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Old 26th April 2007, 12:21 PM   #11
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Quote:
Originally posted by el`Ol
The MBL loudspeakers are multi-way systems, i.e. for higher frequencies you have narrower stripes, so the stripe width stays small compared to the wavelength.
My personal belief is that an important point about omnidirctionals is that they have the same acoustic center (in projection on the horizontal plain) over the whole frequency spectrum and this is relevant because the wavefront radius is audible in form of interaural time delay.
Actually, while the MBL Radialstrahler's drivers are physically concentric, they do not share the same acoustical origin over the audio spectrum. Like so many other multi-way speakers, the MBL is not time coherent. The combination of multi-way drivers and cross-overs causes the tweeter output to arrive first, followed by the mid-range, etc. Look at Fig. 6 in this Stereophile test report.

MBL 101E step response - see Fig. 6

Don't get me wrong; I'm a fan of the MBL 101E. It is one of the finest speakers I've ever heard, but not because of time coherence.

Quote:
Originally posted by el`Ol
One conclusion of previous discussions was that a cage with vertical bars is not practicable because every element would beam and there wouldn`t be a homogeneous sound field.
I would respectfully suggest that this should not be accepted as a final conclusion. With enough facets in 360 degrees, beaming can be brought under control. A faceted cylinder will beam less at high frequencies than will a single facet (of the same size). Each adjacent facet in a cylinder (or partial cylinder) contributes a vector to the phase front in such a way as to smooth out the beaming or lobing. As an example, listen to the Sound Labs speakers, which are only a quarter cylinders, but they illustrate the point. They are made of numerous faceted planar ESL drivers. There is relatively little lobing as you move around in front of the speakers. If you were to listen to just one of the 4” to 6” wide drivers by itself, you would notice that the highs would disappear quickly as you moved off to either side, which is to be expected of a single large planar tweeter.

A faceted cylinder must be analyzed by antenna/optical techniques. At high frequencies, there will be gentle lobing, but less so than you might think. The more facets that comprise the 360 degree cross-section circle, the closer the approximation becomes to an ideal circular cross-section cylinder that would have no lobing. As to the RTR DR-1, I don’t know how many ESL panels it uses in 360 degrees. If it uses very few, it will have some lobing.
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Old 26th April 2007, 12:31 PM   #12
SY is offline SY  United States
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Quote:
It is one of the finest speakers I've ever heard, but not because of time coherence.
Why, then?

BTW, I'm in 100% agreement- based on a few quick listens, these were superb speakers.

I think the toughest nut to crack with cylinder ESLs is not lobing but rather what to do with the backwave.
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Old 26th April 2007, 01:00 PM   #13
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SY,

I wish I had the definitive answers; I don’t. But there are at least two things going for the MBL - very smooth response across the critical central octaves (quite an achievement with these oddball drivers), and that nearly perfect azimuthal omnidirectionality. I've never been a believer in adding “echoes” to a musical source that should already have the venue’s natural reverb recorded-in (or the recording studio’s artificial reverb added-in). In fact, I’ve always preferred fairly dead and large listening spaces. But there is something about a true omni source that is arresting in its ability to throw a soundstage. My jaw dropped the first time I heard the big MBLs in Miami. I’d never heard a soundstage like that before, or since.

Whatever reflections there are would have the same character as the direct sound since the response is virtually the same at all angles. But I’m guessing that the MBLs would also do very well in a large and well damped room that would reduce and delay the reflected sound. In other words, I’m not convinced that the great sound is dependent upon adding lots of new reverb energy off the walls, just that the reverb power must be is very “flat” (the so-called “power response” is flat).

