Room modes: why bass changes from seat to seat

Sit in one seat and the bass thumps. Move one seat along and it thins out. Nothing about the system changed — the room did the rest.

This is the most common low-frequency complaint in small rooms, and it comes from room modes.

What a room mode is

Low-frequency sound has long wavelengths — metres rather than centimetres. When a wavelength fits neatly between two parallel surfaces, the sound reflecting back and forth reinforces itself and a standing wave forms.

A standing wave has fixed high-pressure regions and fixed low-pressure regions. Sit in a high-pressure spot and that frequency sounds loud; sit in a null and the same frequency nearly disappears. The pattern is set by the room's dimensions, so it does not move when you change the speaker.

Why small rooms suffer most

In a large hall, the modes are close together in frequency and blur into something smoother. In a domestic room, the dimensions are close to the wavelengths of musical bass, so the modes are widely spaced and individually audible.

That is why the same subwoofer can sound tight in one room and bloated in another. The room, not the box, is doing most of the talking below a few hundred hertz.

What does not fix it

A bigger subwoofer raises the level everywhere, including in the peaks. Equalisation can pull a peak down, and that helps — but it cannot fill a null, because at that position the energy is cancelling itself. Boosting a null only heats the amplifier.

Thin foam panels do nothing here either. Absorption at these frequencies needs depth, which is why corner bass traps are physically large compared with the panels used for treble.

• A larger subwoofer: raises peaks as much as everything else
• Equalisation alone: can cut peaks, cannot fill nulls
• Thin absorption: too shallow to affect long wavelengths

What does help

Geometry first: room proportions decided at design stage set where the modes fall, which is why modelling a room before it is built is worth the effort. After that, the positions of the seats and the subwoofers matter enormously — moving a seat out of a null is free and instant.

Then multiple subwoofers, placed to excite the room's modes more evenly rather than for convenience, deep corner absorption, and finally equalisation to correct what remains. In that order — correction last, because it works best on a room that is already behaving.