port length problems

Here I am again asking questions.  I'm trying to get a ported box going for a brahma MKII 10.   It will get 400W and will be in a .85 ft3 box tuned to 30hz.  I'm getting two different air speed calculations from unibox and winisd pro (alpha).  I've read that winisd doesn't always calculate ports and such right.  The thing that also makes me wonder is the length of the ports.  61 inch long 4.85" diameter port!?  That makes the port bigger than the enclosure.  Could that be right?  Any input is appreciated.

https://www.the12volt.com/caraudio/boxcalcs.asp#por

try a 2"x9.87" long, that should do it.

it would have alot of port noise, but being the box he described is only .83 cu. ft it's gonna be hard to tune  it that low, unless he uses a small  port, or get's an elbo fitting.

As far as air speed calcultations go:  by default, WinISD Pro 0.50a7 has rear port velocity set by default to "peak" instead of RMS.  Here is a thread where this was discussed.  Here's what I did:  in WinISD, go to the top pull-down toolbar, select File, Options.  Place the cursor over the word "peak" next to Air Velocity, click to change to "rms".  Also make sure it is set to "ft/s".  In this dialogue box you can customize the graphs the way you want to see them.

Keep in mind, fugit, that when you ask the program to respond to the inputs you put in for that particular woofer, it will automatically attempt to find the "flattest" response possible.  It pays no mind to the consequential port length, whether it's a feasible length or not.  It's up to you as the designer to work with the space you have, the volume of the enclosure, and your accumulated knowledge to use the program to arrive at a workable enclosure.  When you are using these two programs (I do the same thing) to work up a solution and then find big differences in the results, it will prove to be an error in parameter inputting...or an error in deciphering the results.

The difference is only if you are doing a comparison of one program to another.  That is, they should both have the same standards. 

You should be able to change peak to rms if you just follow what I wrote above.  When you are, again, doing a comparison of programs they both have to have the same standard.  This is usually why some people think that WinISD gives false air velocity numbers.  If it is calculating peak and the other program is using RMS, they will differ.

55 ft/second is .05 Mach.  I generally use the 10% guideline when designing a subwoofer enclosure for auto use, which lets me get to 110 ft/sec at highest RMS (make sure the power is inputted into the program).  This keeps the port opening at a reasonable size, thus affecting the port length.  If I worried about the peaks, the port would often get too big for the project.

The same standard makes sense as far as being easy to read.  I had looked up what m/s and mph were each at 5% of mach.  I changed the peak to rms but still get the same air velocity problem with it.  Everything else is the same.  As far as volume, thats minus sub and bracing and of course the monster port.  I wanted the tuning low so I could get a more sql type setup.  Yes, the slot port seems like it would be much easier to get inside if (when) I have to snake it around all that way.

I think I may have found a compromise though, especially with stevdarts info on using 10% instead of five.  If the port velocity is only slightly over at fb , and the rest is under then wouldn't most of the noise not be a problem?  I'm not too sure about what freq most music stops their lows, and I VERY limited expreience with ported enclosures, basically I have heard them only enought to know they are louder than sealed. 

This project of mine is going into my little commuter accord and I haven't had any "good" experience with a sub in the trunk.  Hence the ported box idea.  Seems like I've almost bit off more than I can chew, but I'll try a ported box and a sealed and see how I like them both.  Thanks again for all your help guys.

So, I got one figured.  I thought I'd post so you can sharpshoot it if you like.  Also if anyone else needed an enclosure for a 10" brahma MKII they can check it out.

..86 ft3 tuned to ~28.25 HZ

D  12

H1  16 

H2  12

w 22.5

brace  11 x 10.5 with 4 3" holes and a 7" hole for the sub magnet

back of port   9 x 12 @ .75 mdf

All of the box is 3/4" mdf except for 4 pieces of 5/8" for the port baffles.