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?
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]

Originally Posted by Soundtrackmixer

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]

Originally Posted by 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

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]

Originally Posted by Silent Screamer

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.