Yet another query re TMM vs.MTM (using DEQX) - diyAudio
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Old 17th April 2013, 01:46 PM   #1
DEQXter is offline DEQXter  United States
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Default Yet another query re TMM vs.MTM (using DEQX)

Having read many of the threads discussing the relative benefits of each, I must admit to some persistent confusion and would much appreciate some clarification as to potential issues I face with my upcoming build.

First I must confess that I am unequipped to do any serious cabinet building and am planning a drop-in approach using a pair of Usher 8948a 7's per side that go into a cubic foot enclosure measuring 22" tall. Were I using a conventional tweeter, I would probably just use an MTM approach and call it good. But I tend to like planar drivers and if I position the port in front, there is precious little real estate available. Even if I go out the rear, things are seriously cramped, particularly if I go with an AMT type device which is my current leaning--maybe Beyma or the Aurum Cantus aerostrictions. (This last preference BTW just is one of those bucket list items that dates back to my first hearing of the ESS Heils and vowing one day to build a speaker based on those and have owned a few magneto-planar based systems, most recently the big B&G 75's).

So I have decided just to build outboard tweeter modules and make those more or less ear level which drops the woofers to something like 2' and 1.5' above the floor. Now after having read up a bit I am stressing about cancellation and floor bounce, whether it should be a 2.0 or 2.5 configuration, etc. Since I plan on using a DEQX for XO and bi or tri-amping the sysem, where I do have a potential advantage is very little overlap between the mids and tweeters and perhaps? the ability to lower the XO point to between 1.5 and 2 kHz. My back of the napkin calcs show a delay of about 0.1 ms worst case for direct arrival times but I know this is making some awfully simplistic assumptions. So looking for some enlightenment that will maybe improve the odds of getting a really good implementation vs using a pair of these:

Thanks in advance for any help to this noob.
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Old 18th April 2013, 12:33 AM   #2
terry j is offline terry j  Australia
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just a tip, when designing your speaker never allow yourself to think 'I'll use the ability of the deqx' to overcome any sort of basic established speaker building principals.

It cannot make a silk purse out of a sows ear matter how much it might make it better.

So now I'll let the technical points be addressed by those that can help you.
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Old 18th April 2013, 02:11 AM   #3
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With 7" drivers and a crossover 1.5k+, you'd want a TMM to be a 2.5-way.
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Old 18th April 2013, 04:29 AM   #4
DEQXter is offline DEQXter  United States
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Quote:
Originally Posted by terry j View Post
just a tip, when designing your speaker never allow yourself to think 'I'll use the ability of the deqx' to overcome any sort of basic established speaker building principals.

It cannot make a silk purse out of a sows ear matter how much it might make it better.

So now I'll let the technical points be addressed by those that can help you.
So terry, appreciative of the advice and hope--really really hope that dropping a couple grand on some respectable drivers/cabinetry is not the proverbial sows ear: I been having to save coin for far too long. But I am also concerned that I am not engaged in wishful thinking here by supposing that at least some of the problems inherent to a 2.0 TMM design might be reduced by using steeper slopes and a lower XO to minimize the region of overlap and the phase errors in just the same way that a higher order passive XO might have an advantage over say a first order XO that has significant overlap among drivers for say a couple of octaves. It other words I hope there is nothing about the DEQX beyond the ability of any high order active XO I am counting on, and if so would want to be called out on it as I have been saving my coin for some time and want to get this right! The DEQX just happens to be one of the few toys from a much more prosperous time I still own, and have become quite attached to it as it is the one purchase that got me out of the vicious cycle of upgrade-itis and paying for the college educations of other peoples kids--namely my favorite audio dealer's, and showed me the potential once and forever of DIY.

As an example: Say I run the filters at 100 dB/octave with a XO=1500, if my math is OK, the f10 points will be like 1450 and 1575Hz. Now for grins compare this to a 4'th order network which many might consider steep, the 10dB down points will be 1225 and 1840--so in one case I have overlap of 100 Hz and in the other, over 600. Granted there are some philosophical tradeoffs (or engineering compromises) and potential irritating discontinuities associated with what might be called a lack of blending, just as there will always be a dyed in the wool camp that insists first order XO's are and have always been the most natural sounding. It all depends and what works for me is to just get it (the transition) over and done with. I listened to the Ushers for quite a while with and w/o DEQX eq before committing, and can only hope that the transition from moving coil to planar bugaboo doesn't bite me in the ****. If so I'll bail and retool my thinking. That's a big unknown as I have not had the opportunity to listen to the newer AMT's.

