That is an option that may be a little on the costly side. Any idea how large of a sealed box I would need to get down to 20hz?
How would the JBL compare to one of the Part Express Titanic 15" drivers?
Size of the box has to be the correct size for the driver. It will not affect low frequency extension, just Q.
Now a sealed sub will not reach 20 Hz without an expensive driver, a lot of Eq and a big amp.
On your budget to get to 20 Hz at any useful spl. a ported enclosure is mandatory.
A sealed enclosure will roll off at 12 db per octave somewhere around an octave above the driver Fs and often higher. For a lot of drivers Eq has to start around 60 Hz and even for the best drivers Eq will start at around 40 Hz almost always above.
A sealed sub driver will have a low Fs high Qts and a huge xmax and be able to handle lots of power. This usually results of a driver of low sensitivity and efficiency. You have to equalize below roll off at 12 db per octave. For every 3db boost at a given frequency power demands are doubled. So very high power amps are mandatory.
Now the situation is very different for a ported enclosure.
The extension of a ported enclosure can extend to very close to the Fs of the driver, usually a little above, sometimes at Fs and occasionally a little below. But basically Fs determines the F3 under most ported models.
Since most of the output of the deep bass comes from the port and not the driver, linear xmax does not have to be as large. Qts of around 0.3 to 0.35 is usually optimal. Drivers for this application can be made higher. Sensitivity and efficiency are higher, and Eq can not be used as below F3 the driver uncouples. So if Eq is used you get large cone excursions, but no sound output. So you can use a much smaller cheaper amp for the same spl.
So to get to 20 Hz a ported design is budget friendly, a sealed design is not.
The same comments apply to ported and passive radiator tuned enclosures.
