A subwoofer’s spec sheet tells you exactly what box it wants. Most of the numbers on it don’t matter for sizing an enclosure, and the ones that do are easy to misread. Here’s what to look at, in the order it matters.
Start here: recommended enclosure volume
Almost every manufacturer prints a recommended sealed volume, a recommended ported volume (with a tuning frequency), or both, usually in cubic feet. This is the single most important line on the sheet. It is the range the engineers designed the driver around. A box inside that range will sound like the product was meant to; a box well outside it won’t, no matter how good the driver is.
Two things to check about that number: whether it’s net (after the driver’s own displacement is subtracted) or gross, and whether the ported figure includes the port’s own volume. Sheets differ, and a 0.2 cubic foot misunderstanding is enough to move a small sealed box noticeably off target.
The Thiele/Small parameters, in plain language
Fs
Free-air resonance, in Hz. Lower generally means the driver can play deeper. It’s a hint, not a verdict; the box decides the final low-end.
Vas
Equivalent compliance volume, in liters or cubic feet. The air volume with the same “springiness” as the driver’s suspension. A big Vas usually means the driver wants a bigger box.
Qts
Total Q. A rough rule of thumb: well below 0.4 leans ported, around 0.4 to 0.7 works in either, above 0.7 leans sealed or free-air. Rules of thumb, not law. The manufacturer’s recommendation already accounts for it.
Xmax
How far the cone can move one way while staying linear, in mm. More Xmax means more air moved before distortion. Matters for how loud and low a given box can play.
Sd
Effective cone area. Together with Xmax it sets how much air the driver can move. It’s why two 10s can out-move one 12.
You don’t need to compute anything from these to buy the right box. They explain why the recommended volume is what it is. If you’re ever choosing between a manufacturer’s recommendation and a number you calculated from Thiele/Small parameters yourself, trust the manufacturer unless you know exactly why you’re deviating.
Power: RMS is the number, peak is marketing
RMS (continuous) power handling is what the driver can take all day. Peak or “max” is a short-burst figure that varies wildly in how it’s measured. Match your amplifier’s RMS output at the driver’s impedance to the driver’s RMS rating, within reason. A sub rated 500 W RMS on a 1,500 W amp with the gain set carelessly is a short-lived sub.
Impedance and voice coils
A dual-voice-coil sub (DVC) can be wired to two different final impedances, which changes how much power your amp delivers. A “dual 2-ohm” sub wires to 1 ohm or 4 ohms; “dual 4-ohm” to 2 or 8. Check that your amplifier is stable at the impedance you end up at. This doesn’t change the box, but it changes whether the system works.
The physical numbers that decide fit
Mounting depth: how deep the driver sits behind the baffle. The box’s internal depth must clear it, with room for air to move behind the magnet. The most common reason an “under-seat” box fails is depth.
Cutout diameter: the hole size in the baffle. Not the same as the nominal size; a “12-inch” sub is not a 12-inch hole.
Outer diameter / mounting hole pattern: whether the flange clears the box edges and nearby trim.
Driver displacement: the driver’s own volume, which you subtract from the box’s internal volume to get net airspace. Our calculator has a field for it.
What to send us
The brand and exact model number is enough; we’ll pull the sheet. If you already have it, the lines we’ll ask about are recommended sealed and ported volume, mounting depth, cutout diameter, displacement, and RMS. With those and your vehicle, we can quote a box in one conversation. Contact us or call (775) 541-2950.
