Designing an MTM for Home Theater

[Siggma]

Hi all. My name is Tom and I'm an Amateur hobbyist speaker builder. I'm working on a project to replace all my current satellites and center speakers in my home theater. I live in an apartment and am currently using a KEF C1 bookshelf speaker set (4 of them) for satellites and a KEF C6LCR for center duty. I don't like the way it sounds. The satellites produce copious quantities of sound below port cutoff that overwhelms the room with LF energy and muddies up the sub crossover. I also don't like the brassy sound of the KEF tweeter. I've already chosen and purchased the woofers I'll use which also pins down the cabinet size as well. I was going to use a Visaton W130S for the woof but my test speaker indicates it will be lacking in midrange clarity. My second choice was an Aurum Cantus AC130MKII, chosen for it's bass extension (85-90hz .2? sealed) and it's low distortion. The choice was also influenced by the fact that I found 6 of them for 1/3 off regular price and shipping on all of them for $25 so I now own them.

I've tentatively decided to go with a Vifa XT25SC90 dual ring tweeter or possibly it's bigger sister the Vifa XT25TG30-04. It has an fs of 530Hz and will hopefully cross with low distortion below 2K. I'm not married to the idea of using these tweeters though. Another possible choice is one of the Dayton Audio RS28xxx series tweeters like the RS28A Aluminum or RS28F Silk dome. The goal is to cross them around 1K, which won't happen, but 1.5K is probably dooable and will probably maintain nearly flat off axis response till the tweeter begins to roll off. But I'm getting ahead of myself. On to the questions. I've never designed or built an MTM speaker either in the D'appolito style or as an MTM center speaker. The crossover questions are driving me nuts.

How do I calculate the gain, if any, of a pair of AC130MKII, 90db SPL1w/1m in a box vs a single driver?
What if I end up with a 3 db gain from the box and only 91 db tweeter?
How do I mount the tweeter to ensure it won't interact badly with one of the woofers, meaning do I go for asymmetric or symmetric tweeter placement?
The woofers are 8 Ohm so the plan is to wire them in parallel, grab an impedance curve, take measurements on tweeter axis and design a single crossover for both as if it's a single speaker. Correct?

Thank you for any help. You're a genius!

 

[Siggma]

BUMP

Bump?
Nobody ever designed an MTM?

 

[GM]

Greets!

You're kidding, right? Search this site and/or Google for 'mtm design' and D'Appolito theory' to get probably thousands of threads devoted to MTM design.

GM

 

[Soundtrackmixer]

I live in an apartment and am currently using a KEF C1 bookshelf speaker set (4 of them) for satellites and a KEF C6LCR for center duty. I don't like the way it sounds. The satellites produce copious quantities of sound below port cutoff that overwhelms the room with LF energy and muddies up the sub crossover.

I am going to seperate this comment from the one that stated you want to replace this setup. Your problem stems less from your choice of speaker(except in regards to the tweeter), but more with a poorly setup bass management settings, or a lack of setup flexibility within the bass management processing.

The 80hz setting of a THX based crossover would have worked just fine on this setup if you had it.

The tweeter is a different story, and a good reason to replace or re-mod the speaker.

 

[Siggma]

I am going to seperate this comment from the one that stated you want to replace this setup. Your problem stems less from your choice of speaker(except in regards to the tweeter), but more with a poorly setup bass management settings, or a lack of setup flexibility within the bass management processing.

The 80hz setting of a THX based crossover would have worked just fine on this setup if you had it.

I do have a THX (active LFE) crossover it and it doesn't work "just fine". It has quite a selection of crossover frequencies but they all have the same slope. The setting each produce a fixed roll off beginning at crossover FR whereas the satellites have a ported bottom end that rolls of sharply at 65Hz and is unloaded below that. It's the unloaded part that's the issue. Below 60 Hz the ported satellites are still very much "in the mix" and dump LF energy into the room that is somewhat amplified by the pressure zone of the corner and wall gain from the wall(s). It sounds muddy and boomy and I doubt it's going to sound any better until and unless I replace the ported satellites with sealed units so I can control the LF more accurately. But that's not the issue. The issue is that I'm gong to be designing an MTM from scratch and could use some experience pointers.

The tweeter is a different story, and a good reason to replace or re-mod the speaker.

I wouldn't even try to "remod" the speaker. They are designed just find the way they are, but, they have too small a woofer and they are ported. They were a poor choice on my part as a satellite. As for the tweeter, you have experience with the KEF 3/4" tangerine loaded tweeter? Have you listened to it? Other's who have heard it don't think it's brassy or harsh at all so any feedback would be helpful.

 

[Siggma]

Greets!

You're kidding, right? Search this site and/or Google for 'mtm design' and D'Appolito theory' to get probably thousands of threads devoted to MTM design.

