Acoustical Dynamic Range of the System

We described the dynamic range of
the program going into the microphone,
and of the electrical signal
through the console and power amps,
but what of the sound coming out of
the loudspeaker system? If you haven't
already guessed, it also must have the
same dynamic range. If the speakers
aren't capable of this range, then
they're probably going to either distort
(or burn out) on the peaks, be incapable
of responding to the lowest power
levels, or experience some combination
of these problems.
What are the actual sound levels
that must be reproduced? That all
depends on the distance between the
loudspeakers and the audience, and
how loud one wants the sound to be at
the audience. Let's assume that we
don't want to shatter eardrums... we
don't want people in the audience to
feel their ears are one inch from the
lead vocalist's tongue during a maximum
shout. The peak sound level we
might accept as a reasonable facsimile
ofthis excitement is 120 dB SPL. Without
going through the math (we cover
some of that in Part I, Section 5), take
our word for it that these particular
speakers must (cumulatively) generate
130 dB SPL in this particular environment.
Well, we know ifthey generate
130 dB SPL on peaks, they're going to
have to generate 40 dB SPL during the
quietest passages, and will have a
90 dB dynamic range.
From this, we also know that ifthe
sound reaching the audience during
peaks was attenuated by air and
distance by 10 dB from 130 dB SPL to
120 dB SPL, the 40 dB SPL generated
by the speakers during quiet passages
will also be attenuated. When the
40 dB drops to 30 dB, it will be below
the ambient noise level in the audience.
This means that the audience
may not hear the very quietest parts of
the show. This illustrates why some
electronic manipulation of dynamic
range is often called for. In this case,
compression of the loudest peaks would
allow the level to be turned up so the
quiet passages are louder.