Why are all the center channel speakers hybrid designs?

hifiaudio2 · 01-19-2009, 05:16 PM

Just wondering... Seems like not having a regular tweeter would be best. Im sure its something that cannot be avoided, just wondering why.

Does anyone feel the hybrid tweeter takes away from the sound?

This is just about the tweeter, by the way. I understand size for the bass drivers would be prohibitive.

amey01 · 01-19-2009, 05:19 PM

None of us are ML designers so we're not really qualified to answer, but I'd assume it is to do with dispersion - a centre channel requires wider dispersion than mains.

sleepysurf · 01-19-2009, 05:37 PM

Quote:

Originally Posted by amey01

None of us are ML designers so we're not really qualified to answer, but I'd assume it is to do with dispersion - a centre channel requires wider dispersion than mains.

That would be my guess as well. For multichannel, wide dispersion takes precedence over not having a crossover (though I'm sure JonFo will argue otherwise

).

amey01 · 01-19-2009, 06:37 PM

Quote:

Originally Posted by sleepysurf

That would be my guess as well. For multichannel, wide dispersion takes precedence over not having a crossover (though I'm sure JonFo will argue otherwise

).

JonFo has got wide disperson AND no crossover - but he's just insane!

hifiaudio2 · 01-19-2009, 07:12 PM

So how does the sound of the stage stack up to something like the Summits? Is it an obvious downgrade in transparency?

What about in the ever elusive ranks of "best center channel". Is it a top notch unit?

FISH_MAN · 01-19-2009, 08:57 PM

my stage is great. It is not as musical as my spire of course but it does a better job on the vocal tracks in the movies its just geared that way! smaller panels need tweeters I think the size is why they tweeters are implemented

roberto · 01-19-2009, 09:06 PM

Hola...the main reason why is because of the size of the stat panel. They need to move a lot of air, and with the size of the center channels the aid of a tweeter is a must. Bigger center channels as the one that Jonfo made, do not need the tweeter of course. Now, for the first time ML has done a new industry standard...the tweeter on the new models, like Stage or Motif, etc, the tweeter is aligned in time with the stat panel. It is built in the center of the stat panel with the dome aligned with the stat membrane. Transparency at the sound and wide dispersion is the clue here...happy listening,
Roberto.

JonFo · 01-20-2009, 02:58 AM

We’ll ya’ll knew the mad scientist would have to chime in on this one

First, the reason for all ML centers having a tweeter is, as some have noted already, dispersion characteristics.

A small (12” high or so) panel element has next to no vertical coverage above 2Khz, so they have to cross over to a transducer that has broader dispersion, which is a regular dynamic tweeter.

The reason for the woofers in an ML center is likewise, the small ESL panels can’t really go below 500hz with any volume ability, so they cross over about that point to a single or usually a pair of 6” dynamic drivers.

The fundamental challenge is that the basic ESL design is really a line-source system, where you have several linear feet of reproduction surface to cover the primary frequency range (~300Hz – 20Khz) as well as handle vertical coverage (= the height of the panel) and because of the curve applied, it can provide the horizontal dispersion as well.

But since few people have acoustically transparent screens and front projection in a darkened room (to place the center behind the screen), the usual form-factor for centers is a small rectangle that goes below or above the screen.

This size and locations are less than ideal regardless of transducer topologies, so even a box full of dynamic speakers is still a big compromise (IMHO). So getting a decent ESL-based result is an even bigger ask.

But ML’s centers are good designs (considering the challenges) and hard to do any better in the form-factor they need to adopt.

So while having a tweeter in the middle of ESL panel seems like a bit of a strange match, it’s absolutely the only way to make it deliver the kind of spatial coverage required of a center.

Now, I felt strongly enough about these compromises that I went my own way on this topic, but then I seem to be in a unique minority at the bleeding edge (of tech or insanity, who knows)

JonFo · 01-20-2009, 03:08 AM

Oh, one more thing …

Regarding sound quality, yes there are noticeable differences between the centers and L/R’s, and much of it is due to the center not being a line-source and even using a point-source (the tweeter) for much of the key localizable frequencies.

Line sources broadcast a deep ‘near-field’ were the decrease in acoustic power drops by only 3db for every doubling of distance, a point source will drop by 6db for every doubling in distance.

So if your speakers are roughly 12 feet away from your listening position, the relative loudness from the center’s tweeter is falling off at twice the rate of the ESL panels in the >2Khz range.

The speaker is designed to overcome some of that, but that means it might be too shrill if you sat at 8 feet from it or too dull if you are at 16 feet.

So one fundamental challenge is the miss-match of volume projection between line source and point source transducers.

Some of this can be overcome by using a good room correction system like the Audyssey MultEQ XT process found in many pre-pro’s these days. It can work wonders not only at correcting the room, but at addressing fundamental frequency balance and loudness challenges in the speaker system. The other huge benefit is absolute time-alignment on all the channels. It’s like getting all new speakers.

FISH_MAN · 01-20-2009, 05:55 AM

I have heard that the transparent screens are not really so good as they still cover the speaker I know that you use one JonFo what do you think?another thing I have heard is that the freq response of an esl panel is not that flat when listened to straight on especially a large one

JonFo · 01-20-2009, 07:28 AM

Fish_Man, it's true that some Screen's are not completely acoustically Transparent, but the latest batch from Stewart and ScreenPix are very transparent.

Here's more on the topic: http://www.martinloganowners.com/~td...8&postcount=62

But even my old Stewart AT screen, which is perforated vinyl and not a woven material screen, still performs well enough.

Here is the post-Audyssey response of one of the front speakers (left graph) vs the center (right graph) which is behind the screen.

Note that both have declining HF curves (and a dip in the midrange) that are there on purpose as that's the Audyssey target curve for HT.

But the center does have a bit more of a droop in the high-end due to both the screen, and the fact that I don't EQ the center independently (like the original SL3 crossover did), I just let Audyssey correct as much as it can and let it go at that. The result is very impressive. One can not tell there's a center there.

I've cheated and told people I'm running Phantom center or pure stereo mode, and they believe me, it's that well integrated

No speaker is flat, so that’s not the issue. Listening to the SL3 straight-on is not that different than from an angle. What’s really different is how the angular placement relative to the room influences the sound.

Also, placing any dipole perpendicular to an untreated wall is bad, and I have measurements to prove it. To correct that, my center is firing onto four MiniTrap HF’s behind it.
So any center placed behind a screen needs to have treatments on the wall behind the screen.