Port and frequentcy

The design I chose requires a 6" diameter port, 33.5 inches long.

I'm having a bit of trouble fiding a 6" diameter PVC pipe.

According to WinISD, I would get the same tunning frequency using two 4" diameter ports, 30.75 inches long. Does this sound reasonable?

If not, What length of the 4" diameter ports would get me the same tuning frequency?

WinISD is fairly accurate in it's port length calculations especially for ports over 10 inches long, so I'd trust it. Make sure you use the proper end correction factor though(flanged or free ends). Although with a port that long an inch in length one way or the other will probably change tuning by an unnoticeable amount.

As long as you have internal volume and tuning frequency to your liking then WinISD will be able to correctly calculate the port length when you specify diameter and the number of ports

Ron E

2002-06-27 10:41 pm
In answer to your question....The two 4" ports will be fine and the length sounds about right compared to the 6" port.

As pinkmouse said, those ports are rather long, what he didn't mention is that you might be better off using a passive radiator for that design, but it is difficult to say without more details of the design.

31" ports will have a organ pipe resonance at about 13562/(2*31)=~220Hz and it will probably be a pretty nasty one. If you are not using the speakers that high, you don't have a problem...
Thanks for all the answers.


Whats the correction for the flared/flat ports?

I did notice that the port doesn't have to be round. Is there any disavantage with square ports?


I'm not trying to fit it in a small enclosure. Its about 6 cu.ft.

Ron E,

Its a subwoofer, so it will be crossed at 80 Hz. I hope that that cross over point doesn't create pipe resonances.


excuse my ignorance, but what are ABS pipes? What material are they made from?

will the two 4" ports take more volume in the box than the 6" port, or about the same? Will the 4" ports have more port distortion than the 6" one?

Thanks again.
To calculate end correction download the newest version of WinISD pro and it should have that option in the "vents" tab to the right of port length it will be labeled "end correction." Click the drop down arrow and it will say two free ends, one flanged end, and two flanged ends. Select your choice.

with an 80hz crossover you should have no problem with the pipe resonance. Is your crossover 24db/octave? Those usually work best for subs. I'd reccomend at least 18db/octave

the two 4" ports shouldnt take up any more volume than the 6 inch one judging by the fact that port length is actually shorter. Enclosure tuning is a factor of two things. Enclosure volume and effective air mass of the port. The effective air mass in the port is determined by the cross section and length. If the cross section increases, length must also increase.

Since a 6 inch port has a cross section of ~28.3 in^2 and a two 4 inch ports have a cross section of ~25.1 in^2 it would make sense that the 2 4" ports would be slightly shorter and take up slightly less volume in the enclosure (theoretically, not taking into account the thickness of the tubing) But since you're adding more PVC they should be close to the exact same


2004-11-26 3:15 am
Ideally you want largest area/perimeter so a single larger port will be superior. A square and round port give same area per perimeter so no real difference in that respect. A rectangular may theroetically be not as good. Rectangular are basically easier to build is the reasoning for them and if near a edge of the box result in less wasted space.


Disabled Account
2002-12-22 2:13 am
Calgary, Alberta
perpetual said:

excuse my ignorance, but what are ABS pipes? What material are they made from?

PVC = polyvinyl chloride = the white or cream plastic pipes you see at the hardware store

ABS = acrylonitrile butadiene styrene = the black plastic pipes you see at the hardware store

My point was just to make sure you hadn't asked the store for 6" PVC specifically and gotten a no while they indeed had a pile of 6" ABS there.

So I won't have to change the volume of the enclosure going with the two 4" ports as oppose to just the 6" port, right?

I'm using my receiver's crossover. I don't know if is 24 or 18db/octave


this is where I'm getting the internal volume of the box and the length of the port. My actual dimensions are different because of the shape I want the enclosure, the bracing and the thickness of the HDF.


Thanks, You've all been very helpful.
Here it is. The port is huge. I'll add a second layer of HDF all around to make all the walls stiffer. I'll also add the rest of the dampening before putting in the driver. I'm not sure how to finish it.


Here is another pic with the front in place. The port is sticking out 3/4" so that when I put the second layer it'll be flat.



2004-11-26 3:15 am
the pvc pipe looks aluminium in the pics just the lighting i guess other than that it looks good. 1.5" thick he he sweet. your going to round the port edges a bit right? This is for your titanic? I looked at your site and I can't say I'd see much panel flex on your boxes.;) you do 2 layers partly so you can have a smooth finish on the outside for painting or thats just a added benifit?
Actually the aluminum is an added support to the port. The braces are holding it too but it is kind of heavy so I added the aluminum for better support.

I'll round the edges. I like the smooth finish. And it is for the Titanic. I'll put a 15" Quatro in the enclosure that is on the web site (if it fits).

I do the two layers to have a smoother finish for painting like you mentioned (you know round the edges, no nails and stuff like that) and also to have stiffer walls for less vibration. I aslo use HDF instead of MDF, which I think is better for subs.