Stuffing and Damping

Fill inside a box can calm it down or choke it. Here is what it really does, and when an SPL box is better off without it.

Sound bounces around inside a box just like it does in a room. Those reflections hit the back of the cone and the walls and can colour the sound with peaks and dips, mostly above the bass range. Damping material (polyfill, fiberglass, wool or acoustic foam) soaks up some of that bouncing energy.

In a Sealed Box

Loose fill does a second job in a sealed box. It slows the way the air inside heats and cools as it is squeezed, so the air gets a little softer, a weaker spring. The result is that the sub behaves as if the box were somewhat bigger than it is, usually around 15 to 25 percent, and the bass goes a touch lower and smoother.

Fill it evenly rather than in clumps. How tightly to pack it depends on the material, so try a little at a time and check the result. Keep it off the back of the cone, where it adds weight to the cone and can get sucked into the sub.

In a Ported Box

A ported box needs its air to move freely to the port. A thin lining on the walls makes little difference to the output, but heavy fill, or fill sitting in the path to the port, soaks up the energy the port is meant to put out. Loose fibres can also get blown out of the port.

That is why most ported SPL boxes run little or nothing inside. If a ported box sounds boomy or honky up higher, line one or two of the walls opposite the sub with a thin layer, and keep the port and the path to it completely clear.

Loose fill suits a sealed box. In a ported box, line a wall or two at most and leave the port alone.

The Battery Test

A quick battery check tells you which way your sub is wired. It needs one ordinary 1.5 volt battery, an AA or a D cell. Disconnect the wires from the amp first and do the test at the amp end of the wire, so you check the wiring as well as the sub. Only ever test one sub at a time, and only with a single small battery.

Hold the battery’s plus end to the plus wire and touch the minus end to the minus wire for a moment, then take it away. Never hold it on.
Watch the cone. With plus to plus it should jump outward. If it pulls in, the wires are reversed somewhere.

Getting this right matters a lot once you have more than one sub. The polarity and phase page explains why.

The battery won’t tell you much about damping. The bounce you hear when you let go is mostly the sub on the box’s air spring, and with no amp connected almost any sub rings more than it does in use. To judge fill properly, do an impedance sweep before and after: added fill lowers and flattens the peak.