GM

I found one (Dr K's MTM) on PE but there are hardly "thousands". What I'm looking for is a discussion of issues specific to the MTM crossover design. I'm not sure how to wire up the crossover. Common sense says to wire the two 8 Ohm woofers in parallel and use a single crossover for both but I may have efficiency issues with these drivers. If they sum as a pair and I get +6 db output I'll end up needing a 95db tweeter or I'll need to gang up two of them as well. In the end it might be better to do just that, mount a tweeter above each woofer. But that increases the complexity of the design unless I can get away with a single parallel crossover for tweet and wire them in series and a parallel on the woof wired in parallel. It gets complicated in a hurry and I could use some guidance.

 

[Silent Screamer]

For satellites any reason you don't just do T-M?

 

[Dissi]

Hi,

I don't think you will get a sensitivity problem. It's right, two woofers wired in parallel give + 6dB. But you also have to consider, that you loose almost 6 dB at low frequencies because of the baffle step.

Unfortunately the TS-parameters given by the manufacturer are inconsistent, but a SPL of 90 dB seems rather unlikely to me. I guess you will actually get 87 -88 db, and that's not louder than your tweeter.

 

[Ahmad_tbp]

i m not sure if I don understand what you r saying because of my english or because of my lack of knowledge , or because you r wrong
anyway , i m not an expert by all means but from what I know you gonna need one crossover for whole system , I mean you have to connect your woofers in parallel and connect them to your designed crossover , again means you have to see the paralleled woofers as one single unit ...!
first , if my memory serves me right you gonna end up with +6db in sensitivity of paralleled woofers ,and you don need to find a tweeter to have the same sensitivity , if you r worried about difference in sensitivity of drivers (your parallel woofers and your tweeter) you don need to double the tweeters either , you need an L-pad (driver attenuation circuit ) to equalize the different driver sensitivities (if there is any!),
this is very brief tutorial on how to design a crossover :
3 Way Crossover

but when you finish with your crossover calculations and design you need to simulate that and start tweaking , and I don think you can just calculate it and then build it ! 90% percent of time I end up with very different component values and sometimes adding or deleting a circuit or component during simulation , but maybe it s just me

 

[Siggma]

For satellites any reason you don't just do T-M?

Yes, I need more output than I can get with a single TM plus a center speaker needs wider dispersion and the more cone area it has the more air will be moved but I don't want more extension, just more efficient power curve. I don't know how to express it other than it being like a mini line array. Greater efficiency per watt, lower distortion. I could do two TM speakers side by side but its more efficient to go MTM because they woofers couple in the enclosure as well as on the baffle.

 

[Soundtrackmixer]

I do have a THX (active LFE) crossover it and it doesn't work "just fine". It has quite a selection of crossover frequencies but they all have the same slope. The setting each produce a fixed roll off beginning at crossover FR whereas the satellites have a ported bottom end that rolls of sharply at 65Hz and is unloaded below that. It's the unloaded part that's the issue. Below 60 Hz the ported satellites are still very much "in the mix" and dump LF energy into the room that is somewhat amplified by the pressure zone of the corner and wall gain from the wall(s). It sounds muddy and boomy and I doubt it's going to sound any better until and unless I replace the ported satellites with sealed units so I can control the LF more accurately. But that's not the issue. The issue is that I'm gong to be designing an MTM from scratch and could use some experience pointers.

Not trying to argue, but I have used the THX filters quite successfully with speaker pretty close to the size you have. If you set the crossover to 80hz, the THX active band pass filter has a complimentary 24db per octave slope on low and high pass. At 40hz, the output of the satellites would be down 24db's. At the resonant point of the port of your satellites, it would probably be down 8-10db. Below 60hz, the speaker is falling out of the mix, not into the mix. As far as making it better, bass traps could have easily done that if you have corner loading issues. I seriously doubt a 4" woofer(port or no port) could load the room in the way you speak, unless its output is still flat at 40hz or so. That would be in the territory of the subwoofer, not the satellites.

While I completely understand you want to design new speakers - the reasoning behind dumping your olds ones flies in the face of room/speaker interaction pretty profoundly.

I wouldn't even try to "remod" the speaker. They are designed just find the way they are, but, they have too small a woofer and they are ported. They were a poor choice on my part as a satellite. As for the tweeter, you have experience with the KEF 3/4" tangerine loaded tweeter? Have you listened to it? Other's who have heard it don't think it's brassy or harsh at all so any feedback would be helpful.

If the speaker is designed just fine, then why are you complaining about the tweeter being brassy, and the low frequency energy mudding the sound. This does not make much since.

Anyway, you are going to design a MTM. Do a search on MTM speakers here or on Google, there is ton's of information.

 

[Siggma]

Hi,

I don't think you will get a sensitivity problem. It's right, two woofers wired in parallel give + 6dB. But you also have to consider, that you loose almost 6 dB at low frequencies because of the baffle step.

