0.04 = 25 Hz
Example calculation 3
The wavelength of a signal on an oscilloscope
screen is 5 cm long. The timebase setting is
0.1 ms/div. (1 ms = 0.001 s)
•Wavelength = 5 cm
•Period = 5 × 0.0001 s = 0.0005 s
•Frequency = 1  0.0005 = 2000 Hz
Summary – Picturing Sound
•A starting pistol can be used to measure the speed of
sound by using the difference in speed between light and
sound, the bigger the distance the better the result.
•An oscilloscope can be used to display the frequency
and amplitude of a sound wave. If the wavelength on the
screen in cm is multiplied by the timebase we can work
out the period of the wave. 1/period = frequency.
•The bigger the amplitude, the louder the sound.
•The closer the waves, the higher the pitch.