What room modes are
In any closed room, sound waves bounce between parallel surfaces. At certain frequencies the reflections line up into standing waves, called room modes. At those frequencies the bass is boomy in some spots and almost absent in others. Modes dominate the low end of every small room, which is why bass sounds so different from seat to seat.
Axial modes
The strongest modes run between two opposite surfaces. Their frequencies are:
f = n × c ÷ (2 × L)
where c is the speed of sound (about 1,125 ft/s or 343 m/s), L the distance between the surfaces and n = 1, 2, 3 and so on. Each dimension (length, width and height) has its own series. Tangential modes (four surfaces) and oblique modes (six surfaces) are weaker.
Example
A room of 20 × 15 × 8 ft:
- length: 1,125 ÷ 40 ≈ 28 Hz, then 56, 84 Hz and so on;
- width: 37.5 Hz, 75, 112 Hz;
- height: 70 Hz, 141 Hz.
These modes are reasonably spread out. If two dimensions were equal, or one exactly twice another, their modes would stack up at the same frequencies and one bass note would boom.
The Schroeder frequency
Above a certain frequency, modes overlap so densely that the room behaves statistically. The Schroeder frequency estimates this transition:
fs ≈ 2,000 × √(RT60 ÷ V)
with RT60 the reverberation time in seconds and V the volume in cubic meters. For the example room (about 68 m³) with RT60 = 0.5 s, fs ≈ 172 Hz. Below that, think in terms of modes and placement; above it, think in terms of reflections and absorption.
Good room proportions
Rooms whose dimensions are not simple multiples of each other spread modes more evenly. Classic recommended ratios (height : width : length) include 1 : 1.14 : 1.39, 1 : 1.28 : 1.54 and 1 : 1.6 : 2.33. Cubic rooms and rooms with equal dimensions are the worst.
Taming modes
- Placement: moving the listening seat away from the exact middle of the room, often to about 38% of the length, avoids the worst nulls and peaks.
- Speaker and subwoofer position: placing a subwoofer at a mode’s null reduces its excitation. Two or four subwoofers smooth the response across seats.
- Bass traps: thick porous absorbers in corners, or tuned membrane traps, reduce mode ringing.
- Equalization: room correction can cut peaks, but it cannot fill deep nulls.