TM 5-805-4/AFJMAN 32-1090
APPENDIX B
BASICS OF ACOUSTICS
B-1. Introduction
reference value, the decibels are standardized to
that reference value. In acoustics, the decibel is
a. This appendix presents the basic quantities
used to quantify sound pressure levels that people
used to describe acoustical properties. For the
hear, sound power levels radiated by sound
purposes of the material contained in this docu-
sources, the sound transmission loss through a
ment perceptible acoustical sensations can be gen-
wall, and in other uses, such as simply "a noise
erally classified into two broad categories, these
reduction of 15 dB" (a reduction relative to the
are:
original sound level condition). Decibels are al-
(1) Sound. A disturbance in an elastic me-
ways related to logarithms to the base 10, so the
dium resulting in an audible sensation. Noise is by
notation 10 is usually omitted. It is important to
definition "unwanted sound".
realize that the decibel is in reality a dimension-
(2) Vibration. A disturbance in a solid elastic
less quantity (somewhat analogous to "percent").
medium which may produce a detectable motion.
Therefore when using decibel levels, reference
b. Although this differentiation is useful in pre-
needs to be made to the quantity under evaluation
senting acoustical concepts, in reality sound and
vibration are often interrelated. That is, sound is
and the reference level. It is also instructive to
often the result of acoustical energy radiation from
note that the decibel level is primarily determined
vibrating structures and, sound can force struc-
by the magnitude of the absolute value of the
tures to vibrate. Acoustical energy can be com-
power level. That is, if the magnitude of two
pletely characterized by the simultaneous determi-
different power levels differ by a factor of 100 then
nation of three qualities. These are:
the decibel levels differ by 20 dB.
(1) Level or Magnitude. This is a measure of
b. Decibel addition. In many cases cumulative
the intensity of the acoustical energy.
effects of multiple acoustical sources have to be
(2) Frequency or Spectral Content. This is a
evaluated. In this case the individual sound levels
description of an acoustical energy with respect to
should be summed. Decibel levels are added loga-
rithmically and not algebraically. For example, 70
(3) Time or Temporal Variations. This is a
dB plus 70 dB does not equal 140 dB, but only 73
description of how the acoustical energy varies
dB. A very simple, but usually adequate, schedule
with respect to time.
for obtaining the sum of two decibel values is:
c. The subsequent material in this chapter de-
Add the following
fines the measurement parameters for each of
When two decibel
amount to the
these qualities that are used to evaluate sound
higher value
values differ by
and vibration.
3 dB
0 or 1 dB
B-2. Decibels.
2 or 3 dB
2 dB
The basic unit of level in acoustics is the "decibel"
4 to 9 dB
1 dB
(abbreviated dB). In acoustics, the term "level" is
used to designated that the quantity is referred to
10 dB or more
0 dB
some reference value, which is either stated or
When several decibel values to be added equation
implied.
B-2 should be used.
a. Definition and use. The decibel (dB), as used
in acoustics, is a unit expressing the ratio of two
quantities that are proportional to power. The
decibel level is equal to 10 times the common
(eq B-2)
logarithm of the power ratio; or
In the special case where decibel levels of equal
magnitudes are to be added, the cumulative level
(eq B-1)
can be determined with equation B-3.
In this equation P2 is the absolute value of the
(eq B-3
L sum = Lp + 10 log (n)
power under evaluation and P1 is an absolute
value of a power reference quantity with the same
where n is the number of sources, all with magni-
units. If the power P1 is the accepted standard
tude Lp.
B-1
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