NEw BOX for the Q18

swimmerdude · Post by **swimmerdude** » Fri Mar 16, 2007 5:46 am

you are going to want the 6 3" ports if not more!!

Bwcad1 · Post by **Bwcad1** » Fri Mar 16, 2007 6:21 am

u want to use less larger diameter ports. not more smaller diameter ports. the more ports of smaller diameter increase the surface area which increases drag. with that many small ports your loosing lots of efficiency your better off going with a slot port. even if you have the material i highly advise against it. do it once do it right. get 2 six inch ports. also don't forget the larger ports also increase tuning. make sure you have the CF to make up for it.

Post by **GlassWolf** » Fri Mar 16, 2007 2:53 pm

told ya so

note also that putting bends in the port tubes will affect port velocity.

subster87 · Post by **subster87** » Fri Mar 16, 2007 7:12 pm

ok well after building the front side of the box i realized 6 3 inch ports is impractical any way... i dont know that i have room for six inch ports with my design, I would rather not have that big of a whole in the box anyway, mainly for the fear of things ending up in the box, I am going with a false floor and will be using half the top of the box for a trunk if something falls in ill be freaking pissed!!! ok slot port. I suck with winsd dont have the patience could someone tell me the port size for an exact 8.25 internal volume box?

subster87 · Post by **subster87** » Fri Mar 16, 2007 8:34 pm

ok so i used a formula to create a slot port someone tell me if this sounds right... ok the port with have a width of 3 inches a heighth of 18.5 and a a lenght of 19.4 to tune to 30hz? Fi says not to tune this sub over 32 they want it tuned at 28 which is so so low i went right in the middle am i close to right on this one?

Fishy · Post by **Fishy** » Fri Mar 16, 2007 9:21 pm

I'm bored. Lemme grab the TS parameters off the Fi site and I'll plug things into Win ISD. 8.25 ft^3 right? and like 1000 watts? Just out of curiosity how much clearance will you have between the port opening and the inside of your trunk?

Fishy · Post by **Fishy** » Fri Mar 16, 2007 10:23 pm

OK. Assuming you're using like 1/2" mdf to build your port and you're not using an enclosure wall as part of the port you get a net internal volume of ~7.4 ft^3 with the sub inverted. According to WinISD this gives you an Fb of ~28.4 Hz.

I did a port velocity plot and things don't look good below about 35 Hz with 1000 watts. To avoid chuffing you want to keep the port velocity below 5% of the speed of sound or about 17 m/s.

http://home.comcast.net/~guppyrig/wsb/m ... te1094.JPG

With 500 watts(a more realistic value unless you're playing test tones) you're fine.

http://home.comcast.net/~guppyrig/wsb/m ... te1095.JPG

Did you ever get any chuffing with the 4.5 ft^3 box?

Oh and Glass is right(as usual :P) about the Q18 being designed for sealed enclosures. An EBP of 37.8 is pretty low for vented use.

Fishy · Post by **Fishy** » Fri Mar 16, 2007 10:57 pm

Are you using a subsonic filter with this setup? Out of curiosity I added a 12 dB/octave butterworth highpass @ 20 Hz and the port velocity plot looks a lot better even at 1000 watts of input power:

http://home.comcast.net/~guppyrig/wsb/m ... te1096.JPG

subster87 · Post by **subster87** » Sat Mar 17, 2007 6:32 am

no subsonic filter the amp produces form 20hz up i beleive its built in dont quote me on that and i am using one of the walls for the port. And yes my ports were horrible on the last box hence the rebuild lol. Thanks alot fishy for your help by the way. And i would have 10 inches of port clearance at the very least depending which side of the box i put the port on i have to go to work be back on tonight thanks alot guys!!! saving my hard work on my box by fixing my F(*&ed up port

Fishy · Post by **Fishy** » Sat Mar 17, 2007 9:33 am

If you're using one of the walls as part of the port then you'll be getting a slightly lower tuning frequency for a couple reasons. One, you'll have a little more net volume and two, using a wall changes the end correction of the internal port opening, which is just a fancy way of saying that the collumn of air vibrating inside thinks its in a longer port.

http://mobile.jlaudio.com/support_pages.php?page_id=165

In your case(h=3", see JL's diagram and simple formula) the effective length, Lv, is increased by 1.5".

I punched the revised numbers in WinISD and it yields a tuning frequency of 27.4 Hz with a 7.46 ft^3 net volume. If you're shooting for a "magical" 28Hz tuning, an 18 1/4" internal length is about dead on in WinISD.

JL also doesn't recommend "thin" slot ports(w to h ratio greater than 9:1). You're at about 6:1 with your port so you should be good in that regard.