question regarding center channel

[mike93lx]

Could someone explain the potential issues with laying a MT on it's size to be used for a center? I know that crossovers are typically redesigned or at least adjusted for sideways use, but i don't know what needs to change.

thanks!

[Geoff Gunnell]

How significant the first problem is, the horizontal interference pattern produced by the drivers, depends on how far off axis you need to sit -- if your primary seating is within +/- 15 degrees you should not have this problem with most MT designs.

Reducing this problem requires placing the drivers close together, and using a low crossover frequency -- usually below 2kHz. If you have an MT with a good bit of space between the drivers and/or a 2.5kHz+ xo frequency its not going to have very good horizontal dispersion when it's turned on its side.

The second problem may arise with any speaker used in near flat boundaries, such as a center channel speaker in 'furniture' and/or near a big flat TV screen -- if you are building a speaker from a design posted on the internet, you my want to use a reduced baffle step crossover design (if one is available) to adjust the frequency response to compensate for the nearby surfaces.

It never hurts to post your layout and ask what speaker / xo would be best.

Do set a protractor on the floor where your speaker would go and sight the angles to your seats. If your seating angles are over 20 degrees either way, you will want to be pickier about MT (or MTM) design, and if they are 30 degrees or more you are going to have to realize that most drivers individually are going to roll off at those angles.

[mike93lx]

Geoff, thanks for the feedback.

The design that I was planning to use is comprised of:

Seas P18RNX/P
Seas 27TDFC
and
Crossover

I may not use that exact, preassembled x-over, but I planned on using it's schmematic as a starting point.

I plan to use a 14L vented cabinet for the L & R and wanted to use the same design for the center. Tweeter and woofer will be as close as possible and I planned to offset the tweeter slightly to the side, as is recommended. Cabinet will be a translam with rounded front "corners", and the baffle will be approx 9" wide.

I am going to make the first cabinet out of MDF as a trial, and if it works, I wanted to use it as a center channel. I do not have testing equipment at the moment.

Thanks!

[Curt C]

Mike,

An MT on its side will also have a non symmetrical horizontal radiation pattern. That is, it will sound different to the left than to the right. It would be better to build an MTM design, which while it also will have its flaws, at least forces a symmetrical lobing pattern. I’d suggest using a couple of 5” drivers (perhaps the CA15RLY) with the H1189 and crossing with something like 1500 Hz 4th order acoustic slopes. Offsetting the tweeter slightly will allow you to keep the center to center distance between the woofers at a minimum, which will also ameliorate the off axis lobing issues.

C

[Ouch_my_ears]

Something to consider... I read somewhere that having a screen smaller then 60" or even 50" doesn't absolutely require a center channel. You might have to let your receiver/amp/decoder that you are not using a center channel to compensate for it. It's something you might want to consider specially if your L and R front speakers are relatively close to you screen.

[Xander]

Something to consider... I read somewhere that having a screen smaller then 60" or even 50" doesn't absolutely require a center channel. You might have to let your receiver/amp/decoder that you are not using a center channel to compensate for it. It's something you might want to consider specially if your L and R front speakers are relatively close to you screen.

Like you said, it depends on how close your front speakers are.

It also depends on how close you're sitting to the speakers, how far off axis you're sitting, and if any seating positions are significantly closer to one speaker than the other. It is rather irritating to hear things from only one side speaker that are supposed to be centered.

[Deward Hastings]

What's the problem with mounting the drivers vertically? They'd fit quite comfortably in a box only 13 inches tall . . . is that so much worse than the 9 inches of your proposed box-on-its-side?

[Ouch_my_ears]

...how far off axis you're sitting, and if any seating positions are significantly closer to one speaker than the other. It is rather irritating to hear things from only one side speaker that are supposed to be centered.

Can't agree more. Some people assume you NEED a center channel at all costs and thought it might be one more thing to throw at the OP.

...a box only 13 inches tall . . . is that so much worse than the 9 inches of your proposed box-on-its-side?

Good question!

