Low frequencies are long enough that facing forward is still close enough to half space radiation.
We put the mic on the ground plane to assure there are no reflections, if the measurements are done out doors far from any reflective surfaces.
They don't.
Electrostatic loudspeakers typically exhibit a dipolar polar response, quite different from "a single flat wavefront".
If the ESL's rear radiation were enclosed, it's low frequency radiation would be that of a monopole with the same radiating surface.
The box baffle limits it's wavefront to hemispherical to the boundary wavelength. The individual point sources will exhibit constructive or destructive patterns depending on the off axis angle relative to vertical and horizontal spacing.
Yes, a theoretical rigid piston diaphragm 60" tall and 12" wide would have a different high frequency polar pattern than four 12" drivers in a vertical array.
At low frequencies, it would have the same polar pattern as the 4x12" vertical array.
Yes, yes, I get that an ESL is a dipole, not really getting to what I don't understand. Question is in what ways are stacked woofers not a single radiating surface, and in what ways are they? It is that nuance I'm trying to understand.
Imagining multiple woofers stacked on a floor which is an infinite baffle, the polar radiation at say 10' away will appear as a rigid piston at low enough wave frequencies... so for this we can imagine a single piston but the reality is that we have multiple hemispherical wavefronts, so each is subject to potential interference from floor reflections at wavelengths far below the frequency at which they start to beam.
So going back to my question, we can approximate the single radiating piston in some ways, but not in others. The wavefronts are not magically planar, they just add up that way when you are far enough away. The reality is that when wavelength >> driver diameter we have multiple point sources. It's only at a distance and in approximation in some senses that we treat them as a single piston.
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I like push-push, and have two different double 18" ported cabinet designs utilizing it. Really helps reduce cabinet vibration on asymmetrical bass transients.
But not for this project where I needed six smaller single sealed 18"s. I need to be able to put two each under LCR speakers, or three each under stereo.
Plus some other experiments...like vertical polars of a stack out on the driveway. If only I could figure out an easy way to stack all six !
Often so. But often not so, too. Depends on the consideration given to achieving line array goals with the stacks..
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To me, the question is when does a stack of radiators behave as a line source, rather than a collection of single radiators that have destructive interference (lobing) between them.
The two overarching guidelines that I know of are: the spacing between individual radiators c2c's determines how high in frequency they act as a line, and the length/height of the line determines how low in frequency they act as a line.
Necessary maximum c2c spacing for high end is generally quoted at 1/4 to 1/2 wavelength. Necessary total line length for low end at about a full wavelength.
I don't think the idea of point source fits into this 'line array rationale'. For me, a point source is a single omnidirectional source...the classic definition.
So in my mind, a beaming driver is the opposite of a point source.
I think in terms of multiple drivers each having their own radiation pattern vs frequency, based on driver size vs frequency. And again, the question becomes when do they act together as a line, and when do they act individually.
Hope that all made sense...
Hi, you could do some simple trigononetry, right triangle. Assume you are in front of the line, head at height of end of the array. Now, calculate distance from closest and furthest driver to your ear and they would sum constructively up to 1/4 wl difference. You'll find out the further you are the higher in frequency this happens. So, calculate at your listening distance to get some useful number.
OK, so lets ask when are they individual pistons and when are they a line source?
It sounds like the approximation of a line source happens because at long distances the interference patterns merge, giving us the behavior of a flat wavefront in a very practical sense, but the waveforms themselves haven't stopped being spherical, just that the effects end up combining.
Jim Griffin's line array white paper covers this and other aspects of line sources/arrays: http://www.audioroundtable.com/misc/nflawp.pdf
I don't think so. I think it's when the drivers act as line source, ala classic Olson....that a non combing summation takes place due to tight (1/4 to 1/2 WL) c2c spacing.
Some food for thought for @eriksquires on how the driver fields interact. These animated GIFs are BEM observation fields for 30cm piston driver(s), stacked and sitting on the floor (IB). The observation freq is indicated in the upper left of the pic.
Attachments
Taylor did it awhile back and don’t forget the Griffin paper.
The one picture story (nice animation ofthis Don):
In a rush, ca’t quickly find the paper, i have posted link often before.
dave
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Hi Mark
How about using a big tripod with a lifting devise?
Then you could stak the cabinets from underneath, ie: Stack two cabs, fasten them together, lift up, put a third cab underneath, lower down, fasten to upper cabs, lift up, put the next cab underneath and on and on til all six cabs are stacked.
As a bonus the tripod can keep the the stack stable while measuring.
Regards, Steffen
Hi Steffen, I was half joking about wanting to stack all six. It wouldn't be very hard for though, I have a pair of Global Truss ST-132 crank stands from my foray into live sound that would make it pretty easy. Plus, the subs only weigh 60lbs..can almost get up em up a ladder safely. Oh, and they have smaller rubber feet and routed indentations on their tops for stacking, so pretty stable.
But yesterday testing out on the driveway, after i stacked three I got to 'what's the point of further stacking' unless i want to measure all around polars...which I don't. Results were too predictable vs modeling (like per DonVK's post) and summation SPL was spot on too. So I hit a who cares further point....
One thing I can say is...damn, even though I full well know it from many sub builds...sealed subs suck when it comes to the very bottom. Indoor measurement with room gain make them look ok, but outside they show their low end struggle.
EQ'ed flat, either indoors or out...they just ain't the same as vented...it's like you can hear the compression at higher drive levels. But I don't think it's something folks can hear / would know, until comparing directly with same driver vented. It simply takes two sealed subs to equal one vented, same driver.
But yesterday testing out on the driveway, after i stacked three I got to 'what's the point of further stacking' unless i want to measure all around polars...which I don't. Results were too predictable vs modeling (like per DonVK's post) and summation SPL was spot on too. So I hit a who cares further point....
One thing I can say is...damn, even though I full well know it from many sub builds...sealed subs suck when it comes to the very bottom. Indoor measurement with room gain make them look ok, but outside they show their low end struggle.
EQ'ed flat, either indoors or out...they just ain't the same as vented...it's like you can hear the compression at higher drive levels. But I don't think it's something folks can hear / would know, until comparing directly with same driver vented. It simply takes two sealed subs to equal one vented, same driver.