MTM -vs- TMM

[Coenlaf]

I am making a new pair of loudspeakers, and I am thinking about an MTM design. A friend told me that the centers of the Mid's have to be a certain distance appart. Is this true? If so, how do I calculate this?
I am using 2 Dynavox 6.5" drivers from Parts express crossed over at 2000htz (tweeter is Morel MDT 33 if that matters.)

Also, I have seen TMM designs with the bottom Mid spaced much lower on the baffle. Some about 6 inches or so below the top mid...some placed near the bottom of the speaker. Is there a reason for this? How do I find the size of the space between mids that works for this design?....Thanks

[sardonx]

If you haven't already found it, the distance between the center of your tweeter and center of your mids should each be equal to one wavelength of your crossover frequency. In your case it's 2000 hertz. To get that you take the speed of sound, 330 meters per second, and divide it by your x-over frequency, 2000 cycles per second. That's .165 meters, or 6.5 inches. That's your distance.

In my opinion MTM sounds "big", while TM sounds more natural.. which is more what properly designed TMM will sound like I assume, but I have no experience with TMM.

[tinitus]

A TMM is best done as a 2.5way

[schmutziger]

Originally posted by tinitus
A TMM is best done as a 2.5way

Why?
Difficult to compensate bafflestep/match sensitivity with two woofers and only one tweeter?
Loobing issues?

Should the lower woofer be tilted upwards to lower floor reflexes and/or increase direct sound?

Why shouldnt you place the woofers as tightly together as possible?

[tinitus]

More than one mid will always present a problem
But ofcourse it relates to woofer size, and bigger being worse
Also xo point will have influence, lower being better

It has since very long been known that a mid should be positioned as closely as possible to the tweeter
Some have even cut the tweeter faceplate, or mounted it behind the mid chassis, to get mid and tweeter closer together
Coax being the ultimate, despite its other issues

So its logic that with a TMM, where the bottom mid is further away from tweeter, there is some phasing issues
I believe that MTM was "invented" fore that reason
But in a MTM the two mids are even further apart from each other, which also present a problem
A low xo point is best fore that configuration

But many people likes the look of a MTM
And Im not saying that it cant work
And it surely have some advantages
But at the same time introduces compromises, that should be known to deal with them

A 2.5way is kind of genious, as simple as it is
I saw it first in french JMlab many years ago, a small TMM with 5" woofers
I first thought it was bunggled cheating, and wasnt proper speaker design
Whether JM((JaqueMahul?) "invented" it or not, I dont know
But it has since proved its own worth

I have never done a 2.5way, and I sure would expect som phase issues, but nothing worse than other xo designs

edit
Fore a good MTM design, I have thought of using 3x 4" TB "ti" fullrange
This means that xo point could be placed very low
I might also use a ribbon supertweeter
But its not really cost effective, and it may actually work better as a 2.5way
The beauty of it is that one could try any of above configurations

[schmutziger]

Interesting stuff tinitus!

I'm very keen on an idea of mine
its a modular satelilite system to be paired with 2 or 4 10" bassmodules

it will consist of 1 1" tweeter
and 1-4 6.5" woofers.

the 2 and 4 woofer versions the outmost woofers will probably be shunted at ~450-500Hz

It will either be vented fullrange or tuned to ~70-75 Hz
or sealed tuned to ~70-75 Hz, not decided yet

The problem is i don't know how to compensate for the back wall gain and the phase issues. amongst other things

I really have to get hold of a good book in the subject and buy a mic!

So if you've got any good reading tips, pleas post!

[ashok]

I really have to get hold of a good book in the subject

.....Absolutely !

...and buy a mic!...

Yes ! If you are serious about what you are building .

Is it going to be easy ?......smooth sailing ? ...........

BUT it's going to be GREAT fun ! We are all here to support you !

[tinitus]

Originally posted by schmutziger


The problem is i don't know how to compensate for the back wall gain and the phase issues. amongst other things

Many things depend on size of your listening room, and listening distance, and distance between L/R speaker

Big speakers doesnt work well in very small rooms with short listening distance

Placed against wall is problematic too
It will compromise the fine aspects of reproduction, such as soundstage etc
Smaller design would do best
Going BIG in a small room, maybe possible with a very tight closed low Q bass design...and response above that will have to be "customised" to the special situation
But much too little lowend is a risk too, which I also think is really awfull...though too much is worse

But my listening distance to a 3way is only 2-2.5 meter, so it is possible
But I still think that a smaller 2way would be better
But up against a wall ?...probably NOT a 2.5way, maybe with the exception of using very small woofers, like 5"
But with a 3way, a very low xo point may help...that would be more like a 2way +sub

But its a very fine line to walk, and exstremely difficult to get absolutely right

Active EQ on woofer might help a lot

Active room EQ might be nice too

[schmutziger]

Originally posted by tinitus

Placed against wall is problematic too
It will compromise the fine aspects of reproduction, such as soundstage etc

I was thinking about 10-20cm from backwall, which is ca 5cm absorbpanel on concrete wall

Originally posted by tinitus

But my listening distance to a 3way is only 2-2.5 meter, so it is possible

I will have 2,5-3 meters

Originally posted by tinitus

But with a 3way, a very low xo point may help...that would be more like a 2way +sub

As i wrote it is a satelite system, I already have the bassmodules.
2x10", active XO @ 80:ish
2 of these:

Originally posted by tinitus

But its a very fine line to walk, and exstremely difficult to get absolutely right

Active EQ on woofer might help a lot

Active room EQ might be nice too

Check
check
check

[kyrie48]

Getting ready to build a 2 way MMT with seated ear-level between the two midwoofers. From what I can figure, with the speakers/drivers that high, the (two seperate) cancellation nodes should still be above and below ear-level. But I can't measure/confirm this.

Kyrie