Sound Bites

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Sound Bites
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Basics

• Sound is a mechanical, longitudinal wave.
• The medium usually associated with sound is air,
  but sound can travel through both liquids and
  gases as well.
• As sound travels through a medium, it
  alternately compresses and expands the medium the
  same way a slinky behaves when you create a
  longitudinal wave in it.
• compressed areas - high pressure area
• expanded areas- low pressure areas

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Drawing a sound wave

• It is somewhat difficult to draw a longitudinal
  wave in terms of high and low pressure areas.
  Instead you will draw a graph of the pressure
  changes themselves. Once you have done that, you
  will have a picture that looks like a transverse
  wave, which is easier to describe.

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Speed of Sound

• The speed of sound depends on the medium that it
  is passing through.
• Generally speaking, the denser the medium, the
  greater the speed of sound. As a result of this,
  sound travels much faster in solids than it does
  in gases.
• Material Speed (m/s)
• Air 343
• Water 1440
• Wood 4000
• Glass 4500
• Steel 5000 explains the ear to the train
  track!

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• The speed of sound also depends on the
  temperature.
• For sound travelling through air, the
  relationship between the temperature and the
  speed is given by
• v (331 0.60 T) m/s
• v speed of sound in air
• T temperature in degrees Celsius

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Give it a go

• What is the speed of sound at room temperature
  (20 C)?
• Use this answer if the temp. is not given

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Example 2

• The speed of sound in air is 350 m/s. To the
  nearest degree, what is the air temperature?

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Echoes

• All the properties of waves apply to sound waves.
  
• reflection,
• superposition,
• refraction
• diffraction
• Reflected sound waves are given a special name,
  echoes.
• The wave equation also applies to sound.
• v ?f

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Example

• Some cameras use very high frequency sound to
  determine the distance from the object being
  photographed. If an object is 2.0 m from such a
  camera, how long will it take for the sound to
  return to the camera?
• You yell into a mine shaft and hear your echo
  3.00 s later. How deep is the well?

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Pitch

• With sound we use specialized terms for the
  general wave properties.
• pitch - frequency of sound.
• The human ear responds to sounds in the range of
  20 Hz to 20000 Hz.
• ultrasonic sound with freq above 20000 Hz
• Bats can hear sounds with frequencies as large as
  100000 Hz.
• Infrasonic - sound with freq below 20 Hz
• created by earthquakes and machinery

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Interference with Sound Waves

• Sound waves can interfere just like other waves.
• An interesting phenomenon occurs if the two
  sound waves that are interfering are fairly close
  in frequency.

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Beats

• The top two green waves have slightly different
  frequencies. When they interfere with one another
  there will be times when they are nearly in phase
  and a loud sound will be heard and at other times
  they will be nearly out of phase and very little
  sound will be heard.The red wave represents the
  result of the interference. Notice that the
  change in the amplitude has a frequency of its
  own.The frequency at which the amplitude
  changes is called the beat frequency. The beat
  frequency is simply the difference between the
  frequencies of the original two waves.

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Example

• tuning fork 1 256 Hz.
• tuning fork 2 ? Hz
• Beat freq 4 Hz
• The two possible answers for the second fork are
  260 Hz AND 252 HZ.
• How can this property be used to tune musical
  instruments?

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Doppler Shift

• You have probably noticed the frequency (or
  pitch) of a siren change as it approached and
  passed you. If either the source of a sound, the
  observer of a sound or both are moving, the
  "observed" frequency of the sound will change.
• Of course, the actual frequency of the sound
  source remains the same. When the source and
  observer are moving towards one another, the
  observed frequency will be higher and when they
  are moving away the frequency will be lower.

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Doppler Shift
Now the sound source is moving to the right at a
constant velocity. As the source emits sound
waves, it "catches up" to the waves in front of
it and pulls away from the waves behind. This has
the effect of decreasing the wavelength of the
waves in front and decreasing the ones behind.
This is a stationary sound source emitting waves
at a constant frequency. Notice that the
wavelengths are equal.
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Doppler Shift

• As sound moves away form you
• ?? then f ? (low pitch)
• As sound moves towards you
• ?? then f ? (high pitch)

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Calculating Doppler Shift

• f' observed frequency (Hz)
• f frequency of the sound source (Hz)
• v speed of sound (usually 343 m/s)
• vo velocity of the observer (m/s)
• vs velocity of the sound source (m/s)

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Example

• An ambulance carrying Ms. Ryan (coffee overdose)
  is approaching you at 30 m/s. If the ambulance's
  siren emits sound at a frequency of 1500 Hz, what
  frequency will you hear as you stand in shock at
  possibly losing your beloved physics teacher?

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Give it a go

• A tornado siren goes off so you jump on your
  bicycle and drive away from the siren at a speed
  of 20.0 m/s. If the siren has a freq of 500 Hz,
  what freq will you hear?
• (471 Hz)

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Sonic Boom

• An interesting thing happens when an object (such
  as a plane) travels with a velocity equal to the
  speed of sound. The sound waves in front of the
  plane can't move away and build up in one very
  large pressure wave. It actually takes a burst of
  extra energy to break through this barrier. Once
  the plane has broken through, it will be
  outrunning its own sound waves and is said to be
  supersonic.
• The sound waves will all line up to
  constructively interfere and to form what is
  known as a shock wave. This shock wave is
  commonly called a sonic boom.

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