Port Calcultaion Question

[b_knockwood]

This isn't a polk question, so i apologize for posting this here. I didn't know where else to turn to, and would greatly appreciate your help.

I am designing a ported custom enclosure for 2 Fusion PowerPlants fpw-1200's.

Here are the specs for 2 subs:
Internal volume = 3.46 ft3
Port Dimensions = (2 Ports) 4" x 16.8"
Port Frequency = 30.1 (in car)

The port length at 16.8"/sub is a bit long. I was wondering how long would i make the ports if I was to put in 4 - 4" ports?

Thanks

B

[PoweredByDodge]

you can do ONE round port with a 4 inch diameter, with a length of 7.88 inches


this was figured out using the following calculator...

http://www.loudspeakers101.com/VentCal1.htm

which is available as a link of my half ass put up website -- www.thetruthisozthere.web.com under "the black book". (whole site as a whole still needs a lotta work though)

i plugged in your box volume, then your tuning frequency, then your number of ports, then your port diameter, and it told me about 18 inches... which is a little longer than 16.8 but its close enough to say its an accurate calculator. 10% error isn't anything huge.

anyway... so then i plugged in HALF of your box volume (as if it were just one sub in a 1.75 cubic foot box with a 4 inch round port) and the tuning frequency and it told me 18 inches again... so now i understand why you want two ports... because you got specs for one sub and just doubled them... doube the box size, double the number of specified ports.

however, by using just one port, in a box that's made for two subs, with twice the internal volume (hence for two subs) you can make just one port -- with the same 4" diameter... and it only has to be 7.88 inches long.

i dont know why this is this way... but it for some reason sort of i think makes sense. the bigger the port is, the less resistance it gives to air evacuation, thus the longer it must be to create more resistance to evacuation. if you have two subs, you can either do two ports of equal resistance or one of half the resistance. double check this with some port box builder guys to be sure.

thom the polk installer dude will probably know better tho - if he doesn't respond to the post, click on his tag and email him.

[b_knockwood]

Thanks a lot dodge for your input. I've also entered those figures in a port length calculator from http://www.the12volt.com/caraudio/boxcalcs.asp , for some reason it gave me a total different port length. ??? something like 10.8" tuned to 30.1 hz.

can you give it a shot and see what you think? Which one should i go with?

Thanks

B

[PoweredByDodge]

ok here's what i'm gonna say -- "GO WITH HALF"...


i tried the 12 volts calculator out and for the 1.73 cubic foot box (half) it gave me a length of twice that 10 figure that you got -- like 21.6

now with the calculator i was using, for the 3.46 box i got like 16 and for the 1.73 box i got like just under 8 (7.88).


in both cases, the 1.73 box is roughly half of the 3.46 box for port length.

so i would say refer back to your original figure of needing a 16.8" port for each sub, and then cut that in half -- which would give you 8.4" for two subs.

lacking any further knowledge, if i was stuck building the thing myself tonight, i'd just do it at 8.4" and call it a day.

u really might wanna get ahold of thom though first.

[b_knockwood]

I talked to the tech dudes, and they said that there would be to much port flutter if i just go with 1 port.

I'm actually thinking of putting 4 of these subs in 1 chamber with 3 ports. I ran the specs on this setup, and the port length of (3) 4" ports would be 12" tuned to 30.1 hz. I took the box specs, less speaker displacemnt and a center brace for my calculations.

Just wondering what are the thoughts of having all 4 of these subs in 1 chamber?

[PoweredByDodge]

I dunno -- i hate shared chambers -- i feel that in general it diminishes the individual performance of any one sub. however, i also see the ease of install.

that's something you're gonna have to really think about and go with what your gut tells u.