Drawbacks to TMMT Center Channel?

[DrDyna]

I've been considering re-designing my center channel, as it's getting up in years and I've got some spare drivers and crossover parts on hand.

I haven't really decided which drivers I'm going to use yet, so I haven't simulated a crossover or done any cabinet calculations, but I'm considering building it T-M-M-T.

Has anyone laid out drivers like this for a center channel? I looked around online and didn't really see anything that looked quite like what I was planning, so I'm wondering if I'm being stupid and there's some really obvious answer as to why nobody does it. Most of what I see are M-T-M or M-TM-M.

[oublie]

are we talking horizontal or vertical?

T
M
M
T

the distance between the tweeters will be too high and introduce a lot of combing.

M
T
T
M

would be more acceptable


however,

this might be possible

T
T
M
M

or

T
MTM


and the following would be bad

TMMT

[DrDyna]

Ah, so it's the comb filtering. Yeah, I was thinking of TMMT horizontally, much like what you'd get if you laid 2 normal speakers down on their sides, bottom touching. I figured there was a reason nobody did it.

[5th element]

I am sure you are aware that a normally configured loudspeaker, such as a

T
M

can have, if properly designed, an extremely flat frequency response on axis that varies very little as you go off axis laterally. This is because as you move laterally along the horizontal plane the phase relationship, providing you don't move closer or further from the loudspeakers, remains pretty much constant between the tweeter and the mid/bass. This is because the drivers do not change in distance, that is, get closer or further away from your microphone or from your ears. As a result the wave produced by the drivers keeps having to travel the exact same distance, so when they sum at the microphone they do so in the same way, regardless of if you're 10 degrees off axis or 50 degrees off axis.

If you then move up or down in the vertical plane however you immediately change the relative distances that the two drivers are away from the microphone. As a result the two waves travelling from the drive units themselves, will now travel different distances and interfere differently at 20 degrees off axis up, then they would 20 degrees off axis down. This is why you get lobing occurring as you go up or down off axis, because the drivers start going in and out of phase with one another, creating peaks and dips. This is why we almost always have drivers configured in a vertical line. We tend to listen at a fixed height and thus we keep the vertical off axis angle pretty constant. The horizontal off axis angle can change quite a lot though, depending on how much toe in you need to use (perhaps to reduce the energy directed towards the side walls to provide a more pleasing tonal balance) or, even if the listening distance is kept the same, you may be forced to place the loudspeakers closer together thus reducing the horizontal off axis angle as compared to the speakers placed further apart.

Of course there are exceptions to this, if you've got two drive units positioned close enough together so that the separation between them is small compared to the wavelength they are producing, they will act as a single source of radiation. Also if you're using a coaxially arranged driver, such as the KEF UNI-Q driver then naturally going vertically off axis is exactly the same as going horizontally off axis if you ignore any changes in diffraction that most loudspeakers would show.

It is for these reasons that anything such as an MT, TM, MTM, WMTMW etc is a bad choice in a centre channel. People tend to sit at different angles off axis laterally when watching a movie because they obviously sit next to each other. If you've got a horizontally arranged set of drive units the axis that you get lobing occuring is in the horizontal one, which is the exact opposite of what you'd want in a centre channel. Someone could therefore end up sitting in a null which would sound particularly bad and could result in a loss of them being able to hear clearly what is being said during scenes where crap has hit the fan.

If you can you should really arrange a centre channel as an

T
M

or as is typically done an

WTW
M

Such as this beast from clearwave audio or something less over the top such as this from CJD and this HTGuide Forum - Modula NeoD CC Introduction and Build Thread from JonMarsh. You will also notice that B&Ws more expensive centre channels use a vertically arranged tweeter and midrange driver too.

Adding in a second tweeter is asking for chaos as the wavelengths at high frequencies are very small and you will get crazy lobing patterns occurring all over the place as the tweeters go in and out of phase with one another.

Centre channels are typically done as MTM because its visually very attractive, but functional it really is not. If done with a small tweeter and small mid an optimally configured three way centre channel does not have to be that big. Say use the scan speak 10F and the SEAS 27TFFNC/G you're looking at a total height of ~150mm which is quite similar in size as standard 5.25" mid/basses.

