Sound

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Sound

• Waves

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Sound Wave Properties
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Sound Waves are Longitudinal Waves
The air molecules shown below are either
compressed together, or spread apart. This
creates alternating high and low pressure.
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Frequency

• The frequency of a sound wave (or any wave) is
  the number of complete vibrations per second.
• The frequency of sound determines its pitch.
• Human Hearing ranges from
• . 20Hz to 20KHz

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The higher the frequency, the higher the pitch
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Wavelength

• Wavelength is the distance between two high
  pressures, or two low pressures. This property
  is dependent on the velocity of the sound and
  its frequency.

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AmplitudeThe Amplitude of a Sound Wave
Determines its loudness or softness
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Velocity of Sound
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The velocity of sound depends on

• The medium it travels through
• In air at room temperature sound travels at
  343m/s (766 mph)
• v 331 m/s (0.6)T
• v velocity of sound in air
• T temperature of air in oC

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Relationship between velocity, frequency, and
wavelength

• V ?f
• V velocity of sound
• ? wavelength of sound
• f frequency of sound

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• Sound travels faster in liquids than in air
• Sound travels faster in solids than in liquids
• Sound does not travel through a vacuum
• there is no air in a vacuum so sound has no
  medium to travel through

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Echoes REFLECTION
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Echoes are the result of the reflection of sound
Sound waves leave a source, travel a distance,
and bounce back to the origin.
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Things that use echoes...

• Bats
• Dolphins/ Whales
• Submarines
• Ultra sound
• Sonar

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REFRACTION OF WAVES
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Refraction of Sound

• Refraction is the bending of waves when they
  enter a medium where their speed is different

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DIFFRACTION

• THE BENDING OF WAVES THROUGH A SMALL OPENING

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BENDING OF A WAVE
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Beats
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A beat occurs when sound waves of two different
(but very much alike) frequencies are played next
to each other. The result is constructive and
destructive interference at regular intervals.
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Sound waves move out like this
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But when they move, the front of the wave gets
bunched up (smaller wavelength) and the back of
the wave starts to expand (larger wavelength)
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Observer C hears a high pitch (high frequency)
Observer B hears the correct pitch (no change in
frequency)Observer A hears a low pitch (lower
frequency)
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Police use the Doppler Shift when measuring your
speed with radar

• A frequency is sent out of the radar gun
• The sound wave hits the speeding car
• The frequency is changed by the car moving away
  from the radar and bouncing back
• The amount the frequency changes determines how
  fast you are going
• The faster you are going, the more the frequency
  is changed.

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When the source goes faster, the wave fronts in
the front of the source start to bunch up closer
and closer together, until...
www.youtube.com/watch?vKg9F5pN5tlI
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The object actually starts to go faster than the
speed of sound. A sonic boom is then created.
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Resonance
http//videos.howstuffworks.com/tlc/29833-understa
nding-tacoma-narrows-bridge-video.htm
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• Resonance happens with closed pipe resonators and
  open pipe resonators
• Resonance occurs when there is a standing wave in
  the tube
• Closed pipe resonators
• open end of tube is anti-node
• closed end of tube is node
• Open pipe resonators
• both ends are open
• both ends are anti-nodes

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Closed pipe resonator
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Open pipe resonator
http//www.youtube.com/watch?v-gr7KmTOrx0
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SOUND INTENSITY

• THE LOUDNESS OF SOUND

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DECIBEL

• MEASURES THE LOUDNESS OF SOUND
• RELATES TO THE AMPLITUDE OF THE WAVE
• EVERY INCREASE OF 20dB HAS 10x GREATER AMPLITUDE

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150dB JET TAKING OFF 115dB CHAIN
SAW 100dB POWER MOWER 80dB NOISY
RESTAURANT 75dB VACUUM CLEANER 50dB AVERAGE
HOME 25dB PURRING CAT 20dB RUSTLING LEAVES
15dB WHISPER 0dB FAINTEST THAT CAN BE
HEARD
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A SOUND 10 TIMES AS INTENSE IS PERCEIVED AS BEING
ONLY TWICE AS LOUD
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NOISE POLLUTION

• Prolonged exposure to noise greater than 85-90
  dBA (decibels measured on the A-scale) may cause
  hearing loss
•   Brief exposures to noise sources of 100-130
  dBA can cause hearing loss
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• A single exposure to a level of 140 dBA or
  higher can cause hearing loss