I wasn't necessarily after an increase in efficiency by wiring them in parallel. The woofers are rated at 90 db 1w/1m. If I put two drivers in the same enclosure wired in parallel or series they "couple" acoustically and the net SPL is often greater than a single driver. If I understand correctly thats partly what Joseph D'Appolito is famous for discovering. He also pointed out that an 18db slope on each driver will produce a complementary phase situation between the drivers and the tweeter reducing horizontal lobing (vertical in a center channel speaker). The theory is the same regardless of gravity. Following his stated design, as I understand it which is barely, an MTM would have smoother vertical response and horizontal response might suffer as a result. Series wiring is guaranteed to produce some phase shift but it might end up being complementary to the roll off and crossover design. A single crossover driving a series wired pair would have different phase relationships between drivers than a single crossover driving parallel wired drivers. Anyway, the point was if the acoustic coupling gain is too high I might not be able to match it with the tweeter I want to use. The Vifa XT25TG30 is rated at 91 db @ 2.83v which is more than one watt into 4 Ohms so it's actually only about 88-89db(guess) 1w/1m. The Dayton Audio RS28A is rated at 88 db 1w/1m which also may not be able to keep up. I'll have to see how much I loose in the crossover and how efficient it really is. It might drop a couple db once I apply more inductance or they may not really be that efficient to begin with. What I was hoping for was more information about how to predict the net output of dual radiators in a single enclosure. I've read that sometimes the phase of a woofer is not complementary with itself over the frequency range causing undesirable issues. Its also possible one driver doesn't phase track the same as the other but I can't imagine that would produce a huge effect. It's just a huge amount of information and I'm' probably over analyzing it.

Unfortunately the TS-parameters given by the manufacturer are inconsistent, but a SPL of 90 dB seems rather unlikely to me.

Many of the high quality mid woofers, especially those with hard cones, have high efficiency although I agree, it's uncommon.

I guess you will actually get 87 -88 db, and that's not louder than your tweeter.

Well, I've seen plenty of woofers rated at 90db although often it's at unusable frequencies. Some of them are rated 90 db from 1K to 8K but 89 below that. This one I think does actually have that much efficiency due to its light cone and strong motor. See HERE The only way to know for sure it so test one, which I'll do in a day or two when they get here.

 

[Siggma]

Not trying to argue, but I have used the THX filters quite successfully with speaker pretty close to the size you have. If you set the crossover to 80hz, the THX active band pass filter has a complimentary 24db per octave slope on low and high pass.

Sounds like you're trying to convince me what I have is just fine, or that if I can't make it work now I won't be able to later either. After reading the whole thing it kind of sound like "shut up and live with it". ??? I haven't done any room correction so much of what you say about correcting the room has merit. But I don't like the metal dome tweeter in the KEF speaker. It sounds brassy and hard on movie soundtracks to me. I don't like the sound of a 4" ported satellite straining to keep up. It sounds mushy and loose and has very poor bass definition. I have not been able to adjust the current system to what I consider a usable state with the surround processor crossover settings alone and I do not believe I will be able to unless and until I replace the satellites. I do not think a satellite should be ported. It's a personal opinion that I formed after I bought the KEF speakers whether ya like it or not. Without going into the rest of this, the thread is about the issues involved in designing an MTM not whether or not I like KEF tweeters or whether they are well designed or even about adjusting THX crossover alone. I'm trying to do my homework ahead of time so I have a better idea what I'm in for when I get drivers mounted and start taking measurements. Even though your comments sound a little out of place, even "KEF-ish.." I do appreciate you taking the time to write. Opinions are what makes the world of society go round, and round, and round...

 

[Siggma]

Not trying to argue, but I have used the THX filters quite successfully with speaker pretty close to the size you have.

Not in my room you haven't. I have an odd sized room. If the surround processor settings worked I wouldn't mention the room or the sats. But they don't work in my room with my sats or my sub.

This is why I'm asking...
http://forum.blu-ray.com/showthread.php?t=89614

I also don't think my current MTM (KEF C6LCR) is designed as a D'Appolito. I think it's designed as a horizontal center channel speaker.

 

[Moondog55]

Have you tried using your KEF centre in the vertical orientation yet?
I never use the C in the horizontal and I use a cheapish Sansui system ( cheap to buy but well designed and very well made ) and they use the same XO in the TM surrounds as they do in the MTM centre
Sansui uses an 8R mid in the 2-Way surrounds and 2*4R mids in series in the centre but the XO is the same in both speakers.
I can't remember the model number but it was in production until last year
Mid drivers are Peerless clones BTW ( 5 inch ) but not as smooth obviously, generic XO first on the mid and second on the tweeter at 4k, 1 resistor on the high end
It isn't perfect but it is OK until pushed really hard.
point of all this is I have never had the problems you allude to. I'm wondering if adding a small .5 woofer to the surrounds would give you better bang for your buck?
I'm thinking of adding a .5 woofer to my front L&R
OH I have my surround system set to Large speakers and I modified them by stuffing and sealing, they sound better stuffed and sealed

 

[wintermute]

Yes, I need more output than I can get with a single TM plus a center speaker needs wider dispersion.

