The special frequencies are also called room modes. Standing waves between parallel walls are called axial modes. Other modes exit. For instance tangential and radical modes. (See the illustration). Normally the axial modes are the strongest.

Axial

Tangencial

Radial

The standing waves are characterized by having a maximum sound pressure at the boundaries of the room. Depending on the frequency there are one or more dips across the room. In a box shaped room the frequencies can be calculated as follows:

where : f = frequency in Hz c =speed of sound (approx. 340 m/s or 1130 ft/s) l = length of the room w = width of the room h = height of the room n = integer from 0 and up