MTM sound characteristics

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I have been reading a lot of older threads, and I have run across a number of negative statements about MTM driver arrangement.

I am considering an MTM layout for a future project. It will be a pair of 5 or 6 inch drivers with a conventional dome tweeter (no waveguide), with active crossovers. For the sake of discussion, we can assume I would use LR4 crossovers at 200 Hz and 2000 Hz. I have dual 12 inch woofer cabinets.

I do my serious listening while sitting in a specific comfortable location. I do not need a big "sweet spot".

I am interested in any opinions about the inherent characteristics of the MTM layout, both positive an negative. Obviously the MTM layout has a more narrow vertical dispersion than an MT layout, but beyond that I am interested in any theories about why an MTM layout would have a characteristic sound quality.

Thanks !
 
The weird thing about MTM is that some very highly regarded speakers like the Olson Ariel and Salk Songtower are MTM and highish crossover on top of that. I've personally liked the presentation of MTM in my home before, it had a sort of focused sound that projected to you, instead of splashing around. My hypothesis why this happens so often has to do with typical listening room with low ceilings. I think the MTM reduces reflection from the ceiling (and hard floors) in the midrange, which can be helpful. If all rooms were fantastic treated listening rooms, then probably MTM wouldn't be preferred as often anymore, because they do have very real polar response issues. But those rooms are rare.
 
I used perform a test with WTW speakers switching on/off the top woofer to compare WT and WTW (with level compensation). In my impression, WTW sounds more dynamic and analytical, but less intimate. For home audio, I would choose WT, and for critical listening WTW.
 

TNT

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MTM has an inherent problem with a none-linear power response. It's off axis FR is krocked and this cannot be avoided. It has do so with 2 drivers playing the same thing that has a c-c distance > 0. And you will listen to the off-axis response as it will mix with the on-axis via reflexions - A. Bose is right - 8/9 are reflected energy. So yes - MTM is faulty at conception. But ohh, don't they look cool. That sells.

//
 
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MT as well has a unique set of power compromises. If there is a hole in the reflected response near the crossover, the ceiling reflection can sound treble oriented, like a dis-jointed crossover may sound, which can draw undue attention to it.
 
TNT is right about the ceiling reflection. MTM is known to be sensitive to ceiling reflection, so it would be better to use them with ceiling cloud or such, like music studios. As I said, I consider MTM for more accurate monitoring, and room has to be treated well anyway.
 
Anyway, I find a real drawback of MTM is M units do not move exactly the same. Moving detached 2 membranes adds very slight something, makes overall sounds a little less organic and less intimate. I don't know how I should call this effect. It's not as bad as 6 string guitar vs 12 strings, but it's audible. For people who prefer full range to multi units, it's would not be trivial, and it should be considered.
 
Some minor points re the above:

1/ 'As bad as 6 string guitar vs 12 strings'? Since when were they intended (or used, by competent musicians) for the same thing?

2/ I design multiway speakers & single-driver types. I like MTM designs and other types running multiple LF units & do not hear the catastrophic 'non trivial' matters that you appear to be stating is some form of universal fact.

More broadly, the polar response of a multiway loudspeaker system heavily depends upon configuration and the on / off axis radiating characteristics of the individual drive units at the chosen crossover frequency, to say nothing of XO order. The terrible flaws claimed above for MTM type speakers quickly start to disappear when wavelengths become long for a given driver spacing. Most MTMs are crossed too high to avoid at least some issues; that does not mean it is invariable.
 
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Scottmoose, I use MTM everyday and it's one of my prefered topology. I was not trying to insist that it's a universal catastrophic fact, but if my post sounded like it, I apologize that it caused misunderstanding. Guitar thing is just an exaggeration, please just forget it.

Have you ever directly compared two units vs one unit? I think you'd hear some difference as I did. I can't tell if it's trivial or not for you, and if you can hear only positive effects from 2 units, that's great. :)

PS: If you would perform the test, its more obvious doing it with mono speaker, not stereo, but it probably depends on the room or speakers...
 
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FWIW, it was quite audible when I've tried dual 15" 515B, 416A Altec in MTM, TMM at 800 Hz/2nd order [500 Hz recommended], so guessing it's their ultra wide BW, modest slope order that does them in.

