'THE
DB;
WHO, WHAT, WHY
No matter what happens to the signal while
it
is
being processed,
it
will eventually be heard once
again by
a
human ear. So the process of convert-
ing a sound to an electrical quantity and back to
sound again must follow the logic of human hear-
ing.
The first group of scientists and engineers to dea1
with the problems of understanding how the ear
works were telephone company researchers, and
the results of their investigations form the
foun-
dation of al1 the measurement systems we use in
audio today. The folks at Bell Laboratories get
the credit for finding out how we
judge sound
power, how quiet a sound an average person
can
hear, and almost
al1
of the many other details
about sound you must know before
you can work
with it
successfully.
From this basic research, Bell Labs developed a
system of units that
could be applied to al1 phases
of the system. Sound traveling on wires as
elec-
trical energy, sound on tape as magnetic energy,
sound in air; anyplace that sound
is,
or has been
stored as energy until some future
time when it
will again be sound,
can be described by using
the human ear-related system of numbers
called
"bels" in honor of Alexander Graham Bell, the
inventor of the telephone.
What
is
a bel and what does it stand for?
It means, very simply, twice
as
loud to the human
ear. Twice
as
loud as what? An obviousquestion.
The bel
is
always a comparison between two
things. No matter what system of units of
meas-
ure you are working with
at
the time, you must
always state a value as a reference before you
can
compare another value to
it
by using bels, volts,
dynes, webers
-
it
doesn't matter, a bel, or ear-
related statement of "twice as loud"
is
always a
ratio, not an absolute number. Unless a zero, or
"no
difference" point
is
placed somewhere, no
comparison is possible.
There are many positive and definite statements
of reference in use today. But before we go over
them, we should divide the "bel" into smaller
units. "Twice
as
loud" will be a little crude to be
used
al1 the time. How about one tenth of a bel?
Okay, the decibel
it
is,
and
O
means "no differ-
ente,
same as the reference".
It
seldom means
"nothing". Now, if you double the power,
is
that
twice
as
loud? No,
it
is
only 3dB more sound. If
you double an electrical voltage,
is
it
twice as
loud? No,
it
is
only 6dB more sound. The unit
quantities must follow nonlinear progressions to
satisfy the ears' demand.
Remember, decibels follow the ears.
All other
quantities of measure must be increased in what-
ever units necessary to satisfy the human
require-
ments, and may not be easy to visualize. Sound
in air, our beginning reference,
is
the least sound
the human ear
(young men) can detect
at
1000
to
4000Hz. Bell Labs measured this value to be
.O002 microbar, so we say OdB
=
.O002 microbars
and work our way
up from the bottom, or "no
perceivable sound to humans" point. Here
is
a
chart of sounds and their ratings in
dB, using
.O002 microbar pressure change in air as our re-
ference
for "OdB".
10,000
22
nch
bass
drum
mic inride drum
pnare druni
l
inch
t140
1
I
powr?Ilc voice
at
1
inch
Ircream)
10
newtoiir
per
100
dvner
per
100
100'
-1
newton
per
1
dyne
per
rquare
rneter
square
cm.
l
niicrobar
Auerage
converratlun
--
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rubwavr. rtreet
nolre)
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Homen citv at night
.O1
microbar
Isoiared recording
or
TV
studio
Open
field, night,
no
wind
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microbar
icricketr.
nrect
noirer,
etc.)
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