Zaph's BMTM vented

[bobhowell]

I built Zaph's BMTM sealed in a 1.3c ft box. I am using it in a small room and want to vent it. I have pluged it into Unibox and can make no sense of what I get. Should I just use his tuning vent for 32hrz? What can I expect?

Thanks

Bob

[Hdale85]

If he chose 32hz probably a good reason for it so yeah I'd say go with the original design.

[ThomasW]

Should I just use his tuning vent for 32hrz?

You can use that Fb but not Zaph's 69mm long port since it's for a 2 cu ft box.

[bobhowell]

How do I calculate the proper length?

[ThomasW]

With Unibox..

Post a screen shot of your sim as an attachment

[bobhowell]

I am not sure how to use this but this is with a F3 of 32hz. Is this the tuning?

Bob

[ThomasW]

Fb is the tuning frequency

[bobhowell]

Here is fb of 32hz. Tube di of 3.27 cm and 27cm long. I think I should stick with di of 2" but how long? Zaph recommended 61cm and 2" for 2 cft. How should I account for my 1.3cft volume?What would be a good length? 61cm< or 61cm>. I see no way to work this out in Unibox.

Thanks,

Bob

[ThomasW]

Zaph's port lengths are mm not cm.

His port diameters are 2" or 5.07cm

[bobhowell]

I see now. So how do I get a new length? Unibox has several areas. I don't see which to use.

[ThomasW]

Scroll down a few posts in this thread, it's basically a Unibox tutorial

http://www.htguide.com/forum/showthread.php4?t=23273&highlight=Unibox+tutorial

[bobhowell]

OK. Now I see how to get the port length. 4" seems to follow Zaph's design.

Now, as I built 1.3 cft rather than 2cft, will I get good results? What change can I expect?

Thanks,

Bob

[ThomasW]

Comparing sealed and vented Unibox sims for your 1.3 cu ft box will tell you the expected differences.

[HareBrained]

Since you've already built the box, you have to subtract the port volume from the total volume to achieve the woofer volume (which is what is entered into B66 of Unibox). It requires iteration as the port length changes, the woofer volume changes and the tuning changes. I set-up a spreadsheet to calculate volumes and port lengths for a fixed sized box. It can be done in Unibox by modeling a smaller than expected woofer volume and then determining the port length. Then calc the final volume, enter into Unibox, and then play with the Fb until the port length returns.

In my playing around, 34Hz with 35L seemed to be the sweet spot for wall covered, but excursion is an issue and a 2" port is too small for 50W. And the FR curve is less then ideal. Not the best alignment for a pair of these drivers. Sealed models much better with little sacrifice of extension.