View Full Version : Port Calculator Discrepencies??? Who Is Right???

03-28-2004, 09:40 AM
I have used WinISD to do most of my box building. I have heard that it is not that accurate and to try using this:


Between loudspeakers101 and WInISD the numbers are pretty different. Another thing I don't understand about the loudspeakers101 calculator, is the slot and square option. I know the difference. But look at the JL web site. (http://www.loudspeakers101.com/VentCal1.htm) It says to add 1/2 the port's width to it's length for the correction factor and loudspeaker101's calculator subtracts 1/2 of the port's width from it's length when the slot option is selected.

So, between these three resources, who is correct?? Am I better off to stop using calculators and go back to using the the formula, pencil, and paper??

03-28-2004, 03:47 PM
well, cant go wrong with pencil and paper... its just a pain in the ***. Try figuring out which program follows the formula and then use that program.

03-28-2004, 06:46 PM
Well, on paper, I have found out that loudspeakers101 is correct as far as their port length claculator goes.

Now, who is correct about the correction factor?? I also think that loudspeakers101 is...after talking to a few people...you DO subtract for correction factor and DO NOT add like jlaudio says.

I know that there are people out there that know a lot more about box building than me. Where are you guys??

03-29-2004, 01:51 AM
It would make sense to subtract rather than to add the correction factor. JL Audio explains that it is because one wall of the port is usually a wall of the box, "effectively adding length to the port". So since having it on the wall adds length, it would only make sense to correct that by subtracting 1/2 of h from the length of the port.

03-29-2004, 07:43 PM
Oh, I see, WinISD gives you the effective port length, whereas that other program gives you the physical length that you should make your port

03-29-2004, 08:35 PM
WinISD also won't compensate and recalculate for driver displacement. I know www.bcae1.com does though. If you use their calc and plug the 'with displacment' volume in, they both churn out nearly identical lengths.

Jim Boom
03-29-2004, 10:02 PM
so which program is right?

03-30-2004, 11:02 AM
Well, after all this I am using loudspeakers101's calc. I tried to use the calc. on bcae but I always got an error message. But whichever, if I have one that works, I'll stick with it.

It not good to see how many people are reffered to the jl site and to see it be incorrect.

03-30-2004, 05:28 PM
well, I don't think the people who programmed winISD would make such a blatant mistake in such a good program... or why a reputable company like JL would give faulty advice? I'd like to hear what someone like Donn has to say about this.

example: box that is 1.5 cubes AFTER displacement tuned with a slot port to 30 hz with the port dimensions being 2"X10"
WinISD gives a port length of 34.75
paper and pencil gives a length of 36.2003
dont know what the other program comes up with or BCAE because I dont have the program and don't know how to use BCAE's calc, so why is there the discrepency?

lets just keep bumping up this post till someone knowledgable answers it.

03-30-2004, 05:30 PM
oh, BTW the formula I used for the "Paper and pencil" calc was from JL's port tutorial

03-30-2004, 05:34 PM
sorry bout so many posts, but I just tried out WinISD pro and it gave a different length for that same box,
WinISD pro 36.07" long

Bill T.
03-30-2004, 06:11 PM
Using loudspeakers101's calc:

For 8 cu/ft after displacement, 35Hz tuning, 100 sq/in of port area: I came up with a length of 14.25".

That sound about right? I keep coming up with low 20"s with WINisd.

Bill T.

03-30-2004, 07:57 PM
Here is the calc. that I used that is dead on with loudspeakers101:

Lv = [(1.463 x 10^7 x R^2) / (Fb^2 x Vb)] - 1.463 x R

If using a square or slot vent then R = sq.rt. (A / Pi)

They also subtract 1/2 the port's width when choosing the slot port option.

I have not yet figured out the exact difference between WinISD and loudspeakers101's calcs. And I also do not know, and con not prove, what formula WinISD uses.

Bill T.
03-30-2004, 10:24 PM
I had much sucess building my first slot port with WINisd. I would recommend using the newest release 50a7. The loudspeaker101 site works but you dont see the gain. But if you know exactly what you want, thats the calculator to use. I typed in my box on loudspeaker101s and WINisd a7.. my actual length is 27", loudspeaker came up with 26" and WINisd came up with 27.5". I had my numbers checked by RE and they said it gains 3.5db at 33Hz. Bingo, WINisd is nearly right.

Whats the relation between net volume and vent gain? Just messing around and accidently typed in 10 cu/ft for a single XXX15" and it was gaining 9db @ 42Hz.

Also, whats the 1st port resonance mean?

Bill T.

03-30-2004, 10:32 PM
well, it sounds like the two programs are using completely different equations, I got a copy of Loudspeaker Design Cookbook by Vance Dickason coming in soon, that's the equation I'll trust.

Bill, net volume and gain on the low end are related, as the box gets bigger you gain low end but it lowers power handling and can cause a "boomy" sound. No clue what 1st port resonance means. might be because you can use winISD for 6th order bandpass, so the tuning frequency for the other chamber would be labeled as "second port resonance" but I am probably wrong.

Bill T.
03-30-2004, 10:42 PM
I thought thats what it meant too. But I dont see a 2nd or 3rd etc etc port resonance. I noticed BIG changes in frequency when the box gets bigger. I wonder if it needs to stay within a certain limit so the sub dont just flop.

Bill T.

03-31-2004, 04:13 PM
I noticed BIG changes in frequency when the box gets bigger. I wonder if it needs to stay within a certain limit so the sub dont just flop.

Bill T.

yeah, you need to stay in/near the manufacturers reccomended size to keep the sub from sounding crappy and so it still can handle some power

Bill T.
03-31-2004, 06:04 PM
My last box was 4.25 @ 33Hz. WINisd showed a gain of 3.5db. My next one (possibly another single XXX15") will be closer to 6cu/ft @ 35Hz, and that shows a gain of 6.2db.

Would that practically be as loud as 3 15" subs in a sealed enclosure? I also thought about 4 XXX10". I heard a set of the SE10s and I dont think thats what Im looking for.

Bill T.