The two first factors to consider when deciding on what type of sub box to buy or build are:
- Do I want the best possible sound or the loudest, lowest bass notes, and
- How much amplifier power do I have, and how close to the speaker's max is it?
Sound Pressure Level vs. Sound Quality Designs
SPL designs typically use a vented, or ported, enclosure. By tuning the port to the resonant frequencies of the speaker cabinet and subwoofer driver, low frequency response is boosted and optimized at what is known as the "port tuning frequency".
SQ Designs typically use a sealed subwoofer box enclosure type. This is because the sealed sub box provides damping - meaning, the sealed design helps control the motion of the speaker cone, causing its movement to follow the input signal more closely. The result is more accurate bass - the tradeoff is less efficiency.
The bandpass subwoofer enclosure attempts to enhance bass response and cancel out higher frequencies using a combination of port tuning and a driver that fires into another chamber rather than into the listener's airspace. They are more complicated, but offer distinct advantages in certain situations if well designed. These sub boxes offer the lowest bass response - and the most nonlinear. Amazing if done right - but horrible if they aren't built to tight specifications.