Port Length Help

[ctmaudi 10+ year member]

Got my box from pro-rabbit today. About to cut my ports and put them in, want to double check before I do so.

Box is 31.25L x 15H x 16D, with double baffle. Two 3in aeros.

So internal dimensions would be 29.75L x 13.5H x 13.75D, which would be 3.19cu before subs.

Using psp-inc calculator, do I use 3.19cu, or subtract the subs first (2x SA-10s = .24cu)?

Using 3.19cu for 33Hz it says 8.77" straight, 9.77" flared. The flares on these total 6in, the tube is 12in. Instructions say to cut 5in short, then add the flares, and it will be 1in longer. So I would cut each of them to 3.77", which would be 9.77" with the flares right?

Do I have to take in to account the double baffle any where besided figuring the internal depth?

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[Phoenix Risen 10+ year member]

you need to subtract sub displacement from your gross area.

im not sure if that calc takes into account port volume or not, maybe someone else can chime in on that one.

 

[CHEMMINS 10+ year member]

IIrc, it does not account for port displacement either.

 

[ctmaudi 10+ year member]

How can you figure port displacement, if you dont know how long the ports are going to be yet?

Also forgot about the 45s in the corners, that should only be about .01cu or so right? The subs are .24, so together .25.

At 2.93cu so far.

 

[Phoenix Risen 10+ year member]

thats what i was thinking but i couldnt actually remember.

to get it to where you actually want it tuned to youll just need to keep adjusting numbers till you get it as close as possible.

 

[Phoenix Risen 10+ year member]

im coming up with 11" with the flares. iirc the tubes with flares are 18'' long so you need to take 7" off of the tubes.

 

[ctmaudi 10+ year member]

It says to make sure there is at least the diamater of the port of space before the back wall, so that means a max length of 12.25in. I was thinking make them 12in, which would be about 32-33Hz at 2.9cu, or a little high if it doesnt take the port displacement in to account.

 

[ctmaudi 10+ year member]

im coming up with 11" with the flares. iirc the tubes with flares are 18'' long so you need to take 7" off of the tubes.

Thats with 2.9cu @ 33hz right?

 

[outsideshot23 10+ year member]

i always tape the flares up and test on tl till i like it then glue em

 

[BlactimusCrime 10+ year member]

you did this backwards. you needed to start with the desired net volume, calculate port length and volume, add the sub volumes to the net box volume and port volume, then design the box so it fits where you need it too.

Its hard to accurately figure tuning now bc when you lengthen the ports it reduces box volume which raises tuning.

 

[Phoenix Risen 10+ year member]

Thats with 2.9cu @ 33hz right?

yep.

and eddy i got plenty of practice with this dance with my 15 box since i only have so much space to work with.

 

[ctmaudi 10+ year member]

you did this backwards. you needed to start with the desired net volume, calculate port length and volume, add the sub volumes to the net box volume and port volume, then design the box so it fits where you need it too.
Its hard to accurately figure tuning now bc when you lengthen the ports it reduces box volume which raises tuning.

I got the box to save space, so he did it to the max dimensions I could fit, and he figured everything else out. He has been busy and not been online so I couldnt PM him to find out exactly where to cut them. He had said about 11in in a PM, but I wasnt sure if that was with the extra inch for the flare, or the 5in extra Im supposed to cut off.

Also, Im trying to figure out where I want the tuning to be. I want to get it as low as they can, which is why I was thinking of cutting them to 12in.

 

[BlactimusCrime 10+ year member]

yep.
and eddy i got plenty of practice with this dance with my 15 box since i only have so much space to work with.

its annoying as fawk. thats why i like my car lul

and BTW, you SPELLED MY FAWKING NAME WRONG!

 

[Phoenix Risen 10+ year member]

its annoying as fawk. thats why i like my car lul

yes it is, and some of us dont have the pleasure of driving a boat.

 

[ctmaudi 10+ year member]

I guess I missed a PM from him or forgot about it, found this one,

The graph I have with your enclosure setup with 34hz tuning would give you a good level output, but would have a peak around 38hz and play in car down to around 26hz. There is a roll off above 40hz and below 32hz out of car.As for the port, i would just set it up at 11.5" long. If you looked at the directions it should say to cut the port itself like 5" shorter to account for the faired ends.

So to get 34Hz with 11.5in flared length, that would be 2.6Cu.