I guess my original question (and dumptruck I would love to hear your rationale), is why the 2.5 might be preferred given the tiny slice of shared bandwidth we are looking at. I truly want to understand as I can easily add an inductor to the 0.5 woofer which seems to have its set of +/-'s and BSC can be dialed in with the DEQX using one of 10 parametric EQ's. Is it something else besides the lower woofer-tweeter collision? Just trying to get a handle on all of this before creating sawdust.... The proof is always in the listening and if someone can suggest a XO point, I'll invest in a couple of coils to do some serious A-B listening.
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Old 18th April 2013, 04:34 AM   #5
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Consider a modular (MT) W design, perhaps with the goal of designing one high'ish efficiency woofer with two different MT tops as time and money allows. AMT planars are typically expensive, and mated to higher quality ~95db/watt midbass like the Lambda TD10M or Beyma 8P300. There are several good DIY Usher 8948a + dome tweeter MT designs that can go above a woofer.

There are several well written, and well reviewed papers on M-T-M spacing with equations plus measurements for both the M-T and M-M spacings required to avoid lobing. These papers prove that an MTM using 7" midbass speakers with any Xover over ~700Hz will have noticable lobing.

Biro Technology

Vertically Symmetric Two-Way Loudspeaker Arrays Reconsidered
---------

If you are set on using just the four Ushers, then a 2.5 way (TM)M would be a good choice, where the low frequency M can have the Xover frequency and height adjusted for baffle step and floor bounce effects.

Don't forget rounded edges for diffraction control.
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Old 18th April 2013, 11:40 AM   #6
terry j is offline terry j  Australia
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maybe you took me the wrong way?

Don't fret, I use two deqxs in my system (four way plus subs), ALL I was 'latching' on to was Since I plan on using a DEQX for XO and bi or tri-amping the sysem, where I do have a potential advantage is very little overlap between the mids and tweeters and perhaps?

Sure, it was a question, yet also a question set against what seemed to be a recognition of possible 'poor' driver layout if you will.

My simple point being, no matter how wonderful a unit like the deqx may or may not be (and obv I think it is a good one) it is best used to maximise a good design rather than 'make amends' for a less optimum one.

I most certainly was not commenting on the quality or otherwise of your driver selections.

Was more a philosophical point if you will.

Sorry if I offended.
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Old 18th April 2013, 02:11 PM   #7
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Quote:
Originally Posted by LineSource View Post
Consider a modular (MT) W design, perhaps with the goal of designing one high'ish efficiency woofer with two different MT tops as time and money allows. AMT planars are typically expensive, and mated to higher quality ~95db/watt midbass like the Lambda TD10M or Beyma 8P300. There are several good DIY Usher 8948a + dome tweeter MT designs that can go above a woofer.

There are several well written, and well reviewed papers on M-T-M spacing with equations plus measurements for both the M-T and M-M spacings required to avoid lobing. These papers prove that an MTM using 7" midbass speakers with any Xover over ~700Hz will have noticable lobing.

Biro Technology

Vertically Symmetric Two-Way Loudspeaker Arrays Reconsidered
---------

If you are set on using just the four Ushers, then a 2.5 way (TM)M would be a good choice, where the low frequency M can have the Xover frequency and height adjusted for baffle step and floor bounce effects.

Don't forget rounded edges for diffraction control.
Linesource--many thanks and will get back to you when I have a bit more time. Actually I started right where you suggest with bass cabs and a Mid/highs module on top. But then on the advice of nearly everyone while lurking decided that a 3 way might be a bit more of a bite than I am prepared to chew: in other words get a 2 way right (or 2.5) then tackle the 3 way. Nice thing about the ushers that the vented alignment and sealed (critically damped) box volumes are the same--so I just plug the vent when I go 3 way and am good to go. Part of using two/side was to push efficiency and max spl so as to kind of keep up with the AMT's should I go there.Any way morre later,appreciate the input.
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Old 19th April 2013, 12:21 AM   #8
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Quote:
Originally Posted by terry j View Post
maybe you took me the wrong way?

Don't fret, I use two deqxs in my system (four way plus subs), ALL I was 'latching' on to was Since I plan on using a DEQX for XO and bi or tri-amping the sysem, where I do have a potential advantage is very little overlap between the mids and tweeters and perhaps?

