Sound waves are longitudinal waves. The wavelength of sound is the distance between 2 consecutive
compressions or 2 consecutive rarefactions. For sound waves, c = f .

Sound is caused by vibrations. When sound waves travel in air, in some places air molecules
are pushed together (compressions), while in other places the molecules are farther apart from
each other (rarefactions).
These compressions travel across the room to one’s ear at the speed of sound. The air molecules do not
travel across the room, they just vibrate to and fro.
Sound Waves
Travelling of sound waves
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Sound Waves
Different speed of sound in different mediums
Sound travels with different speeds in different mediums.

The denser the medium, the faster sound travels through it.
In fact, sound travels fastest in solids, followed by liquids and slowest in gases.
Bell jar experiment
-
Switch
If inside the bell jar there is air,
when the switch is closed, the
hammer is seen vibrating and the bell
is heard ringing.
-
Rubber Bung
If the air is pumped out of the bell jar,
then vacuum results. This time, when
Thick glass bell jar
the switch is closed, the hammer is
still seen vibrating, but no sound is
Fine flexible wires
Rubber cord
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Battery
heard.
Vacuum
Vibrating hammer
To vacuum pump
This experiment shows that:

Sound needs a medium to travel in. Sound does not travel in a vacuum.
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
Our ears are made to hear a range of audible frequencies. The full human hearing range is
about 20 Hz to 20 kHz. The upper limit decreases with age.

Sounds with higher frequencies cannot be heard, and they are known as ultrasound.
Uses of Ultrasound
-
Bats use ultrasound to avoid flying into objects and to detect prey. They send out high-pitched
the reflected signal and turn it into a picture of what’s around. So bats ‘see’ with sound waves.
Ultrasound can be used in industry for cleaning and quality control. Because ultrasounds have such
high frequencies, their vibrations can be used to shake dust from surfaces that need to be clean;
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Dentists use ultrasonic tools to easily and painlessly remove hard deposits of tartar which build up
on teeth and which would lead to gum disease.
-
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squeaks (ultrasound) and pick up the reflections with their big ears. Their brains are able to process
-
Sound Waves
Audible Frequency & Ultrasound
Ultrasound is used in medicine for pre-natal
scanning. An ultrasonic scanner sends pulses of
ultrasound into the patient’s body. At the
boundary between different types of tissue, such
as muscle and bone, the waves are reflected.
These echoes are then processed by a computer to
create an image on a screen. Using this technique
for monitoring the growth of a baby in the
mother’s womb is ideal as it presents no threat to their health. X-rays cannot be used as they might
affect the unborn baby.
-
Fishing boats use echoes (sonar) to detect shoals of fish and depths.
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Sound spreads around corners.
Sound does not come out through
an open door in a narrow beam,
but it fans out so that it can be
heard in any direction.
Sound waves are reflected well from hard, flat surfaces such as walls and cliffs. The same laws of
reflection are obeyed.

The reflected sound forms an echo (reverberation).
Example:
The speed of sound of ultrasound in water is 1500m/s. Calculate the depth of the water if the time
interval between the sending of a pulse of ultrasound and its echo arriving back from the sea bed is
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Reflection of Sound Waves - Echoes
Sound Waves
Diffraction of Sound
0.8s.
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
Refraction occurs when the speed of sound changes. The speed of sound in air is affected by
the air temperature, so if sound waves pass through layers of air at different temperatures they
will be refracted or turned in a different direction.
across the coutryside. The sound waves are refracted down towards the ground.
Interference of Sound
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The diagram illustrates how on a summer’s evening refraction makes it easier to hear distant sounds
Sound Waves
Refraction of Sound
When a person walks slowly as
indicated in the diagram, he
hears variations in the loudness
of the sound as he moves across
the room. A loud sound is
produced where the waves from
the two speakers interfere constructively. A quiet sound is heard where destructive interference occurs.
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Windows or seats in a bus rattle when the engine speeds up to a
certain frequency (their natural frequency).
-
If soldiers march in step over a bridge, then they can make the
bridge vibrate so much at its natural frequency that it may break.
Pitch
The pitch of a note depends on frequency. The greater the frequency of a sound wave, the
higher the pitch of the sound produced and the smaller the wavelength.
-
High frequency sound waves sound high-pitched like a squeaking mouse.
-
Low frequency sound waves sound like low pitched like a mooing cow.
Energy Carried by a Wave
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
Sound Waves
Resonance (not in syllabus)
The amplitude of a wave gives an indication of the amount of energy that the wave carries.
The greater the amplitude, the more energy the wave
carries and the louder the sound produced.
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A microphone changes the sound
wave
into
a
varying
electrical
signal. The variations in the current
carry the information. The currents
from a microphone are very small
and are amplified into much bigger
signals by an amplifier. These
Sound Waves
Oscilloscopes, Microphones, Speakers and Amplifiers (not in syllabus)
signals from the microphone can
through speakers. Speakers turn
electrical signals into sound waves.
The speaker and the microphone have opposite functions.
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be recorded and played back
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