Help with Centre Speaker

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Hello ,
I'm new to this forum but am looking to build my own centre
speaker for Home Theatre.
I want to use 2 Vifa P17SJ-00-08 mid drivers and 1 Vifa D25ASG-35-06 shielded tweeter in a bass reflex enclosure. The situation is that due to 'spouse factor' and existing A/V cabinet dimensions , my
box needs to be around 650mm wide x 420 mm deep x 250 mm high (this height measurement is variable a bit)
I'm using 18mm MDF .
I'm getting varying results using WinISD & LSPCad software.

Can someone suggest what size & length port I need ??
What internal volume would be appropriate given that my box
internal dimensions come to around 49litres ?? The computer programs suggest around 33 litres ! Do I fill the box with foam / padding of some sort to take up the remaining ~ 16litres ??

I'm confused ! Help !

Beware the MTM center!

Hello Andrew,

Welcome to the forum.

Since you're building your own center, you might want to consider the following fact: MTM (D'appolito) configurations are fine when the drivers are arrayed vertically, but when they're turned on their sides they are become problematic.

I have never understood why 99% of center channels out there have this topology. I suspect their popularity is driven by factors more aesthetic than acoustic.

The problem: Comb filtering and beaming. When two drivers operating in the same band illuminate a listening area, their separate wavefronts interfere with each other. This causes response peaks (where the wavefronts combine constructively) and response nulls (where they cancel) along the axis of their alignment. This lobing/comb filtering becomes pronounced when the wavelength nears the distance between the centers of the drivers. In addition to peaks and nulls, there will also be a narrowing of directivity (beaming) along the array axis.

When the array is vertical and the lobes are radiating up and down and the directivity is narrowed vertically, there isn't much of a problem. Carpet helps quell the lower lobes and there is still good horizontal dispersion. But lay that array on its side and you've got narrowed horizontal dispersion with frequency-dependant response lobes on either side. (If you have an SPL meter, this is easy to prove with test tones.) This is unfortunate when your center channel's goal in life is to evenly illuminate a broad horizontal area of couch. The simplest fix would be to just rotate that center channel 90 degrees, like your other channels.

However, if you simply must have a center channel broader than it is tall, there are a couple of ways to make it work and recover broad, even horizontal response. I have seen horizontal MTM configurations with tapered crossovers, that is, one midwoofer rolls off before the other. This way, one midwoofer is dominant when wavelengths shorten to the point that beaming/lobing would otherwise become a problem. The other answer is to use a mid, a tweet, and two woofs. Array the mid and tweet vertically. Place the woofers one either side and be sure to cross them over low.

You're probably sorry you read this, since it sorta spoils your ability to ever respect a run-of-the-mill center channel again. If you sit in the middle of your couch, you may have been blissfully unaware of the off-axis problems. But for the sake of your family/friends, give it some consideration!

Bill F is absolutely correct regarding the off axis behaviour of MTM array center channels. I have never even liked them in vertical arrays for left and right speakers because for typical arrays, the interference nulls start at around 1-2kHz at 15 to 30 deg off axis, and in this frequency range the carpet is not sufficiently absoptive. This results in a floor reflection that is highly coloured in spectral balance. Much research has shown that we are sensitive to the effects of reflections in the vertical direction and much less so to those coming from side wall reflections. It is important therefore to minimise these, but this should be done in an even manner across the spectrum, not in the haphazard way that MTM arrays produce.
When we combine a horizontal MTM array for the center channel with a vertical array for the fronts we get the worst of all worlds. Since voice (or timbre) matching is critical in a multi-channel system, we cannot ever achieve this with such an arrangement if we sit other than directly on axis with every speaker. As we move off axis, the response at say 15 deg is different for the vertical array compared to the horizontal array, no matter how well they were matched on axis. Ergo they will sound different. Tapered crossovers on the bass drivers helps, but is not a complete answer.
Therefore the center channel directivity should be engineered to match the front speakers. Bill's suggestion of a vertical array of midrange and tweeter in a three way sytem is a step in the right direction, but has a flaw..unless the fronts are also three way, the phase responses will not match. In a situation where the sound is panned such that it comes from say both left speaker and center (and there are many exmples of recordings that do this), the different phase responses will make it very difficult to create a properly positioned apparent sound source.
The best solution is to use identical speakers for front and center.
The reason most manufacturers center speakers are sideways MTM arrays is aesthetic, the customers do not want tall center speakers, hence two small drivers side by side still with the cone area of a larger single driver. However, we are all above this aren't we :) We agree to suffer for our art. Go for a tall center :p
For these and other reasons, I'm pretty sold on the concept of coaxial/dual concentric drivers, if a single transducer doesn't offer sufficient BW for ya. There's good reason why concentrics remain popular for critical professional monitoring, though they've slipped out of sight in the great unwashed consumer market.

My next serious system will feature 'em.


Oh, and constant-directivity horns, ala Unity. I think they're a hop, skip, and jump closer to the ideal.
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