Aero port question

[Sweety]

Hi guys, i'm about to build a new 1.77ft^3 enclosure for my JBL GT3 12" sub, I was going to do a slot port but I think it'll take up to much room in the trunk of my motor as it often gets filled when we go away with the caravan not only that I will be changing my motor later this year so have decided to do a tube ported enclosure instead. Now the ports we have are 2.8" on the inlet side & taper to 3" as the exit part. I'm not sure which measurement I should use to calculate the length needed to tune the enclosure to 34hz, i'm going to have to cut 2 of them to get the length needed which I think will be around 7" long & also so there is a small flare either end

Cheers for any help

[Sweety]

When I cut both port's to get the length needed it'll have an opening of 3" either end & taper to 2.8-2.9" in the middle of the port, would that work ok

[Joe X]

Even a non tapered 3" port would cause very noticeable port noise, get some 4" raw PVC pipe to do this.

[Sweety]

Thanks for your help Joe X, my other option is to maybe build this enclosure I found on here http://www.stevemeadedesigns.com/board/topic/149509-box-design-help1st-time-box-builder/ only thing I'm not sure on is the port length as I normally do a double baffle on the front, would removing 3/4" off the end of the port wall to compensate for the extra added by a double baffle & add 45' bracing to all of the corners work? I was looking for a 34-35hz tuning point really as it's for my every day motor but a little lower at 33hz shouldn't make that much difference would it ?

By the way many thanks for your help with the plans I put up a while ago on a 3.5ft^3 enclosure for my Son, got it built & it sounds nice & he's really pleased with it

[Sweety]

My other option is to make up 2x 2.5' ports from some pvc tubing I have & put a 90' bend in them & flare out the ends. Sadly 4" pipe is hard to get hold of here in the UK & it's quite costly

[Jessica]

what the heck is under your toilets?

[Sweety]

what the heck is under your toilets?

It's not cheap to buy over here & would have to buy a 90' bend the fit it in the box, reason I mentioned the 2.5" pipe is I have about 6ft of it plus a couple of 90' bends so it would cost me nothing to use it. If the 2.5" pipe is ok to use am I right in thinking each port would need to be 11" long to tune a 1.77ft^3 enclosure to 34hz ?

[Jessica]

you would be on the very very low end of port area, You might be happy with it. your numbers are ok otherwise. Just make sure to take into account the volume of the port when you design the box.

since its only a 250 watt sub you probably wont have much port noise.

[Sweety]

The JBL GT3 12 is 225w rms & is powered by a 220w rms JBL amp, I could tune a bit lower say 33hz then the ports would be a bit longer or even make the box a little bit bigger if that would help but sadly I have no idea how much volume 2x 2.5" ports at around 11" long would take up 0.25ft^3 ?

[Jessica]

go find a area of circle calculaor on the internet. plug in your port diameter take the result X length of port. take that time number of ports. if you have any port that will be external dont count that.

edit:

use the OD of the pvc