What is sound ?
Sound is a pressure wave which is created by a vibrating object. This vibrations set particles in the surrounding medium (typical air) in vibrational motion, thus transporting energy through the medium. Since the particles are moving in parallel direction to the wave movement, the sound wave is referred to as a longitudinal wave. The result of longitudinal waves is the creation of compressions and rarefactions within the air.
Measuring the Intensity of Sound
The softest audible sound modulates the air pressure by around 10-6 Pascal (Pa). The loudest (pain inflicting) audible sound does it by 102 Pa. • Because of this wide range it is convenient to measure sound amplitude on a logarithmic scale in Decibel [dB]. • Decibel is not a physical unit – it expresses only a ratio for comparing the intensity of two sounds: 10 log10 (I/I o) where I and I o are two intensity/power levels (I~P 2 , P is sound pressure) • One can say e.g. a channel is amplifying the sound by 3 dB, meaning the output is 3 dB louder than the input. • In order to make it interpretable as a real unit, a fixed pressure P 0 = 2*10-5 Pa is defined (the reference of 0db corresponds to the threshold of hearing) and the absolute sound pressure P in Decibel is defined as: 20 log10 (P/P 0 ) • Thus +20 dB means an increase in pressure by a factor of 10