The handful of numbers on a spec sheet decide what box a subwoofer needs and what amplifier suits it. Here is what each one means.
The parameters that decide the box
Spec
What it means
Fs
Free-air resonance, in hertz: the frequency the cone moves most easily on its own. Lower usually means deeper potential bass.
Qts
Total Q, how damped the driver is at resonance. Lower values (around 0.4 and under) lean toward ported boxes; higher values toward sealed.
Vas
Equivalent compliance, in liters or cubic feet: the volume of air with the same stiffness as the suspension. It drives the box size.
Xmax
Linear excursion, in millimeters: how far the cone moves in one direction before distortion rises. More Xmax means more air moved at low frequencies.
Recommended box
The maker’s sealed and ported volumes (and port tuning). Start here.
A quick test some builders use is the efficiency bandwidth product, EBP = Fs divided by Qes. Under about 50 suggests sealed, over about 100 suggests ported, and in between can work either way. It is a pointer, not a rule; the maker’s recommendation comes first.
The numbers for the amplifier
RMS power is the continuous power the subwoofer handles. Match your amplifier to it. Peak power is not a planning number.
Impedance and voice coils. A single voice coil (SVC) sub has one coil; a dual voice coil (DVC) sub has two that can be wired in series or parallel. A DVC 4-ohm sub wires to 8 ohms (series) or 2 ohms (parallel). Two DVC 4-ohm subs can be wired to 4 ohms or 1 ohm. Wire to a load your amplifier is stable at.
Sensitivity, in dB, is how loud the sub plays with a reference input. A few dB matters: 3 dB more sensitivity is roughly like doubling the amplifier power.
The numbers for fitting it
Mounting depth: how deep the sub sits behind the mounting surface. It must be less than the box’s inside depth, and behind-seat boxes often limit it.
Cutout diameter: the hole size in the baffle.
Displacement: the volume the sub takes up inside the box. Subtract it to get net volume (see the volume calculator).
Send us the model number and we will read the spec sheet for you: custom order or call or text (785) 430-0822.
