The Physics Of Sound

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The Physics Of Sound

• Chapter 9

(Turn on your speakers)
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Sound is made when something vibrates.

• The vibration disturbs the air around it.
• This makes changes in air pressure.
• These changes in air pressure move through the
  air as sound waves.

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Feeling Sound

• When something is making a steady sound then it
  must be vibrating to push the air back and forth.
• Place your hand
• On a purring cat
• On a ringing telephone
• On a radio or speaker that is playing loud music
• On your throat and count to ten

Demo Tuning fork in water
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Nature of sound in air

• Compression and rarefaction
• When the door is opened, a compression travels
  across the room.
• When the door is closed, a rarefaction travels
  across the room.
• It is not the medium itself that travels across
  the room, but the energy-carrying pulse. The
  pulse (disturbance) travels from the door to the
  curtain.

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Nature of sound in air (cont.)

• When the prong of the tuning fork next to the
  tube moves toward the tube, a compression enters
  the tube.
• When the prong swings away in the opposite
  direction, a rarefaction follows the compression.
• The frequency of the vibrating source and the
  frequency of the wave it produces are the same.

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• The sound waves cause pressure changes against
  our ear drum sending nerve impulses to our brain.

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Pitch

• how high or low
• Frequency
• human hearing range
• frequencies between 20 Hz and 20,000 Hz
• ultrasonic
• frequencies gt 20,000 Hz

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• When the frequency of a sound doubles we say that
  the pitch goes up an octave.
• We can hear a range of pitches of about ten
  octaves.
• Many animals can make sounds and hear frequencies
  that are beyond what we can hear.

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Loudness

• To create vibrations energy is used.
• The greater amount of energy used the louder the
  sound.

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Transverse Waves

• side to side vibration in a direction
  perpendicular to the wave's motion
• Examples
• water waves
• waves on a rope
• string musical instruments

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Longitudinal Waves

• back and forth vibration in a direction parallel
  to the wave's motion
• Examples
• slinky waves
• sounds waves

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INTERFERENCE

• Constructive or destructive interference results
  when waves add.
• Examples
• Noise cancellation headphones
• Poorly placed stereo equipment

http//www.kettering.edu/drussell/Demos/superposi
tion/superposition.html
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Click here to run WWW applets of interfering
sound wavesthat result in discernable beats.
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DOPPLER EFFECT

• the change in wavelength due to motion of the
  source
• "Wheeeeeeeeeeee.Oooooooooooooo
• Examples
• moving cars and trains

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http//www.kettering.edu/drussell/Demos/doppler/d
oppler.html
http//www.phy.ntnu.edu.tw/java/Doppler/Doppler.ht
ml
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Sound

• If a bell rang in space, would you hear it?

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Nature of Sound in Air

• Sound requires a medium.
• solid, liquid or gas
• Demo Bell in a evacuated Bell Jar
• Sound waves have compression and rarefaction
  regions.

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Speed of Sound in Air

• 340 meters/second
• 760 miles/hour
• Mach 1

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• What is the approximate distance of a
  thunderstorm when you note a 3 second delay
  between the flash of the lightning and the sound
  of the thunder?
• Answer 3 seconds ? 340 meters/second
• 1020 meters

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YOU DONT NEED TO KNOW THE REST OF THIS FOR THE
EXAM

• But it is still interesting

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SPEED OF SOUND
How it varies
increases with humidity increases with
temperature increases with density
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Example Sound Speeds
http//hypertextbook.com/physics/waves/sound/
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Lightning and Thunder
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Acoustics...

• ...the study of sound properties.
• When a sound wave strikes a surface it can be.
• (a) reflected.
• (b) transmitted.
• (c) absorbed.
• (d) all of these.

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Reflection of Sound

• e.g. an echo
• Reverberation - re-echoed sound, multiple
  reflections of sound waves from walls
• Compare reflections from a hard wall with that
  from a carpet wall.

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Refraction of Sound

• Refraction - the bending of a wave
• Sound travels faster in warm air than in cool
  air.
• Sound waves bend toward cooler air.

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Forced Vibrations...

• the setting up of vibrations in an object by a
  vibrating force.
• Examples of Forced Vibration
• A tuning fork touching a wood surface
• Sounding boards for stringed instruments

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• During forced vibration sound is intensified
  because a larger surface area is available to
  vibrate air molecules.

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Natural Frequency...

• the frequency at which an object naturally tends
  to vibrate.
• At this frequency, a minimum energy is required
  to produce a forced vibration.

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Natural Frequency Examples

• Ringing Small and Large Bells
• Xylophone
• Rubbing a Wine Glass
• Aluminum Rods

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Resonance...

• is the result of forced vibrations in a body
  when the applied frequency
• ...matches the natural frequency of the body.
• The resulting vibration has a high amplitude and
  can destroy the body that is vibrating.

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Examples of Resonance

• swinging your legs in a swing
• breaking a wine glass using sound
• a singing rod caused by forced vibration
• a tuning fork exciting a guitar string
• a truck driving on a rough road
• In 1940, the Tacoma Narrows Bridge was destroyed
  by wind-generated resonance.

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Tacoma Narrows Bridge

• Resonance allows energy to be transferred to a
  vibrating object efficiently if the energy is
  delivered at the natural frequency of vibration.

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http//www.pbs.org/wgbh/nova/bridge/tacoma3.html
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SOUND WAVES
Experiments

• Make a big bang by bursting an air-filled paper
  bag. The air trapped inside the bag will send a
  powerful sound wave through the air that reaches
  our ears.
• Set up a row of dominoes by spacing them fairly
  close. Knock the first one over and watch how
  the wave travels.
• Sprinkle a few grains of uncooked rice on a paper
  and lay that paper over a radio and watch the
  rice jump as you turn up the sound.
• Place a ruler on the edge of a table and bend the
  ruler. Watch the ruler vibrate. Try changing
  the length that is over the edge. The shorter
  the length, the quicker the vibrations, the
  higher the sound. The number of sound waves a
  second is called the frequency of waves.

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The End
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