Sure, it was a question, yet also a question set against what seemed to be a recognition of possible 'poor' driver layout if you will.

My simple point being, no matter how wonderful a unit like the deqx may or may not be (and obv I think it is a good one) it is best used to maximise a good design rather than 'make amends' for a less optimum one.

I most certainly was not commenting on the quality or otherwise of your driver selections.

Was more a philosophical point if you will.

Sorry if I offended.
None taken whatsoever. What I was trying to get at is that I suspect many of the reasons that a TMM 2.0 is traditionally rejected in favor of a MTM or 2.5 implementation has to do with are 1) The overwhelming popularity of a D'Apppolito config whose engineering strengths may have more to do with minimizing the shortcomings of relatively shallow passive XO's than an absolute superiority given 2 midwoofers and a tweeter with no real constraints on XO topology. (I don't know the answer to this obviously but being the kind of guy that believes more often than not "conventional wisdom isn't," questioning whether it makes sense to at least reexamine the alternatives) and 2) that again most MTM configurations employ tweeters with relatively high XO's so that even small path length differences between the two woofers can result in nulls where the tweeter ain't present to fill in the gap. 3) Many audiophiles seem to be in love with visual symmetry even when they know it may be for the worse.

So in short, the question I was trying to phrase and did a lousy job of is given some major technical differences such as a tweeter comfortable at 1500 Hz and ultra steep XO's are 2.5 or MTM systems necessarily superior? Which is sort of the inverse position of starting with a flawed design and counting on the DEQX to bail me out. So no offense taken--just trying to clarify the question, which after reading the link provided to Biro Audio seems more open than ever.
Cheers,
John S
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Old 19th April 2013, 01:52 AM   #9
terry j is offline terry j  Australia
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I'm glad then that I was not taken the wrong way.

I can't help you too much with any lobing technical questions, tho I wonder if it is actually true re the favored position of MTM. Maybe it is only 'lately' but I have the impression I am seeing quite a few TMM around. Perhaps the pendulum has swung a bit on that one?? (my gut feel is that ten-twenty years ago MTM were definitely the majority??)

Personally, I would question the 'superiority' of MTM, I really don't 'like' them at all. Not that that is saying TMM is superior, don't have much experience with them. I find TM (W I guess) to have a much larger soundstage.But then again my room has very high ceilings, and maybe one of the advantages of MTM is to reduce ceiling reflections (dunno if that is even true mind)

I think you could be right about the possible reasons you gave about MTM being so popular, 'they are everywhere' (becomes 'so that is what we all should do'..ie fashion) and visual symmetry.

I once read on the net a persons description of 'why I don't like D'appolito' or somesuch (am mentioning this because for the life of me I cannot find it again...this might jog someones memory) and as I read it I found myself nodding and saying 'yeah! that's right!' with every point.

So perhaps the upshot of all of that is, like everything, there are differing viewpoints and hence yes, you wondering if MTM is inherently superior is a valid one.

Any thoughts on adding subs eventually?
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Old 19th April 2013, 04:09 PM   #10
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Terry,

Hi again. Subs? Definitely and would love to get your take on getting the most out of the DEQX as my former attempt not so good with music-used a pair of ridiculously long throw 18's in an IB enclosure with the back half directed into the crawl space. Enough output at 15 Hz to break out the windows but also good at exciting the entire flooring of the house-a brand new house at that. But a major whoopee for those into seat of the pants tactility, but I digress.

So will be looking for subs in the future once I get the mains dialed in. As I mentioned, at that time I will likely seal the enclosure which gives a critically damped alignment with an f3 of around 85 Hz. Likely run a stereo pair of subs and look for 10's or 12's that can be stuffed into a small enclosure. Tc Sound has something good IIRC.

And you may be absolutely right on regarding more recent trends. I'm just getting back into serious audio after a slumbering interest of a few years. What I did was just google MTM vs 2.5 or variations thereof and read all I could find--which really wasn't all that much... and which prompted this thread in the first place.

So at this point I'm feeling pretty good about the choice of running a TMM config. Will start with 2.0 listen, measure, tweak and if I don't like then get a couple of coils.

And after some soul searching, am gonna go with AMT's--just a matter of choice between Aurum Cantus (which I can find precious little about in DIY circles except they measure well) and the much more prevalent Beyma's which cost a bit more but aint Chinese. Of course I would love to get my hands on some of the ADAM's but don't see that as an option just as there are some damn fine ribbons that can't be had.
Cheers
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