Speaker boundary interference response: the dips a speaker's own reflections cause. Bass leaving the back and sides of the woofer bounces off a nearby surface and returns late. Where the extra distance is half a wavelength, it arrives inverted and cancels part of the direct sound.

How is the frequency calculated?

The speed of sound divided by four times the distance from the woofer to the surface. At 20 °C that is about 343 m/s, so a woofer 1 m from the wall has its first null near 86 Hz. Measure to the woofer itself, not to the back of the cabinet.

Can EQ fix a boundary dip?

No. The dip is two copies of the same sound cancelling. A boost raises both, so they still cancel, and the amplifier and woofer work harder for nothing. Moving the speaker or the seat moves the dip; EQ does not.

Where should my speakers go, then?

Usually close to the front wall, which pushes the first null above 300 Hz, or far enough out that it drops below 40 Hz, which few rooms allow. Keep the distances to the front wall, the side wall and the floor different from each other, so their dips do not stack.

No measurement needed

Speaker boundary interference calculator

Every surface near a woofer sends its bass back late and cancels part of it. Enter where the speakers and the seat are, and this shows the frequency each surface cancels, which of those dips land on top of each other, and how far to move the speakers to get them out of the way.

Measure to the inside faces of the walls. Getting within about 5 cm is close enough.

Room

Length

front wall to rear wall

Width

side wall to side wall

Height

floor to ceiling

Listening seat

From front wall

to your ears, not the sofa back

Ear height

seated

Speakers

Assumed symmetric. If yours are not, measure the one that sounds worse.

From front wall

to the woofer, not the grille

From nearest side wall

Woofer height

centre of the bass driver

I have a subwoofer

Advanced

Room temperature (°C)

Sets the speed of sound, which shifts every predicted mode by about 0.2% per degree.

I will mount panels with an air gap behind them

Where the dips land

The first cancellation from each surface

Each dot is the first null one surface causes, on a scale from 20 to 500 Hz. Shallower nulls follow at three and five times that frequency.

Speakers

2050100200500

ceiling 1.60 m53.6 Hz
side wall 0.90 mstacks95.3 Hz
floor 0.90 mstacks95.3 Hz
front wall (behind speakers) 0.70 m123 Hz

Your seat

2050100200500

side wall 1.90 m45.2 Hz
rear wall (behind listener) 1.80 m47.7 Hz

Each value is the quarter-wavelength null for that surface on its own. For the wall behind the speakers it is close to exact. For the floor, the side walls and the ceiling it estimates the loss in the total bass the woofer puts into the room; the dip at your seat lands somewhat higher, because those reflections reach you at an angle.

Which of these does your room actually have?

A calculator predicts. A measurement shows which dips are really there, and how deep. Export your REW measurement as text, and the analyser matches each dip it finds against these surfaces and against the room's own modes.

Analyse a measurement

Common questions

What is SBIR?: Speaker boundary interference response: the dips a speaker's own reflections cause. Bass leaving the back and sides of the woofer bounces off a nearby surface and returns late. Where the extra distance is half a wavelength, it arrives inverted and cancels part of the direct sound.
How is the frequency calculated?: The speed of sound divided by four times the distance from the woofer to the surface. At 20 °C that is about 343 m/s, so a woofer 1 m from the wall has its first null near 86 Hz. Measure to the woofer itself, not to the back of the cabinet.
Can EQ fix a boundary dip?: No. The dip is two copies of the same sound cancelling. A boost raises both, so they still cancel, and the amplifier and woofer work harder for nothing. Moving the speaker or the seat moves the dip; EQ does not.
Where should my speakers go, then?: Usually close to the front wall, which pushes the first null above 300 Hz, or far enough out that it drops below 40 Hz, which few rooms allow. Keep the distances to the front wall, the side wall and the floor different from each other, so their dips do not stack.

The full explanation, with the two measurements that tell a boundary dip from a room mode: Speaker boundary interference: the bass dip EQ cannot fill