tags: #publish links: [[Acoustics]], [[Acoustic room treatment]], created: 2021-10-29 Fri --- # Speaker boundary interference response AKA _speaker-boundary interference response_ (SBIR), _listener-boundary interference response_ (LBIR), the _boundary effect_. It is basically **reflection-induced _comb filtering_**. (See also [[Room modes]], which involves resonance of the entire room.) Excellent article: http://arqen.com/acoustics-101/speaker-placement-boundary-interference/ https://www.gikacoustics.com/speaker-boundary-interference-response-sbir/ **Summary:** speakers are "coupled" to walls and all other reflective surfaces, due to constructive *or destructive* interference between the direct and reflected wave. The solutions are speaker positioning and acoustic treatment. The destructive interference is the main problem as it can't be corrected with EQ - that sound is just gone. If you boost the signal you just boost the intering wave too. Destructive interference (cancellation) happens when the speaker is 1/4 wavelength from the wall, because the reflection is then 1/2 wavelength out of phase - inverted. And of course the 1/4-wavelength distance is frequency-dependent. ## Front wall > The first cancellation notch caused by SBIR is usually about two thirds of an octave wide. So you want to place your speakers so that this notch is either: > - At a high enough frequency so it can be treated using absorption. (High frequencies are easier to absorb at the reflection point.) > - Outside the frequency range that your speakers reproduce. **Quarter wavelength cancellation frequency (Hz)** = `343 / (4 × distanceFromWall)` (metres) Solutions are all about speaker placement. In preference order: * Mount the monitors flush in the wall, so there's no reflection! Great if you're building a studio from scratch, I guess. * Or place close to the wall (e.g. 0-20cm) ideally with absorption material behind (works best if close = higher frequency) * **Note this also boosts the bass response** but you can EQ that. * Or place far away from the wall so the cancellation is outside the speakers' frequency range - e.g. like 2.2m-3m away depending on low-frequency cutoff. Typically you ***really* need to avoid coupling at bass frequencies** e.g. 40-200Hz, because it's hard to fix and takes out a large range of sound. ## First reflections Rear wall, side walls, ceiling, floor, even the desk in front of you. There's interference between reflected and the sound direct from the speaker. **Half wavelength cancellation frequency (Hz)** = `343 / 2 × (reflectedPathDistance - directPathDistance)` (metres) ## Rear wall You get cancellation of bass frequencies. > You get peaks at 1⁄2, 1, 3⁄2, etc wavelengths from the back wall, which tend to be just under 6 dB (bass ringing) > You get nulls at 1⁄4, 3⁄4, 5⁄4, etc wavelengths from the back wall - large bass dips / suckout. > Ideally, your listening position should be at least **3 metres from the rear wall** behind you, placing the quarter wavelength cancellation frequency below 30 Hz. Absent that (or perhaps in addition), the only solution is bass trapping at the rear, to reduce rear-wall reflections.