TM 5-805-4/AFJMAN 32-1090
EQUIVALENT DISTANCE FROM ACOUSTIC CENTER OF A
NONDIRECTIONAL SOURCE;. "D" (FT.)
Note: T h i s f i g u r e h a s b e e n a d j u s t e d t o t a k e i n t o a c c o u n t l a r g e
obstacles or large pieces of equipment distributed about the room.
Therefore, the curves for large values of R do not agree with
similar textbook curves that tend to ignore such obstacles.
Figure 3-1. Approximate Relationship Between "Relative Sound Pressure Level" (REL SPL) and Distance to a Sound Source
for Various "Room Constant" values.
absorption coefficients of the acoustic- materials at
e. Simplified table PWL to SPL. As a conve-
various specified frequencies. The calculation is
nience, table 3-2 presents the REL SPL data of
summarized in equation 3-3:
figure 3-1 for a number of distance and Room
Constant values. This table is for use only in
calculating SPL from PWL; it does not give the
where R is the Room Constant (or "room absorp-
difference between two REL SPL values, as is
tion" as it is often called), S1 is the total area of all
given in table 3-1.
the room surfaces having "sound absorption coeffi-
S2 is the total area of all the room
cients"
3-3. Room Constant.
surfaces having sound absorption coefficient
etc.
a. Calculation of room constant. The room con-
The areas S1, . . .Sn are expressed in ft.2, and the
stant is a measure of the amount of sound absorp-
sound absorption coefficients
are dimensionless.
tion that exists within a room. Most current
The resulting Room Constant R is also expressed in
acoustic textbooks give details of a conventional
ft2 The term "sabin" is used in the literature as a
calculation of the Room Constant for any specific
unit of room absorption or Room Constant, where
one sabin is the absorption provided by 1 ft2 of
room, when the following facts are known: (1) all
material having perfect absorption; i.e.,
having a
the room dimensions, (2) the wall, floor, and
2
value of 1.0. In the manual, 1 ft of absorption and
ceiling materials, (3) the amount and type of
1 sabin are used synonymously.
acoustic absorption materials, and (4) the sound
3-2
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