Hi,
Date: Sun, 20 Mar 2011 15:52:55 +0000
From: Fons Adriaensen <f...@linuxaudio.org>
On Sun, Mar 20, 2011 at 11:21:41AM +0100, J?rn Nettingsmeier wrote:
and the superposition
principle does not invalidate the concept of "coupling".
What is this 'coupling' ? You mean one driver's response
being modified by the presence of others ? This is again
a purely linear effect (if not you have other problems).
It has to be taken into account when finding the indivual
responses of each unit. Again nothing magical, just the
correct way to do superposition.
but the huygens principle holds, and of course a line of point
sources
can be viewed as coupling to form a linear source, or vice versa.
Up to some frequency (determined by the spacing of the point
sources) that is a valid approximation. It's just superposition,
nothing else. Again what is this magical 'coupling' ?
they contradict their own argument by stating further down that line
array elements should not be used as singletons. it is of course
safe to
assume that a single line array element does not create a "slice of
cylindrical wave", but some vendors have waveguiding tricks up their
sleeves that do just that, although single elements still fail
because
you are more or less always on the boundary. but it is possible to
use
horn reflections in such a way that you will get a hf sound field
that
would appear to be made of several hf tweeters, spaced more
closely than
they actually are. a hack that does not automatically a perfect
cylindrical wave make, but a damn clever one, and one that moves the
upper coupling frequency a lot higher than the spacing of the
tweeters
would imply.
Yes, such techniques do exists. Assume for a moment that they
work as claimed, and that the complete 3.5 meters of stacked
drivers do indeed behave as real line source and produce
cylindrical waves in the listening area.
It would be unusable: extremely bad FR, even on axis and
dependent on distance. And even 10 degrees of-axis it gets
much worse again - you'd have a real pencil beam at HF.
It's OK-ish exaclty on-axis and at a distance of 40 m
or so, but no amount of EQ or whatever is going to make
it usable at closer range.
To be usable, you'd have to 'splay' the array (and put it
on a tower). This will provide a uniform response if done
correctly, and at close range it will still drop off less
than 6 dB when distance is doubled. But not because of any
cylindrical waves or any 2-D effect, but just because
such a splayed array emulates a more distant point source.
It still needs EQ, close to +3dB/oct. In practice a large
part of this is taken care of by making individual drivers
more directional as frequency goes up. Which does indeed
mean they wouldn't make very good speakers when used
individually.
The whole PA world has gone rather line-array mad since the idea was
re-introduced and re-engineered by L'Acoustique and Intellivox (Duran
Audio) some 10-12 years ago. Now, every manufacturer has to have a
"line-array" system, though the use of the term is often barely
appropriate. Initially the systems were for very large spaces, with
long lines, but over time there has been a desire for smaller
systems, sometimes to almost ridiculous extremes (see K-Array).
L'Acoustique produced well engineered, powerful, and good sounding
systems that were scalable, and everyone else (apart from some like
Tony Andrews of Function One) has followed. The fact that they were
well engineered, powerful, and good sounding probably convinced the
world that this was "a good thing" almost as much as the controlled
vertical dispersion (at mid to HF) which resulted in reduced
reflections from unwanted directions. L'Acoustique had made a range
of good sounding, compact and powerful PA speakers for some time
under the Heil brand name, though these were not really known outside
France. They claim 5 degree vertical dispersion for their mid/HF
boxes, something I have doubts about for an individual element though
the arraying and 'coupling' may contribute to this claim.
The arrays were always what became termed "J" arrays, nearly vertical
at the top, with more boxes, and curved towards the bottom to cover
"the near field" with fewer boxes. Thus there was more energy at
higher frequencies radiated to more distant listeners, otherwise
known as 'good throw". Each box had its own amplifier, and current
systems have amplifiers with DSP to provide cross-overs, equalisers
and delay in the cabinet, all controllable by a laptop over a
network, to allow "steering" of the entire array.
The term "line array" also covers a 2D implementation of speaker
(or microphone) array under DSP control.
Smaller "line array" systems are a response to the demand for buzz
word technology on a smaller scale. d&b systems, for example, are
sold as stackable and scalable, and individual elements are widely
used singly as well as in larger arrays. They are very usable.
I too, like J?rn, am curious to know at what point does ambisonics
break due to the non-simultaneity of arrival of sound from widely
spaced sources. Obviously lower order components will suffer most
from this, With a widely spaced array, the "sweet spot" will be
correspondingly larger, and thus a bigger audience can be within it,
but the ultimate constraints will be size of venue and environmental
noise concerns.
Ciao,
Dave
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