Sealed Subwoofer Box Qtc Calculator
Enter the driver Thiele/Small values and the target system Q. The calculator returns the required net air volume, gross build volume, ideal response points, and practical rectangular dimensions.
Required net air volume (ft³)0.83
- Required net volume (L)
- 23.6
- Gross internal build volume (ft³)
- 0.94
- Gross internal build volume (L)
- 26.57
- System resonance Fc (Hz)
- 53.03
- Estimated F3 (Hz)
- 53.03
- Response peak (dB)
- 0
- Peak frequency (Hz)
- Not applicable below 0.707 Qtc
- Suggested internal dimensions
- 9.32 W × 11.66 H × 14.92 D in
- Estimated external dimensions
- 10.82 W × 13.16 H × 16.42 D in
- Alignment warning
- No major sealed-box alignment warning for these inputs.
Inputs are interpreted in US units: 0.938 ft³ gross before rounding dimensions.
The dimensions are internal airspace estimates; confirm driver cutout, mounting depth, and bracing clearance before cutting panels.
How to use this calculator
- Choose US or metric units, then enter Vas, displacement, and panel thickness in that same system.
- Enter the driver Fs and Qts from the published small-signal specifications.
- Choose the target Qtc for the sealed alignment you want to model.
- Select a box proportion to turn the gross internal volume into suggested dimensions.
How the sealed box volume is calculated
A sealed subwoofer box raises the driver's resonance and total Q because the trapped air acts like an additional spring. The small-signal closed-box relationship is Qtc = Qts × sqrt(1 + Vas / Vb), where Qts is the driver's free-air total Q, Vas is equivalent compliance volume, and Vb is the net air volume inside the finished enclosure.
This calculator solves that relationship backward from the target alignment:
Vb = Vas / ((Qtc / Qts)^2 − 1)
The target Qtc must be greater than the driver's Qts. If it is equal or lower, the denominator is zero or negative, so there is no finite positive sealed box volume that can produce that target with the entered driver.
After the net air volume is found, driver displacement and bracing or hardware displacement are added to get gross internal build volume. The suggested dimensions divide that gross volume by the selected width, height, and depth proportions, then add twice the panel thickness to estimate outside dimensions.
Fc, F3, and response peaking
The sealed system resonance is Fc = Fs × sqrt(1 + Vas / Vb). For frequency response, the ideal second-order high-pass magnitude is modeled with x = f / Fc and M² = x^4 / ((x² − 1)^2 + x² / Qtc²).
Solving M² = 1 / 2 gives the theoretical -3 dB point:
F3 = Fc × sqrt(((1 / Qtc² − 2) + sqrt((2 − 1 / Qtc²)^2 + 4)) / 2)
A Qtc near 0.707 is the classic maximally flat Butterworth target. Lower Qtc values need larger boxes and roll off more gradually. Higher Qtc values need smaller boxes, but they can create a bump near resonance. When Qtc is above 0.707, the calculator reports the ideal peak level and the frequency where that peak occurs.
What moves the result most
Vas and the gap between Qts and target Qtc dominate the box size. A target only slightly above Qts can produce a very large enclosure, while a high target Qtc shrinks the box and raises response peaking. Driver and bracing displacement do not change the required net air volume, but they do change the gross volume you must build.
What this model leaves out
The math assumes the published driver parameters are accurate, the enclosure is rigid and airtight, and the signal level is small enough that the driver behaves linearly. It does not model leakage, stuffing, losses, panel flex, parameter drift, voice-coil heating, cabin gain, room gain, excursion limits, amplifier filters, or the extra effective volume sometimes attributed to polyfill.
Worked example
Suppose a driver has Fs = 30 Hz, Vas = 1.77 ft³, and Qts = 0.400, and the target is Qtc = 0.707. The volume denominator is (0.707 / 0.400)^2 − 1 = 2.124, so the required net air volume is 1.77 / 2.124 = 0.833 ft³.
With 0.070 ft³ for the driver and 0.035 ft³ for bracing and hardware, the gross internal build volume is about 0.938 ft³, or 26.57 liters. The system resonance is about 53.03 Hz, and the ideal F3 is about 53.03 Hz. With compact 1 : 1.25 : 1.6 proportions and 3/4 in panels, the suggested internal dimensions are about 9.32 W × 11.66 H × 14.92 D in, with outside dimensions about 10.82 W × 13.16 H × 16.42 D in.
Common questions
What Qtc should I choose for a sealed subwoofer?
A Qtc near 0.707 is a common starting point because the ideal response is maximally flat. Lower values such as 0.6 need a larger box and have a gentler rolloff. Higher values such as 0.9 or 1.0 make the box smaller but add response peaking near resonance.
Why must target Qtc be greater than driver Qts?
A sealed box can only raise the total Q of the driver system because the trapped air adds stiffness. If the requested Qtc is equal to or below the free-air Qts, the reversed formula has no finite positive box volume. That target would require changing the driver, adding equalization, or using a different alignment.
What is the difference between net and gross box volume?
Net volume is the air volume the driver sees after solid objects inside the box are subtracted. Gross internal build volume is the physical internal volume you build before subtracting the driver basket, magnet, bracing, terminal cup, and amplifier. This calculator sizes the acoustic net volume first, then adds displacement for the build volume.
What are Fc and F3 in a sealed enclosure?
Fc is the resonance frequency of the driver once it is mounted in the sealed box. F3 is the frequency where the ideal sealed-box response is 3 dB below the passband reference. For a Qtc near 0.707, F3 is very close to Fc.
Does polyfill change the effective box volume?
Absorptive stuffing can make a sealed box behave as if it is somewhat larger, but the amount depends on material, density, placement, and losses. This calculator does not apply a stuffing multiplier. Treat the result as the geometric air volume unless you have measurements or design data for your damping material.
Why might measured performance differ from this calculation?
Real drivers vary from their published small-signal parameters, and parameters shift with temperature, break-in, and drive level. Leaks, panel flex, damping, room gain, cabin gain, crossover filters, and amplifier equalization also change the measured response. The calculation is best used for first-pass enclosure sizing before simulation and measurement.