Sub Box Designer

Qtc is the damping of the sealed system. The driver's own Qts sets the floor. Shrinking the box raises both Qtc and Fc together. You are always trading extension for size.

• 0.50 to 0.60: tight, dry, best transient behavior. Rolls off early and gently. Big box.
• 0.577 (Bessel): best time-domain accuracy. The "one note per note" alignment.
• 0.707 (Butterworth): flattest possible amplitude response. The textbook target.
• 0.80 to 0.95: a little bump before rolloff. Punchy and warm. Fine in cars when the box has to be small.
• 1.0 and up: a real peak at Fc. Boomy, one-note bass, sloppy decay. The peak also stacks on top of cabin gain.

Key sales floor line: a smaller box does not make bass tighter past a point, it makes it peakier. Q is the honest way to say how small is too small.

Below roughly 50 to 75 Hz the cabin pressurizes and adds about 12 dB per octave of boost. A response that measures flat outside sounds bloated inside. A response that rolls off at 12 dB/oct (which is exactly what a sealed box does) can come out nearly flat in the car.

That is the whole reason sealed low-Q boxes have the SQ reputation: the anechoic rolloff and the cabin boost are complementary slopes. Toggle the black in-car trace on and off and watch it happen.

Smaller cabins start gaining higher. Trucks and SUVs gain later and leak more, which is why they usually want more cone area and lower F3.

On this tool, the target is the black in-car trace, not the red anechoic one. The shaded gray band on the graph is the optimal zone drawn for you: flat through the crossover region, rising gently about 2 dB per octave below 80 Hz toward the bottom, within about plus or minus 2 dB. Keep the black (or green, with the DSP fix on) trace inside that band and the bass will sound right to almost everyone. You want it:

• Flat or gently rising toward 25 to 30 Hz. A slight downward tilt from 30 Hz up to 80 Hz sounds balanced to most ears.
• No hump between 45 and 70 Hz. That hump is what customers call boomy. It usually comes from high Qtc stacking on cabin gain.
• Staying under the amber dashed line across the range you will play it at. Above that line the cone runs out of Xmax and distortion takes over before the coil ever heats up.

Then check the per-driver watts badge. Green means the amp and thermal rating agree. If the badge is red, either add drivers, drop power, or pick the driver with the bigger motor.

At a fixed amplifier power, going from 2 to 4 drivers adds about 3 dB (same watts spread over double the cone). Doubling power on the same drivers also adds 3 dB. Doing both adds 6 dB, and that is the difference people actually hear as "way more output."

More drivers also means double the displacement, so the excursion ceiling moves up 6 dB. In practice, quantity buys clean low-frequency headroom even more than it buys sensitivity.

Sizing rule: each added driver needs its own share of box. Four drivers at 0.42 ft³ each is 1.7 ft³ net before driver and bracing displacement. That is the conversation to have before promising four twelves under a rear seat.

Wiring is where delivered power actually gets decided. Two dual 4-ohm subs land at 1, 4, or 16 ohms depending on how you strap them, and the same amplifier delivers wildly different power into each. Pick the wiring first, then the amp; the wiring card does that math for you.

Q is fundamentally a statement about how fast the box stops moving, and the group delay plot is where you see it. Group delay is how many milliseconds each frequency lags behind the rest of the music.

The dashed guide on the plot is the one-cycle line (one full period at each frequency). Delay under one cycle in the bass region is generally inaudible; delay well past it starts reading as slow, thick, or detached bass. Sealed low-Q boxes hug the bottom. High-Q sealed boxes rise near Fc, and that rise is the "slow bass" people describe.

This is also why the DSP fix on a small sealed box genuinely works: the sealed response is minimum phase, so cutting the amplitude peak with a parametric EQ pulls the group delay peak down with it. Amplitude and time fix together.

This tool runs textbook Thiele-Small math on the parameters Focal publishes (Fs, Qts, Vas). Focal's printed suggested-application tables come from their lab, and lab boxes have stuffing, leakage, real port losses, and drivers whose actual parameters vary from the printed ones by 10 to 20 percent, which is normal production tolerance.

Where you will notice it: on some drivers the printed table shows F3 dropping as the box grows, while pure T/S math on the printed Qts says F3 bottoms out near Butterworth volume and creeps back up in bigger boxes. Both can be honest; they are answering slightly different questions. When the numbers disagree by a few Hz, trust the direction of the tool (bigger box, lower Q, gentler knee) and trust the printed sheet for Focal's warranty-backed recommendation.

The reliable parts of this tool are the comparisons: config against config, box against box, before DSP against after. Treat any single absolute number as a model, because it is one.