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Sound
The Wave Model of Sound
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Sound travels in
longitudinal waves.
Sound waves can only
move through matter, not
empty space.
Several things change the
speed of sound--the kind
of medium, temperature,
elasticity, and density of
the medium.
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The higher the
temperature, the faster
sound travels.
If the medium is very
elastic, sound travels
faster.
If the medium is very
dense, sound travels faster.
The Sound Barrier
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When a plane travels
slower than the speed of
sound, its wave crests do
not overlap.
When a jet travels faster
than sound, it moves
past its wave crests.
Wave crests
shock wave
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The wave crests overlap,
causing constructive
interference. Where the
wave crests overlap a
cone-shaped shock wave
forms. This is the sonic
boom you hear.
Properties of Sound - Intensity
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The more energy a sound
has, the more amplitude
the sound wave and the
louder the sound. The
loudness of sound is
called intensity.
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Sound intensity is
measured in units called
decibels (dB).
Sounds louder than 120
dB can cause pain and
permanent hearing loss.
Properties of Sound - Frequency
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The more sound waves
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pass a point each second,
the higher the sound’s
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frequency, and the higher
the sound’s pitch.
Pitch describes how high or
low a sound is.
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Frequency is measured
in units called Hertz (Hz).
The range of human
hearing is from low
sounds of 20 Hz to high
sounds of 20,000 Hz.
Many animals can hear
sounds that humans
cannot hear.
The Doppler Effect
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The Doppler Effect is a
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change in the pitch and
frequency of a sound caused
when either the sound
source or the listener moves.
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When an ambulance comes
toward you, its sound waves
are bunched together, but when
the ambulance passes you, its
sound waves spread out.
Bunched together waves
produce a high pitched sound,
but spread out waves make a
low pitched sound.
Sound Quality
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Sound quality, or timbre
makes the different
instruments in a orchestra
sound different even
when they are playing the
same note.
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Musical instruments
make sounds of several
different frequencies at
the same time. The
blending of these
frequencies is timbre.
Sound Wave Interactions Reflection and Diffraction
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When sound waves hit a hard
flat surface, they are reflected
back. This causes an echo.
To prevent echoes, theaters
have heavy curtains on the
walls to absorb sound.
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Why can you hear sounds
around a corner?
Sound waves are diffracted
around edges of barriers and
fan out in all directions.
Low frequency sounds
diffract more than high
frequency sounds.
Sound Wave Interactions Refraction and Interference
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Sound waves travel faster in
warm air than in cool air.
You can hear sounds better
at night than in the daytime
because as air cools at night,
the sound waves are
refracted back towards the
earth.
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Sound waves can interfere with
each other.
In some places in a stadium or
auditorium, it can be harder to
hear than in others. This is
because of “dead spots”
produced by destructive
interference.
Sound Wave Interactions Resonance
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The vibration of an object at
its natural frequency is
called resonance.
A tuning fork vibrates when
a musical instrument plays
at its natural frequency.
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Many objects have a natural
frequency that can be
matched by other things.
Wind can make windows
vibrate.
If wind blows at the natural
frequency of the concrete in
a bridge, the bridge can
vibrate so much it collapses!
How We Hear
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Sound waves travel through
your ear canal to the
eardrum, which vibrates
when the sound waves hit it.
The eardrum pushes on the
hammer, anvil, and stirrup,
making them vibrate.
The stirrup pushes on the
oval window of the cochlea.
Auditory nerve
semicircular canals
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Oval window
Inside the cochlea, hair cells
of the same frequency are
made to vibrate, and they
change the vibrations into
nerve signals.
The nerve signals travel by
the auditory nerve to your
brain, which interprets the
sound.
ear canal
eardrum
cochlea
Stirrup anvil hammer
Eustachian tube
Sounds You Hear - Music
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Musical sounds are pleasing.
They have pitch, intensity,
rhythm, harmony, melody,
and quality.
Rhythm is a repeating
pattern of beats or accents.
Each piece of music has a
different series of pitches, or
melody, accompanied by
other notes played together
(harmony).
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Musical instruments produce
several sounds at once, their
main tone, and higher
frequency tones, called
overtones.
The combination of overtones
makes each instrument sound
different.
There are three main groups
of musical instruments: wind,
string, and percussion.
Sounds You Hear - Noise
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Noise is sound that is
annoying and unwanted.
Sound waves from music
are regular and predictable
but sound waves from noise
are random and irregular.
Rustling leaves 10 dB
Whispering 10-20 dB
Talking 60-70 dB
Vacuum cleaner 75-85 dB
Jet engine 170 dB
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Sounds between 60-100 dB
can be annoying, but
sounds above 100 dB can
damage your hearing.
Sounds above 120dB can
cause pain.