SBIR and Boundary Effects

What is SBIR?

SBIR occurs when sound from your speaker reflects off a nearby wall and combines with the direct sound. At certain frequencies, the reflected sound arrives exactly out of phase with the direct sound, causing cancellation.

Unlike room modes (which are a property of the entire room), SBIR is specific to each speaker's position relative to nearby boundaries. Move the speaker, and the SBIR frequencies change.

Diagram showing direct and reflected sound paths from speaker to listener

SBIR occurs when the reflected path is half a wavelength longer than the direct path

SBIR vs Room Modes

Room modes affect the entire room at fixed frequencies. SBIR creates dips that depend on speaker placement and primarily affect the frequency response at the listening position.

Calculating SBIR Frequencies

The first SBIR dip occurs when the path difference between direct and reflected sound equals half a wavelength:

f = c / (2 × d)

Where:

f = frequency of the dip in Hz
c = speed of sound (~343 m/s)
d = distance from speaker to wall in meters

Example Calculations

Distance to Wall	First SBIR Dip
0.5 m (20")	343 Hz
1.0 m (39")	172 Hz
1.5 m (59")	114 Hz
2.0 m (79")	86 Hz

Multiple Boundaries

Each nearby surface creates its own SBIR effect. The front wall, side wall, floor, and ceiling all contribute, creating a complex pattern of dips.

Boundary Loading vs SBIR

Boundaries don't just cause problems—they also provide helpful bass reinforcement called "boundary loading" or the "half-space effect."

Boundary Loading (Helpful)

When a speaker is placed near a wall, low frequencies below the SBIR frequency are reinforced. Each nearby boundary adds roughly +3 dB of bass output:

Free-standing: 0 dB reference
Near one wall: +3 dB
In a corner (two walls): +6 dB
In a corner on the floor (three surfaces): +9 dB

SBIR (Problematic)

Above the boundary loading region, SBIR creates a dip. The challenge is that moving speakers to reduce SBIR often reduces beneficial boundary loading.

Graph showing boundary loading boost and SBIR dip

Boundary loading provides bass boost below the SBIR frequency

Mitigation Strategies

Speaker Placement

Against the wall: Pushes SBIR to higher frequencies where room treatment is more effective
Far from wall (1m+): Moves SBIR to lower frequencies but sacrifices boundary loading
Flush mounting: Eliminates front-wall SBIR entirely (if possible)

Room Treatment

Absorptive treatment on the front wall behind your speakers reduces the strength of the reflection, minimizing SBIR. However, effective bass absorption requires thick panels (4"+ of fiberglass or rockwool).

Digital Correction (Sounn)

Sounn can partially correct SBIR dips. However, there are limits:

Boosting to fill a dip uses significant amplifier headroom
Deep, narrow dips may not respond well to correction
SBIR varies with listening position, so correction at one seat may not help at others

Correction Limits

Sounn's Maximum Boost setting limits how much the correction will try to fill dips. The default 6 dB is a good starting point. Higher values may cause distortion or speaker damage.

Best Practice

For optimal results, combine physical placement optimization with digital correction:

Start with speakers placed to minimize the worst SBIR effects
Measure with Sounn to see the actual frequency response
Generate correction filters with conservative boost limits
Listen and adjust placement if needed, then re-measure