confused on instruction manual port dimensions (JBL GTO804)

[dwcopple 10+ year member]

http://www.jbl.com/resources/Brands/jbl/Products/ProductRelatedDocuments/en-US/BoxesandParameters/GTO%20804D.pdf

they show in a .6 cu ft box a 2.5" port 8-13/16" long being tuned at 40Hz. All of the calculators I have typed this into say this is IMPOSSIBLE. What should I do if I want to tune the port to 32-35Hz? The box will be 3/4 13 ply birch and trying to stay around .6-.67 cu ft.

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[dwcopple 10+ year member]

anyone?

 

[sbelew 10+ year member]

It is possible to do, depending on; How much power? Amp? Your max enclosure dimensions? These are great little subs. I've got two I'm experimenting with.

 

[dwcopple 10+ year member]

I want to keep it as small as possible as I use my trunk often. I will be sending it about 200-250w RMS @ 4ohms. I was thinking something like 14x12x10. The port is gonna be the issue. I want to use PVC and not slotted. I'd like to tune around 32-35Hz.

 

[dwcopple 10+ year member]

the point I am trying to make is that JBL's (Harmon's) port of 2.5" x 8 13/16"'s in a .6 cu ft box cannot possible be tuned for 40Hz. The calculators I use put it at 11.9-12.3 inches in length.

 

[quackhead 10+ year member]

the point I am trying to make is that JBL's (Harmon's) port of 2.5" x 8 13/16"'s in a .6 cu ft box cannot possible be tuned for 40Hz. The calculators I use put it at 11.9-12.3 inches in length.

yup...just a quick run through Precision Port's calculator put it at 12" long for a 2.5" straight port.

 

[dwcopple 10+ year member]

so would a 2" port from PVC be sufficient? It is the only size I'd be able to fit in the box to be able to go as low as I want.

 

[sbelew 10+ year member]

a 2" port would make your port velocity a bit higher than I'd care for. You would need a 2.5" port 14.75" long in .65 cu ft to get a 35 Hz tune. If you could get one of your dimensions to accomodate internal 17.25", (14.75" plus 2.5" for end clearance). a 32 HZ tune would make your port length almost 18". Go take some max measurements that you could live with and post back.

 

[dwcopple 10+ year member]

well, lengthening the box will be fine as long as that reduces its depth. I'd like to keep height at 12" for ease of cutting. I plan on buying a 2'x4' sheet for this box. if I cut it lengthwise, I can make all of my 12" pieces LOL.

 

[dwcopple 10+ year member]

the calculator on the12volt says a 19x12x7.5 @.75 (which the wood is more like .68 or .7) makes a .638 cu ft box

 

[sbelew 10+ year member]

ok, let me take a look at this for a bit and I'll get back with you.

 

[dwcopple 10+ year member]

cool, thx

 

[sbelew 10+ year member]

Ok, here is what I would go with. 12"h x 18"w x9.5"d 2.5" diameter port, 14.75" long. Sub firing to the rear, port firing to the side. Will give you a tuning of 33Hz and keep your port velocity in acceptable range. That is approximately .70 cu ft after port and sub displacement. It models better in a slightly larger enclosure then JBL recommends. Just make sure you have 2.5" of clearance (or more) on the outside by the port, and be try to round off the inside edge of the port at the outside of the box. This is assuming you are going to use PVC for your port.

 

[dwcopple 10+ year member]

I can probably swing that. where did you get sub displacement specs? the .58 they have listed can't be right can it? maybe I just don't know how to figure it. Also, does it matter where on the side I place the port. Probably closer to the back panel opposite the sub mount huh? top bottom or middle on that side?

 

[sbelew 10+ year member]

JBL doesn't publish the displacement spec and when I called them, they couldn't even seem to tell me, so the general rule of thumb for 8" sub displacement is .03 cu ft.

Just try to keep the port 2.5" away from the back, top and bottom wall if at all possible which with these box specs shouldn't be a problem at all. If you need a sketch, let me know.