Sound Production and Spread
Sound is produced by mechanical vibration.
Only the sound with vibrative frequency between 20~20000Hz can be heard.
Sound can spread in the media: air etc. with sound waves.


Avoiding Interference


Sound Speed, Frequency, Wavelength, Phase


Several Representative Sound Pressure Levels (SPL)
SPL meter can tell __ (dB)
0 dB for sound only heard, 120 dB can make ears pains.
Background yawp in silence is about 50 dB.
Face to face talk is about 60~70 dB.
Cinema dialogues are about 80 dB.
Sound in a disco hall is about 110 dB.
Site sound for a jet plane taking off is about 140 dB.