How to model a dual chamber ported(Weems) enclosure in WinISD? Please help >>

[beady]

I’m modeling different subwoofers in different enclosure types trying to decide which I want to build. I’m using WinISD for my modeling. Can someone describe to me how to model, or simulate, a Weems type dual chamber ported arrangement please? I understand how to set up the design for the “overall” box, solve for two identical ports in a regular ported enclosure. That’s all fine and well, and I can see the plots for that arrangement just fine.

But, since there are two tuned frequencies, how do I get WinISD to show me this?

I tried modeling the same driver in an enclosure volume 1/3 of the original ported volume using 1 port of the same diameter as solved for above, then altering the enclosure tuning frequency to match the previous port length. This does indeed give me a higher tuned frequency, and when I plot both enclosures on the same graph I can see the two peaks and the dip in between. However, the tuned frequencies are not an octave apart as I have read they should be.

Please explain what is wrong with this approach, and how I can model the Weems type enclosure in WinISD somewhat successfully.

Thanks very much for any enlightenment,

Mike

 

[kelticwizard]

To my knowledge, WinISD doesn't model it. I don't know if the Beta version models it, but the regular freeware version does not.

At any rate, you don't bother with specialized tuning, as I have read. You just calculate for the two vents, then make a third vent identical to the other two, and put the third vent between the larger section and the smaller section.

Anyway, here is an earlier discussion on the matter, with material included by Weems himself.
http://www.diyaudio.com/forums/showthread.php?postid=93570#post93570

 

[beady]

Sorry, make that I used the second calculation based on 2/3 of the box volume and a single port.

Also, the peaks seem to be the same basic shape, and are under each other, not next to each other, as my post above had modeled the driver in the smaller airspace accidnetally. The two tuned frequncies are not an octave apart either. One is 35 Hz and the other is 40 Hz.

Sorry for the confusion, can someone help to clarify this for me?

Mike

 

[beady]

Thanks kelticwizard, I had actually already seen that post, and searched for any other information on this site before posting.

I understand that the second "tuned frequency" just falls out naturally once you calculate the overall ported box with two ports.

I was hoping to be able to plot the double peakek curve so I could physically have an idea of it's shape, that's all.

Thanks,

Mike

 

[thoriated]

I am also interested in the double chamber br approach for an upcoming project, but it seems from what I've seen that the Fostex variation which only has one external port also doesn't exhibit the passband notch that the Weems version is supposed to have. Does anybody have an idea what any resulting tradeoff might be - perhaps less effective reinforcement at F3, and if so, would increasing the external port area help here?