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Sound Propagation in Different Environments

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

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What is Sound?• Alternating condensation and rarefaction of

molecules that are propagated across a medium.

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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.

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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.

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Effect of Medium on Speed of Sound

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Inverse Square Law

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Inverse Square Law• http://hyperphysics.phy-astr.gsu.edu/hbase/

acoustic/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 doesn’t

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

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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).

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Constructive and Destructive Interference

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

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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