We should also make
a
point of mentioning that
the maximum number on this chart represents
"peak power" and not average power. The
rea-
son? Consider if even some monetary part of
your recording is distorted, it will
force a re-re-
cording and
it
is
wisest to be prepared for the
highest values and pressure even if they only
happen "once in
a
while". On this point, statistics
are not going to be useful, the average sound
pressure
is
not the whole story. 'rhe words them-
selves
can be used
as
an example. Say the word
"statistics" close to the
mic while watching the
meters and the peak LED level detector. Then
say the word "average". What you are
likely to
see are two good examples of the problems
en-
countered in the "real world" of recording. 'rhe
strong peaks in the
"s"
and
"t"
sounds will pro-
bably cause the LED's to flash long before the
VU meter reads anywhere near "zero" while the
vowel sounds that make up the word "average"
will cause no such drastic action.
To
allow peaks to pass undistorted through
a
chain of audio parts, the individua1 gain stages
must
al1
have
a
large reserve capability. If the av-
erage
is
X
than
X
+
20dB is usually safe for
speech, but extremely percussive sounds may
re-
qui re
as
much
as
90dB of "reserve" to insure good
results. Woodblocks, castanets,
latin percussion
(guido, afuche) are good examples of this short
term
violence that will show
a
large difference
between "LED flash" and actual meter movement.
When you are dealing with this kind of sound,
believe the LED,
it
is telling you the truth.
Since the reference is assumed to be the lowest
possible audible value,
dB spl is almost always
positive, and correctly written should
have
a
+
sign in front of the number. But
it
is
frequently
omitted. Negative
dB spl would indicate so low an
energy value
as
to be of interest to
a
scientist try-
ing to record one cricket at 1,000 yds. distance,
and
is
of no significance to the multichannel
recordist. Far more to the point
is
the question
"What is
a
microbar?" It
is
a
unit of measure-
ment related to atmospheric pressure and al-
though it
is
extremely small, it must be divided
down
quite
a
lot before
it
will indicate the mini-
mum pressure change in air that we consider
minimum audible sound. This will give you
a
better idea of the sensitivity of the human ear.
One microbar of pressure change is
slightly less
than one millionth of an atmosphere, and you
can find it on our chart as 74 dB spl. It is not
terribly loud, but
it
is certainly not hard to hear.
As
a
matter of fact,
it
represents the average
power of conversational speech
at
6
feet. This
level is
also used by the phone company to define
norma1 earpiece volume on
a
standard telephone.
Now think about that minimum audible threshold
again:
.O002 microbar.
That's two ten thousandths of
a
millionth part of
one atmosphere
!
This breakdown of one reference
is
not given
just to amaze you, or even to
provide
a
feel for
the quantity of power that moderate
levels of
sound represent. Rather it is intended to
explain
the reason we are saddled with
a
ratiollogarithm
measurement system for audio. Adding and sub-
tracting multi-digit numbers might be easy in
this age of pocket calculators, but in the 1920's
when the phone company began
its
research into
sound and the human ear,
a
more easily handled
system of numbers became an absolute necessity.
Conveniente
for the scientist and practical en-
gineer, however, has left us with
a
system that
requires
a
great dea1 of complex explanation be-
fore you can read and correctly interpret
a
"spec
sheet" for almost any
piece of gear.
Here are the formulae for unit increment, but
they are necessary only for designers. And unless
you build your own gear, you won't
have to dea1
with them. For power (watts) increase or loss,
calculate by the following equation:
10
LOG,,
For voltage, current or pressure calculations:
20
LOG,,
v2
v1
=
N
(dB)
One whole atmosphere, 14.70 pounds per square
inch, equals 1.01325 bars. So one whole
atmos-
phere in microbars comes out to be 1,013,250.
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