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Sound Propagation in Different Environments

Sound Propagation in Different Environments. What is Sound? Free Field Sound Field Rooms Sound in Motion. What is Sound?. Alternating condensation and rarefaction of molecules that are propagated across a medium. Properties of a Sound Source. Mass Elasticity

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Sound Propagation in Different Environments

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  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

  6. Inverse Square Law

  7. Inverse Square Law • http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/invsqs.html

  8. 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.

  9. 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.

  11. Sound Fields

  12. Sound Fields

  13. Sound Fields

  14. Sound Fields

  15. Sound Fields

  16. 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.

  19. Standing Waves

  20. 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.

  21. Doppler Effect

  22. Summary

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