Sound Propagation in Different Environments

1
Sound Propagation in Different Environments

• What is Sound?
• Free Field
• Sound Field
• Rooms
• Sound in Motion

2
What is Sound?

• Alternating condensation and rarefaction of
  molecules that are propagated across a medium.

3
Properties of a Sound Source

• Mass
• Elasticity
• E.g., tuning fork. As you vary the mass and
  elasticity you will change its natural frequency.
• All sound sources have mass elasticity.

4
Properties of a Medium

• Mediums (e.g., air molecules) also have mass and
  stiffness.
• Medium is not displaced over an appreciable
  distance.
• Sound needs a medium, it cannot propagate in a
  vacuum.

5
Effect of Medium on Speed of Sound
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Inverse Square Law
7
Inverse Square Law

• http//hyperphysics.phy-astr.gsu.edu/hbase/acousti
  c/invsqs.html

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Inverse Square Law

• Bottom Line
• In a free field, the sound pressure will decrease
  6.02 dB for every doubling of distance.
• In a sound field, the inverse square law doesnt
  hold because of reflection and diffraction of
  sound waves.

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Free Field

• Any acoustic field that is free of reflective
  surfaces.
• Anechoic Chamber

10
Sound Field

• Any field that has reflective surfaces.
• Virtually all acoustic fields are sound fields.

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Sound Fields
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Sound Fields
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Sound Fields
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Sound Fields
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Sound Fields
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Constructive and Destructive Interference

• Involves addition of two or more waveforms.
  Waves can be from different sources or one source
  where sound has been reflected.
• Constructive Interference - 0 to 120 degree phase
  difference
• Destructive Interference - 120 to 180 degree
  phase difference
• If waves are 0 degrees phase and are of the same
  amplitude you will see a 6 dB increase in sound
  pressure.
• If waves are 180 degrees phase and are of the
  same amplitude you will have silence (Remember
  silence ? 0 dB).

17
Constructive and Destructive Interference

• In this example, the reflected (secondary) wave
  arrives out of phase from primary wave.

18
Standing Waves

• Reflection of waveforms (i.e., standing waves)
• Results in areas of constructive and destructive
  interference, and occurs in a system that is
  enclosed at both ends
• e.g., ear canal with hearing aid or earphones, or
  sound treated room.

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Standing Waves
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Doppler Effect

• As object approaches, the frequency is higher
  than expected. (e.g., 1000 Hz source might be
  perceived 1500 Hz).
• When object recedes, the frequency decreases.
• The effect is directly related to speed of the
  object.

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Doppler Effect
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Summary