Port Tuning

[bjackson]

Is it possible to experimentally tune a port, because in all of the simulations are a bit different and very short (2 inches?) for a 28hz tuning?

So could I try a bunch of different tunings and see where it is the most agreeable?

[Ron E]

When ports are that short, there can be significant error in the calculations due to the approximate nature of the end corrections. You would be better off using a larger diameter port - which would require a longer tube.

The proper formula (from first principles except for the end corrections) is:
Lv = 2361*Dv^2/(Fb^2*Vb)-K*Dv
where K is 0.85 for a tube flanged at both ends (or shorter than ~50mm)
and K=0.73 for a tube flanged at one end and free on the other.
Lv is port length in mm and Dv is port diameter in mm - Vb is box volume in liters.

If you want to tune by listening and experiment - go ahead....

[bjackson]

I am using a 4" flared port, I don't think it gets much larger than that (that I have seen).

Maybe there is something wrong with my calculations?

[Svante]

If your box is about 270 litres, the calculation are correct. Still, such a short port is very easily overloaded. Try using several ports instead, maybe 4 of them. Then the port lengths should be 280 mm, which is far better in terms of overload margin.

[Ron E]

Originally posted by bjackson
I am using a 4" flared port, I don't think it gets much larger than that (that I have seen).

Maybe there is something wrong with my calculations?

To help you design your box, you really need to give more information, like what teh woofer T/S parameters are at the very least. Especially important in choosing ports is Sd (cone area) and Xmax (excursion).

Wiht a box that large, you are probbaly using a large woofer or several of them. There is more to port design than just figuring length - it is best to model port velocity and try to make sure it will stay below ~17 m/s at the highest listening level you will be using. As SVante says, it will probably take multiple ports.

[bjackson]

I am going to have 2 10" Dayton RS270s in a 214 liter box, with around 15 liters going into a subenclosure for the mid, and has 5 pounds of polyfill in it.

I have a hole made for the 4" flared port. which is around 6.5" at the outside end around 5 or 5.5 inch diameter on the inside flare.

The T/S parameters are found at http://www.partsexpress.com/pdf/295-368z.txt
at the bottom.

Thanks for all of your help!

[bjackson]

Does anyone have an idea for me?

I think the port mach for a single 4" should be fine, however it's just so short, is there anyway I can fix this without purchasing another port? I already have enough budget overruns.

[Ron E]

Originally posted by bjackson
Does anyone have an idea for me?

I think the port mach for a single 4" should be fine, however it's just so short, is there anyway I can fix this without purchasing another port? I already have enough budget overruns.

You may be right, one port will be close - best if rear-firing, though. I would forget about stuffing teh box - stuffing is not required in ported boxes. Line the walls with ~2" of foam or fiberglass instead - or put the stuffing in a net and staple it to teh walls..

If it were me, I'd make the port about 5" long. But leave room to experiment.

[bjackson]

My port is rear firing, but all of my calculations say like 2.5 inches, where are you getting 5" from?

[bjackson]

Oh and the boxes are already stuffed, there is nothing I can do about that, they are sealed up.