I have 4 woofers and 2 tweeters and 2 crossovers from a Sony sound bar and they are built for stout! I am going to use them in a pair of MTMs made from cinder blocks for computer desk duty.
Is there a prescribed geometry for the tweeter offset from the center line of the two woofers?
Is there a prescribed geometry for the tweeter offset from the center line of the two woofers?
When did that offsetting start? The earlier MTMs I saw were all in-line vertically.
No doubt this isn't a simple question, it has to do with the mounting diameter of the drivers (how close together you can put the woofer and tweeter - I've seen tweeter flanges with cutouts for closer mounting) as well as crossover frequency.
No doubt this isn't a simple question, it has to do with the mounting diameter of the drivers (how close together you can put the woofer and tweeter - I've seen tweeter flanges with cutouts for closer mounting) as well as crossover frequency.
I always try to get the midbasses as close as possible without the tweeter "cone" edge moving very far outside the midbass cone. The closer the midbasses the lower you can cross.
In an ideal world the XO frequency at or less than the ¼ wavelength of the M-M centre-to-centre. Hard to do in the real world.
dave
In an ideal world the XO frequency at or less than the ¼ wavelength of the M-M centre-to-centre. Hard to do in the real world.
dave
You need the three MTM drivers in a line as close together as you can physically fit them.
The distance apart of the two Ms determines the lobe width/angle.
The distance apart of the two Ms determines the lobe width/angle.
I was once enchanted by the MTM configuration by its aesthetics, but when you start do the math one will realize it's nearly impossible to do any MTM because the x-over frequency would have to be VERY low in order to avoid lobing from the two MM drivers interaction, and so low can't normal tweeters do.
And if I recall my memory right, I belive the MM need preferably a 3 order filter to cut them of sharply to minimize the effects from lobing.
It's not answering your question and not very encouraging, but I consider the MTM a wet dream.
But if I may suggest, how about doing a 2,5-way system in a TMM configuration, something like this:
T = full tweeter range
M = full bas midrange
M = the lowest driver running up to maybe 100-200 Hz
And if I recall my memory right, I belive the MM need preferably a 3 order filter to cut them of sharply to minimize the effects from lobing.
It's not answering your question and not very encouraging, but I consider the MTM a wet dream.
But if I may suggest, how about doing a 2,5-way system in a TMM configuration, something like this:
T = full tweeter range
M = full bas midrange
M = the lowest driver running up to maybe 100-200 Hz
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You just need to choose a wider bandwidth "tweeter".
This is designed to XO at ~250 Hz. ¼ wavelength spacing and a convienient place to XO to deal with bafflestep.
dave
This is designed to XO at ~250 Hz. ¼ wavelength spacing and a convienient place to XO to deal with bafflestep.
dave
Third order is recommended. The quality of phase quadrature on axis offering flat power and pressure response is wanted. Lobing is allowed, as the lobes in a theoretical odd order filter are tilted and the mirror image second woofer fills in the vertical nulls.
Would it be more effective to butt the woofers together and put the tweet on top, in a TWW configuration?
By the way, these are 3" woofs and .75" dome tweet with a already developed crossover.
By the way, these are 3" woofs and .75" dome tweet with a already developed crossover.
If the tweeter has less than a standard size face place, lining up the drivers vertically, crossing as low as you can with 3rd order acoustic, you should not have any issues. I've never been a fan of offsetting the tweeter due to uneven off axis response.
Here is a helpful program for this application: Tolvan Data
Here is a helpful program for this application: Tolvan Data
Thanks Face. I don't know the crossover freq or order but will use what came with it. Just an experiment.
Hi,
Offset the tweeter to 0.6 of baffle width.
Move the bassmids a little closer together.
rgds, sreten.
Offset the tweeter to 0.6 of baffle width.
Move the bassmids a little closer together.
rgds, sreten.
Huh?? Offset anything by .6, or even .5, of its width and you're off the baffle.
I'm not disagreeing. I do not understand, and I am very interested because I'm just beginning an MTM design.
If my baffle is 10" wide, what should be the amount of tweeter offset?
Peace,
Tom E
On the (vertical) lobing issue one thing to consider is the use case. If you will always be seated, position speakers so tweeter is at ear level and it becomes a largely mute point.
On the using existing crossovers.... may not be good. Probably optimised for horizontal layout and may not be good for vertical....
Tony.
On the using existing crossovers.... may not be good. Probably optimised for horizontal layout and may not be good for vertical....
Tony.
In a MTM, lobing is a feature not a defect. Basically the off axis nulls reduce the amount of sound going into the floor and the ceiling, which improves imaging.
On the downside, the vertical beamwidth is narrow.
As far as xover slope and xover point, that's a tricky one to answer. Because the lobes and the spacing will be different with various combinations of slopes.
For the MTM that I've been experimenting with, I've been using first order low pass on the woofers and 3rd order highpass on the tweeter. This creates deep strong nulls off-axis.
By the way, I wouldn't try an MTM without some type of simulation software, the response can vary a LOT.
In particular, note that the Z axis is REALLY important. IE, if you move the tweeter forward or backwards by a single inch, the response shape changes dramatically, because you have three drivers interacting at the xover point, not two.
I am using John Kreskovskys ARPE to do the sim.
After reading all this good advice maybe it would be easier to put one of the woofers on the back side. What do you all think?
ARPE = Asymmetrical Response Pattern Estimator
https://web.archive.org/web/20110502051949/http://www.pvconsultants.com/audio/radiation/arpe.htm
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