By Zencalcy Editorial TeamLast reviewed About us
A rectangular room supports standing waves between opposite walls. The axial modes along one dimension occur at multiples of c/2L, and they dominate the low-frequency response of small listening rooms. Spacing modes evenly and avoiding coincident dimensions reduces boomy or null-heavy bass.
Low-frequency axial resonances of a rectangular room.
f_n = n c / 2L
This is the one-dimensional axial form. A full three-dimensional room has oblique and tangential modes as well, but the axial series is the strongest and the one most often used for speaker placement.
| Symbol | Meaning |
|---|---|
| f_n | Frequency of axial mode n. |
| n | Mode number, a positive integer. |
| c | Speed of sound. |
| L | Room dimension between parallel walls. |
f₁ = 343 / (2 × 5) = 34.3 Hz, with modes every 34.3 Hz above that. A 3.5 m width would put its fundamental at 49 Hz, close enough to interact strongly with the length modes.
Continue with the Reverberation Time Calculator, Sound Intensity Calculator, or the Sound Intensity Level Calculator — or browse more acoustics and sound tools and all physics calculators.
At a mode frequency the room stores energy and the pressure varies strongly with position. Moving a speaker or listener a quarter wavelength can turn a boom into a null.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Content updated: September 2026