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Chapter 42: Sound
What you need to know
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What is sound?
How is it produced?
How does it travel?
How quickly does it travel?
What are echoes?
How does the ear work?
How do we measure noise?
What is sound?
Sound is a form of energy which is caused by
vibrations
e.g vocal chords vibrate – voice
strings vibrate - guitar.
Sound is a form of Kinetic Energy
Transmission of Sound
Sound needs a material to pass
through
In space there is no sound………………because
there is no medium – Astronauts need to
communicate with each other using radios.
Sound moves more quickly through solids than
liquids and more quickly through liquids than gases
How it works
Sound is carried through the air by vibrations of air
particles.
If you speak into a loudspeaker it vibrates the air
particles near you.
The air particles beside them vibrate also. These
vibrations travel through the air until they reach
your ear and cause your eardrum to vibrate. If
there was no air (i.e. medium) the vibrations
could not travel
How quickly does sound
travel?
• Light travels faster than sound
• We know this because we see lightening
before we hear thunder
Speed of Sound
Sound travels very fast
It travels at 340 m/second
Over long distances there is a lag time e.g. from
seeing the light to hearing the sound e.g.
fireworks.
Experiment to Show that Sound
Cannot Travel through a Vacuum
What are echoes
Echoes are sounds that
are reflected from a
surface
Echoes
Advantages of Echoes:
Ships uses echo sounding
to find the depth of the sea.
Disadvantages of Echoes:
Reflected sound can get mixed up with new sound
and arrive together to the audience in a theatre.
Example 2
• Sound is sent down to the seabed by a ship. A
receiver on the ship picks up its echo 1.2 seconds
later. If the speed of sound in water is 1500m/s
calculate the depth of the water beneath the ship.
Distance = speed x time
Distance travelled by sound = 1500 x 1.2
= 1800m
But depth of water is half distance travelled by
water (ie up and down) 1800 /2 = 900m
Example
Suddenly a shoal of fish swims under the ship at a
depth of 250m below the surface. How long does
it take for the receiver to pick up the signal now?
This time the sound travels 250m down and 250m
back a total distance of 500m
Time = distance / speed
= 500 / 1500
= 0.33 seconds.
Sound Detection in the Ear
The outer ear is shaped to collect sound.
The eardrum is like a thin drum skin and it
vibrates when sound hits it.
The vibration is carried through your middle
ear into your inner ear.
Nerves pick up the vibration and send
electrical signals to your brain
Our Ear
Our Ears
Our ears are extremely sensitive
to sound.
A sound level meter is used to
measure sound.
The decibel scale is used to
measure sound.
Sounds above 85 decibels can be
harmful to your ears.
Decibels
• On the decibel scale, the smallest audible sound (near total
silence) is 0 dB
• * A whisper - 15 dB
• * Normal conversation - 60 dB
• * A car horn - 110 dB
• * A rock concert or a jet engine - 120 dB
• * A gunshot or firecracker - 140 dB
• Any sound above 85 dB can cause hearing loss, and the loss is
related both to the power of the sound as well as the length of
exposure. You know that you are listening to an 85-dB sound if
you have to raise your voice to be heard by somebody else. Eight
hours of 90-dB sound can cause damage to your ears; any
exposure to 140-dB sound causes immediate damage (and
causes actual pain)
Decibel Scale
Noise Level & Damage
Sounds about 85 Decibels (dB) can be harmful to
your ears.
The threshold of pain is about 130dB
Hearing Protection
1. Obey warning signs.
2. Wear ear muffs.
3. Move away from the
source of the sound.
4. A high wall or trees will
reduce the noise from a busy road.
Summary of Chapter