Subwoofer placement: the corner and the striped field

The corner: free level, dearly paid for

Every solid surface nearby reflects sound and adds it back. The floor alone gives you something, floor plus wall more, the corner most — a good 6 dB in total, sometimes more. That is real level you do not have to pay for, which is why the sub sits in the corner almost everywhere.

But it only holds below the frequency at which the wall is closer than a quarter wavelength. At 50 Hz that is 1.72 m, at 80 Hz only 1.07 m. So the gain is not a flat pedestal, it runs out towards the top — exactly in the range where the crossover to the tops sits.

You pay for it in evenness. In the corner all three room axes meet, and the sub excites every one of them as hard as it possibly can. You get the 6 dB and with it the deepest peaks and the deepest holes the room has to offer. Half a metre out of the corner costs a little level and takes the edge off the modes.

Two subs left and right: the striped field

Putting one sub left and one right looks symmetrical and is the most common mistake in large-scale sound. On the centre line both arrive together and add — that is where it is loudest. Off to either side, the path difference between the two grows.

Where that difference reaches half a wavelength, they cancel. At 60 Hz that is 2.86 m, at 40 Hz 4.29 m. Beyond it they add again, then cancel again — a fan of loud and quiet stripes settles over the audience area, and it sits differently for every frequency. Whoever stands in the middle hears too much bass; three metres further along it is missing.

This is not an equaliser matter. A filter acts the same at every seat; the striped field does not — what you cut in the middle is missing twice over at the edge.

What actually helps, ordered by effect

1. 1Both subs together in the centre. No spacing means no path difference means no striped field — at any seat. That is the most effective change on this list, it costs nothing, and it gives you another 6 dB from putting them together.
2. 2If the centre is impossible: not symmetrical, but both on one side or close together. Stripes come from the spacing — the smaller it is, the further the cancellations slide out of the audible range.
3. 3Out of the corner if the bass booms. Half a metre to a metre is often enough. You lose a little level and gain evenness — and level is the cheaper part, the amplifier gives that back.
4. 4The equaliser only after that. Whatever is left as a single room resonance after placement can be cut narrow and then works throughout the room. What was there before mostly was not a resonance at all.

How to tell what you changed

Measure twice from the same spot, once before and once after moving, and compare. A one-metre move makes differences of 5 to 10 dB in the bass — that is not a subtlety, you see it immediately.

And measure at two seats, not one. A dip that has vanished three metres to the side was a cancellation and not a room problem. One that stays is a resonance — and only that is worth a filter.