Boundary Interference (SBIR)
A wall or console behind the speaker cancels part of the low end; work out the frequency before moving anything.
Related
The low end collapses in a narrow band, and a different pair of speakers collapses in the same place. The drivers are usually fine; the wall behind them is not. A reflection arriving just after the direct sound subtracts from it, and which frequencies suffer depends on how far the speaker sits from the boundary.
Distance sets the notch
A gap of d metres puts the first deep notch near the speed of sound divided by four times the distance: about 86 Hz at one metre, 172 Hz at half a metre, 43 Hz at two metres. Close to the wall the notch lands in the low midrange; far out it drops lower, where treatment rarely reaches. Corners add the side wall and floor, so more than one notch appears.
Four boundaries, four bands
| Boundary | Typical distance | First notch | What it sounds like | First move |
|---|---|---|---|---|
| Rear wall | 0.3–1 m | 86–286 Hz | Thin voice, bass loses a layer | Move or flush-mount |
| Side wall | 0.6–1.5 m | 57–143 Hz | Image pulls, left and right differ | Symmetrical layout |
| Floor | 1–1.2 m | 70–86 Hz | Woolly low end, no footing | Stands and decoupling |
| Console top | 0.2–0.4 m | 215–430 Hz | Hollow low midrange | Angle the surface |
Move first, absorb second, equalise last
Pull the speakers away from the wall until the notch misses the notes that matter; flush mounting removes the rear reflection but needs a sealed joint between cabinet and wall. Thick absorbers in the corners work on the top end; the bottom needs real depth or a resonant structure. EQ can cut a peak, but boosting a null feeds the cancellation and makes the hole wider.
When the wall is not the first suspect
In a small room with less than a metre of listening distance the direct sound dominates, and modal behaviour usually outweighs boundary interference; treat the modes first. A flush-mounted main monitor has no rear-wall notch left, so look at the side walls and the console instead. Not every dip is boundary interference: crossover behaviour, standing waves and desk reflections all leave marks on the same curve. Move the speaker and watch whether the dip travels with it.
Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Along the same line
Sources
- Everest, F. Alton，Master Handbook of Acoustics（McGraw-Hill）— 扬声器边界干涉与低频抵消
- Newell, Philip，Recording Studio Design（Focal Press）— 控制室低频与扬声器嵌入
- Toole, Floyd，Sound Reproduction（Focal Press）— 房间边界与低频响应