SPEAKER PLACEMENT

Setting up a Sound System:

The exact arrangement for a sound system in a theatre will depend greatly
on the nature of the theatre and its program.

Theatre systems have to be flexible, because every show is different.  A
show might need only special effects, or voice reinforcement, or a mic and
podium PA system.  Each system may require a different arrangement of
elements, especially of speakers.  


Most theatres have both permanent speakers and portable speakers to allow
variable placement.

One problem faced in theatres is that you must give all seats in the
audience as close to the same show as poss. This can be difficult.

You also will want the sound to seem to be coming from the proper place,
that is, on stage. The simplest way to do this is to put the speakers on
stage, but that is often difficult.

Consider the question of Mono vs. stereo

Stereo's advantage over Monoral is that it can simulate the difference in
timing that one ear gets vs. the other.  This is how one locates sounds in
real life.  But to work properly, the speakers must accurately portray what
you would hear if you were at the source of the original sound.  That means
your ears must be about half way between the spearkers.

For any given seat in the aud., the person there will naturally hear the
closest speaker best.  The further speakers are not heard as well.  This is
why true Stereo systems rarely work well in a theatre.  The stereo imaging
effect only works along the center line of the audience.



Stereo requires that you be equidistant from both speakers to that the
built in delays that create stereo imaging will be correct.  Being closer
to one speaker than another changes the timing and so distorts the sound
image.

One solution to this problem is to forgo the stereo effect all together.
All speakers are placed in a center cluster located over the center of the
procenium.

This places the sound in the center of the stage.  Even though the sound is
actually 20 feet above the apparent source, it works because we are better
at localizing sounds horizontally than vertically, so we notice the
centered location but not the height of it.  It also has the advantage of
allowing everyone in the audience to hear exactly the same thing.


Some theatres are also built using stereo speakers located in the prosceniums.

This actually has more problems than the mono system.  Stereo only works
along the center line

To either side, the closer speaker is better heard, and the sound therefore
comes from the proscenium rather than the stage.

However, in live mic situations, this arrangement will cause fewer feedback
problems.  The speakers are further from the mics and less likely to be
picked up.


A variation of the Mono approach is to place multiple speaker clusters
across the top of the proscenium.

Varying proportions of the left and right channels are fed to the
appropriate speakers.  This requires a separate proportional mix for each
channel, usually from a custom designed mixer; no off the shelfstuff here.


Another arrangement is the distributed speaker system. 

In this approach, many small speakers are placed throughout the room,
usually in the ceiling.  To avoid phase problems caused by the time delay
of far speakers reaching distant listeners after the near speakers,
sophisticated delay circuits must be built in to compensate.

This system removes the problem of near seats being overpowered by the
sound while far ones strain to hear.  However, all directionality is lost.
Perceived sound comes from the nearest speaker, rather than the stage.

This system may be the only way to overcome problems in large theatres with
poor accoustics or everhanging balconies that create sound shadows, where
architecture blocks the sound path.


The current favorite system is the Left-Center-Right system.

This combines Stereo and Mono.  Left channel is sent to left cluster, Right
to right, and the Center is a mix of both.  You get the advantage of the
Mono system with some stereo imaging.

This does require a mixer with three output channels.  Traditional mixers
are Mono or Stereo, so work-arounds must be done.  Most modern mixers come
with the L/C/R option.


Special effects speakers are placed as close to the apparent source as
possible.  Backstage, and/or onstage near and upstage from the "source".

One caveate, don't block the speaker with accoustically opaque materials or
the sound will be blocked.


In Arena theatres, you will find two variations commonly used.

First, the Center Cluster, adapted to Arena.

Center cluster hung directly overhead.
	Especially suited to large arenas, like large sports arenas.

	Takes less rigging, cabling, etc.  and reduces chances of phase
cencellation, but actually difference in perceived location more extreme
than other option.  

Distributed system adapted to Arena

	Suitable for smaller spaces where line of sound is actually lower than
center cluster.

	More control over where sound is coming from when each speaker is
controlable separately.

	Some difficulty in mic use as chance of feedback is greater.


So,
Setting up a Sound System involves:

Analyzing the accoustics of the room to determine how sound behaves.

Determining what sort of speaker arrangement is needed in the space to cope
with these problems

Deciding what is possible with available equipment, budget, and time.

Assembling the equipment and balancing the system, EQing the space, etc.

©2000, Mick Alderson