Why MTM's?

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charmerci

Why MTM's?
« on: 3 Jul 2009, 04:33 pm »
Jim, Dennis,

If they sound not-so-good off the tweeter axis, -

"For MTM's, if you're above or below that point, the arrival times of the two woofers will be different and they will start to cancel out in the midrange.  Check out the last graph at the bottom of this link.  http://murphyblaster.com/content.php?f=gem_mod.html "

why don't you build the Songtowers like the one-off S2's?

http://www.audiocircle.com/index.php?topic=65726.0

zybar

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Re: Why MTM's?
« Reply #1 on: 3 Jul 2009, 04:35 pm »
What's so hard about listening at tweeter level?   :scratch:

George

charmerci

Re: Why MTM's?
« Reply #2 on: 3 Jul 2009, 04:49 pm »
What's so hard about listening at tweeter level?   :scratch:

George

Hmm, you never do anything else while listening to your stereo? 

Anyway, sitting for hours can be difficult.  :)

zybar

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Re: Why MTM's?
« Reply #3 on: 3 Jul 2009, 04:56 pm »
What's so hard about listening at tweeter level?   :scratch:

George

Hmm, you never do anything else while listening to your stereo? 

Anyway, sitting for hours can be difficult.  :)

I am being serious and not trying to be a smart-ass with this question...

Why is it difficult to sit for hours?  Do you have a comfortable listening chair?

I love sitting for hours (with breaks of course) in front of my stereo listening to music.   :dance:

If I am seriously listening to my stereo, I am seated.  I will most likely have a laptop open as well, but that's it. 

I don't expect, nor require, optimal sound for other scenarios.  I am also not concerned with optimizing my sound for more than single chair. 

I realize others might have different requirements and that their needs are equally important.

George

jsalk

Re: Why MTM's?
« Reply #4 on: 3 Jul 2009, 05:11 pm »
Jim, Dennis,

If they sound not-so-good off the tweeter axis, -

"For MTM's, if you're above or below that point, the arrival times of the two woofers will be different and they will start to cancel out in the midrange.  Check out the last graph at the bottom of this link.  http://murphyblaster.com/content.php?f=gem_mod.html "

why don't you build the Songtowers like the one-off S2's?

http://www.audiocircle.com/index.php?topic=65726.0

First, it is not that an MTM arrangement "sounds not so good."  Joe D'Appolito made this arrangement famous and the design does offer increased sensitivity and lower distortion due to the fact that dual woofers are utilized.

With MTM's in a vertical format, as long as your ear is at a constant level with regard to the tweeter, you will not notice any change in response due to the effect you mention.  As you walk around the room, your ears are at a constant level with respect to the tweeter, so it is generally not an issue.

With a 2.5 arrangement, you will also have comb filtering where the woofer signals intersect. 

The important thing to realize is that the "perfect" speaker does not exist and ALL designs involve trade-offs.

The perfect speaker would have no more than one driver.  That is why single driver speakers have a following.  Unfortunately, there is no single driver that can handle mids, high treble and low bass effectively at the same time.  So multiple drivers are usually the answer to this deficit.  But the more drivers you add, the more difficult it becomes to tie them together into a seemless fashion.

That is the nature of speaker design.  There is no free lunch and ALL approaches have benefits and limitations. The MTM format has proven itself to be an effecive way of creating a 2-way design with higher sensitivity and lower distortion, as evidenced by the wide acceptance of this basic design format.

- Jim
 

charmerci

Re: Why MTM's?
« Reply #5 on: 3 Jul 2009, 05:12 pm »
As a fellow audiophile, I do understand the sweet spot - so I wasn't attacking you - just being silly. This be who I am.

But the question is basically asking why they chose an MTM instead of the TMM. I'm sure that there are compromises between the two types and so I'm just curious. I posted here instead of a PM because I thought the response would be of general interest to people.

Peace.

charmerci

Re: Why MTM's?
« Reply #6 on: 3 Jul 2009, 05:22 pm »

First, it is not that an MTM arrangement "sounds not so good."  Joe D'Appolito made this arrangement famous and the design does offer increased sensitivity and lower distortion due to the fact that dual woofers are utilized.


Well, I guess my wording was "not so good" as I guess I should have simply asked why he chose the MTM design over the TMM design given two identical speakers and cabinets and what were the advantages of each.

