Acoustics - General equations

The title "Acoustics" here refers to the study of elastic waves and, more specifically, their modes of propagation. The qualifier "elastic" is more general than the qualifier "acoustic" which, in principle, refers to audible phenomena, i.e. with a frequency between 20 Hz and 20 kHz, but "Acoustics" also has the advantage of being a noun.

The frequency of the waves studied here (which naturally include both infrasound and ultrasound) is not a priori limited. These waves are mechanical disturbances of a medium's state of equilibrium. They propagate only in material media: gas, liquid or solid. The structure of these media imposes an upper limit on the frequency; the wavelength must remain large in relation to the characteristic length of the medium (mean free path for a fluid, interatomic distance for a solid). Since wave frequencies are well below this limit, the medium is considered continuous. In addition, attenuation, which increases with frequency and with the disorder of the medium, must allow propagation over several wavelengths.

The phenomena studied are macroscopic: we are not considering the individual movement of the molecules making up the medium, but that of a fluid or solid particle. This term designates an element of infinitesimal volume on the scale of the medium's physical dimensions, yet containing a large number of molecules. Acoustics draws on fluid mechanics and the mechanics of deformable solids.

The "Acoustics" article is the subject of several booklets:

AF 3810 General equations

AF 3812 Propagation in a fluid

AF 3814 Propagation in a solid

The subjects are not independent of each other.

Readers will need to refer back to the other issues often enough.