Speaker boundary interference: the bass dip EQ cannot fill

Below a few hundred hertz a woofer radiates in every direction, backwards included. The sound that goes backwards hits the wall behind the speaker and comes forward again, having travelled an extra distance of twice the gap. At the frequency where that extra distance is half a wavelength, the reflection arrives upside down relative to the direct sound and the two subtract. At the frequency where it is a whole wavelength, they add.

So the frequency depends on one number: the distance. The first null is at 343 divided by four times the distance in metres, with further nulls at three, five and seven times that frequency. The higher ones are shallower, because by then the woofer is beaming forwards and sends less energy at the wall.

The front wall is usually the worst offender because the speaker is closest to it and the reflection comes straight back past the speaker towards you. For that wall the arithmetic above is close to exact, and it barely depends on where you sit.

The floor, the side walls and the ceiling each produce a null of their own, set by their own distance. Their reflections reach your ears at an angle, so the dip at the seat lands higher than the simple formula says and shifts as you move. What the formula still predicts is a loss in the total power the woofer puts into the room, which every seat hears.

• Equal distances stack. A woofer 80 cm from the front wall, 80 cm from the side wall and 80 cm off the floor puts three dips on the same frequency and turns them into one deep hole. This is known as the Allison effect, after the engineer who described it in the 1970s. Make the three distances clearly different and the dips spread out into shallow ones.
• The listener has boundaries too. A seat with your head 1 m in front of the back wall produces its own dip near 86 Hz at your ears, by the same arithmetic. Sofas pushed against the back wall are the usual case.
• Two speakers, two sets of dips. If the left and right speakers are different distances from their walls, their nulls land at different frequencies, which is one common reason the two channels measure differently in the bass.

On a single measurement a boundary null and a seat sitting in a room mode's pressure minimum look the same: a narrow, deep notch. They respond to different changes, and that is how to separate them.

1. 1.Do the arithmetic. Measure woofer to wall, multiply by four, divide 343 by the result. A dip within about 10% of that number is the prime suspect.
2. 2.Move the microphone half a metre sideways and measure again. A modal null changes depth dramatically, because you have walked out of the mode's quiet plane. A front-wall boundary null largely stays put, because the extra path length is set behind the speaker, not at the seat.
3. 3.Move the speaker instead. Pull one speaker 25 cm further from the wall and measure it on its own. A boundary null moves down by the amount the formula predicts: from 86 Hz at 1.0 m to 69 Hz at 1.25 m. A modal null stays at the mode's frequency, because the room's dimensions have not changed.

1. 1.Move the speaker much closer to the wall. With the woofer under about 30 cm from it, the first null rises above 280 Hz, where the woofer is already directional and the dip is shallower. The wall now reinforces the bass below that instead, and too much bass is a peak, which EQ cuts cleanly. Check the manual first: speakers with rear ports often need 10 to 20 cm of clearance.
2. 2.Or move it much further out. Beyond about 2 m the first null drops below 45 Hz, among the room modes and below where many speakers produce much. Few domestic rooms allow it, which is why the first option is the usual one.
3. 3.Hand the low bass to a subwoofer near the wall. A subwoofer with its driver 20 cm from the wall has its first null above 400 Hz, far above any crossover. Combined with main speakers moved close to the wall, it takes the problem out of the bass entirely.
4. 4.Use thick absorption behind the speaker, and expect less from it than from moving. A porous absorber works where air moves, and air moves most a quarter wavelength from the wall, which is exactly the distance involved. A 5 cm panel on the wall behind a speaker 1 m out does almost nothing at 86 Hz. It takes material 20 cm or more deep to reduce a dip below 150 Hz by a few decibels.