Why do I need a subsonic filter?



The reason you need a subsonic filter is a really interesting phenomenon.

A subwoofer cone moves in and out very, very rapidly. 40hz is 40 "in and out"s per second!
A port is an air-resistive device... it slows the air from exiting and entering the box.
With the sub moving rapidly, and pressurizing and depressurizing the air inside the box so rapidly, an air mass ends up getting pushed and pulled in and out of the port - but moving so fast, the mass doesn't have time to get to the end of the port before it gets pulled back in again, if you picture a "cloud" of air getting sucked in and out of the port.

(the reason it does this is to "Recycle" the otherwise wasted sound pressure energy off the back of the cone - which if not for that slight resistance and delay, would just cancel out the sound energy outside the box, rather than helping increase the dB levels).

The lower the note you play, the fewer cycles per second (20hz is only 20 "ins and outs" per second, for example, compared to 40hz, or 80hz).

There's a point, depending on your port "tuning frequency", where the cone slows down enough that it can't support that moving mass of air anymore...
...and at that point, the port just acts like a gaping hole in the box... making the box not act like a box at all. At that point, there's no air pressure inside the box helping control cone excursion - and it's easy to bottom the sub out with just a fraction of your wattage!
Blown sub... overexcursion damage.

But the good news is, you might not need one. If you are tuned at 30hz or LOWER (EDIT: oops! I said "higher" originally!), you won't need one. I even had one tuned to 32hz on my old car, and I beat on that one all the time, no issues. So don't let it keep you up at night, breaking out in sweats...

It's not instant, below your tuning frequency... I usually set my subsonic filter about 1/2 octave below my tuning frequency.

...but if you are using those F-mods, the slope isn't exactly steep - I'd get one right near my tuning frequency, as close as I could. And I'd seriously stack like two of them in-line, at least.


© Copyright WolfWare, Ltd.. All Rights Reserved.