Tuning & QTC Calculator

Port Dimensions

Port Length (inches)

Measured inside the box

Port Type

Port Diameter (inches)

Number of Ports

Box Volume

Net Box Volume (ft³)

— or — Net Box Volume (liters)

1 ft³ = 28.317 L

Formula: Fb = (c / 2π) × √(Ap / (Vb × Leff)) where c = 13,526 in/s, Leff = port length + 0.732 × √(Ap / π)
Tip: For slot ports, Leff = port length + 0.732 × √(Area / π). Tuning typically targets 10–15 Hz below the driver's Fs for music, or equal to Fs for SPL.

Driver T/S Parameters

Qts (total Q)

From manufacturer spec sheet

Vas (ft³)

— or — Vas (liters)

1 ft³ = 28.317 L

Box Volume

Net Box Volume (ft³)

— or — Net Box Volume (liters)

—

QTC

System Q

—

α (Vas/Vb)

Box stuffing ratio

—

Recommended Vb

for QTC = 0.707

QTC Value	Sound Character	Transient Response	Bass Extension
< 0.5	Very overdamped, weak bass	Excellent	Poor
0.5 – 0.65	Tight, punchy, SQ-oriented	Very Good	Limited
0.65 – 0.75	Maximally flat — ideal SPL/flat	Good	Good
0.75 – 1.0	Slight midbass boost, musical	Moderate	Better
> 1.0	Boomy, ringing, overdamped bass	Poor	High (colored)

Formula: QTC = Qts × √(1 + Vas/Vb) | Optimal box for QTC = 0.707: Vb = Vas / (0.707/Qts)² − Vas
Tip: QTC 0.707 gives maximally flat response (Butterworth). For car audio where cabin gain adds low-end, targeting QTC 0.6–0.7 often sounds best.