Enclosure response at low frequencies
Fondamental : Hypothesis
We assume here that the largest dimension L of the enclosure is small comparing to the wavelength. This can be written as kL << 1 (where k is a wave number) or
(where V is an enclosure volume).
In practice: For "column" enclosures for which the largest dimension may be greater than the wavelength, it is often necessary to consider a waveguide element along its length. The reasoning remains valid for the lowest frequencies.
Movement of a vibrating surface changes the volume inside the enclosure, which reacts by a pressure variation. At low frequencies, the air inside can be approximated to a spring (flexibility of the air inside a closed volume, part 2.3.2.).
In the vicinity of the membrane, the velocity distribution leads to a discontinuity mass, which depends on geometry of a problem (part 2.3.2.).
