MCM 21" tuned LOW thoughts? - Page 3 - diyAudio
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Old 29th March 2013, 09:38 PM   #21
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Nice!
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Old 16th April 2013, 10:10 PM   #22
Decker is offline Decker  United States
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I just bought a pair of these (MCM 21") to try in open baffle. Very interested in your experience using them. How big are your baffles? Did you use any LF boost to get the frequency plot you posted? What kind of crossover are you using? (frequency and slope) I am thinking about a pair of H-frames, 24" square by 16" deep.
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Old 16th April 2013, 11:41 PM   #23
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There is zero EQ on the 21's - simply a 18db/octave crossover @30Hz. I have tried the 21s in four configurations. The first was all four on the floor with pieces of wood holding up the magnet - very impressive. I then had them stacked (2 per side) with no baffle - still not bad. I then added a ~2'x4' baffle - a pretty good gain in performance. Current setup is 2'x4' baffles suspended from the ceiling - mainly space restraints and the wife likes them. There is minimal baffle movement at large (relative anyway - my best guess is 5-6mm) excursion.

The crossover does eat a lot of headroom before the amplifier - I'm using a behringer A500 (4ohms/channel) with the gains all the way up (almost). I am also using a consumer to pro-level booster with it's gain almost maxed.

With this setting they blend very well with the mid/tweeter combo. Note - above ~60hz the 21s need very little power. It is not until the 30-40Hz range where the amplifier really starts pumping out the juice - and that's if I'm listening at high(er) volumes. FWIW - the four of them have handled almost club level volumes without over-excursion with the exception of a select few songs that dip into the 20Hz range. I can activate the 40hz high-pass filter on the receiver and volumes can get into the ridiculous very easily.
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Old 17th April 2013, 01:58 AM   #24
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Man thats cool!
I remember seeing my first 21! A Tekton 21" woofer it was $2100 ea. They were using 2 in a in-wall theater install and had put 2 in a truck for someone, that was like 22 years ago.
Now they're available at 5% of the price, wow.
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Old 17th April 2013, 02:54 AM   #25
a.wayne is offline a.wayne  United States
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MCM ..!

Dont those things have an Xmax of about zero .... they may very well be non linear at 6MM...

Regards ...
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Old 17th April 2013, 09:46 AM   #26
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Could be non linear, but thankfully at those low frequencies, non linearities are much harder to hear Most of the time, even at moderate volumes excursion is less than 1mm - very little movement.
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Old 25th April 2013, 10:51 PM   #27
3GGG is offline 3GGG  Australia
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Any progress?
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Old 29th April 2013, 11:50 PM   #28
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Sorry no progress. This was a mere fun simulation run - and maybe a build in the future. As of current, my focus is on finishing up a 1973 BMW to bring it back on the road.

Ben K.
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Old 4th March 2014, 10:17 PM   #29
a.wayne is offline a.wayne  United States
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Finish the 2002 yet , what about the 21 inch sub ...?
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Old 4th March 2014, 10:46 PM   #30
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I'm glad that this old thread popped up. I am very interested in something like what is shown hanging from the ceiling in this pic from post 15:
Click the image to open in full size.

The reason that this is interesting to me is not because they are using the MCM21" drivers but because this kind of mounting of an open baffle near a wall is of interest to me. Once you start to bring the wall into the equation, you get something that behaves more like a line than an open baffle. In the picture, the baffle is far away (e.g. I am guessing 12"/30cm or more) and all sides are open. This means that the Q of the line will be low. But you still get delay from front to back. Delay is the reason that the low frequencies do not suffer from a complete acoustic short circuit and cancel out. When the line becomes smaller and sides are closed, it will tend to have a resonance. John Kreskovsky has explained that for a U-frame this resonance causes a negative group delay that tends to cancel out the delay. Adding stuffing helps to damped the resonance which reduced the group delay, and the low frequency behavior is better.

I have a couple of drivers that I have wanted to try like this, in a large open baffle near (and parallel to) a wall. I need to experiment to see how close you can get to the wall before the resonance of the cavity starts cause problems. This space could also be stuffed, although I am not sure how much good that does at low frequencies. It may reduce higher modes, however.

The advantage of this approach is that a rather large open baffle subwoofer can be placed out of the way, at the wall, but you still get the benefits of an open baffle system. Compared that to a U-frame, where a long length would require it to stick out in to the room, and you can see the benefits right away.

I will probably work up a prototype in the next few weeks time and see what happens. I will probably use something along the lines of the "slot loaded open baffle" project that Nelson Pass did a couple of years ago, except as a subwoofer only:
http://www.enjoythemusic.com/diy/091...le_speaker.htm
If you read the link and see the pic of the finished system you can see why putting a huge baffle out into the room can be a problem... thus the idea to see how close to the wall it can be, to help the spousal approval factor improve a bit.

Last edited by CharlieLaub; 4th March 2014 at 10:58 PM.
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