Does WIN ISD Alpha provide proper port shelf dimensions?

[dug dog]

I'm trying to do some designing of my own and I'm getting confused by the port shelf calculation stuff. (I searched for all thread titles containing WIN ISD and didn't find my answer).

What's mucking me up is that when I look at the fEarful 15/6, for example, and plug in the dimensions from the plans, Win Isd shows that the box is tuned to around 53hz- I'm sure GB said they're tuned around 43. Then I read this post by GB on calculating port shelf length and I'm scratching my head:

fEarful™ 12/6 & 15/6 enclosure designs - FREE! (Part III) Getting Green!

Can I assume that there's something wrong with the way Win Isd is making this calculation?

For reference, I punched up the 3015LF using Quasi butterworth and adjusted the net volume to 108 litres (I'm pretty sure that was what GB stated) and then set the vents to 2 square vents @ 9.5 inches by 3 by 9 inches long as in the plans.

Thanks to anyone who can deconfuse me.

[dug dog]

Surely someone can shed some light on this....

I've found that WinIsd has an online calculator that gives a much different result for vent mach... Hmmm....

What other sims have you guys used with success? Preferably freebies.

[rpsands]

No, it does not. The general advice I think is to divide Winisd's port shelf by 1.5 or so? And then calculate the volume of the cabinet with the additional volume behind the port added.

[NoGraveConcern]

WinISD provides accurate round and square or rectangular port dimensions which assumes the port is a nominal distance from any adjacent wall. A shelf port, however, shares 3 walls with the sides of the cabinet which changes the tuning, thus the advice to divide by 1.5.

[Mr. Foxen]

Think it's called 'end correction' or something.

[dug dog]

Thanks, guys. I was having a real beach of a time finding any info on that...

[rpsands]

Just be mindful it's not exactly /1.5, because you then have to add the volume of air behind the port and recalculate tuning based on that, iteratively. It's pretty irritating process. I did maybe 3 iterations before I got tired of it and went with "close enough"