Reading a Sub's Spec Sheet

Every serious sub comes with a list of short codes like Fs, Qts and Vas. They look like alphabet soup, but they are the sub's measurements, and they let you work out the right box before you cut any wood.

Where the Numbers Come From

In the early 1970s two engineers, Neville Thiele and Richard Small, worked out that a sub in a box behaves in a predictable way, and that a handful of measurements of the sub on its own are enough to predict it. Those measurements are called the Thiele/Small parameters, or T/S specs for short.

Think of a sub as a weight on a spring with a motor attached. The specs describe how heavy the weight is, how stiff the spring is, how strong the motor is and how much the whole thing holds itself back. Put that together with the size of a box and the maths can tell you how loud and how low it will play before you build it.

The Six That Matter Most

Most box design starts and ends with these.

FsHz: The note the sub bounces at on its own, hanging in open air.; Roughly how low it can go. A lower Fs usually means a deeper sub.
Qtsno unit: How much the sub holds itself back at that note. Low means tightly controlled, high means loose and bouncy.; What kind of box it wants. Low Qts suits ported and bandpass, higher Qts suits sealed.
Vaslitres or cu ft: How stiff the cone's suspension is, said as a volume of air. It is the box size whose air would be exactly as springy as the surround and spider.; How big a box it needs. A big Vas wants a big box.
Sdsq cm or sq in: The area of the cone that pushes air, including about half the surround.; How much air each millimetre of travel moves. It also decides your class by cone area.
Xmaxmm: How far the cone can travel one way before the motor starts to lose its grip and the sound gets dirty.; How loud it can get before it runs out of travel, especially below tuning.
Reohms: The voice coil's plain wire resistance, measured with a meter.; Part of how much power the amp actually delivers. Always a bit under the rated ohms.

Qts in Pictures

Qts is the one people find hardest to picture. It is about control at the sub’s own note. A low Qts sub is held firmly and stops dead. A high Qts sub keeps bouncing a little after the music stops, which shows up as a bump in the bass just before it drops away.

Three versions of the example 12-inch sub, sealed in the same 1.5 cubic foot box and changed only in motor strength, worked out with the site’s box maths. Each line is shown against its own level at 120 Hz.

The low Qts sub starts rolling off early and gently. That is why it suits a port, which fills in the low end. The high Qts sub makes a hump and then falls off fast. In a sealed box in a car, where the cabin lifts the low notes, the middle one is often the sweet spot.

The Rest of the Sheet

These fill in the detail, and some calculators ask for them.

Qesno unit: The part of Qts that comes from the motor. The magnet and coil brake the cone electrically.; A strong motor gives a low Qes. Fs ÷ Qes gives EBP, below.
Qmsno unit: The part of Qts that comes from the surround and spider soaking up energy.; Usually much bigger than Qes, so it matters less.
BLtesla metres: The strength of the motor: magnet strength times the length of wire in the gap.; How hard the motor can shove the cone for each amp of current.
Mmsgrams: The weight of everything that moves, plus the air that moves with the cone.; Heavy cones go deeper but take more power to get moving.
LemH: The voice coil's inductance, which makes it harder for the amp to push current at higher notes.; Matters more for midbass than for deep competition bass.
SensitivitydB at 1 W, 1 m: How loud the sub is on one watt, measured one metre away.; A quick comparison, but check how it was measured. In a box, in a car and in open air give very different numbers.

EBP: A Quick Check

Divide Fs by Qes and you get a number called EBP. It is a rough guide to the box a sub was built for. Around 50 or under points to sealed. Around 100 or over points to ported or bandpass. In between, either can work. It is a starting point, not a rule, but it takes ten seconds with any spec sheet.

Why Box Maths Needs Them

A box adds a second spring of air behind the cone. How much that changes things depends on how the box’s air compares with the sub’s own spring, which is why Vas and the box size always go together. Fs and Qts then say where the bass starts dropping and what shape the drop takes. Sd and Xmax say how much air the sub can move, and so how loud it can get before it runs out of travel.

With those numbers, a calculator can try box after box on paper in a fraction of a second. Without them it is guesswork. The Box Designer asks for Fs, Qes, Qms, Vas, Sd, Xmax and Re, and its Find My Sub tab works the other way round, telling you which numbers to look for to fit the space you have.

Spec Sheets Aren’t Perfect

The numbers shift as a sub breaks in, as it warms up and with how hard it is driven. Two samples of the same model can differ by 10 to 15 percent, and different makers measure in different ways. Some sheets are measured carefully, some are copied from a similar model.

Treat a spec sheet as a good starting point. Build from it, then check the finished box with an impedance sweep and adjust from there. The sub’s own note, Fs, is also covered on Resonance and Impedance.