Sealed Design

• A sealed enclosure provides an airtight space behind the subwoofer speaker itself. This sealed space acts as an acoustic damper through the pressure changes as the speaker cone itself moves back and forth. These pressure changes aid the return the cone to its rest position, resulting in improved accuracy of sound reproduction, but coming at the cost of low efficiency. Sealed designs require more amplifier power to match sound pressure levels with other designs. However, sealed designs can be effective in smaller enclosures.
  
  

Vented Design

• Vented cabinets introduce an opening for air and sound to escape. This means that more sound energy is projected forward from the speaker enclosure. The size of the vent has a bearing on the overall sound of the subwoofer, as it defines the frequencies that the enclosure reproduces, a process called tuning. Below this tuned frequency, a vented cabinet has little output. The volume of space behind a vented speaker must be larger as well, so low-frequency waves can develop.
  
  

Ported Design

• Porting a speaker is similar to venting, but rather than simply cutting a hole in a subwoofer enclosure, a tube called a port is added. The tube has a resonance that aids the tuning process. Baffles inside a speaker cabinet can be used in a similar manner. As with vented speakers, sound pressure level is more easily created than with sealed designs, so lower amp power is required, at the expense of accurate sound reproduction.
  
  

Other Designs

• Subwoofer cabinets are not limited to sealed, vented and ported, though these designs comprise the majority. Dipole subwoofers are essentially exposed speakers, front and back. Bandpass subs essentially combine the benefits of sealed and vented/ported design using a dual chamber, with a sealed sub enclosed in a ported chamber. This design is harder to develop accurately, but it is capable of excellent performance when done well.