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Old 30th January 2005, 02:58 AM   #1
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Default new morel MTM project: rantings

OK I'm starting this thread to dump stuff in with regard to my new MTM project I'm working on. Prompted because I was discussing something with kelticwizard and pretty much starting to thread jack.

Keltic, here is the nearfield measurement of one of the woofers (with both being driven)

I was expecting from the sim, that freq response would start to drop off at 200Hz and the -3db to be about 80Hz, but it's doing the opposite (and if I take 12db as the reference point 500Hz) the -3db point is about 64Hz.... now from memory the nearfield data probably isn't much use above about 500Hz, but even so, this is really not what I expected.

Tony.

edit: the big dip at about 2Khz is baffle diffraction..... need to do something about that!! it corresponds directly to the width of 175mm of the baffle.... Not so sure about the big peak, probably not to worry as this is nearfield and it's probably nonsensical anyway
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Old 30th January 2005, 03:17 AM   #2
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Just for the heck of it, here is a picture of my test environment yesterday not ideal but not to bad either. The pc under the speaker was purely for elevation. gave me a bit longer gate time, but unfortunately I couldn't quite push it far enough to get more resolution in speaker workshop.

The front baffle is very rough and was never originally intended to be used. It was more to sus out driver mountings. One thing I learned from my testing yesterday was that my idea of resessing the tweeter back into the baffle with rounded over edges was a really bad one!!!! huge trough in response between about 4.5 and 5.5K due to diffraction! (unfortunately I didn't save the response graph)

This is only a test box, final one will be different, but it has proven invaluable as a learning tool. Learn't quite a lot yesterday from my testing with the mic too.

Tony.
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Old 30th January 2005, 03:17 AM   #3
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Okay, so the Vas of the woofers is 9.8 liters each, and the the volume of the box is 6.6 liters, that is the same as one speaker with the Vas of 19.6 liters. Do I have tha correct?
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Old 30th January 2005, 03:23 AM   #4
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Ahhhh one important fact that I didn't mention. There is only one driver in that 6.6L the drivers have separate cavities.

Note I learn't something very usefull with my very dodgy braces that can be seen in this pic. The one directly behind the driver caused nast reflections, which caused a very nasty irregularity in the FR at about 700Hz! I'm now thinking that dowel may be a better thing to use as bracing!!!

The sealing foam is only for this test box, the final ones will be properly glued.

Tony.
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Old 30th January 2005, 03:30 AM   #5
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I did the math, according to the formula below.

I used the following parmeters:
Vas = 19.8 liters, (for the two together)
Fs = 34 Hz
Qts = 0.41


I don't need to sim when I have the formula. In fact, the sim is based on Small's formula, which this is.
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Old 30th January 2005, 03:33 AM   #6
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Quote:
Originally posted by wintermute
Ahhhh one important fact that I didn't mention. There is only one driver in that 6.6L the drivers have separate cavities.

Now you tell me, lol.

Let me recalc, back in a minute.
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Old 30th January 2005, 03:35 AM   #7
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Doh, all the sims I did were for a single driver since I was modelling them in separate box volumes. I guess I should do one with two drivers and just double the volume.

Tony.

edit: just realised part of the problem, the drivers have quite different specs, I guess I should model based on the average specs of all the drivers and just double it. Actually I did do the average, that is how I came up with 6L (then added 10% as is the norm), what did I do with those average specs......
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Old 30th January 2005, 03:35 AM   #8
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Know what? I messed up the calculations in that post. So it is just as well for me to recalculate anyway!
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Old 30th January 2005, 03:44 AM   #9
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I get:

Fc=53.6 Hz
Qtc=0.65

And since, according to the formula below, the output is -3.79 dB at resonance when Qtc = 0.65, then the -3 dB point is probably just above Fc, (53.6 Hz). Let's call it 56 Hz.

F3 = 56 Hz.
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Old 30th January 2005, 03:50 AM   #10
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From your Speaker Workshop graph, I think the midpoint is probably about +12 dB. So the -3 dB down point seems to be just under 70 Hz.

Looks like either the Thiele-Small parameters or my calculations are a little off.
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