10. Room Acoustics & Environment · Concept 9 of 13
Boundary Interference Response
The dip and bump in your bass when a speaker sits near a wall or floor and its reflection clashes with the direct sound.
A speaker fights its own wall reflection: paths arriving a half-wavelength late cancel into a deep bass notch (f = 343 / 4d), while full-wavelength delays add up to +6 dB.
What it is
Bass peaks and dips caused when a speaker's reflection off a nearby wall or floor stacks with or cancels its own direct sound.
Key facts
Speed of sound c = 343 m/s in air at 20 degrees Celsius (rises ~0.6 m/s per degree)
Wavelength (metres) = c (343) / frequency (Hz). So 100 Hz = 3.43 m long
Reflection ADDS when its extra path = one full wavelength; it CANCELS at half a wavelength (180 degrees out of phase)
First deep notch: f_dip = c / (4 x d), where c = 343 and d = speaker-to-boundary distance in metres
Example: sub 0.86 m off a wall -> first notch at 343 / (4 x 0.86) = ~100 Hz
Cancellation depth can hit -20 to -30 dB at the notch; full reinforcement maxes at +6 dB (pressure doubles)
+6 dB = double the pressure; -3 dB = half-power point; +10 dB sounds roughly twice as loud
Closer to the wall = higher notch (less bass hit); far from wall = low notch that guts musical bass
Flush-mounting the speaker IN the wall (d=0) removes the rear notch - why cinema subs are soffit-mounted
How it works
Sound leaves the speaker as direct sound straight to the listener.
Some of it travels backward/down, hits the wall or floor, and reflects.
The reflection arrives late because it travelled a longer path.
Where the extra path = half a wavelength, direct and reflected cancel -> a dip.
Where the extra path = a full wavelength, they add -> a bump (+6 dB max).
Move the speaker and every dip/bump frequency shifts - placement IS the EQ.
Real examples
Sub jammed in a corner sounds thunderous (corner = +9 dB coupling) but one dropped mid-floor goes thin.
DJ sub 1 m from the back wall has a brutal ~85 Hz notch that no EQ boost can fix.
Floor monitor on a stage deck gets a +3 dB low-mid boost plus a comb dip higher up.
Two subs spread along a wall sound fuller than one in a corner because their notches differ.
Pulling a sub out to 0.3 m from the wall pushes the notch up to ~285 Hz, clearing the kick band.
How it helps in live sound
Get subs OFF the wall by a consistent distance, or push them flush against it (d near 0) to kill the rear notch.
Corner-load one sub for max output (+9 dB free below 100 Hz) when you need brute SPL.
Hear a fixed bass dip? MOVE the sub before reaching for EQ - a cancellation notch can't be EQ'd back.
Keep ground-stacked subs on the floor (clean +3 to +6 dB coupling) rather than up on risers.
Measure with mic + RTA/REW: the notch frequency reveals the distance (d = 343 / (4 x f_dip)).
Spread multiple subs at different wall distances so their notches land on different frequencies and average out.
Everyday analogy
Two mates yell the same word a split-second apart: sometimes the words pile up into a roar, sometimes they collide and mush into nothing, just like a speaker arguing with its own echo off the wall.
Watch out
Myth: a bass dip just needs a big EQ boost. Reality: a cancellation notch is the speaker fighting its own reflection - boost just wastes amp power; you must physically MOVE the speaker to shift the notch.
Fun fact
The same physics is called Speaker Boundary Interference Response (SBIR), the reason mastering studios soffit-mount monitors flush into the wall so the back-wall notch literally cannot exist.
Key takeaways
Direct sound + its own delayed wall/floor reflection = bass peaks and dips.
First notch f = 343 / (4 x distance to boundary); move closer = higher notch.
Cancellation can be -20 dB and is NOT fixable with EQ; reinforcement maxes at +6 dB.
Boundary coupling is separate free gain: floor +3, wall+floor +6, corner +9 dB.
Placement is your most powerful bass control - it sets where every dip lands.