Oldschool Fosgate XLC 18; thoughts on enclosure please

[beady]

Hi all, not new to audio stuff, but it’s been a good while. My son wanted a sub, so I am back dabbling. Got him some Kicker C12a’s and picked up a 4-ohm 18” Rockford Fosgate RFP-2418 on a whim for me. 

I measured the XLC parameters with a parts express DATS V2, and tried modeling a largish 4-th order bandpass for it (going in an oldschool OBS suburban, so have room). The weird thing is that it doesn’t model wider, deeper, or more efficient on the whole than a plain ported enclosure of similar volume.  And the 4th order I had for the Kickers waaay back in the day models a decent gain in efficiency while still going nice and low.  

I guess I’d rather lean towards SQ than SPL at this point (whatever that means for this sun, like I said I bought it on a whim without researching) as I’m rarely ever going to run it full tilt. We have a small farm and 3 kids, so quite often there’s a ton of crap in the back of the suburban, hay, feed, lumber, etc., which is partly why I want to go 4th order (physical protection). Also, I had a 4th order with pair of Kicker C12’s back in college which sounded killer. 

Going to run the 4-ohm 18 off one of several amps, Punch 30HD (would be cool to shock ppl with what “30 watts” can do), Punch 75HD, Punch 250.2, or maybe MTX 3002. So can either lean towards super efficient, or less so, and still be OK.

I’m attaching pics of the measurements, and the simulation I’m thinking of going with graphed with sealed and ported enclosures of about the same size for comparison.

Oh yeah, I like deep bass, electronica, old school bass like dj magic Mike, techmasyer peb, etc. Would also like to have good kick drum kick if possible, is why I’m thinking larger, lower efficiency, and wider pass band may be where I need to be.  Constructive input is welcomed. 

Thanks,

Mike

[Joe X]

These are the official params for your sub:

 

 

From what you measured it looks like your sub is stiff either because of not getting broken in or due to possibly aging.

 

I came with 2 specs, a 4th order bandpass ( in blue ) and a ported ( in orange )

 

 

Specs for each are:

 

BP4:      4, 6@47Hz 50 in2

 

Ported:   6@30Hz 30 in2

 

Ported would be the enclosure of choice delivering a nice relatively flat performance with low group delay, good for sound quality.

 

The bandpass is just too big with just marginal gains in output.

 

My impression: I wouldn't bother to build a big box for a 250W woofer, also it is an old sub and  might fail at any time due to that. You should keep power low to help it last more.

 

The sub is highly efficient as is the case with old subs so you will get relatively loud on very low watts at the price of a giant enclosure.

 

What I would do is build a ported enclosure but with a modern sub port area of about 90-100 square inches so that you can upgrade later to a high power sub that make it worth carrying around such large enclosure.

 

 

Edited February 11 by Joe X

[beady]

Thanks Joe.  Why does the frequency response for the 4th stay so flat, so low?  Is cabin gain added in that curve?

 

Since the sub models well ported, what about a 6th order?  

 

I probably won’t run the box forever, I’ve just never had a 15 or 18, so just wanted to try it.  Could be the shop sub in a corner or HT sub in the room over the garage eventually and I could put a more modest sub in.  
 

I have an old EQX I could use to bring down some peaks…

 

Mike

[Joe X]

Yes exactly, cabin gain is added to those curves, it is a generic form so do not expect exceptional accuracy, still, adding a generic cabin gain results in a better representation of what you will be getting than adding nothing. Anything below 20Hz in those plots you can ignore.

 

6th order would be much larger than ported and 4th bandpass even if that happened to work out it would not be worth to build, modern subs work in so much lower airspace and yet you don't see everyday someone building a 6th order with 18 subs.

 

Now you couldn't use as HT box a car audio box, you would need to re tune the box to the lower 20s to work as HT.