Sealed Box Qtc Calculator
In a sealed loudspeaker, the trapped box air stiffens the driver suspension, raising both the system resonance and its total Q (Qtc). Qtc sets the character of the bass: 0.707 is maximally flat, lower is tighter, higher is boomier. This calculator applies the standard Thiele-Small sealed-box equations to find Qtc, the system resonance fc, and the compliance ratio from the driver Qts and Vas plus your chosen box volume. Qts and Vas are driver-specific manufacturer parameters and are user inputs.
Sealed box equations
compliance ratio alpha = Vas / Vb
resonance factor = sqrt(alpha + 1)
Qtc = Qts * sqrt(alpha + 1)
fc = fs * sqrt(alpha + 1)
Both Qtc and the system resonance fc scale with the same factor, the square root of the compliance ratio plus one. A smaller box raises that ratio, lifting Qtc and fc together.
Sealed alignment facts
- Qtc 0.707 is the maximally flat Butterworth alignment with no bass bump.
- Qtc 0.5 is a critically damped, very tight alignment with gentle roll-off.
- Qtc above 1.0 produces a peak at resonance and audible ringing.
- A smaller box raises both Qtc and the resonance frequency.
- Vas and Qts are Thiele-Small parameters published per driver.
Sealed box Qtc: frequently asked questions
What is Qtc in a sealed speaker box?
Qtc is the total Q of the driver mounted in a sealed enclosure. It describes the damping at the system resonance: low Qtc gives a tight, gradual roll-off, while high Qtc gives a peaky, boomy response. A Qtc of 0.707 is the maximally flat Butterworth alignment.
How is Qtc calculated?
Qtc equals the driver total Q (Qts) times the square root of (Vas / Vb + 1), where Vas is the driver compliance equivalent volume and Vb is the box volume. A smaller box raises Qtc because the trapped air stiffens the suspension.
What is a good Qtc value?
A Qtc of 0.707 is maximally flat (Butterworth) with the best transient response. Values around 0.5 to 0.7 are tight and accurate; values above 1.0 produce a bass bump and ringing. The target depends on the listening goal.
How does box size change the sound?
A smaller box increases air stiffness, raising both the resonance frequency and Qtc, giving more output near resonance but less deep extension and looser transients. A larger box lowers Qtc and resonance for tighter, deeper bass.
Where do Qts and Vas come from?
They are Thiele-Small driver parameters published by the manufacturer or measured from the driver. They are user inputs here because they are specific to your driver. Box volume Vb is your design choice.
Official sources
- National Institute of Standards and Technology: SI units and frequency.
- Audio Engineering Society: loudspeaker measurement standards.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.