In article , Ewar Woowar
wrote:
jim, thanks for that, i thought it might be to do with phase and polar
patterns.

i guess a wide dispersion pattern flat over frequency, same signal
coming from 2 speakers would mimic similar sound waves to a single
actual source located in the middle. thinking back to o'level physics,
i guess you could get a tray full of water and compare two pulsating
bobs (two omnidirectional speakers) with one pulsating bob in the middle
and compare the waves arriving at an equidistant point (the listener).

Yes, however this does not necessarily mean that 'point sources' that
radiate uniformly in all directions are always ideal. Speakers that
approach having a 'planar' radiation pattern which 'beams' the signals
towards the listener may be better. Depends upon the details.

The problem is that when speakers radiate sounds into the room, some then
reflects from the walls, furniture, etc. If the speaker's polar pattern
varies in a complicated, uncontrolled, way with frequency, the result is a
speaker-imposed frequency-dependent variation in the amount of indirect
sound reaching the listener. This can have the effect of shifting the image
in a complex frequency-dependent manner.

Bit like looking at a photo where the colours are all 'out of alignment'.
:-)

Having speakers whose patterns are relatively frequency independent means
the speakers are not having the above effect as they are distributing the
power in much the same way at all frequencies. There may still be effects
due to the room, etc, having a frequency-dependent behaviour, though. Hence
the problem may be reduced, but not removed.

By having sources that tend to 'beam' at the listener, you get a higher
direct-to-indirect ratio, so can reduce the effect.

The above is argued to be one of the reasons why speakers like the Quad
electrostatics can provide good imaging. However other types can also
deliver good performance provided that they, and the room, etc, all are
doing a decent job. Hence there may be more than one way to deal with this
problem. There are various reasons why 'beaming' is problematic in
practice, so it can have drawbacks that may mean another approach ends up
being preferred.

And in the end, of course, the source material has to provide the signals
that can then be used to create a decent image. If the recording/broadcast
is crap in this respect, then even excellent speakers can't give you a good
image. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html