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In Reply to: RE: Mmmm "it already is phase-coherent resulting from the drivers all being on the same physical plane." ... posted by Davey on April 02, 2015 at 22:43:25
I don't think there is a vertical component to the output of a line source as far as near field listening is concerned. Meaning that the contribution of the output from the top of the driver and the bottom of the driver does not reach the listener at significant contribution to SPL Those portions simply maintain the cylindrical waveform.
Griffin discussed this in a post here a decade ago.
http://www.audioasylum.com/cgi/t.mpl?f=mug&m=67308
Here is a live URL of his paper
http://www.diy-audio.narod.ru/litr/nflawp.pdf
This means that within the near field, which would be down to 500-600hz on a maggie at 10' there should be the possibility to obtain time coherence within the frequencies important for time domain perception (localization and thus imaging). If successful array extension by reflection off the ceiling and floor are contributing (which in most cases you don't have since the drivers have so little output in the vertical dimension) then you simply have a lower transition freq from near to far field.
As Helmhotz and Guitar Slim demonstrated with their DEQX time alignment of drivers to get rather nice time alignment with the impulse response within 0.1 ms for all the drivers together. The time smear of the line source is negligible and needs no correction by concave shaping of it. The departure from line to concave line produces more of a point source effect which works against image height rendition. One of the advantages of a line array over a box full of drivers covering different freq., time aligned or not
http://www.audioasylum.com/cgi/t.mpl?f=mug&m=192759
Follow Ups:
Well, the 300us exists. You can't explain it away. :)I understand Jim Griffin's take on this aspect of line-source radiation and chatted with him a few times regarding this. And I understand where you're coming from with your take this, but this is an inherent physical characteristic of the transducer.
A Stanley tape measure will show the relative differences in path length (time smear) do, in fact, exist. No acoustic measurements are necessary to observe/prove that.
Whether our human perception/processing masks this characteristic...our measuring techniques can't fully display it....or how much relative weight we place on this aspect of the radiation, is another topic. A pseudo-line-source like a Maggie speaker will exhibit a comb-filtering aspect to its response if measured in a free-field environment. I don't care what kind of user impulse-response measurements "disprove" that theory. :)I'm not making any bold statements here....just pointing out the obvious.
I believe "image height rendition" of this sort is "large"ly unnatural and that a point-source radiation is more realistic in a domestic environment. So, where does that leave us? :)
A left turn here (as you like to do often) to a different aspect of this. Obviously Maggies don't have this option, but you might consider why the power-tapering wiring configuration was adopted for the line-array design Jim put forward. :) I know he explains his reasoning in his paper, but think about that a little more. :)
Cheers,
Dave.
Edits: 04/03/15
I am not arguing that they don't exist, I am saying that for our psychoacoustic purposes the magnitude of the contributions from the time smear are not sufficient to overcome the precedence effect for localization, but that that it might contribute to the sense of "ethereal" images that line source planars often induce rather than the "solid" images from a simulated point source like a Vandersteen. Should have raised that when we were discussing it with Josh a few months back.
I should point out that Beveridge's elaborate waveguides solve the residual vertical radiation problem pretty much entirely and I don't recall their having an ethereal feel to their images.
What the impulse measurements show is that the time smeared contribution is not at significant levels.to contribute to SPL so would likely be ignored by our hearing mechanism's localization process. Fortunately the time smear is too short to affect our pitch determination, which is a far longer process.
If the image height rendition of line sources is "artificial" it sounds to me far more realistic than anything I have heard from a box - even concentric KEFs and Tannoys.
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