1. Physical characteristics of sound

Sound is a physical phenomenon of mechanical origin, consisting of a very slight variation in pressure, vibratory velocity or fluid density, which propagates by progressively modifying the state of each element of the medium considered, thus giving rise to an acoustic wave. Sound is also the sensation provided by this wave, which is received by the ear, then transmitted to the brain and deciphered by it. Of all acoustic waves, only some can be perceived by the ear: these are waves with frequencies between 20 Hz and 20 kHz. Below 20 Hz, we speak of infrasound, and above 20 kHz of ultrasound. Sound results from a periodic variation in air pressure. TheL _p sound level (expressed in dB) is directly related to the amplitude of the pressure variation.