Subsonic filter?...


New member
Username: Newbie47

Post Number: 6
Registered: May-04
I have a JBL bp.1200 amp, which does not have a built in subsonic filter. i have always used sealed boxes, but am thinking about porting. will playing subs in the ported box without the subsonic filter damage them, or will they sound bad? if so, where would i be able to maybe find a subsonic filter?

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 3111
Registered: Dec-03
the subsonic filter protects the sub in a ported box.
in a ported box, if the sub tries to produce frequencies below the tuned frequency, the sub loses it's damping, which is essentially the resistance to movement that prevents the sub from over-extending with the given amount of power (this is why we don't use most subs without a box.. ever.) so when you hit a lower note, the sub can over extend, and pow.. ya got an ate up sub.
go to
look at FMOD high-pass 20Hz filters.
that's what ya want.

Bronze Member
Username: Hiphopanonymous

Post Number: 57
Registered: Dec-03
but it seems that when speakers are in ported boxes, they don't have that much more excursion than speakers in sealed boxes. when my speakers are in sealed boxes, they move like hell. but in a ported box, they don't seem to move a lot, since there's not that much pressure behind it, but still produces massive air movement. i don't get it. the sub wouldn't be moving as much, but it will still be over-extended?

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 3140
Registered: Dec-03
the speaker doesn't move as much due to the damping from the enclosure.
when a sub is forced to play below the tuned port frequency, it can lose that damping, and over-extend.
when that happens, you get coil separation, or other nasty things.

It's when that damping is lost that you have problems.
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