1 Sound Propagation in Different Environments What is Sound? Free Field Sound Field Rooms Sound in Motion.

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Presentation transcript:

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 astr.gsu.edu/hbase/acoustic/invsqs.html 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 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