Sound and Hearing

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Sound and Hearing
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How is sound made?
Sound is made whenever anything vibrates. For
example, the strings of a guitar are made to
vibrate when they are plucked, and a drumskin
vibrates when it is struck by a drumstick. The
chords in our voicebox vibrate when we speak
you can feel the vibrations if you press your
fingers to your throat as you talk.
The cones of a loudspeaker are made to vibrate by
an electric current.
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How does sound travel?
Sound travels as a wave. Whenever an object
vibrates, it makes the air around it vibrate.
This spreads the sound out. These vibrations,
which we call waves, travel very quickly in air -
330 m/s, which is about one kilometre every three
seconds. This is faster than a jet
airliner. Sound waves can also pass through
liquids and solids. Evidence for this is that
sound passes through walls, and whales and
dolphins communicate under water using
sound. The speed of sound increases with the
density of the material it is passing through. It
travels at about 1,500 m/s in water, and can
reach a speed of 5,000 m/s in steel.
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Looking at Sound
We can look at sounds if we use a microphone and
an oscilloscope. This lets us see the shape of
the sound wave.
Musical sounds usually have quite a smooth shape.
The distance from the centre to the top of the
wave is called the amplitude. The wider the wave
is, the lower its frequency.
amplitude
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Controlling Waves - Frequency
The pitch of a sound depends on its frequency.
The frequency is a measure of how fast the wave
is vibrating. The faster the vibration, the
higher the frequency, and the higher the
pitch. Frequency is measured in hertz (Hz). One
hertz is one wave per second. Frequency is
sometimes measured in kilohertz (kHz), where 1
kHz is 1,000 Hz, or in megahertz (MHz), where 1 M
Hz is 1,000,000 Hz. The human ear can only hear
frequencies from about 20 Hz up to about 20,000
Hz. This is called the audible range. As you get
older, the highest frequency you can hear
decreases. Hearing damage also often leads to a
loss of higher frequency hearing.
Low frequency, low pitch
High frequency, high pitch
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Controlling Waves - Loudness
The loudness, or volume, of a sound wave depends
on the amplitude. The bigger the amplitude, the
louder the sound.
Large amplitude, loud sound
Low amplitude, quiet sound
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Detecting Sounds The Ear
Our ears detect sounds. There are several parts
to the ear. We usually talk about the outer ear,
the middle ear and the inner ear.
Outer Ear This is called the pinna, it collects
sound waves and directs them into the ear.
Middle Ear This consists of the eardrum and the
three smallest bones in the body. The eardrum
detects the sound, and the bones pass on the
vibration to the cochlea.
Inner Ear. This consists of the cochlea and the
auditory nerves. The cochlea is a coiled tune
filled with liquid. When the liquid vibrates,
tiny hairs inside the cochlea vibrate and send
electrical signals along the auditory nerves to
the brain, which we then perceive as sound.
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How the Ear Works
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What can go wrong?

• The ear is a very delicate part of the body, and
  can be damaged if we are not careful.
• The most easily damaged parts are
• The eardrum, which can be burst by sudden loud
  noises
• The hairs in the cochlea, which are damaged by
  loud sounds, or even by sounds that do not seem
  to loud, if we are exposed to them over a long
  period.

Loudness is measured in decibels (dB). A quiet
room has a sound level of 50 to 60 dB. Sounds
above 90 dB start to have an effect on our ears.
The louder a sound is, the shorter the time you
are allowed to be exposed to it. In the UK,
workers are not allowed to work in a sound level
of 100 dB for more than 40 minutes. Some personal
stereos can reach a level of 100 dB.
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Using Sound
We use sound for many different things. One of
the most useful is to find distances. If we see
a flash of lightning, there is usually a delay
before we hear the thunder. This is because light
travels a lot faster than sound. In fact, light
travels at 300,000,000 m/s, nearly 1 million
times faster than sound.
If there is a delay of 10 seconds between seeing
the flash and hearing the thunder, how far away
is the storm? (Speed of sound in air 330
m/s). Distance speed x time
330 x 10 3,300 m The storm is
3,300 metres away.
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Using Sound (2)
A ship sends out a sound signal. It receives an
echo from the sea bed 0.2 s later. How deep is
the sea? (speed of sound in water 1,500 m/s)
Distance speed x time 1,500 x
0.2 300 m This is the distance
to the sea bed and back again. So the depth of
the sea is half this distance. The sea is 150
metres deep.
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You have now reached the end of the revision
presentation for the Sound and Hearing
topic. I hope you have found it useful. If you
have any ideas to improve this presentation
please let Mr. Turner know. Thank you for
looking at this work, and good luck in your test.
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