On Fri, 05 Aug 2011 15:43:11 +0100, Jim Lesurf
wrote:
In article , Don Pearce
wrote:
On Fri, 05 Aug 2011 09:43:55 +0100, Eiron
wrote:
On 04/08/2011 16:28, Jim Lesurf wrote:
In aweb.com, Rob
wrote:
I recognise what you mean. Even optimally set up I experience
something similar. I tend to regard them as the 'biggest pair of
virtual headphones in the world' in some ways. The best result means
having your head in about the right location to get your ears in the
right places wrt the invisible large headphones floating in space.
So the 'virtual point source' doesn't really work?
The virtual point source was an interesting claim, but technically
naiive. Had the speaker been a complete sphere, the virtual point source
at its centre would have been spot on.
IIRC Quad were said at one time to be working on a 'ball' ESL. But hardly
surprising that this didn't lead to a product. Not an easy thing to make,
even compared with the 63!
With a speaker of limited size, the effect is the same as a true point
source in another room, and a speaker-sized hole in the intervening
wall. It is easy to see how things go wrong off-axis. In other words, if
you aren't perfectly lined up, the point source is simply not visible
through the hole, and you lose all the highs.
Its a bit more complex than that for the ESL63 and its children,
essentially for two reasons.
Firstly, the phasefront is quite distinctly curved over much of the
freqency range. So the 'apparent point source' center is close to the
'hole'.
Any idea how close they got it? I know they used annular panels and a
tapped delay line, but I've never found out how much delay they used.
Secondly, the size of the hole also scales with the wavelength, and the
hole has graduated edges.
That helps a lot.
Plus, of course, the fields then exhibit what people useually think of as
diffraction spreading to a wider angle as they move away from the array.
Well, that would be true of the hole too.
The result isn't much like 'geometric ray' behaviour. Nor like a
rectangular aperture with a uniform (plane wave) field.
As a result you don't lose 'highs' as quickly as you might expect if
assuming behaviour like a rectangular hole the size of the speaker passing
sound from a distant pointlike source. And the change in behaviour as you
go off axis is much smoother.
My own experience is that the highs roll off quite progressively - but
also rather quickly compared to a dome tweeter.
FWIW I did once visit Quad and discuss with PJW having a go at modelling
the speaker radiation in terms of Gaussian Beam Mode analysis. This is a a
form of analysis for field propagation I and others used a lot for the beam
diffraction behavour of EM radiation and antennas, arrays, etc. However I
never got around to this as other applications of the method provided more
money at the time. ;-
These days it would be done with a 3-D array of points covering the
entire field together with finite element analysis. You can achieve an
arbitrary degree of accuracy, but you do need a lot of computing
power.
Somewhere or other I've got some plots that PJW gave me showing the
dispersion versus frequency of the 63. This is perhaps rather more
revealing than the usual set of polar plots when considering the overall
behaviour. I'll see if I can dig a plot out and shove a copy onto the
website so people can see what the behaviour is like.
[I've just done this and the result is at
http://jcgl.orpheusweb.co.uk/temp/half.png
note this is a 160kb png.]
I don't think there are too many of us left who would be daunted by a
160kb png ;-)
Kind of difficult to interpret though. Can you shed some light on what
we are looking at?
d