TM 5-805-4/AFJMAN 32-1090
pressure, equals 1 Newton/m2). This reference
c. Decibel subtraction. In some case it is neces-
sary to subtract decibel levels. For example if the
pressure represents approximately the faintest
cumulative level of several sources are known,
sound that can be heard by a young, sensitive,
undamaged human ear when the sound occurs in
what would the cumulative level be if one of the
the frequency region of maximum hearing sensi-
sources were reduce? Decibel subtraction is given
tivity, about 1000 Hertz (Hz). A 20 uPa pressure is
by equation B-4.
0 dB on the sound pressure level scale. In the
strictest sense, a sound pressure level should be
stated completely, including the reference pressure
base, such as "85 decibels relative to 20 uPa."
(eq B-4)
However, in normal practice and in this manual
d. Decibel averaging. Strictly speaking decibels
the reference pressure is omitted, but it is never-
should be averaged logarithmatically not arithmet-
theless implied.
ically. Equation B-5 should be used for decibel
c. Abbreviations. The abbreviation SPL is often
averaging.
used to represent sound pressure level, and the
notation Lp is normally used in equations, both in
this manual and in the general acoustics -litera-
ture.
d. Limitations on the use of sound pressure
levels. Sound pressure levels can be used for
evaluating the effects of sound with respect to
B-3. Sound Pressure level (Lp or SPL).
sound level criteria. Sound pressure level data
taken under certain installation conditions cannot
The ear responds to sound pressure. Sound waves
be used to predict sound pressure levels under
represent tiny oscillations of pressure just above
other installation conditions unless modifications
and below atmospheric pressure. These pressure
are applied. Implicit in these modifications is a
oscillations impinge on the ear, and sound is
sound power level calculation.
heard. A sound level meter is also sensitive to
sound pressure.
B-4. Sound power level. (Lw or PWL)
a. Definition, sound pressure level. The sound
Sound power level is an absolute measure of the
pressure level (in decibels) is defined by:
quantity of acoustical energy produced by a sound
source. Sound power is not audible like sound
pressure. However they are related (see section
B-6). It is the manner in which the sound power is
(eq B-6)
radiated and distributed that determines the
Where p is the absolute level of the sound pressure
sound pressure level at a specified location. The
and pref is the reference pressure. Unless other-
sound power level, when correctly determined, is
wise stated the pressure, p, is the effective root
an indication of the sound radiated by the source
mean square (rms) sound pressure. This equation
and is independent of the room containing the
is also written as:
source. The sound power level data can be used to
compare sound data submittals more accurately
and to estimate sound pressure levels for a variety
of room conditions. Thus, there is technical need
(eq B-7)
for the generally higher quality sound power level
Although both formulas are correct, it is instruc-
data.
tive to consider sound pressure level as the log of
a. Definition, sound power level. The sound
the pressure squared (formula B-6). This is be-
power level (in decibels) is defined by:
cause when combining sound pressure levels, in
almost all cases, it is the square of the pressure
(eq B-8)
ratios (i.e. {p/Pref)2}`s) that should be summed not
the pressure ratios (i.e. not the {p/Pref}`s). This is
Where P is the absolute level of the sound power
also true for sound pressure level subtraction and
and Pref is the reference power. Unless otherwise
averaging.
stated the power, P, is the effective root mean
b. Definition, reference pressure. Sound pressure
square (rms) sound power.
level, expressed in decibels, is the logarithmic
b. Definition, reference power. Sound power
ratio of pressures where the reference pressure is
level, expressed in decibels, is the logarithmic
20 micropascal or 20 uPa (Pascal, the unit of
ratio of the sound power of a source in watts (W)
B-2
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