Once we have this chart, we can see the differ-
ente
between the way humans perceive sound
and the amount of
force it takes to change air
pressure. Unfortunately, the
result is nota simple
"twice
as
much pressure" of sound to be heard
as
"twice
as
loud". If you plot decibels
as
the even
divisions on
a
graph, the unit increase you need
is
a
very funny curve.
VOLTAGE,
CURRENT
/
OF
PRESSURE
v1
dB
=
20
iog,,
E
wav
5 10
15
20 25 30
-
increase
RIS~
in
even
1 dB
Unit
This is how the ear works, and we must adapt our
system to it. We
have no choice if we expect our
loudspeaker to produce
a
sound that resembles
the
original sound we begin with. The high sensi-
tivity to sound of the human ear produces
a
strong "energy" illusion that has confused listen-
ers since early times. How powerful are the loud-
est sounds of music in
real power? Can sound be
used
as
a
source of energy to do useful work,
such
as
operating a car? For any normally "loud"
sound the answer
is,
regrettably, no! Perhaps not
so regrettably, consider what would happen
if
one pound of pressure was applied not to your
head, but directly to your inner ear. One pound
of air pressure variation
is
170dB spl
!
This
a-
mount of "power" might do some useful work
-
but not much,
it's
still only one pound and to
make use of it you will
have to stand one mile
away or you will go deaf immediately.
If we reduce our sound power to realistic musical
values, we will not be injured, but we will
have
almost nothing (in real power terms) to run the
mic with! This low available energy is the reason
that high gain amplifiers are required for
micro-
p hones.
When we take
a
microphone and "pick up" the
sound, we do
have some leeway in deciding how
much energy we must
have in order to operate
the electrical part of our system.
If wecan decide
that we don't
have to truly hear the signal while
we are processing
it from point to point and we
can wait until the electronic devices have done
L
al1 their routing and switching before we need
audible sound, we can lower the power of the
signal. What
is
a
good value for
a
reference here?
Well, we need to have enough energy so that the
signal
is
not obscured by hiss, hum, buzz or other
unpleasant things we don't want, but not so high
that
it
costs
a
fortune in "juice" or electrical
power. This was
a
big consideration for the tele-
phone company.
They now
have the world's biggest audio mixing
system, and even when they started out,
electric-
ity was not free. They set their electrical power
signal reference
as
low
as
was practical
at
the
time, and
it
has lowered over the years
as
elec-
tronic equipment has gotten better. In 1939 the
telephone company, radio broadcasting, and
re-
cording industry got together and standardized 1
milliwatt of power
as
OdBm, and this is still the
standard of related industries. Thus,
a
OdBm sig-
nal at
a
600ohm line impedance will present
a
voltage of 0.775 volts.
Once again, we owe you an explanation. Why
does
it
say ZERO on the meter? What isan ohm?
Why 600 of them and not some other value?
''
What's
a
volt? Let's look
at
one thing at
a
time.
1. The logic of ZERO on the meter is another
hangover from the telephone company
prac-
tice.
When you start
a
phone call in Califor-
nia, the significant information to
a
telephone
company technician in Boston is
-
did the
signal level drop?
If so, how much? When the
meter says ZERO
it
indicates (to the phone
company) that there has been no loss in the
transmission, and
al1
is
well. The reference
level is one milli-watt of power, but the gain or
loss is in the information the meter was
sup-
posed to display, so the logic of ZERO made
good sense, and that's what they put on the
dial. We still use it even though
it's
not logi-
cal
for anything else, and the idea of
a
refer-
ence level described
as
a
"no loss" ZERO, no
matter what actual power
is
being measured
is
so firmly set in the minds of everyone in the
audio world that
it
is
probably never going to
change.
2.
One ohm
is
a
unit of resistance to the passage
of electrical energy. The exact reasons for the
.
choice of 600 ohms
as
a
standard are con-
nected to the demands of the circuits used
.
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