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.
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.
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.
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:
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/0911/slot_loaded_open_baffle_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.
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/0911/slot_loaded_open_baffle_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.
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The 2002 is not done
There "only" thing left is to finish the wiring under the dash and exhaust work. Sadly the wiring is taking much longer than anticipated and I haven't had much dedicated time at all lately to work on it.My 21's are still hanging from the ceiling. I love them there as they do not take up any floor space. I did draw up some more OB designs, however I do not have any plans to build them as of right now. The ported 21 idea was just that, an idea and was mainly looking for feedback. I feel that you can get much more bang for your buck and space by going with some SI HT 18s or some FI subs. I think those would be better choices. If I had to do it all over again, 4 18s from either SI or FI would be the ones I'd get.
@CharlieLaub
The OBs are about 24" out from the rear wall. The right panel is only a few inches from the wall and the left is about 6-7" away. The total open area is rather large, so I was not expecting any resonances to occur. If my room was larger, I would have much liked to keep the drivers on the floor to get some good "bounce" and a bit further from the rear wall.
I'm looking forward to your prototype.
The OBs are about 24" out from the rear wall. The right panel is only a few inches from the wall and the left is about 6-7" away. The total open area is rather large, so I was not expecting any resonances to occur. If my room was larger, I would have much liked to keep the drivers on the floor to get some good "bounce" and a bit further from the rear wall.
I'm looking forward to your prototype.
Well, I bought the woofers in April of 2013 and now it is July of 2014 and the boxes are finally done and I am getting ready to put the woofers in them. I decides to go with 24" square by 16" deep U-frames. The actual cone of the woofer is 18" across, so that is the size of the mounting hole for the woofers. I have taken pictures of the build as it progressed. After they are running I will post the photos and comment on the success or lack thereof.
WOOHOO! Finished the boxes, mounted the woofers and plugged them in. (Pardon my exuberance, but I am always amazed when something I build works the first time Sensitivity is about the same as the Alpha 15s on open baffle that they are replacing. Didn't get much chance to play them last night. Put on the Stereophile test CD #2, bass tracks, and got solid response down to about 30 hz or so. Put on Jennifer Warnes "The Hunter" and actually got a favorable response from the significant other, who normally couldn't care less about what we listen to music through. It was getting late, so I didn't want to annoy my apartment neighbors. Will post pics soon. I did want to mention that I tried a different (for me) box building technique. For this build I used "System 3" 15 minute epoxy, using their cartridges and mixing tubes in a caulking gun. I was originally going to just butt join the boxes, but got cold feet and put in wood bisquits at the joints. Turns out these were not necessary. After doing a dry fit to make sure of my assembly sequence, I squeezed glue on the mating surfaces with the caulking gun, spread it a bit, assembled the first box using a metal strap box clamp. By the time I got the second box set up to assemble, the first box was rigid enough to take the clamps off. By the time I got the second box glued and clamped, the first box was set enough to lift and move around. I didn't use epoxy for the strength, but for the fast setting time. System 3 also makes a 5 minute epoxy, but I don't think I could assemble the boxes fast enough to use it. 2 big boxes assembled and ready for finishing in 30 minutes. I will definitely use this glue for the next set of boxes, which will be new cabinets for my Fostex 166en full range drivers that match the MCM boxes and will sit on top.
Sensitivity is about the same as the Alpha 15s on open baffle that they are replacing. Didn't get much chance to play them last night. Put on the Stereophile test CD #2, bass tracks, and got solid response down to about 30 hz or so. Put on Jennifer Warnes "The Hunter" and actually got a favorable response from the significant other, who normally couldn't care less about what we listen to music through. It was getting late, so I didn't want to annoy my apartment neighbors. Will post pics soon. I did want to mention that I tried a different (for me) box building technique. For this build I used "System 3" 15 minute epoxy, using their cartridges and mixing tubes in a caulking gun. I was originally going to just butt join the boxes, but got cold feet and put in wood bisquits at the joints. Turns out these were not necessary. After doing a dry fit to make sure of my assembly sequence, I squeezed glue on the mating surfaces with the caulking gun, spread it a bit, assembled the first box using a metal strap box clamp. By the time I got the second box set up to assemble, the first box was rigid enough to take the clamps off. By the time I got the second box glued and clamped, the first box was set enough to lift and move around. I didn't use epoxy for the strength, but for the fast setting time. System 3 also makes a 5 minute epoxy, but I don't think I could assemble the boxes fast enough to use it. 2 big boxes assembled and ready for finishing in 30 minutes. I will definitely use this glue for the next set of boxes, which will be new cabinets for my Fostex 166en full range drivers that match the MCM boxes and will sit on top.Ah, I had forgotten about this thread, and my post. Just re-discovered it now, so I will give a follow up.
Initially I was thinking of a large open baffle subwoofer, something on the order of 2x3 or 2x4 meters (!) with four 12" high-excursion subwoofers mounted around the center line of the baffle and near the floor. But after thinking about it for awhile I became concerned that I would go through an expensive construction project (for such a large structure) only to end up with a resonating system that would not work well as a subwoofer. This is because I was envisioning something placed relatively close to the wall compared to your panel system (see post #15), e.g. on the order of 12" or less. I fear that such a small separation will make it behave like a transmission line or long U-frame, and that has certain drawbacks and introduces additional complexity. For the location in which I want to install the sub, you approach the room from the direction that would be looking at the "side" of the large panel, and something sticking out into the room 24" or more (to reduce the tendency of the rear space to resonanate) would not be well received by my wife! I have now decided to use these drivers for large sealed subs, since they are a perfect match for four plate amps that I current own.
But the original idea of an OB panel sub has not left me, and I may try something along the lines of what you have constructed except on the floor. I have a pair of 18" Peavey LowRider drivers that I can use and have modeled these in a wide floor standing OB (e.g. 30"H x 96"W). The response looks promising. This would also be a lot simpler to construct, even just with some temporary material for the baffle, since it would largely be along the floor. So this is the way that I will probably go with the OB sub concept. Even that might not pass the sniff test by wifey, but it would be something that I can easily pack up and bring to DIY events.
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