I realise that this is quite different to what you perhaps had in mind, but it's just some extra food for thought and depending on what tweeters and mid/basses you currently have perhaps only a small format mid range driver would be require for an optimum design.

[DrDyna]

Thanks for explaining that, it makes perfect sense. My current center is (I'm not sure how to say it) woofer left, tweet top center, mid bottom center and woofer right, with a cobbled together crossover that looked "acceptable" in speaker workshop. My older Eton 7-360's are getting a little rough though, and some of the rubber surrounds have started to crack and the one that I did try to repair threw the suspension out of alignment and it rubs the coil a little if pushed. It seems like repairing them would be a pain, as neither Madisound nor Eton have any suggestions besides buy new ones :P

I've thought about just cloning what I have for left and right mains as one of the ideas I've tossed around, which at the moment, are basically Carmody zx spectrums until I get my hands on a couple of RS180s.

Thanks for clarifying that though, it's going to help a lot.

[5th element]

Well it seems like you've already got a centre configured in one of the optimum ways so going to something less then optimum would seem like a step backwards. However as you've already got a cabinet built for that geometry and driver layout, perhaps finding some different drivers to replace the etons and redesigning the crossover would be a good idea.

The thing I often find annoying about replacing drivers is that the rebates are always going to be different. A decent solution I've found is to make a new front out of solid wood. Also if the new rebate needs to be slightly bigger then the previous one you can glue in a temporary block of MDF to the cut out that would serve as a pivot for which a circle cutter can attach to.

That Eton driver certainly is quite unusual in having such a large diameter frame, 187mm in diameter! The scan speak illuminators come close, but still not quite!

[DrDyna]

Originally Posted by 5th element

That Eton driver certainly is quite unusual in having such a large diameter frame, 187mm in diameter! The scan speak illuminators come close, but still not quite!

Yeah, they do have a pretty good size basket, there's about half an inch between the basket edge and the edge of the surround. I always wondered why they did that. It looks good because the driver appears to be larger than it really is.

I do like the advice of using solid wood baffles, as I've been doing that for quite a while with other speakers, building boxes with mdf or ply and then using an elastic type sealant with screws to affix a hardwood baffle so it can expand and contract without breaking the seal. I've been using oak panels from lowes for quite a while cause I like how it looks with a simple beeswax oil au naturale, and I can use my same cabinets for different drivers.

My center channel has an mdf baffle at the moment, but part of what I was planning was to cut it out and use the edges to anchor t-nuts and slap one of the oak panels on it. Perhaps a new baffle with a couple of Hi-Vi M6A will fit the bill, the M6A that I've had in my main speakers have held up very well for about 3 years now and the surrounds are in great shape, despite living in a humid climate, plus it will match the center's mid (Hi-Vi B4N).

I wish I could get another pair of 7-360 though, I love their sound (almost tubey warmth), but their hefty pricetag puts them out of reach for the foreseeable future.

Edit: couple of pics.

[5th element]

Wow those eton drivers certainly haven't weathered the climate there well at all. Were the cones really that colour when you first bought them? They don't look anything like the colour I've seen on eton drivers before.

If you like the M6A it sounds like it would make a nice replacement part and keep the visuals appealing too. The T/S parameters of the two are reasonably similar also.

[DrDyna]

Originally Posted by 5th element

Wow those eton drivers certainly haven't weathered the climate there well at all. Were the cones really that colour when you first bought them? They don't look anything like the colour I've seen on eton drivers before.

If you like the M6A it sounds like it would make a nice replacement part and keep the visuals appealing too. The T/S parameters of the two are reasonably similar also.

They've always been a little dark, they have a sort of "sticky" coating on them, which I've lovingly dubbed "magic shiny sh*t". They are about ...12 years old now, my oldest drivers that I haven't tossed by a big margin. The listening area I had when I first ordered them had one wall that was open to sunlight for most of the day as well, which may have contributed. It probably doesn't help that the light in this room right now sucks and so does my camera :P

[5th element]

A little dark? I thought they looked way to bright. The eton cones are usually quite dark, like a cross between some brown and graphite black. Maybe the cone material has changed over the years.



This is the colour I'm used to but a quick google search did show up some much lighter coned units, maybe eton changed the mix/colour at some point.