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Sound – Part 2
Year 7 Science
Review
• In our last lesson we learnt about what sound is.
We found that sound is a form of energy caused by
vibrations.
• We also discovered that sound needs a medium to
travel in that contains particles (air, water, steel
etc.) and hence will NOT travel in a vacuum.
• We found out that sound travels at about 330
metres per second (or 1225 km per hour) and
travels at different speeds in different materials
Speed of Sound
Review
• We also learnt what the meaning of pitch
and frequency is and how they are related.
• We learned about hertz – the unit used in
measuring frequency.
• Finally, we learnt that humans having a
hearing range from approximately 25 hertz
to 20,000 hertz. However, animals have a
greater hearing range with bats able to hear
sounds as high as 150,000 hertz.
A Sound Spectrum
Reflections and Echoes
• Sound energy can also be made to reflect.
• An echo is hearing sound for a second time because
the sound is reflected.
• Because sound takes about a second to travel 300 m,
shouting loudly some distance in front of a cliff or
large building will produce an echo seconds later.
• We can estimate the distance of sound reflecting
surfaces by counting how long it takes to hear the
echo. Try this for yourselves !!!
• In the remainder of today's lesson we are
going to learn about human hearing and
noise pollution.
Hearing sounds - The human ear
• Sound waves cause small but rapid pressure changes on
anything they strike due to the vibrations of particles.
• The human ear turns these pressure changes into nerve
impulses which can be sensed by the brain.
• The human ear contains three distinct parts
• The outer ear
• The Middle ear
• The Inner ear
The Human Ear
Let us look at each of these parts in detail…
The Outer Ear
• The fleshy outer part of the ear is
called the auricle.
• Some animals have large auricles
(such as cats) and hence are able to
detect very small levels of sound.
They can also twist their auricles
to collect sounds coming from a
variety of directions. However, we
have to turn our head to collect
sounds from different directions.
The Outer Ear
• The auricle collects sounds in
the outer ear and funnels the
air vibrations through a
passage called the ear canal.
• At the end of the ear canal
there is a thin sheet of muscle
and skin called the ear
drum. This ear drum
membrane vibrates in
response to the vibrating air
particles.
The Middle Ear
• Behind the ear drum lies
an air-filled space
containing three small
bones or ossicles. These
three bones are called
the hammer, anvil and
stirrup.
The Middle Ear
• As the ear drum vibrates,
these bones act as levers and
carry the vibrations to
another sheet of skin called
the oval window.
• The middle ear also contains
the eustachian tube. This
tube is used to keep the air
pressure in the middle ear
the same as atmospheric
pressure.
The Inner Ear
• Behind the oval window is a
fluid filled ‘snail-shaped’ tube
called the cochlea. When
vibrations are passed through
the oval window by the stirrup,
the fluid moves tiny hairs
inside the cochlea. These hairs
are attached to the nerve
receptor cells that send
messages to your brain
through the auditory nerve.
The Inner Ear
• The inner ear also contains
the semicircular canals.
These fluid-filled organs
are not involved in hearing
sound. They are used to
provide your brain with
information associated
with balance.
Sound Intensity
• Now, we found the rate at which particles vibrate
affects the pitch of the sound and frequency.
• The magnitude or amplitude at which the
particles vibrate affects the loudness of sound.
• We hear different intensities of sound as different
levels of loudness. High intensity sounds are very
loud. Similarly low intensity sounds are often
quiet.
Sound Intensity
• Sounds are all around us and we live in a noisy world !!
• Our hearing organs are very delicate and can be easily damaged
by high intensity sounds. Sometimes the damage is temporary and
at other times it can be permanent - resulting in deafness.
• For those whose jobs are at airports there is a real risk of
becoming deaf.
• So we need to be able to measure sound intensity so we know
when to protect ourselves in dangerously noisy environments.
• How do we do this???
Measuring Sound
• Scientists measure sound levels with a special unit
called a decibel – pronounced “dess-i-bell”.
• The decibel is written as dB.
• The softest sound that an average ear can hear is
called 0 dB. This is called the threshold of
hearing.
Decibel (dB) levels of various sounds
A safe work environment
• In the workplace it is
important to create a safe
and comfortable
environment in terms of
sound.
• The law limits both the
intensity and amount of
time we should be
subjected to sound for a
safe environment.
Breaking the sound barrier
• When an aeroplane moves faster than the speed of
sound, it pushes aside air particles with great force
– creating a shock wave. It forms a cone of highly
pressurised air particles.
• A sonic boom is heard when waves at the base of
this pressurised cone strike someone's ear.
• Sonic booms are so loud they can shatter windows
and burst ear drums.
Sonic Boom due to supersonic jet