Calculating the enclosure for an MTM speaker

[WhiskeyRebel]

I've got a book that has formulas for sealed enclosures and ported enclosures using the driver parameters. If I want to design a D'Appolito type enclosure do I need to have two mirror-image chambers that are sealed off from each other, or do the two woofers share an air space? If the second, do I calculate based on a single woofer? Or, do the two woofers combine to have cumulative parameters that I'd use to calculate the enclosure? The book shows how to figure the cumulative parameters of using two drivers in an isobaric arrangement but does not say anything about the effect of setting up an MTM arrangement.

 

[jerrymrc]

Same space for both in a MTM config. general rule of thumb is you can calculate for one woofer then double the size of the enclosure. Hope this helps.

 

[Bigyank]

Ever thought of using sealed back mid's for your design. Makes calcuations much simpiler, in my opinion.

Yank

 

[Billfort]

MTM is just driver location on the baffle - same rules apply as for any dual woofer box (accept isobaric which is a different animal). As Jerry pointed out, just double the volume for 2 woofers sharing the same space in a sealed box but if vented, you need double the volume with the same tuning freq. so you'll need 2 of the vents calculated for 1 woofer or recalculate a single vent to suit 2 woofs.

This is pretty easy to play with using WinISD which you can get as a free download here .

 

[WhiskeyRebel]

I found an online cabinet calculator that confirmed th double rule of thumb.
http://www.kbapps.com/audiospeakers.html
For pairs of the 8" woofers I have on hand it recommends an internal volume of 2724 cubic inches. I'd thought of sizing them to sit on top of my CD shelves which are 30" wide and 5" deep. With 3/4" material, that would mean an internal height of around 19". I'm thinking that would have a very low WAF.
My second though was to make cabinets that will sit on the floor under the CD shelves instead, and angle the baffle face upward like a stage monitor.

That arrangement sounds fine a few feet in front of me aimed up at my face, but how would something like this sound in a living room? I don't know of any home speakers arranged like this. I imagine being on the floor against the wall will affect the bass. Maybe I could get a rough idea of the effect by placing my existing speakers there and propping them up at an angle?

 

[WhiskeyRebel]

So much for that concept improving the WAF. Marie pointed out that if I locate them under the CD racks, the right speaker will get whacked by the bottom of the front door. D'OH!
So that leaves me different options. I could use just one woofer per vented box which would bring my internal dimensions to 28.5"x 5.5"x8.7" which is not unreasonable, but not have the dispersion pattern of the twin woofer design. It was the discussion of this pattern in the crossover section of the book that got me interested in this design, and anyhow I'm sitting on six of these woofers I got for an earlier project that remains unfinished. Sounds like a recipe for left-middle-right to me.

The EBP for the driver is 101 which is supposed to be great for vented boxes but not as well suited for sealed. I don't know the explanation for that.

I could make a two-chamber sealed box. According to my spreadsheet, a chamber of 13 7/8"x5.5"x8" will give me a Qbox of 0.707. The online calculator agrees with this. Where they differ is that the book formula tells me the F-3dB is 52 Hz (equal to the driver's free air Fs), a Fbox of 84 Hz and essentially no dB rise at Fbox. The online calculator says peak dB at resonance is indeed 0 dB but says my F-3dB would be 84 Hz. I don't know which one is right.

I could see if the larger boxes would be acceptable. Not really betting on that one. Unless I reduce the height and make them deeper. 28.5Wx8Hx12 1/4D would give me the right volume but A. might be unsteady on the shelves and B.might become a doorstop. Ugh.

I could make dual-woofer vented boxes to fit the space and have the resulting performance. The online calculator says a box of 28.5"x5.5"x9" would have a 2dB rise at 57.7 Hz and an F-3dB of 61 Hz. Down and then back up? Odd.

Any advice gents?