Troubleshoot a 40 Hz bass null

A dip near 40 Hz that appears at your seat but not a metre away is usually a standing-wave null: the seat sits where a low room mode has little pressure, most often the first mode along a room about 4.29 m (14.1 ft) long. It can also come from two sources cancelling each other, or it may not be a null at all but the subwoofer running out of output. Measure several nearby spots before you change anything.

Is it a null or a roll-off?

At 40 Hz the wavelength is about 8.57 m (28.1 ft), longer than many listening rooms. Two very different problems look the same on a single measurement:

• A null depends on position. Move the microphone half a metre along the room and a null gets shallower, deeper or shifts in frequency.
• A roll-off is the subwoofer, the content or a filter. Many compact sealed subwoofers are already losing output near 40 Hz, and a high-pass or bass-management setting can cut it too. A roll-off is roughly the same at every seat and continues downward below 40 Hz instead of recovering.

So the first test is spatial, not electrical: if every seat shows the same shortfall, look at the subwoofer and its settings. If the dip comes and goes across the room, read on. See how to read a subwoofer frequency response for what smoothing and single-point measurements hide.

What can cause a 40 Hz null?

The first length mode, with the seat near mid-room

The lowest mode between two parallel walls has its pressure maxima at the walls and a null halfway between them. Its frequency is set by the distance between the walls:

f₁ = c / 2L = 343 / (2 × 4.29) ≈ 40 Hz

A room around 4.29 m (14.1 ft) long therefore puts a 40 Hz null across its middle. A sofa placed halfway down the room, which is a common layout, sits right in it.

The second length mode in a longer room

In a room about 8.57 m (28.1 ft) long, 40 Hz is the second length mode. Its nulls sit at one quarter and three quarters of the length, so a seat roughly two metres from the front or back wall can lose 40 Hz even though mid-room is fine. The same arithmetic applies across the room: a room about 4.29 m wide has a 40 Hz width mode with its null along the centre line. A height mode cannot reach 40 Hz under a normal ceiling, which would have to be over four metres high.

Boundary cancellation, which needs a lot of distance

Sound reflected from a wall behind the subwoofer or behind the listener cancels the direct sound where the extra path is half a wavelength. The first cancellation lands at c / 4d, so a 40 Hz dip needs about 2.14 m (7.0 ft) between the subwoofer (or your head) and that wall. It is possible in larger rooms, but less likely than the same mechanism at 60 Hz or above, where far smaller spacings do it. The 60 Hz guide covers this case in detail.

Two sources out of step

Two subwoofers, or a subwoofer and main speakers playing below their crossover, can cancel at a seat when their arrival times or polarity disagree. A wrong distance setting, a 180° phase switch, or a crossover set much higher than intended can all create a dip that looks like a room null.

How do I tell which one I have?

1. Measure a line of positions. Measure at the seat, then 0.5 m and 1 m forward and back along the room's length. A dip that fills in or moves points to a length mode.
2. Check the numbers. Enter your room into the room mode calculator and look for a length mode near 40 Hz. A match is supporting evidence, not proof: real walls flex and leak, so measured modes rarely land exactly on the ideal value.
3. Turn the main speakers off. Measure the subwoofer alone. If the dip disappears, the cause is sub-to-mains interference; revisit distance, phase and crossover settings.
4. Move the subwoofer, not just the seat. A length mode is excited weakly by a subwoofer sitting near that mode's own null. Moving the sub from mid-wall into a corner, or from the front wall to a side wall, changes how strongly the mode is driven.
5. Measure again, with the same method. Compare before and after at the same positions. One change at a time.

Can EQ fix a 40 Hz null?

Usually not. In a null the direct and reflected sound cancel; boosting the signal raises both, so the cancellation stays while the subwoofer works harder, reaches its excursion limit sooner, and the extra energy shows up as a peak at seats outside the null. EQ is good at taming peaks. A null is moved or filled by changing positions: the seat, the subwoofer, or adding a second subwoofer placed to drive the mode differently. The bass dead spots guide explains why, and the placement method shows how to test positions efficiently.

Seeing the null across the room

BassMap for iPhone plays a continuous low-frequency multitone through your chosen audio output while you walk the room, and maps the level of each tone at every spot you covered. Set the frequency to 40 Hz and you can see whether the weak area is a narrow band across the room, a single spot, or the whole room. After moving the subwoofer, scan again and compare the two maps, or inspect the frequency response at any measured spot.

Further reading

• Floyd E. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms — chapters on low-frequency room modes and multiple subwoofers.
• F. Alton Everest and Ken C. Pohlmann, Master Handbook of Acoustics — room modes and modal response.
• Also on this site: room modes explained.