get a subsonic filter on there there, taking out all frequencies below 28Hz.

thanx m8

it will look like its flapping cos 20Hz is such a low frequency that the subs moves slowly to produce the note.

20Hz = 20 cycles per second, so not as quick as say, 60 Hz, which is 60 cycles per second.

As for it sounding poo, it will, unless you use some genious(sp) engineered enclosure.

Rockford's demo pick up truck can go as low as 10Hz, something to do with the fact the the opening for the face of the enclosure is smaller in surface area than the total surface area of all the subs put together.

I'll see if i can find a pic of it somewhere.

your ears cant hear below 20 hz so um...what's the problem?

You got it in the right box? Most sealed subs can go down to 20Hz but a LOT tend to struggle as this is the lowest of bass notes. My 15" type R used to flap arpound 20Hz, i never really found out what caused it but I think i had too much power to it.
To be honest i wouldnt be bothed about a sub playing down to 20Hz as that note is kind of useless, maybe 40-30Hz is low enough? Eq it down a bit at 20Hz and see if that helps?:)

its a rockford fossgate rfr 3115 it can go as low as 20hrz its not the car if i open the boot and look at it the sub is moving a lot but there ant much bass
if u blow a raspberry it sounds like that

what sub is it? u made sure it can go low enough?

could be vibration from the car it self.

sound proof your car to get the best from your sub.

at the moment you could be losing out on a few dbs

i have a 15" 2000w max 1000w rms sub its running off a 1000w rms amp

when it hits the realy low Hrz (30-20) it seems to sound like its flaping

anyone got an idea