Room Modes and Standing Waves

Parallel walls trap low frequencies in standing waves, boosting some notes and cancelling others.

Related

Room ModeBass TrapRoom TreatmentDouble Bass

The problem

One bass note booms, the next seems hollow, and moving thirty centimetres changes the verdict. The speaker is not broken; the room has standing waves at fixed frequencies.

Physics or room

Parallel walls form axial modes at n times sound speed divided by twice the spacing. A four-metre room has modes near 43, 86 and 129 Hz. Sound is loudest at pressure antinodes and weakest at nodes; small rooms crowd problems into fewer notes.

Practice and judgement

Measure the room, calculate axial modes and list them. Move speakers and listener away from the centre and walls to avoid nodes; put bass traps where pressure peaks. Confirm by sweep, and never boost a deep null with EQ.

What to listen for

Play a slow bass sweep, or only bass and kick. When one note thickens and rings longer, note the frequency; move your head. A large level change means a standing wave. Repeat in another corner.

Common misunderstandings

Foam is not a bass trap; thin foam does almost nothing at tens of hertz. EQ can cut a peak but rarely fills a null. Large rooms have modes too, just lower and denser.

Sources

• Everest, F. Alton，Master Handbook of Acoustics（McGraw-Hill）— 房间模式章节核对，未引用
• Newell, Philip，Recording Studio Design（Focal Press）— 小房间低频处理章节核对，未引用

Used for fact-checking only; all prose is original.