What is the best sealed box volume for a subwoofer?

It depends on the driver's Vas and Qts. For Qtc 0.707, use Vb = Vas / ((0.707/Qts)^2 - 1). A 12 inch driver with Qts = 0.35 and Vas = 45L gives about 19 litres net. Simulate in RokketBox for your specific driver.

Does a bigger sealed box mean more bass?

Bigger sealed boxes lower Qtc and extend the low-frequency roll-off point. But beyond the optimal volume, you trade sensitivity for extension. There is no benefit to going excessively large.

How do I calculate sealed box volume?

L x W x H (interior dimensions in cm) divided by 1000 = gross volume in litres. Subtract driver displacement and bracing for net volume. RokketBox calculates this from your entered dimensions automatically.

Free Online Tool

Sealed Subwoofer Box Calculator

A sealed subwoofer enclosure is the most forgiving design to build - but sizing it wrong produces a box that either sounds boomy and underdamped, or rolls off too high and loses all low extension. The sealed box calculator takes your driver's Thiele-Small parameters and finds the volume that hits your target system Q.

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How It Works

1
Enter driver specs
Type in your Thiele-Small parameters — Fs, Qts, Vas — or pick from the built-in driver database.
2
Simulate frequency response
The full circuit-domain engine calculates system Q, resonance, and plots frequency response across your target volume range.
3
Export cut sheet
Lock in your box dimensions and export a panel-by-panel cut sheet ready to take to the table saw.

How box volume affects frequency response

The Thiele-Small relationship between box volume and system resonance is deterministic. A smaller sealed box raises the system Q (Qtc) and pushes the resonance frequency up. A larger box lowers Qtc and extends the low-frequency response but reduces sensitivity at resonance. The target for flat response is Qtc near 0.707 (Butterworth alignment). Most car audio builds accept Qtc 0.7 to 0.9 for extra punch; home audio builds often target 0.5 to 0.7 for accuracy.

The sealed box volume formula

The relationship: Qtc = Qts x sqrt(1 + Vas/Vb), rearranged to: Vb = Vas / ((Qtc/Qts)^2 - 1). For a driver with Qts = 0.4 and Vas = 40L targeting Qtc = 0.7: Vb = 40 / ((0.7/0.4)^2 - 1) = 40 / 2.0625 = approximately 19.4 litres net. This is net volume - the enclosure must add driver displacement, bracing volume, and any other internal structure.

Net volume versus gross volume

The formula gives net acoustic volume - what the driver actually sees. Your gross internal enclosure volume must account for driver displacement (typically 1 to 4 litres for a 12 to 15 inch woofer), internal bracing panels, and ports. RokketBox applies all these corrections automatically from your box dimensions and driver spec.

When to choose sealed over ported

Sealed boxes are right when accuracy matters more than peak output, when the box must be compact (sealed achieves good Qtc in 50 to 70 percent of the volume ported needs), or when the driver has high Qts (0.5 or above) which makes porting difficult. Ported boxes beat sealed on output efficiency near tuning, but trade that for higher distortion near port resonance and the need for subsonic filtering.

Frequently Asked Questions

What is the best sealed box volume for a subwoofer?: It depends on the driver's Vas and Qts. For Qtc 0.707, use Vb = Vas / ((0.707/Qts)^2 - 1). A 12 inch driver with Qts = 0.35 and Vas = 45L gives about 19 litres net. Simulate in RokketBox for your specific driver.
Does a bigger sealed box mean more bass?: Bigger sealed boxes lower Qtc and extend the low-frequency roll-off point. But beyond the optimal volume, you trade sensitivity for extension. There is no benefit to going excessively large.
How do I calculate sealed box volume?: L x W x H (interior dimensions in cm) divided by 1000 = gross volume in litres. Subtract driver displacement and bracing for net volume. RokketBox calculates this from your entered dimensions automatically.

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