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wintermute 21st November 2005 12:19 PM

My Morel MTM Project
 
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OK so Iíve been working on this for what seems like forever. I recently built a gainclone in what for me was record time. I attribute that mostly to the fact I put up a post showing what I was doing, and felt motivated to post progress reports ;) so Iím hoping if I start a thread on my speaker project maybe it will magically accelerate in its completion ;)

On the how long I have been doing this project well, I started thinking about it back before I joined up here at diyaudio (and I joined in Aug 2003). The first post I made here was a question on the focal 6W4311 which was what I was thinking of using for the mid in a WMT config. Iíd already decided I wanted to use a morel tweeter at that stage, but was thinking an MDT33 at the time, and the woofer was already decided as I had two perfectly good Vifa M26WR09-08ís. I had some concerns about difficulty of crossing over the focal, as this was to be my first foray in to xover design, and I was heading straight into the newbie trap of going for a 3 way ;) (though in my defence did have a lot of practical, experience (without the theory behind it) at that stage ;) )

I hadnít considered the Morel MW144 at that point because I thought its sensitivity was too low, that was before someone suggested an MTM to bring up the efficiency level. After reading up on MTMís I decided the MW144 was a good choice.

Though not strictly shielded it has a very low magnetic field due to the dual magnet setup, and these speakers will be in close proximity to the TV (and I may decide to make a centre and surrounds at a later date), this limited my choice of drivers somewhat dramatically (or so I believed at the time).

So what did I decide on in the end?? After contacting the Australian Morel distributor, I decided on an MTM with MW144ís and DMS37s. The DMS37 is a horn loaded tweeter, which was designed by the Australian Distributor of Morel. He designed it so that if you flush mount the tweeter it is time aligned with MW144ís which are surface mounted. As I was initially thinking Iíd try a 1st order crossover (which I now think is impractical with these drivers, though I havenít yet completely ruled out the possibility, some notch filters may make it workable) this was a huge incentive to use the DMS37 over the MDT30 or MDT33 (and the DMS37 is shielded too). I also decided to make the MTM's stand alone, and make separate cabinets for the Vifa 10" drivers, crossing over somewhere between 200 and 300 Hz with a bi-amped setup.

I purchased the drivers around Nov 2004, I made a test baffle over Christmas, whilst I had access to my Fathers tools, (mostly to test the circle jig I made, and make sure I could accurately cut rebates)Ö shortly after this I made a test box to match the baffle (the baffle was made with no original intention of making a box so the box shape was somewhat different to my final design). I did lots of tests and didnít achieve much at all ;) apart from learning that rebating the tweeter into the baffle with a flare (because at that stage I wanted to rebate the woofers and maintain the time alignment) was a bad idea!!!!

Skip forward to August 2005 and I went to my parents place with a couple of sheets of 25mm MDF and the drivers and made extensive use of my Fathers triton workbench and router. Lots of activity for a couple of weeks and then back to not much, which is where I am now. Iíll post some pics from along the way starting with the drawings I did in Aug this year of the cabinets.

Well this has ended up a mighty long post so Iíll give it a rest for now ;) well I might post the second dawing...

Tony.

wintermute 21st November 2005 12:31 PM

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Ok here is the second cabinet drawing. I actually changed the positioning of the woofers slightly the dimention showing the distance between the center of the tweeter and the center of the woofer is 122mm which allowed for 4mm between the edge of the woofer and the edge of the tweeter, I cut it to 120mm giving a 2mm distance between the edges of the drivers.

I decided to not offset the drivers due to stuff I read on Duntechs web site on what this does to the dispersion of the sound. After simulating in BDS I decided I could live with the extra ripple that I'd get, and thought I could try the felt trick to reduce diffraction effects, I do plan on chamfering the edges of the baffles.

Also not shown in the plans is corner bracing, I put quarter round in some of the corners, this was a last minute decision and could have been done much better (mitering etc) but it adds some strength :) I also glued a 25mm square MDM block along the front baffle below (or above) the drivers, I've since decided this was not such a good idea, and have cut it back somewhat with the dremel ;)

Tony.

wintermute 21st November 2005 12:50 PM

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OK and one post to show I actually have built something ;)

The partitions between the midbass' and tweeter are made from two sheets of 12mm MDF laminated together. The reason I did this was so I could get the MW144's close to the tweeters without being too close to the edge of the partition. By using two pieces of 12mm MDF I could make the tweeter side peice a bit shorter and hae room for the tweeters basket, the partitions also serve the purpose of bracing, and allowed me to use golden ratio (one of not the) dimentions for the midbass enclosures dimentions.... funny thing that, I actually decided to make it using all prime numbers, when I did a search to find out whether this was a standard practice, I discovered that my particular primes were one of the acoustic golden ratios :)

I actually did all of the gluing in one go for what you see in this pic... was quite frantic, and I was sweating quite a lot by the time I finished, probably took an hour from start to finish...

