Troubleshoot a 60 Hz bass null

A dip near 60 Hz often comes from a nearby surface rather than the room as a whole: a wall about 1.43 m (4.7 ft) behind or beside the subwoofer, or behind your seat, reflects sound that arrives half a cycle late and cancels the direct sound. A width or height mode, or interference with the main speakers near the crossover, can produce the same dip. The tests below separate these before you reach for EQ.

Why does 60 Hz behave differently from deeper bass?

At 60 Hz the wavelength is about 5.72 m (18.8 ft). That is short enough for ordinary furniture-scale distances, a subwoofer pulled a metre or so out from the wall or a sofa set in front of the back wall, to matter, and long enough that the room's dimensions still form strong modes. Deeper problems such as a 40 Hz null are usually about the room's length; at 60 Hz the list of suspects is longer.

What can cause a 60 Hz null?

Boundary cancellation from a nearby wall

Sound from the subwoofer reaches you directly and also via the wall behind it. Where the reflected path is half a wavelength longer, the two arrive out of step and cancel. With the wall a distance d behind the source, the first cancellation sits near:

f ≈ c / 4d = 343 / (4 × 1.43) ≈ 60 Hz

The same applies to the wall behind your head and to a side wall next to the subwoofer. Real rooms soften the effect because walls absorb some bass and several surfaces contribute at once, but a subwoofer pulled out to roughly 1.43 m (4.7 ft) from the front wall is a classic source of a dip in this range. Roy Allison described this mechanism for loudspeakers near room boundaries in 1974.

A width, height or second length mode

A first mode lands at 60 Hz when two parallel surfaces are about 2.86 m (9.4 ft) apart. That is a typical width for a narrow room and close to some ceiling heights. Its null runs through the middle: the centre line of the room for a width mode, or mid-height for a height mode (about 1.43 m up under a 2.86 m ceiling, a little above seated ear height). In a room about 5.72 m (18.8 ft) long, 60 Hz is instead the second length mode, with nulls one quarter and three quarters of the way down the room. Check your dimensions in the room mode calculator.

Subwoofer and main speakers cancelling at the crossover

Crossovers between 60 and 80 Hz are common, which means 60 Hz is often played by both the subwoofer and the main speakers. If their arrival times or polarity disagree at your seat, they cancel there. Wrong speaker distances, a phase control set at random, or a crossover on the subwoofer stacked on top of the receiver's own can all do it.

Seat height and the floor

Your ears and the subwoofer both sit near the floor and below a ceiling, so moving the measurement point up or down changes the level of height-related effects. A microphone held at standing height can tell a different story than one at seated ear height.

How do I tell which one I have?

1. Confirm it is a null. Measure several spots around the seat. A dip that changes depth or frequency over half a metre is position-dependent; one that is identical everywhere is more likely a setting or the subwoofer itself.
2. Mains off, then mains on. Measure the subwoofer alone. If the dip only appears with the main speakers playing, work on distance, phase and crossover settings first.
3. Flip polarity or sweep phase. With everything playing, change the subwoofer's phase control and measure again. A dip that moves or fills is interference between sources.
4. Change the subwoofer's distance from the wall. Push it close to the wall, within about 0.3–0.4 m (1 ft or so), which moves the first cancellation above 200 Hz and out of the subwoofer's band. Then try it at a different spot along the wall. If the distance change matters most, it was boundary cancellation; if moving along the wall matters most, a mode is the likelier suspect.
5. Change the measurement height. Measure at seated ear height and about 30 cm above and below. A strong change suggests a height mode or floor-to-ceiling interaction.

Will EQ fill it in?

Rarely. A cancellation removes energy by adding two sounds out of step, and a boost raises both by the same amount. The dip stays, the subwoofer and amplifier work harder, and other seats get louder at 60 Hz. Moving the subwoofer, the seat, or adding a second subwoofer is the reliable fix; EQ is better used afterwards to tame peaks. The bass dead spots guide explains the reasoning, and the placement method shows how to compare positions.

Checking the result across the room

BassMap for iPhone plays a continuous low-frequency multitone through your audio output while you walk the room, then maps the level of each tone wherever you went. Set the frequency to 60 Hz to see whether the weak area hugs the seat, runs along the centre line, or covers the whole listening area. Scan again after moving the subwoofer and compare the two maps, or inspect the frequency response at any measured spot.

Further reading

• Roy F. Allison, "The Influence of Room Boundaries on Loudspeaker Power Output", Journal of the Audio Engineering Society, 1974.
• Floyd E. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms — low-frequency room effects and subwoofer integration.
• F. Alton Everest and Ken C. Pohlmann, Master Handbook of Acoustics — room modes.