Room modes explained

Every closed room resonates. At certain frequencies sound waves bounce between the walls and reinforce themselves, creating standing waves or room modes. They are the main reason why bass sounds uneven from one listening position to another.

How modes form

Between two parallel walls a distance L apart, standing waves appear at

f = n · c / (2L), n = 1, 2, 3, …

For a 5 m dimension and c = 343 m/s, the first mode is at 34.3 Hz. A rectangular room has such modes along all three dimensions and in combinations.

• Axial modes bounce between one pair of walls. They are the strongest.
• Tangential modes involve four surfaces.
• Oblique modes involve all six surfaces and are the weakest.

The room mode calculator lists every mode and plots how they are distributed.

What causes problems

• Clusters: several modes close together create a boomy band.
• Gaps: few modes in a region leave a hole in the bass.
• Equal dimensions: a cube or dimensions in simple ratios stack modes at the same frequencies.

The Schroeder frequency

Above a certain frequency modes overlap so densely that the room behaves statistically. This is the Schroeder frequency, approximately 2000 · √(RT60 / V). Below it the room is modal and position matters; above it the sound field is diffuse.

What can be done

1. Choose listening and speaker positions away from modal peaks, e.g. avoid the exact middle of a dimension.
2. Add broadband bass trapping in corners, where all modes have high pressure.
3. Use equalisation for the lowest modes as a supplement, not as a replacement for treatment.

Draft: written by an AI assistant, awaiting technical review.