Wall Distance (SBIR) Calculator
Find the frequency at which reflections from a wall behind your subwoofer cancel its output, and where to place it to avoid it.
Inputs
Measure from the center of the driver to the nearest hard surface behind it.
1.0 = perfectly hard wall; 0.5 = heavy absorption.
Typically your crossover frequency.
Plan view
How it works
A subwoofer radiates sound forward, and backward, toward the nearest wall. That wave bounces off the wall and joins the direct sound. When the extra distance the reflected wave travels equals half a wavelength, it arrives exactly out of phase and cancels the direct sound: a deep notch. This is Speaker Boundary Interference Response (SBIR).
Extra path = 2 · d First null f₁ = c / (4 · d) Response (dB) = 10·log₁₀( 1 + R² + 2R·cos(2π·f·2d / c) ) Max distance for a null above F: d < c / (4 · F)
If the first null sits well above your crossover frequency, you never hear it: that’s why subs are often placed very close to walls, and why the corner is a popular spot. Pull the sub 1 m away from the wall and the first null drops to ≈ 86 Hz, right in the sub’s range.
The notch gets deeper as the wall reflects more strongly (R → 1). Absorbing material behind the sub reduces reflections. Remember this is a one-wall model; real rooms have three nearby surfaces (back wall, side wall, floor) producing three sets of notches, a sub in a corner is as close as possible to all three.