A sealed enclosure is the most straightforward of all subwoofer enclosure types. The driver is mounted in a completely airtight box, and the trapped air behind the cone acts as a pneumatic spring that works alongside the driver’s own mechanical compliance. This combination produces a system with a gradual, predictable rolloff below the resonance frequency, excellent transient accuracy, and tolerant behaviour across a range of enclosure volumes.
The key design parameter for a sealed enclosure is the system Q factor, designated Qtc. A Qtc of approximately 0.707 — the Butterworth alignment — produces the flattest possible frequency response with no peak before rolloff. A slightly higher Qtc around 0.9 to 1.0 produces a small bass boost that can be useful in acoustically dry environments or when added warmth is desired. Lower Qtc values produce a flatter, more extended response that suits music with a wide dynamic range. The Qtc is determined by the combination of the driver’s parameters and the enclosure volume — a smaller box yields a higher Qtc, a larger box a lower one.
Construction follows the same principles as any enclosure fabrication: accurately calculated dimensions, tight and glued joints, internal caulking for airtight integrity, and panel bracing to prevent resonance. The simplicity of the sealed design means that construction quality is the primary variable — there are no ports to model or tune, and the driver’s well-defined T/S parameters make the outcome highly predictable from the design stage.
Installing the fabricated enclosure in the vehicle involves securing it so that it cannot shift during normal driving or hard braking. Unsecured enclosures become projectiles in a collision. Mounting brackets, L-channel straps bolted to the trunk floor, or purpose-built anchor systems are all appropriate depending on the enclosure size and vehicle type.
Connection of the subwoofer’s terminals to the amplifier’s speaker outputs follows standard speaker wiring practice, with polarity confirmed before the system is powered for the first time.