Altec's dual [merely] wide range 12" TMM on the OTOH is a 'treat', ditto their horizontally opposed 9844 studio monitor, so 'go figure'. :(

GM
 
Sounds likely. With the kind of BW those units are capable of, you're going to need the XO right down low & probably 4th order acoustic to keep the transition band reasonably narrow even with close spacing.

Interesting how TMM can work out better though. I've run variations that sounded more or less like a regular TM, with added dynamic range. If I had to guess, I suspect the precidence effect kicks in & we key off the upper unit. I'd draw the line at the Bottlehead Straight 8 though. Pass re the 9844 though -it'd take somebody a whole lot brighter than me to figure that one. ;)
 
Have you ever directly compared two units vs one unit? I think you'd hear some difference as I did. I can't tell if it's trivial or not for you, and if you can hear only positive effects from 2 units, that's great. :)

PS: If you would perform the test, its more obvious doing it with mono speaker, not stereo, but it probably depends on the room or speakers...

Yes, I've done it plenty of times. ;) I even had a commercial range of speakers once upon a time that scaled with additional LF units, tracking the same XO frequency & order, just level variation. I'd still be making them, but my friend & cabinet-making business partner retired, so that, as they say, was that. Granted, I cross lower & steeper than many, which significantly reduces the issues involved with multiple drivers, albeit at the price of a rather expensive filter.
 
At first i liked mtms but then the flaws started to show up so i went back to TM. The increased cone area for mtms are a huge plus but not so the offaxis cancelation, this might be a problem only if you listen close to your speakers which i do. Despite uneven freq response, coaxial drivers are the very best way to go, the sensation of a cancelation free speaker output are rewarding
 
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I find all of my TW or TMW speakers sound like a single point source, that for the most part unmasks(doesn't color) the recorded detail and IME makes them True Hi Fidelity. I can appreciate the advantages with MTM and Planning an MTM 2 woofer 3 way, but thinking they will be best suited for Home Theater because I don't believe that my definition of True Hi Fidelity can be realized, unless there's more to discover that will prove my believe to be wrong.

"M units do not move exactly the same" - Possible Cause for masking effect!

Is there more reflections or diffraction with the tweeter stuck between M and M versus TM with Tweeter at the top, that also allows for rounding or beveling the top of the baffle.
 
Yes, I've done it plenty of times. ;) I even had a commercial range of speakers once upon a time that scaled with additional LF units, tracking the same XO frequency & order, just level variation. I'd still be making them, but my friend & cabinet-making business partner retired, so that, as they say, was that. Granted, I cross lower & steeper than many, which significantly reduces the issues involved with multiple drivers, albeit at the price of a rather expensive filter.

Hello Scottmoose,

I'm starting to experiment with MTM with digital xo, active system. How steep and what type of xo have worked best for you? I have seen odd-order xo mentioned as preferred.

FWIW, listening at 7 feet away from speakers, midranges are 8" and hoping to xo somewhere in 1.2-1.8 kHz. Low, cloud ceiling. And a large tweeter in between...I realize I'm asking for trouble :eek:

Thank you!
 
Hello Scottmoose,

I'm starting to experiment with MTM with digital xo, active system. How steep and what type of xo have worked best for you? I have seen odd-order xo mentioned as preferred.

d'Appolito initially went with 3rd order Butterworth; generally speaking even order is now favoured including by Joe for most 'real world' conditions.

No single answer to the question I'm afraid, since it depends on the drivers -size, position / spacing, on baffle on + off axis responses & also what you're willing to put up with. ;) Some aren't as bothered by a degree of polar shift as others. For MTMs, I won't use more than 1 wavelength C2C separation between the midbass drivers at the XO corner frequency, and when possible lower. You mention 8in drivers & a 'large tweeter', so assuming regular cone & dome units, that presumably means you've got at least 12in C2C spacing. In that case, I wouldn't want a corner frequency > 1.13KHz, LR4. Lower would be better from the POV of the polars, but increasingly impractical with regular tweeters. Steeper would also improve this side of things, but puts more demands on the tweeter as the transition band is narrower (i.e. its forced to stay flatter for longer). So ultimately it's a question of balance & where your priorities lie -power-handling & distortion performance or a more even polar response. YMMV as ever.
 
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