The latter part (wider dispersion) is something that an MTM center (unless you have it vertical) will not give. If you do an MTM on it's side it will actually have very narrow dispersion, as the high frequencies get channeled by the flanking mids.

Tony.

 

[Siggma]

Have you tried using your KEF centre in the vertical orientation yet?

I am now... It sounded odd turned horizontally, it ruined the stereo image of the front speakers, now it sounds brite and nasal. The C1's are fairly flat, I've measured them. I don't know how the C6LCR measures.

From what I'm reading I won't be doing the standard MTM for center speaker. The center speaker needs to be a point source so it images between the left and right sats in stereo space. That means good horizontal phase tracking which won't happen with an MTM.

It isn't perfect but it is OK until pushed really hard.
point of all this is I have never had the problems you allude to. I'm wondering if adding a small .5 woofer to the surrounds would give you better bang for your buck?

I'm selling them, period. I already bought the drivers I'm going to use. But, as someone else pointed out, I do need to work out the room acoustics or it won't sound any different. What do you mean a .5 woofer?

I'm thinking of adding a .5 woofer to my front L&R
OH I have my surround system set to Large speakers and I modified them by stuffing and sealing, they sound better stuffed and sealed

If I modify them in any way I won't be able to sell them again. Since both the C1 and C6LCr are current designs I can get most of what I paid for them back. Plus they have too small a woofer. My philosophy for sats is either make them full range and set them to "Large" or limit them to 80 hz and cross them with a sub.

Seems the MTM is not the best design, at least not when turned sideways. I wasn't paying attention when I bough the KEF's. Not that they are a bad speaker, they seem to be a decent design and measure flat but I want a sealed enclosure. With the metal dome tweeter they sound harsh on my receiver.

 

[Siggma]

The latter part (wider dispersion) is something that an MTM center (unless you have it vertical) will not give. If you do an MTM on it's side it will actually have very narrow dispersion, as the high frequencies get channeled by the flanking mids.

Tony.

That's one of the reasons I started this thread, to discover the general opinion and the actual facts of an MTM outside advertising. Seems an MTM may not be the best center speaker. I might be better off using a 6" two way or I might want to think about a T or Cross arrangement like this:

--M--
-MTM-
--M--
​

or this:

--M--
-MTM-
​

I could buy a .5 cu ft knock down cabinet, cut a hole in the top and add a .25 cu ft to it for a three driver T arrangement. I'm getting weird now...

Probably I'll build a regular D'Appolito, phase align it on Tweet axis and mount it vertically under the TV. If I can't get a tweet to cross low enough I could add a 2" midrange and make it a 3 way. That or I could build two separate speakers and mount one on top and another below the TV.

See: Horizontal Center Channel Speakers and Why They Should Be Avoided - Blu-ray Forum for my reasoning.

 

[Siggma]

Not trying to argue, but I have used the THX filters quite successfully with speaker pretty close to the size you have.

Perhaps you could help me work on my room? I haven't ever used bass traps. I could take some pictures and measure room acoustics if you're willing to help me.

 

[sreten]

Hi,

Designing a x/o is not a trivial matter and ideally the MTM for
the centre would be different to the other MTM's somewhat.

look at the discussions here :

Zaph|Audio - ZA5 Speaker Designs with ZA14W08 woofer and Vifa DQ25SC16-04 tweeter

Also Zaph|Audio

The Aurum-AC130F1 is quite a nice driver suiting sealed and well
stuffed boxes. MTM 2 way is better than MTM or TMM 2.5 way.

With a baseline Qts of 0.7 you going to have to accept some
bass peaking and control this with offset AV x/o settings.

Information as to how to design a proper x/o is in the links below.

Implement some baffle step and choose your x/o points and tweeter wisely.

rgds, sreten.


http://sites.google.com/site/undefinition/diy
(see if nothing else, the excellent FAQs)
http://techtalk.parts-express.com/sh...d.php?t=219617
Zaph|Audio
Zaph|Audio - ZA5 Speaker Designs with ZA14W08 woofer and Vifa DQ25SC16-04 tweeter
FRD Consortium tools guide
http://web.archive.org/web/200909021...esigningXO.htm
RJB Audio Projects
http://web.archive.org/web/200909022...ve99/Spkrbldg/
Speaker Design Works
http://www.htguide.com/forum/showthread.php4?t=28655


Great free SPICE Emulator : http://focus.ti.com/docs/toolsw/fold...t/tina-