DMurphy

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Re: Why MTM's?
« Reply #7 on: 3 Jul 2009, 06:15 pm »
This is a complicated subject, and not everyone agrees on the advantages of an MTM.  The original theory was to use 3rd order acoustic slopes in conjunction with symmetrically mounted woofers to provide a very focused forward sound with controlled vertical dispersion.  Joe D'Appolito later gave up on the 3rd order slope requirement, and it's not clear how important the driver configuration is for vertical dispersion.  But with your ear on the tweeter axis, you avoid any comb filtering between the woofers.  In an MMT crossed above 1500 Hz (and virtually all are), the woofers will not be at equal distances to the ear (unless you listen between the woofers, which would be weird), and response irregularities are difficult to avoid, whether you're standing up or sitting down.  When you stand up listening to an MTM, comb filtering will occur, wherever you ear is relative to the tweeter.  But if the drivers are fairly close together, this will only be a serious issue if you stand very near the speakers (the alarming dip I posted a link to was made with the mic 1 meter away and considerably above the tweeter),  At normal distances, standing and walking around the room will simply produce a slightly more recessed sound, and it might be quite subtle.  I have ST's as my main speakers, and I'm not bothered when I stand up.  But it's best sitting down. 

Jeff B.

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Re: Why MTM's?
« Reply #8 on: 3 Jul 2009, 06:47 pm »

First, it is not that an MTM arrangement "sounds not so good."  Joe D'Appolito made this arrangement famous and the design does offer increased sensitivity and lower distortion due to the fact that dual woofers are utilized.


Well, I guess my wording was "not so good" as I guess I should have simply asked why he chose the MTM design over the TMM design given two identical speakers and cabinets and what were the advantages of each.


Let me answer that one. The same sensitivity improvement exists if the design is an MMT vs an MTM, so why go MTM? The answer has to do with what is called "lobing". I do not know if you are familiar with this term or not. But here's how it works:

Any time you use two drivers that have to cross over between each other at a certain frequency, and if the sound from these drivers does not originate from the same point in space, then you have to account for the differences in path length between them in the design.

As a result any midbass - tweeter combo that is designed to sum flat on a particular axis will have changes in this summation as you move off-axis because as the path lengths change so does the phase between the two drivers (due to delay from the change in path length). The result is that all two-way speakers have what is called lobing where the summation is perfectly in phase on one axis and "lobe" to a cancellation on another axis creating a null off-axis somewhere.

Since stereo is based on horizontal separation (L + R) and we tend to move around some horizontally, but not so much vertically, we tend to arrange drivers vertically to reduce the impact of this lobing. You do not want horizontal lobing if it can be avoided because it really messes with stereo imaging, however, vertical lobing is much easier to live with, and this is why nearly all speakers are arranged vertically.

So this brings us to the main difference between an MMT and an MTM. In an MT there will be one predicted vertical pattern of lobes and nulls along the vertical axis. However with an MMT the separation between the two midbasses results in two different drivers both crossing over to the tweeter but with different path lengths. This means that there are now two independent sets of nulls and lobes, which begins to take the form of some complex comb filtering, and can be quite audible in the midrange where wavelengths are getting shorter and the effect is getting greater. If you use a 2.5Way and roll one midbass off earlier, then you may not get the sensitivity you need in the crossover region.

However, with an MTM arranged vertically the path length changes between the upper and lower midbasses will always be the inverse of each other anywhere along the vertical axis, and the lobing will remain symmetrical at all points (mean always the same above and below the tweeter axis). We can not eliminate lobing, but we can make it much more manageable by making it symmetrical. If you are careful with the crossover design you can actually design it so the lobe of one woofer begins to fill in the null from the other woofer and you can really reduce the impact significantly. This was the basis of D'Appolito's 1984 AES paper that started all of this and he and I have discussed it in depth. This control of lobing and the fact that it is always symmetrical is the reason so many speakers use the MTM arrangement, and it is advantage it has over the MMT.

Jeff B.

Jeff B.

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Re: Why MTM's?
« Reply #9 on: 3 Jul 2009, 06:50 pm »
This is a complicated subject, and not everyone agrees on the advantages of an MTM.  The original theory was to use 3rd order acoustic slopes in conjunction with symmetrically mounted woofers to provide a very focused forward sound with controlled vertical dispersion.  Joe D'Appolito later gave up on the 3rd order slope requirement, and it's not clear how important the driver configuration is for vertical dispersion.  But with your ear on the tweeter axis, you avoid any comb filtering between the woofers.  In an MMT crossed above 1500 Hz (and virtually all are), the woofers will not be at equal distances to the ear (unless you listen between the woofers, which would be weird), and response irregularities are difficult to avoid, whether you're standing up or sitting down.  When you stand up listening to an MTM, comb filtering will occur, wherever you ear is relative to the tweeter.  But if the drivers are fairly close together, this will only be a serious issue if you stand very near the speakers (the alarming dip I posted a link to was made with the mic 1 meter away and considerably above the tweeter),  At normal distances, standing and walking around the room will simply produce a slightly more recessed sound, and it might be quite subtle.  I have ST's as my main speakers, and I'm not bothered when I stand up.  But it's best sitting down.

Ah.... we both answered. I hadn't seen yours first, but it appears they both go together well.

Jeff B.