Quote:
Originally Posted by nani
What is the difference between an open sub box and a sealed one?
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Open sub box, meaning free-air mount? Or open sub box meaning vented/ported?
Subs work based off air pressure - a magnet energizes a
coil, moving the
cone, changing the air pressure in front and behind that cone, making sound. The speaker is brought back to rest by the "
suspension", meaning the rubber surround. Still with me?
Most speakers have more powerful voice coils than suspension, so they need the surrounding air to help hold the cone in the magnetic gap for optimum performance. Almost there...
Free air (or
infinite baffle, as it's sometimes called) is where you mount the speaker with no enclosure, and therefore no suspension help from the air. This is fine for small drivers (tweeters, midrange, small subs) but doesn't work well when you want to play low. Don't free-air mount your sub.
A
sealed box uses the air behind the speaker as a spring, making it less efficient, but allowing a smaller amount of air to do more "suspending"... i.e. smaller box size. Also, its kind of foolproof (just about any shape works, as long as the volume of air in the box is somewhere near correct).
A
ported (or
vented) box works on the principle that the speaker makes sound in front
and behind the cone, so why waste the sound energy behind the cone by muffling it in a sealed space? You still need the air to act as suspension, though, so what you do is make a small hole that lets a specific volume of air work as suspension, while still allowing certain frequencies of sound waves behind the speaker to exit the box - the center of the frequency range is the
tuning frequency of the box. Naturally, these boxes are more complicated, but are
ALWAYS more efficient (more volume with less power) and almost always
sound better.
Bandpass boxes are just ported boxes with another tuned/ported enclosure attached to the front of the speaker. More efficient still, but not as acoustically pleasing. dB drag pros use bandpass. SQ pros use vented/ported.
Edit: Here's a pic of my last ported enclosure for my SQ system in the 240. Soundstream Reference 10" (from back when Reference meant Audiophile) and a tiny Class A/AB hybrid (A on the hot side) Soundstream amp... note the sanded smooth ABS pipe, and shape of the box (wide, so the pipe/port has adequate clearance on the inside).
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