On Fri, 05 Aug 2011 17:36:53 +0100, Jim Lesurf
wrote:
In article , Don Pearce
wrote:
On Fri, 05 Aug 2011 15:43:11 +0100, Jim Lesurf
wrote:
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.
I'd need to go back and reexamine the data. This was something I looked at
*decades* ago, so have forgotten all the real details I'm afraid!
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.
Yes. However the advantage of GBM is that for beam-like situations you
actually get results much more easily, quickly, and more accurate for many
cases. In effect one computation specifies the entire 3D complex field
pattern along and across the beam.
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?
Consider the top graph as an example. The horizontal axis is fairly
obviously frequency. The vertical is the angle between the listening
(measurement) location and the speaker axis. i.e. 'how far off axis' you
are.
The lines are contours of sound level wrt the on axis sound level. So for
example the line with '-3' at the lefthand end is for where the off axis
level is 3dB down on the level on boresight axis at that frequency.
At 315 Hz the -3dB lines (note they are paired either side of the axis) are
about 35 deg off axis. i.e. at 315 Hz the speaker's radiation patten has a
3dB half-width angle of about 35 degrees.
At 5kHz the -3dB lines are about 30 degrees off axis. So at 5kHz the
halfwidth is 30 degrees.
This indicates that over a frequency range from a few hundred Hz up to
around 5kHz the angular width of the radiation pattern (to the -3dB points)
stays remarkably similar at about +/- 30 degrees. The change in beamwidth
is only about 10 percent for more than a factor of 10 change in frequency.
In effect at each frequency a vertical 'slice' though the plot shows you
the shape of the radiation pattern. It is rather more uniform with
frequency than a a large hole with a plane field, and also rather more
uniform that many conventional speakers. TBH I doubt many speaker makers
would like people to see their equivalent results! :-)
This means that one feature of the 63 design is that the radiation off at
angles (which you hear later via transverse room reflections) is tonally
more similar to the direct sound near axis than is the case for many
conventional speakers. In particular, no 'jump' in the axial/surround ratio
at the crossover point between a woofer and a tweeter.
The behaviour breaks down about 5kHz because you are now just using the
central element of the array (the center disc) and so the angles then fall
with frequency in the usual way.
Slainte,
Jim
OK, got that. Why didn't they think to use a log horizontal scale
rather than that muddle?
It is the bit over 5kHz that is the problem though. That centre panel
is a fair bit bigger than your average tweeter and will get rather
beamy.
d