Why Subs Need a Box

A sub works like a pump. When the cone moves forward it squeezes the air in front of it, and at the same moment it pulls the air behind it thin. When it moves back, the opposite happens. So the front and the back of the cone are always making opposite sounds.

If nothing separates the two sides, the squeeze from the front and the pull from the back meet around the edge of the speaker and cancel each other out. You lose the very bass you were trying to make.

High notes have short waves. By the time the back wave has travelled round the edge it is badly out of step with the front, so the two don’t line up to cancel. Bass notes have long waves, many feet long, that bend around anything in their way. At 40 Hz a single wave is about 28 feet long. A sub that is only a foot across is nothing to a wave that size, so the back wave wraps round with ease.

That is why a bare sub sounds thin. The higher notes come through, and the low ones are the first to cancel.

One fix is a big flat board with the sub in the middle. The back wave now has to travel round the edge of the board before it can meet the front. If that trip is long enough, it arrives late enough to stop cancelling. For that, the board needs to be roughly half a wavelength across.

Try a few notes and see how fast the board grows as the note gets lower.

For 40 Hz the board would need to be about 14 feet wide. Nobody is fitting that in a car, which is why we use boxes.

Each way of mounting a sub deals with the back wave in its own way:

• Free air mounts the sub in a wall so the back wave is shut away in a big space, like a trunk sealed off from the cabin.
• Sealed traps the back wave in a closed box so it can never reach the front.
• Ported traps it, then lets it out through a port at a chosen note, turned round so it adds to the front instead of cancelling it.

The next page, Sealed, Ported and Free Air, compares them side by side.