It is a Highpass Crossover that is set at a very low freq. It allows you to filter out the lowested freq. That way you can control the subs better and have less wasted power (on the sub-25hz freq).
"Infrasonic Filter: The infrasonic (or subsonic) filter is a 12dB/octave high-pass filter designed to prevent frequencies below 28 Hz from being applied to the subwoofer from the amplifier. When activated, the infrasonic filter eliminates super-low frequencies that are potentially damaging to the subwoofer and require tremendous amplifier power to reproduce. Consequently, the filter improves performance and power handling, especially with vented enclosures."
so here's my next question then. i installed my alt, did the big3, and put the RF t3000.1bd in, hooked it up to my 9515 @ 1ohm. My deck is a pio 5800mp. The bass is hitting the lower notes but cutting off on some of the higher notes, so where my comps should pick up, the bass rolls off....could this be due to the infrasonic filter? if not, is there a setting on my deck that i dont' knwo about? it's been bothering me for a couple of days now...i just dont' wanna turn off that infrasonic filter....dont' want my sub to bottom out.
Also remember not to set the HU and amp cross over to the same settings, this increases the amount of cutoff the effected frequencies will see. Use one or the other. IE- If you have your LP on your HU set to 80hz with a rolloff at 12db/octave and you have your amp Lp set to 80hz with a rolloff of 12db/octave, the total cutoff will be 24db/ocatave making it extremely peaky at the centered frequency. This is why sometimes I can't stand when the HU crossovers aren't defeatable... Polo.
B!tch of it is some of these HU you cannot shut the crossover off. In order to negate one of them you need to go to the amp and turn your LP as far as it will go around 200hz and just use the HU's crossover. Polo..
^^Yep. I have a deh-480mp. You either have the sub on with the filter or have the sub off competely. I turn my amp to 120Hz and use the H/U as the cutoff at 80Hz.