Speaker Box Calculator — Sealed Enclosure Design
Enter your driver's Thiele-Small parameters (Qts, Vas, Fs) and target system Q (Qtc) to get the optimal sealed box volume, system resonance, and −3 dB frequency — plus the required box depth for your chosen width and height.
L
Hz
cm
cm
cm
Net internal air volume for the target Qtc
14.1 L
- 1
Qtc ÷ Qts ratio
0.707 ÷ 0.4 = 1.7675 - 2
Denominator (ratio² − 1)
1.7675² − 1 = 2.1241Derived from Qtc = Qts × √(Vas/Vb + 1), solved for Vb. - 3
Optimal box volume
30 ÷ 2.1241 = 14.1
How does this calculator work?
Optimal sealed box volume is Vb = Vas / ((Qtc/Qts)² − 1). For a typical mid-bass driver with Qts = 0.4, Vas = 30 L, Fs = 35 Hz and Qtc = 0.707: Vb ≈ 30 L, Fc ≈ 62 Hz, f₃ ≈ 62 Hz. Higher Qtc gives a smaller box with a slight bass peak; lower Qtc gives a larger, flatter box.
Formula
How this is calculated
A sealed (closed) enclosure is the simplest speaker box design: it's an airtight box that adds acoustic mass to the driver, raising its effective resonance frequency and shaping the roll-off below it. The key parameter is the system Q (Qtc), which determines the shape of the bass response. Qtc = 0.707 (Butterworth alignment) gives the flattest possible response with no peak; Qtc ≈ 1.0 (Chebyshev-like) adds a slight bass boost at the cost of audible resonance ringing; lower values (0.5–0.6) give a faster, tighter roll-off used in car audio and professional subwoofers.
The required box volume is Vb = Vas / ((Qtc/Qts)² − 1), derived from the relationship Qtc = Qts × √(Vas/Vb + 1). The system resonance Fc = Fs × Qtc/Qts marks the frequency where output is 3 dB below the passband for a Butterworth alignment, and is related to other alignments by the second-order high-pass transfer function. The −3 dB cutoff f₃ is solved numerically from that transfer function.
Box dimensions are calculated by solving Vb = inner_W × inner_H × inner_D for the depth, given the outer width and height you enter (adjusted for panel thickness). All values assume a rigid, non-resonant enclosure with no internal losses — in practice add acoustic damping material (fibre fill) which effectively increases the box volume by 10–15%.
Frequently asked questions
Qtc is the system Q of the loaded driver in the box. 0.707 is the Butterworth (maximally flat) point — the textbook standard for hifi. Values 0.5–0.6 give a leaner bass better suited to vented or car-audio applications. Values above 0.9 give audible bass emphasis and ringing. For home subwoofers, start with 0.707.
These are Thiele-Small parameters found on the driver's spec sheet. Qts (total Q) characterises how strongly the driver is damped. Vas (equivalent compliance volume) is the volume of air that has the same stiffness as the driver's surround and spider — a larger Vas means a more compliant driver that needs a bigger box.
No — this calculator is for sealed enclosures only. Ported alignments require an additional port-tuning frequency parameter and different Thiele-Small alignment tables. As a rough guide: if your driver has a low Qts (< 0.4), it is often better suited to a ported enclosure.
Also known as
TG we-Calculate Editorial Team. (2026). Speaker Box Calculator — Sealed Enclosure Design [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/speaker-box-calculator
TG we-Calculate Editorial Team. "Speaker Box Calculator — Sealed Enclosure Design." TG we-Calculate. 2026. https://we-calculate.com/calculator/speaker-box-calculator.
TG we-Calculate Editorial Team, "Speaker Box Calculator — Sealed Enclosure Design," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/speaker-box-calculator
@misc{wecalculate_speaker_box_calculator, title = {Speaker Box Calculator — Sealed Enclosure Design}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/speaker-box-calculator}}, year = {2026}, note = {TG we-Calculate} }
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