Port calculations

I've been reading many threads about calculating the diameter and length of ports. I feel a bit overwhelmed. Originally, I had the overly simplistic plan to keep the port length at .75 inches, which is the thickness of my building material, and calculate the diameter from there. Apparently, this is a bad idea, both for the turbulence that occurs at the flat edges, and the possibility of the air speed being in the audible frequency range. WinISD doesn't seem to be much help. What formula should I use to calculate port length and diameter?

Dave R

2006-08-09 5:30 am
port calculations

the diysubwoofers.org site has a lot of info on speaker calculations, including port calcs here:


In general, pvc pipe is a very adequate (and cheap) material for making a port. Sometimes you can find a plumber who has leftovers, and a lot of hardware stores sell by the foot. It comes in a lot of standard diameters, so the vent length is easy to calculate.

Dave R
Thankyou, Dave R, for that excellent reply. That website answered my question, and also gave me some ideas I hadn't thought of. I didn't realize that fiberglass stuffing will INCREASE the effective volume of the enclosure. Might you have any idea how to calculate how much of an increase a specific amount of insulation would make? I'm scouring the internet for an answer right now.


2002-01-07 6:02 pm
Austin Duggan said:
Thankyou, Dave R, for that excellent reply. That website answered my question, and also gave me some ideas I hadn't thought of. I didn't realize that fiberglass stuffing will INCREASE the effective volume of the enclosure. Might you have any idea how to calculate how much of an increase a specific amount of insulation would make? I'm scouring the internet for an answer right now.
Use a design program like Unibox rather than doing it manually.

Re stuffing ported boxes

Dave R

2006-08-09 5:30 am
ported calculations

In general, stuffing is used to "increase" box volume for sealed boxes only.

It doesn't work that way for vented boxes. It can be used for lining the internal walls to alleviate internal reflections however.

hope this helps. :)


2002-01-07 6:02 pm
dublin78 said:
When using champfered edges to straight ports to reduce the edge effect, where is the length and width measured from for tuning purposes?
I use the port end correction factors in Unibox to get it close, then build it and measure the Z and adjust on test. Easy for tube ports, a bigget PITA for slots.
I've calculated the minimum port diameter to eliminate noise using the equation found here: http://www.diysubwoofers.org/misc/portcal.htm But it's huge! 7.47 inches for one port, 5.28 inches for two, with the length being around 11 inches long. The length is alright. But the diameter seems ridiculously large for one 15" speaker (I used 14" as the "Effective diameter of driver.") Am I doing something wrong? When I plug these calculations into WinISD the tuning frequency goes much higher than it's supposed to. So something is amiss. Also, the "flare-it" software I found online claims that one 4 inch, properly flared port is enough to eliminate port noises. Do flares really make that much difference? Or am I being misled? Just for fun, here is a picture of the box in mid-construction:

Dave R

2006-08-09 5:30 am
For a 15", it's not uncommon to see something larger than 6-inch diameter recommended by the formulas or the programs. For the most part, these formulas may be conservative. If you plan on "cranking it up", you may encounter some air noises if it is too small.

I would think that (2) flared 4-inch ports would be adequate. Start with the length a little bit longer than the calcualtion for a straight vent, and the tuning can be accomplished at the assembly level.

There is a flared 4-inch port made by precision port, that is carried by several on-line stores (PE and Madisound for example). It is easy to work with.

If you have a roundover bit for a router, you can make a flare yourself. Make a slight counterbore on the interior surface, for the straight vent to fit into. Then from the outside (vent opening), use the roundover to create the flare. I'd guess a .50 inch or .75 inch radius roundover would be good for that large of a woofer. (larger if you have a heavy duty shaper, for example)

Hope this helps.

Dave R
Minimum Port Diameter

Ok, let's start it here. I don't have experience in this. I am also looking, experimenting and modelling. First of all "Flare-it" can not "claim" because if you give the allowed speed of air (max.) it will "recomend" two diametres one for Chuffing or noise/hiss in the port and another for Core Limit that would be distortion/compression in the limit. So if you bring in the velocity to the program you will have it lower (for required velocities - m/s) or "Required" for a determined number of ports and port diameter. This inner diameter does not correlate with length. A bigger length lower Fb or the Tunning Frequency. After all if you know the speed of the air (like from WinISD) from the driver you will get an X size to flare and maybe reduce it's diameter.

Usually from what I see I agree that vents are very large, but you don't want to hear noise after all. And if it's very large the noise from the back of the speaker will easily also pass through.

The bigger diameter is usually safer but then you have a bigger lenght for the same tunning frequency.

One website,
Now, this site shows a simulation with two formulas one is for bandpass enclosure.

Also here a Port Calculator.

But what I consider the gem of them all is a formula based on experience done by Notax two years ago (2006, in the diyAudio Forum and he is out of now since) and that was derived from T/S parameters. You can see it here.
I passed it to excel (hope he doesn't mind). I am very pleased with it and guess is of very good use.


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Thanks for the great responses guys. Learning a lot. I think my final question before cutting all the holes and making this box a done deal is...how do I calculate the minimum diameter of a flared port? Or, how do I calculate the diameter equivalence between a flared port and a straight port? I ask this question again because precision ports claims a minimum diameter of 7.25" is equivalent to a 4" flared port. If this is the case, then making room for ports just got a lot easier. Thanks.
Austin Duggan said:
p.s. I have scoured the internet, including this message board for an answer to this question. There's obviously an equation for this. It just hasn't been posted anyplace that I can find.

It's based on experience and off course Flare-it must have it nailed. So, it's based on the velocity (of the air in the port) of the driver. What driver did you say you have and it's T/S specs??? size of the enclosure and tunning frequency? (Try to work w/your head a little... npi, I know it can be confusing but don't give judgements if you don't give the details...)
I'm using the eminence kappalite 3015. All the date can be found here: http://www.eminence.com/pdf/kappalite-3015.pdf After factoring in the displacement of the speaker and the bracing, I'm working with about 102 liters. Tuning frequency is 45hz. Don't know what you mean by giving judgments. I'm just getting different answers from different programs, so would like the equations to work with myself. Trust me, I'm grateful to have all these resources.
Ok Austin. Let's make a first approach. (I don't know what you have?)
First I went to NotaxNotes and it said a working diameter of 212,8mm (this is based on T/S-derived and experience as he said). The formula is there for you to see. I am not working with the formulas as I use the software(s). This is my methodology. Ok, then I went to WinISD, did you checked it(?). All the softwares might have the formulas somewhere or then email their authors asking for that imformation. I don't know what your objective is, but you decide if you want to go the scientific way.
In WinISD it gave me a first alignment of 81L. I'm going to insert here:


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Port/ Driver placement is crucial if low ripple FR is a goal and IMO if XO: ed above 250 Hz, all the internal walls should be covered with at least a ¼” wool felt or similar in order to prevent internal reflexes to leak through a short wide port at high signal levels causing vertical image shift and larger MAA.

OTOH if XO: ed below 250 Hz (>= 20 dB/octave) the MAA or Location issue can be ignored as long as the driver is a low distortion type.




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I used a second alignment, that you told me (102L) here:

As the first is with one vent of 10cm, I used a second try with 2 vents to be close to NotaxNotes 20cm.
Also note that this is for flared tubes on one side, because we can choose it, as this parameter was set first (assumed by the software) - one, two or none.


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