This was originally posted by me in response to a query in the "monster beginners guide to bass"
Located at http://www.caraudioaustralia.com/forums/in...showtopic=58374

A subsonic filter is basically a High Pass Filter.
That is, whatever you set it at, only frequencies ABOVE that setting go through. Anything BELOW that setting is "filtered out".
Only frequencies HIGHER than your setting pass, hence HIGH PASS.

There are two situations a subsonic filter is used

One, in a sealed box. The human ear cannot detect sound below around 20hz. You MAY be able to feel it, though.
The lower the frequency, the more excursion your subwoofer needs to have in order to play it loudly or accurately. The subsonic frequencies can potentially damage your subwoofer because they make it expend a lot of energy, and travel right to the limits of its excursion. And for what? to make sound that you can't hear anyway. So for safety, we want to cut those frequencies out. You'd set your subsonic filter to 20-25hz, to stop those low, unheard, potentially damaging frequencies being played.

Not that you really have to worry about them, VERY FEW songs are recorded with these frequencies. They are either usually filtered out at the studio, or (in the case of live music) there aren't many instruments that can play subsonic frequencies loudly enough to cause damage, or microphones capable of picking it up. Synthesised music (dance, r'n'b) etc, are the main dangers.

Now the second scenario, in a ported box.

In a ported box, you tune the port to a certain frequency. I wont go into what that means right here, but basically if your sub tries to play much BELOW the ports frequency, it will need massive excursion to play that note loudly. Say your ports set to 45hz, if you try and play lower than that (30hz) your woofers going to "unload"...that is, its power handling goes down, the excursion goes up, and damage can occur. The reasons why are complex, but for now, thats a basic description of what happens.

So, if you were running a ported setup, you might want to set your subsonic filter to something near the ports tuned frequency, so that way, if there's music below the ports frequency, it gets filtered out. This protects your woofer.

Keep in mind that the subsonic filter is NOT a cut-off. It has a "slope"...that is, whatever frequency you set it at will be muted...it cuts off more and more sound from than point down. So setting it near your ports frequency wont destroy the music below it, just mute it to a level that shouldnt be damaging.