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Guide

Why bass disappears in some spots

Updated

Bass disappears in some spots because two or more copies of the same sound arrive there out of step and cancel. In a room the copies come from reflections: off the walls, floor and ceiling, and from the room's resonant modes. Where they cancel, a note can drop by 10 dB or more, while a seat a metre away hears it fully. The fix is usually to move the seat or the subwoofer, not to turn the bass up.

What is a bass dead spot?

A dead spot is a place where one bass frequency, or a narrow band of them, is much quieter than elsewhere in the room. It is rarely all bass at once. More often a kick drum sounds thin while a bass line one octave lower is fine, or a particular note in a film score seems to vanish. On a frequency response measured there, it shows up as a narrow, deep dip, called a null.

Bass wavelengths are long, several metres at 40–80 Hz, so reflections from every surface of a room overlap at the listening position. Whether they add up or cancel depends on how far each one travelled. Shift the listener or the source, and every distance changes, which is why dead spots are tied to positions rather than to the room as a whole.

Two main causes: modes and boundary cancellation

At a room mode, sound bouncing between parallel surfaces settles into a standing wave with fixed pressure nulls. A seat on one of those nulls hears little of that frequency. The classic case is a seat in the middle of the room, which sits on the null of the first length mode. Modal nulls depend on where you sit and on where the subwoofer is, and their frequencies follow the room's dimensions.

Boundary cancellation

The second cause involves a single nearby surface. A subwoofer some distance in front of a wall radiates sound toward the listener and also toward the wall. The reflection returns and follows the direct sound, delayed by roughly twice the distance to the wall. When that delay equals half a period, the two cancel. This is known as speaker-boundary interference (SBIR), and its first null falls at roughly:

f ≈ c / 4d

Boundary cancellation from a wall behind a subwooferIdealized model. A subwoofer sits 1.43 metres in front of a wall. Sound reaching the listener directly is joined by sound reflected from the wall, which travels about 2.86 metres further. When that extra distance is half a wavelength the two arrive out of step and cancel, here near 60 Hz.wallsubwooferd = 1.43 mdirect soundreflection: about 2d furtherlistenerfirst null ≈ c / 4d = 343 / 5.72 ≈ 60 Hz
Model, not a measurement: a subwoofer 1.43 m from the wall behind it. The reflection arrives about 2.9 m later than the direct sound and cancels it near 60 Hz. In a real room the reflection is weaker than the direct sound, so the dip is real but not total.

With d in metres and c = 343 m/s, a subwoofer 1 m (3.3 ft) from the wall cancels near 86 Hz, and one 2 m (6.6 ft) away near 43 Hz. The same applies to the listener: a seat with its back near a wall gets a reflection from behind. Moving the subwoofer close to the wall, within about 0.3 m (1 ft), pushes this first null up to roughly 286 Hz, above a typical 80 Hz crossover, which is one reason subwoofers are often placed against walls or in corners.

Can EQ fix a dead spot?

Usually not. A null is not a shortage of energy; it is energy cancelling itself. Boosting that frequency raises the direct sound and the reflection that cancels it by the same amount, so the dip remains. Meanwhile each 6 dB of boost needs four times the amplifier power and roughly twice the cone movement from the subwoofer, so the attempt can lead to distortion, a limiter engaging, or a driver bottoming out. The extra output also lands at every other seat, where the frequency may already be loud.

Peaks behave differently. A boom caused by a mode is too much energy at one frequency, and cutting it with EQ usually works well, at least for the seat you measured. That asymmetry, cut peaks but do not chase nulls, is a central rule of bass EQ. More on reading these shapes in how to read a subwoofer frequency response.

What actually moves a dead spot?

How to confirm it is a real null

A single microphone reading can mislead. Before moving furniture, rule out the measurement:

  1. Measure again at the same spot. A dip that comes and goes is noise or a test problem.
  2. Measure at several positions 20–50 cm (8–20 in) apart around the seat. A real null shifts or fades gradually across them; you will see where it is deepest.
  3. Note whether the dip moves when you move the subwoofer, the seat, or neither. A dip that follows the seat's distance from a wall points to boundary cancellation; one that tracks the room's dimensions points to a mode.
  4. Compare its frequency with the room's predicted modes in the room mode calculator and with c / 4d for nearby walls.

Specific frequencies

Two frequencies come up often in home rooms. See the guides to a 40 Hz bass null and a 60 Hz bass null for step-by-step diagnosis. Neither frequency is a universal problem; whether you have one depends on your room.

Mapping dead spots across the room

Checking positions one at a time is slow. BassMap for iPhone plays a continuous low-frequency test signal through your system while you walk, and records each tone's level with its tracked position. The heatmap for a given frequency then shows where it is strong and where it disappears, and you can open the frequency response at any measured spot. Measure, move the subwoofer, measure again and compare the two scans. Levels are relative, not calibrated SPL.

Sources