There may also be some advantage to how each ear can hear both channels of a true omni. The omnidirectional wave front passes over the head in a natural way, so that both ears can hear each channel with the response only delayed and shaped by the head’s natural modification to the response, which is how we are accustomed to hearing the world around us, and how we estimate angle of arrival and distance in routine sounds. Beamier speakers (ALL other speakers) present different amplitudes and phase fronts to the two sides of the head, and this interferes with the ear/brain’s ability to decode angle of arrival and distance, I believe. This last factor may the most critical. I’d love to hear the MBLs in an anechoic chamber or in the middle of a huge room to see how that would affect the soundstaging. I’m guessing they’d still sound great. (As you may know, I’ve been quietly working on cylindrical ESLs.)

What's your view on why the MBLs work?

PS: Firesign fan?
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Old 26th April 2007, 01:19 PM   #14
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Sorry, I was upstairs helping Porcelain make the beds.

Honestly, I don't have a view on why they sound so good. Probably a bunch of things like you outlined. I'd love to have them for a week and measure the hell out of 'em. But there's little question in my mind that the azimuthal omni character was a major contributor to the spooky-solid imaging. And I'll bet the driver material/shape/tension/whatever was very cannily chosen to get rid of the sort of resonances that let us "hear" what the driver material is in so many speakers; acoustic instruments had a very natural and unforced timbre.
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Old 26th April 2007, 01:22 PM   #15
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I noticed you skated past my backwave comment!

That's a very non-trivial issue. The strong point of the ESL is a very thin, light membrane. Such membranes are also almost perfectly acoustically transparent.
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Old 26th April 2007, 01:33 PM   #16
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Quote:
Originally posted by SY
I noticed you skated past my backwave comment!

That's a very non-trivial issue. The strong point of the ESL is a very thin, light membrane. Such membranes are also almost perfectly acoustically transparent.
Intentionally. My anti-skating adjustment is out of whack. I've been working on that very problem (slowly, steadily, as my meager time permits) and I’m not ready to claim victory yet. All speakers can retransmit the reflected backwave, but as you say, ESL diaphragms have higher transmissibility.

BTW, as a tube bigot, I had to grudgingly admit that those big MBL amplifiers were pretty good too. I'd love to borrow a complete MBL system (cost = a small house in Texas) for long-term listening. Would it hold up to long hours of listening?

Another bozo on the bus apparently…
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Old 26th April 2007, 02:03 PM   #17
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Quote:
Originally posted by Brian Beck

As to the RTR DR-1, I don’t know how many ESL panels it uses in 360 degrees. If it uses very few, it will have some lobing.
I believe the RTR DR-1 has 29 ESL panels.
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Old 26th April 2007, 02:15 PM   #18
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29? Seriously? That's a lot, and an odd number. Are all 29 arranged circumferentially?
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Old 26th April 2007, 02:25 PM   #19
enilsen is offline enilsen  Sweden
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Sorry!! - 27 element circumaxial, variable area radiator.

This is what was mentioned in the ebay posting, other than that I can't find out much more about these speakers or even some good close-up pictures.
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Old 26th April 2007, 02:32 PM   #20
el`Ol is offline el`Ol  Germany
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Quote:
Originally posted by Brian Beck
There may also be some advantage to how each ear can hear both channels of a true omni. The omnidirectional wave front passes over the head in a natural way, so that both ears can hear each channel with the response only delayed and shaped by the head’s natural modification to the response, which is how we are accustomed to hearing the world around us, and how we estimate angle of arrival and distance in routine sounds. Beamier speakers (ALL other speakers) present different amplitudes and phase fronts to the two sides of the head, and this interferes with the ear/brain’s ability to decode angle of arrival and distance, I believe.

A very good description and it goes in the direction I meant. Same acoustic center (its axis in the case of the MBL) doesn`t necessarily mean time coherence, since the sound is produced by elements grouped around the axis in different distances and hights, not by the axis itself.

The reason why I find the same center condition so important is:
I am currently listening with the Ciare HX201 fullrange driver facing the ceiling (and just the ceiling as reflector) at exactly 90° to my ears. When I gradually tilt them into this position the omni-typical spacial image doesn`t come gradually (as comes the increase of indirect sound), but it comes quite suddenly around the 90°-position.
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