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-   -   Port calculations (http://www.diyaudio.com/forums/multi-way/120887-port-calculations.html)

Austin Duggan 7th April 2008 02:13 AM

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 7th April 2008 02:24 AM

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

http://www.diysubwoofers.org/misc/portcal.htm

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

Austin Duggan 7th April 2008 06:29 AM

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.

Brett 7th April 2008 06:37 AM

Quote:

Originally posted by Austin Duggan
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
http://sound.westhost.com/articles/boxstuff.htm

Dave R 7th April 2008 02:05 PM

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. :)

dublin78 7th April 2008 03:00 PM

When using champfered edges to straight ports to reduce the edge effect, where is the length and width measured from for tuning purposes?

Brett 7th April 2008 04:20 PM

Quote:

Originally posted by dublin78
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.

Austin Duggan 13th April 2008 07:52 PM

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:
http://i10.photobucket.com/albums/a1...l/DSC00049.jpg

Dave R 13th April 2008 08:08 PM

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

Inductor 13th April 2008 11:32 PM

Minimum Port Diameter
 
2 Attachment(s)
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,
http://www.ajdesigner.com/phpvent/su...m_diameter.php
Now, this site shows a simulation with two formulas one is for bandpass enclosure.
http://www.carstereo.com/help2/Articles.cfm?id=31

Also here a Port Calculator.
http://www.mhsoft.nl/PortLength.asp

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.
http://www.diyaudio.com/forums/showt...threadid=78862
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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