View Full Version : 4" Plastic/PVC port help - The port's tapered?

12-15-2009, 08:46 AM
Hey guys. I'm not new to car audio or subs, but I am new to custom box design. I've got two JBL GT5-12's, and as their spec sheet explains my box is nearly exactly 1.75ft^3.

The problem I have is that the ports that I've bought are too short, and also tapered. They start at 100mm, then taper down to 90mm. The port is also only 200mm long, where as the spec sheet says I need a 254mm port for a tuned frequency of 34Hz.

I planned to take the existing port, and heat up and stretch some PVC pipe to the end of it, then glue it on. Basically, having a port that's 200mm long tapered, then a constant 90mm (diameter) PVC pipe that's around 54mm long attached to the end of it. Would this taper have any bad effect to the bass produced, or could it screw my subs? And also, as the spec sheet for the JBL subs recommends a 254mm port at 102mm diameter, and mine will be 254mm at 100mm down to 90mm diameter - should I make it longer than 254mm?

JBL GT5-12 spec sheet: http://manuals.harman.com/JBL/CAR/Boxes%20and%20Parameters/GT512Tech_050108.pdf

To give you a better idea, here's the setup:

Lastly, for an off-topic question. If the sub spec sheet recommends 1.75ft^3, can this be in any dimensions? My box will be fairly narrow and tall to fit into my car (RAV4 3-Door), would this be ok?

Cheers :cool:

12-15-2009, 09:49 PM
Does anyone know anything about this? I'm building the box today as it's my only day off work for the week, and need to know whether you guys think this port will work ok :confused:

12-15-2009, 10:28 PM
Keep Bumpin', fellow RAVer! 2dr 4wd RAVs are great offroaders (especially 98-00 with limited slip). Please excuse my "Yankee" imperial units.

If you're using 2 12's, you can use a common chamber, or separate.

The box design is 1.75 cubic feet for 1 sub. If you're doing a common chamber, that's 3.5 cf.

Long story short, is that you'll need 2 of those tapered ports. If it's in common chamber, they will tune the box to 40 Hz without extension, or 34 Hz with the 100 mm extensions. If it's separate chambers, put one in each. Same tuning and lengths. I personally wouldn't spend the extra time extending it... but I'm lazy.

To test out some possibilities, you can plug into Port Calculator (http://www.carstereo.com/help/Articles.cfm?id=31), at the bottom of the page. Use D = 3.75 (since it's tapered), V = 1.75 (separate) or 3.5 (common), and adjust the Tuning Frequency.

As far as box design, just make it fit. You don't have much space back there. Make subs face the back seats (with an inch of clearance). The back door rattles horrendously if you fire rearward. I'd fire the ports up, if you can. If you can't, make them face the sides (one per side) and keep them high. You'll damage the rear speakers if they fire directly into them.

Let me know if you need help with box design. I'm drive a 4 door, but it's similar.

12-15-2009, 11:06 PM
I've taken my RAV4 down to the beach a few times, even on the dunes, and it goes pretty well for an AWD. I was racing against a Jeep Wrangler and keeping up on the dunes lol.

Alright, you've helped me think that it'll work. I had planned to make two separate chambers. The thing is that the only way I can get my subs to fit is by pointing both the subs and ports upwards, like this:


Here's my box design (Fb = 34Hz I hope):


If there's anything that you can see wrong with my box design, let me know. That's the biggest depth and width that I can fit into my 'huge' car (it'll take up the entire space behind my two back seats). The only adjustment that I can make is to the height. Also, I'm using 0.62" (16mm) MDF to build the box, and have taken this into account with the dimensions.

If it's in common chamber, they will tune the box to 40 Hz without extension, or 34 Hz with the 100 mm extensions.

Do you mean 100mm extensions on each port? Or 100mm of overall extension? I plan to have 2 ports, one per chamber.

12-15-2009, 11:09 PM
yes both ports
good looking out tim hellofguy ;)

12-15-2009, 11:16 PM
Thanks guys, I'm about to start building my box, I'll let you know how the build goes.

12-15-2009, 11:20 PM
My only problem is the 310mm (where the subs go) is almost exactly 12"... and that won't fit, G-money.

That's why I suggested subs forward. It'll fit that way, as long as the subs are near the top... too low, and the magnets will scrape the bottom of the box.