TM 5-805-4/AFJMAN 32-1090
techniques. This restriction applies equally to both
relative to the sound power reference base of
sound pressure and sound power measurements.
terminology, the reference power in this country
B-5. Sound Intensity level (Li)
was 10-13W, so it is important in using old data
Sound intensity is sound power per unit area.
(earlier than about 1963) to ascertain the power
Sound intensity, like sound power, is not audible.
level base that was used. If the sound power level
value is expressed in dB relative to 10-13W, it can
It is the sound intensity that directly relates sound
be converted to dB relative to 10-12W, by subtract-
power to sound pressure. Strictly speaking, sound
intensity is the average flow of sound energy
ing 10 dB from the value. Special care must be
through a unit area in a sound field. Sound
used not to confuse decibels of sound pressure with
intensity is also a vector quantity, that is, it has
decibels of sound power. It is often recommended
that power level values always be followed by the
both a magnitude and direction. Like sound power,
notation "dB re 10-12W." However, in this manual
sound intensity is not directly measurable, but
this notation is omitted, although it will always be
sound intensity can be obtained from sound pres-
made clear when sound power levels are used.
sure measurements.
c. Abbreviations. The abbreviation PWL is often
a. Definition, Sound Intensity Level. The sound
used to represent sound power level, and the
intensity level (in decibels) is defined by:
notation Lw normally used in equations involving
power level. This custom is followed in the man-
(eq B-9)
ual.
Where I is the absolute level of the sound inten-
d. Limitations of sound power level data. There
sity and Iref is the reference intensity. Unless
are two notable limitations regarding sound power
otherwise stated the intensity, I, is the effective
level data: Sound power can not be measured
root mean square (rms) sound intensity.
directly but are calculated from sound pressure
b. Definition, reference intensity. Sound intensity
level data, and the directivity characteristics of a
level, expressed in decibels, is the logarithmic
source are not necessarily determined when the
ratio of the sound intensity of at a location, in
sound power level data are obtained.
watts/square meter (W/m2) relative to the sound
(1) PWL calculated, not measured. Under the
power reference base of 10-12W/m2.
first of these limitations, accurate measurements
and calculations are possible, but nevertheless
c. Notation. The abbreviation Li is often used to
there is no simple measuring instrument that
represent sound intensity level. The use of IL as
reads directly the sound power level value. The
an abbreviation is not recommended since this is
procedures involve either comparative sound pres-
often the same abbreviation for "Insertion Loss"
sure level measurements between a so-called stan-
and can lead to confusion.
dard sound source and the source under test (i.e.
d. Computation of Sound power level from inten-
the "substitution method"), or very careful acous-
sity level. The conversion between sound intensity
tic qualifications of the test room in which the
level (in dB) and sound power level (in dB) is as
sound pressure levels of the source are measured.
follows:
Either of these procedures can be involved and
requires quality equipment and knowledgeable
personnel. However, when the measurements are
carried out properly, the resulting sound power
(eq B-10)
level data generally are more reliable than most
where A is the area over which the average
ordinary sound pressure level data.
intensity is determined in square meters (m2).
(2) Loss of directionality characteristics. Tech-
Note this can also be written as:
nically, the measurement of sound power level
(eq B-11)
Li + 10 log{A}
LW =
takes into account the fact that different amounts
of sound radiate in different directions from the
if A is in English units (sq. ft.) then equation B-11
source, but when the measurements are made in a
can be written as:
reverberant or semireverberant room, the actual
Li + 10 log{A} - 10
(eq B-12)
LW =
directionality pattern of the radiated sound is not
Note, that if the area A completely closes the
obtained. If directivity data are desired, measure-
sound source, these equations can provide the total
ments must be made either outdoors, in a totally
sound power level of the source. However care
anechoic test room where reflected sound cannot
must be taken to ensure that the intensity used is
distort the sound radiation pattern, or in some
representative of the total area. This can be done
instances by using sound intensity measurement
B-3
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