Sound

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Chapter 15

• Sound

2
The Origin of Sound

• All sounds are produced by vibrations of material
  objects.
• Pitch- how high or low the sound is.
• It depends on how fast the molecules vibrate.
• A high pitch sound has a high vibration
  frequency.
• A low pitch sound has a low vibration frequency.

3
Sound Cont.

• Sounds audible to humans range from 20 - 20,000
  Hz
• Frequencies below 20 Hz are called infrasonic.
• Frequencies above 20,000 Hz are called
  ultrasonic.
• We cannot hear infrasonic or ultrasonic sound
  waves.

4
Sound in Air

• Travels as a longitudinal wave
• Clap your hands and you produce a pulse. The
  pulse vibrates in the air.
• Each particle moves back and forth along the
  direction of motion of the expanding wave.
• This pulse of compressed air is called a
  compression.
• When the pulse travels back towards the source
  refraction occurs.

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Media that Transmit Sound

• Most sounds we hear are transmitted through air.
• But sound also travels in solids and liquids.
• Solids and liquids are better conductors of
  sounds than air.
• Sound is transmitted faster in a liquid than gas,
  and even faster in solids.
• But sound cannot travel in a vacuum, there must
  be a medium present to transmit sound.

6
Speed of Sound

• Speed of sound depends on medium and temperature.
• Speed of sound depends on an objects elasticity.
• Elasticity is a measure of an objects ability to
  stretch and then return to its original size.
• The less elasticity an object has the better
  sound travels.
• speed of sound in air _at_ 0 ºC is 330 m/sit
  increases by 0.60 m/s for every 1 ºC increase.
• Speed of sound at room temperature of about 20 ºC
  is about 340 m/s.

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Loudness

• The intensity of sound is proportional to the
  square of the amplitude of a sound wave.
• Sound intensity is objective and is measured by
  an oscilloscope.
• Unit for sound intensity is the decibel (db)
• Loudness is a physiological sensation sensed in
  the brain, meaning it differs for different
  people.

8
Natural Frequency

• Drop different objects on the floor and they each
  make a distinct and different sound.
• This is because each object vibrates at a
  different frequency when dropped.
• The certain frequency that an object vibrates at
  on its own is called its natural frequency.

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DOPPLER EFFECT

• Refers to the change in frequency when there is
  relative motion between an observer of waves and
  the source of the waves

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• The doppler effect is evident when you hear a
  siren as it passes you.
• When the siren approaches you, the pitch sound
  higher than normal, because the sound waves
  crests are encountering you more frequently.
  (Increase)
• When the siren passes and moves away, you hear a
  drop in pitch, because the wave crests are
  encountering you less frequently. (decrease)

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Bow Waves

• Bow wave- is when the crests of a wave overlap at
  the edges, and the pattern made by these
  overlapping crests is a V shape.
• An example of this would be the bow wave created
  by a speedboat knifing through the water .
• Bow wave is created because the speedboat is
  traveling faster than the waves being created.

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Shock Waves

• A speedboat knifing through the water generates a
  two dimensional bow wave.
• A supersonic aircraft similarly generates a
  three-dimensional shock wave.
• Just as a bow wave is produced by overlapping
  circles that form a V, a shock wave is produced
  by overlapping spheres that form a cone.

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Shock Wave
The cone-shaped wave made by an object moving at
supersonic speed through a fluid. (Here, the
source is moving faster than the wave speed,
which is the speed of sound!!) (Super-sonic
speed)

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SHOCK WAVES

• There are two booms, one from the front of the
  flying object and one from the back.

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Shock Wave
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Sonic Boom
The loud sound resulting from the incidence of a
shock wave. (This is the result of the pile up of
many wave fronts which produces a sonic boom)

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Subsonic

• - slower than the speed of sound

- faster than the speed of sound

• Supersonic

speed of object

• Mach Number

speed of sound

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Resonance

• Condition that exists when the frequency of an
  applied force is the same as the natural
  frequency of vibration of an object or system
• Example of resonance would be some one on a
  swing. When pumping a swing, you pump in rhythm
  with the natural frequency of the swing.
• More important than the force you apply is the
  timing. If pumps are delivered in frequency with
  the natural frequency of swinging motion, produce
  larger amplitude