This was the box that I remembered to drill the holes for the speakers, and insert my hex nuts into before I glued it together ;)

Tony.

wintermute 21st November 2005 01:17 PM

and the current status
 
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just to be complete I thought I better post a pic of what it looks like at the moment......

the straps you see are holding the rear baffle on.... I haven't glued it on yet as I'm not yet 100% happy with my driver fixing bolts and nuts... I've been doing tests with not 100% sealed boxes, but the tests I've done have been promising. Will hopefully get the bolts sorted soon, and then glue the backs on, and start applying the veneer, and finally champher the front baffle.

Then the serious tests for the crossover design can begin!!

Tony.

wintermute 21st November 2005 10:55 PM

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Oh and another thing that deviates from the plan, I rebated the sides where the partitions fit in. Not really visible in the pic that I posted of it being glued together.

the rebate itself is only about 3mm deep, but it made it much easier to slide the internal baffles in and make sure they were straight!! My prototype box I just but jointed them and getting them square was a real pain. Had some problems with the fence on the Triton workbench not being 100% square which resulted in about .5mm difference front to back (between the two sides) but it wasn't enough to cause any problem.

Tony.

wintermute 22nd November 2005 11:07 PM

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Yesterday for whatever reason (procrastination??) I decided to revisit my BDS sims as I had never been that happy that my measured response seemed to agree with the simulated baffle diffraction response.

What I found by accident was that when I used the lowest complexity source (ie point source) but still used quadruple sampling, I did infact get what appears to correlate with my measured response. The magnitude is different, but I think this is driver related, but there is a definite (I belive) correlation between the sim and the measured result. Note that the scales aren't the same, but by skewing them like this it is easier to see a correlation. The thing I'm happy about is that when I sim with a 22mm chamfer (the size I'm probably going to use) the response flattens out significantly, how much this will help in the real world I guess I'll have to wait an see :)

attached sim and actual measured (gated) repsonse of the woofers.

so at least with this project it appears that the best model to use for the drivers is the point source one!! Interestingly the tweeters seem to match more closely when using the combined source model, I would have thought the tweeters would be closer to a point source than the woofers :rolleyes:

Tony.

wintermute 22nd November 2005 11:10 PM

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With the 22mm chamfer the response after the dip flattens out quite a bit in the sim, will be interesting to see what the actual effect is :)

Tony.

exhausted mule 23rd November 2005 01:23 AM

what program are you using to simulate this box of yours?


sounds interesting.

wintermute 23rd November 2005 02:02 AM

I'm using BDS (Baffle diffraction simulator) for the sim graphs I posted, for the actual modeling of the enclosure I used unibox. Both are excel spreadsheet based, which unfortunately rules some people out.

BDS here ---> http://www.pvconsultants.com/audio/d...ownloadbds.htm

Unibox here ---> http://home20.inet.tele.dk/kou/ubmodel.html

there is a lot of good stuff on the FRD consortium page ---> http://www.pvconsultants.com/audio/frdgroup.htm where I found both of these modelers :)

The magnitude of the difference between the simmed and the actual is quite a bit different (unfortunately worse in the real measurements) but the shape of the curve is quite similar, too similar me thinks to be a coincidence :)

I've just been cutting up bits of MDF to use as load spreaders for doing the veneering, I wish my speaker terminals would hurry up and arrive!! I now really appreciate my Dad's Triton workbench, using a jigsaw is not quite as accurate ;) at least it's not critical for this application.

Tony.

wintermute 24th November 2005 01:26 AM

No excuses now!!
 
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Well my order from from Brass and Granite audio just turned up :) very happy!!

appart from the need to go out and buy a router and some bits (as my Dad's is 7 hours drive away) I don't think I have any excuses left!! (oh and I need to go get my M4 Button heads which should have come in today too :)

as you can see from the pic I bought a few extra terminals (though I will be using 6 pairs on these MTM\'s I\'ll have enough for the Woofers too, a center and two rear effects speakers if I ever get around to doing a full HT setup... thought I might as well get the most of my shipping $

Glad I didn\'t glue the backs on before getting the terminals, they have 30mm shafts so I will need to countersink the inside of the shaft hole to take the first nut.... I\'ll have to be very carefull on the depth too... hmmmm I\'ll need to think about that one, as I don\'t have a drill press... might be time to get the cheapy one from bunnings that you can mount a handheld drill in.

I\'m off to do some work!!

Tony.


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