Sealed Box Qtc Calculator

Qtc is the total Q factor of a driver mounted in a sealed (closed) box. It describes the damping of the system resonance: Qtc near 0.5 gives a critically damped, very tight response; 0.707 (Butterworth) gives maximally flat response and the lowest F3; values above 1.0 give a peaky, boomy response. Qtc is always higher than the driver's free-air Qts because the sealed air adds stiffness.

First find the compliance ratio alpha = Vas / Vb, where Vas is the driver's compliance volume and Vb is the box volume. Then Qtc = Qts * sqrt(alpha + 1), and the box resonance Fc = Fs * sqrt(alpha + 1). A smaller box gives a larger alpha, raising both Qtc and Fc.

Qtc 0.707 is the classic maximally flat target with the deepest extension for a given box. Qtc 0.5 to 0.6 gives tighter, more accurate transient response favored for music monitoring, at the cost of a higher F3. Qtc up to about 1.0 to 1.2 trades accuracy for a fuller, warmer bass and is sometimes chosen for car or home theater. Above that the response becomes audibly boomy.

You need the driver's free-air resonance Fs in Hz, the total Q factor Qts, and the equivalent compliance volume Vas in litres, all from the manufacturer datasheet, plus your intended sealed box internal volume Vb in litres. These are measured properties of your specific driver, so use the published figures rather than generic values.

Sealing a driver in a box adds the stiffness of the trapped air to the driver's own suspension. A smaller box traps stiffer air, which raises the system resonance frequency Fc and increases the total Q (Qtc). That is why very small sealed boxes tend to sound peaky and have higher cutoff, while larger boxes approach the driver's free-air behaviour.