(mostly 600 ohms, with some at 135 ohms). It is important to note that nowhere
in table I is an actual level of power indicated.
This emphasizes the fact
that a decibel represents a ratio, not a power level. Although not shown in
the table, a ratio of 1/1 is 0 db, since there is neither gain nor loss.
b. Reference Levels.
Both the telephone industry and military
communicators have standardized all sound level measurements on 1 milliwatt
(1 x 10-3 watt), assigning the symbol: 0 db - 1 mw in 600 ohms.
Other
industries connected with sound transmission, such as broadcast, noise
abatement, stereo components, etc., assign other values or standards.
A few
are illustrated below.
0 db6m = 6 milliwatts (6 x 10-3 watt)
0 dbl2m = 12 milliwatts (12 x 10-3 watt)
0 dbw = 1 watt
0 dbrn = 1 picowatt (1 x 10-6 watt)
c. Instrumentation.
All test instruments must carry some indication of
the reference used. Otherwise level readings have little useful significance.
Unfortunately, most level-measuring instruments show DB on the meter faces and
are therefore called decibel meters. However, if you look closely you may see
in fine print on the meter face an expression similar to "0 db = 1 mw in 600
ohms." Such a meter actually reads levels in dbm referred to its calibration
at 1 mw. If a measured level is given to a circuit conditioner simply as so
many db's, as is so often the case, he should immediately inquire as to the
reference level used by the instrument.
d. Rules for Use of Decibels.
(1) A decibel value may be either added to or subtracted from another
decibel value to indicate either gain or loss respectively.
(2) A decibel value may be either added to or subtracted from a level value
in a circuit to indicate the signal level resulting from the gain or
loss.
(3) Decibel figures must never be multiplied or divided.
(4) A level value must NOT be added to another level value.
To do so is
the equivalent of multiplying power instead of adding it.
(5) Examples of correct and incorrect usage are given below.
Example 1: Assume that a circuit contains three amplifiers and
an
attenuator.
If the gain of the first amplifier is 5
db,
the second is 12 db, and the third 17 db, and
the
attenuator loss is 20 db, what is the overall gain of
the
combination circuit?
Add 5, 12, and 17, and subtract 20.
5 + 12 + 17 - 20 = +14 db, a gain of 14 db.
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