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