[servicetech]

IMHO the center is the most important speaker. You could play the center info through the mains in mono, but it's just not the same IMHO. Of course I have a 65" TV and about 12' between my mains. Perhaps with a smaller set and the speakers close together it might be OK not to do a center channel.

Have you considered building a WMTW design? That seems to be the trend among high end HT speakers.

[mike93lx]

great question.

I am building translam cabinets out of BB ply



I am going to build one cabinet out of MDF as a proof of concept/trial and wanted to use that as a center when I construct the mains out of the baltic birch. The cabinets will be 15.75" high, which just won't fit in my setup as a center.

Since posting my original question, I have considered using a Seas Loki Coaxial for the center and avoid this issues all together, but I would prefer to use the same drivers all the way accross the front.

At the moment, this will be for a bedroom, so I will be on axis at all time. Later on, I may use them in my living room, at which point, I could probably orient it vertically. Considering I will be on axis, do I need to adjust anything at all?

[Deward Hastings]

So why not use the same drivers and make a shorter, wider cabinet for the center?

Or just make them all shorter and wider?

Or deeper . . .

[mike93lx]

The sizing of these speakers goes a lot deeper than that. I am actually making a total of 3 different pairs of speakers in order to minimize the waste generated, and overall, I like the proportions of this design.

I want to use the same profile for the front three in order to minimize my cost of CNC work. I need to have the initial design cut on a CNC so I can duplicate it at home. Each different design costs me $40-50 in CNC work, and that adds up quickly.

[peter_m]

Mike93lx,
wow, nice translam. If that's what you have planed for the bedroom, I can't help but wonder what will be in the HT room..

How did you decide on the final contour of your translam?

[mike93lx]

peter, thanks. It's funny you say that...the HT has some "sweet" cerwin vegas. Mains are the three way towers with 12" woofers. those need to be upgraded at some point but they are great for the room and TV setup.

These may be moved to the living room instead of the bedroom. I'm not sure on that yet.

deciding on the contour was far from scientific. I have been playing with it for a month or so in Sketchup, tweeking as i went. I started with a circle, elongated it, chopped off at the baffle and then rounded everything from there. I just placed an order for the first pieces to be cut on a CNC, but I am already thinking of changes I want to make. It's a never ending process.

[peter_m]

deciding on the contour was far from scientific... ...I am already thinking of changes I want to make...

Dude, you have cohones of steel.... I would be nervous as hell... I AM nervous for you! LOL do let us know how it turns out!

[jquin]

great question.

I am building translam cabinets out of BB ply

This is an excellent idea. How are you planning on doing it.?
I'd be looking to make a template out of sheet steel or aluminium and then getting a template jig for the router.
I've tried freehand routing and it's a joke.
I'd love to see the results.

[mike93lx]

First off, I need to give credit for the idea and rough design to gmed (here on the forums) and Keith Kidder. Their translam designs inspired me to try it myself.

I sent out the plans for the design to a shop with a CNC and I am going to have 2 pieces cut: the piece with the integrated brace and the open piece. THey are cutting them out of MDF. From that, I am going to duplicate, one by one, each piece thereafter on my router table with a flush cut bit.

If you trim back the pieces of BB with a jigsaw or band saw to get close to the profile, the routing isn't so bad, but this project is still going to take a ton of time.

thankfully, by making three cabinets at once, I won't be generating much of any waste and I'll end up with enough cabinets for a couple 2.1 systems or a 5.1 system.

[peter_m]

ah, so the CNC is just for your templates.... get a good vacuum!!!

Don't mean to steel your thunder, but last year I bumped into a site where the guy was building a translam where every piece had it's own unique shape. Once all put together, the cavity formed behind the woofer was a cone curled up on it's tail. From the out side you would never guess it was anything special going on inside. That has to be 100% CNC cutting... anyone seen this site or remember the address?

Peter

[peter_m]

https://web.archive.org/web/20070510170645/http://home.wanadoo.nl/dezaire/ultiem/ultiem.htm

Found the link. Checkout this translam!



This is the same translated by the friendly people at google

https://web.archive.org/web/20100203220222/http://home.wanadoo.nl/dezaire/ultiem/ultimate.htm