The simplest of all loudspeaker designs, the sealed enclosure system consists of a driver mounted on one side of a sealed box. The sealed enclosure system is characterised by excellent transient response, good low frequency power handling, smaller box size and lower sensitivity to misaligned parameters when compared to other alignments. However, sealed enclosure systems tend to suffer from higher cutoff points and lower sensitivity than the other low frequency systems.

There are two types of sealed enclosure systems: the infinite baffle (IB) system and the air suspension (AS) system. The IB system normally uses a large enclosure where the compliance (or "springiness") of the air within the enclosure is greater than the compliance of the driver suspension. The AS system normally uses a small enclosure where the compliance of the air within the enclosure is less than the compliance of the driver's suspension by a factor of 3 or more.

Sealed enclosure systems are probably the best starting point for the beginner DIYer because of the relative ease in achieving the desired frequency response. They are usually the subwoofer system of choice for audiophiles because of their excellent transient response (i.e. no boomy sound) characteristics when designed and built properly.

Designing a Sealed System
The "Qtc" of a sealed subwoofer system is a measure of the system's tendency to resonate at its resonance frequency. Recommended values for Qtc are from 0.6 to 1.0, as transient response degrades with higher Qtc values. A Qtc of 0.7 will usually give pretty good results, but you can use a higher figure if the subwoofer has a low resonant frequency (<20 Hz) or if it's being designed for car use. The diagrams below shows how to use Hornresp to design a sealed subwoofer system. Basically you enter some starting values on the "Parameters" screen and then adjust the value for Vrc (the rear box volume) until the combination of Qtc and response curve is to your liking.

Stuffing
You can decrease the volume required for a particular sealed alignment by using a stuffing material such as fiberglass, wool, or polyfill in the cabinet. Reductions of 25% to 24% in volume requirements are possible. Make sure that you add the volume displaced by the driver and bracing to arrive at a final enclosure volume. If you plan to stuff the enclosure, use 0.75*Vb as the net volume for the enclosure. When you add stuffing to the enclosure, the resonance frequency should decrease. Continue adding stuffing until the resonance frequency stops decreasing.

Sealed System projects on the internet

• Dave Platt's speaker project page
• Bill Eckle's NHT 1259 Projects
• Bill Eckle's Multimedia System
• Sean Collins NHT 1259 Subwoofers
• Zachary Paisley's "Project 'Cube 2007"
• Yuval's active subwoofer