question relating to ports.

[garfa]

hi all,
ok i have a pretty good question for you all, mainly joe, ddphil, phreddy and ray (after all the edgar that had/has happened).
i have just finished designing a box for my DLS MW12 sub.
now i will have 2 ports in it but unfortunately i decided to just get the ports from ebay and forgot that the seller did not specify if the ports were 75MM inside or outside dia. so i assumed inside, but i was wrong ofcourse.
now i bought a bit of PVC pipe to extrend them to the required lenght but its 75mm inside dia, so it fits OVER the port. now since the extension of the port is a larger dia than the port how exactly do i calculate port length?
should i work off the smaller or larger dia or just find the average size?
i will be calling a few plumbing stores to see if i can get my mitts on some 75mm outside dia pipe to perfectly match the port instead of the stuff i currently have.

thankyou any help much appriciated,

Garth.



(PS: or just direct me to some 75 mm O.D. 160mm long flaired ports......)

[Joe Peeps]

hmmm stumbled across this. How'd you go brother?

I have no idea, but would also like to know out of curiosity. I googled but couldnt find the actual equation that they use to work these things out - not that id be able to understand it! CAA maybe?

[| evaN |]

would that extra millimetre or so in diameter make a big difference? wouldn't that just tune the box a few hertz away from what you originally wanted or am i no where near lol

[garfa]

yeah i am just going to flair out some pvc.
cant be ****ed with it any more.
evan as for the tuning changing, its running a pretty fine line as it is so i sorta want to know exactly what the effect would be.
increased port noise probably!

[Tasmaniak]

Throw away eBay port and get new PVC piping and start again. Tune it to length and be done with it. Don't bother with trying to join extra pieces on there. If you are as finicky as I am then you will be bothered by the fact that you have now just changed the internal volume of the enclosure and send yourself around the twist.


Calculation for port length. Numbers in parenthesis are actually the prior numbers power. eg 10(7) is equal to 10 to the 7th power.

1.463x10(7)R(2)
Lv= --------------- – 1.463R
Fb(2)Vb

[garfa]

yeah going out to buy a heat gun 2day probably.
thnx