I have an 8" JBL sub with an Alpine amp. Alpine head unit CDA 9833. Head unit x-over for sub is set at 120Hz with a 6db/oct slope.

When a song comes on with very low bass, I get distortion. Is there a way to cut out the very low frequencies say at arount 40Hz to eliminate the distortion?

 

Subs typically are set low passed around 80Hz with an 18 or 24 dB slope.

If the amp has a subsonic filter then that is used to remove low frequency, 40 hz is not low for a sub.

I suspect you have some setting wrong or probably a poor box.

 

I have the Alpine MPR-M450 momo amp.
don't know if it has a subsonic filter.
I have it set at 120hz because my door speakers are crossed over at 120hz, they are small and can't handle anything lower. If i set the sub at 80hz, i will be missing everything between 80 and 120.

I bulit the box myself and although it is well built structurally, it is possible that the dimensions are not ideal for my sub.

 

It's supposed to have a 15hz subsonic filter which could help but not by much.

Is the box sealed or ported or? What are the dimensions of it, and the port (length and width of the opening, and how long the port is)

 

Yes, some info on the box would help.

 

The box is ported, i will post pics and dimensions tomorrow.

 

width 13.5''
depth 10''
height 11''

 

Inner diameter of the port, and it's length?

The width of the box looks smaller then 23.5" to me..

 

the port is 4'' long, i cannot remember the inner diameter

you're right, sorry, its 13.5'' in width

 

Whats the thickness of the wood, 3/4" MDF or 5/8 or 1/2?