Axial room modes can be predicted from room dimensions alone

Evidence

“Back of the envelope” acoustical calculations. To compute the frequencies at which axial standing waves occur, simply measure the distance between the walls (this is one-half wavelength of the lowest resonance frequency), multiply it by two (to get the wavelength) and divide the speed of sound by that number in whatever units the measurements were done (1131 ft/s [345 m/s]). The result is the frequency of the first-order mode along that dimension. All higher-order modes are the result of multiplying this frequency by 2, 3, 4, 5, etc. For example: a room is 22 feet long. The first-order length resonance occurs at 1131/44 = 25.7 Hz. Higher-order resonances exist at 51.4 Hz, 77.1 Hz, 102.8 Hz and so on. Repeat this for the width and height of the room, and all of the axial modes will have been calculated. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1514

Categorization

• DEFINE: A concrete, worked example of how to compute axial standing-wave frequencies from room dimensions.