TM 5-805-4/AFJMAN 32-1090
CHAPTER 3
SOUND DISTRIBUTION INDOORS
3-1. General.
where D1 and R1 are the distance (in feet) and
Room Constant (in ft.2) values for the measured or
This chapter provides data and procedures for deter-
known sound pressure level LpD1Rl; D2 and R2 are
mining sound pressure levels in enclosed rooms due
the distance and Room Constant values for the
to sources of sound contained within the room.
new set of conditions for which the new sound
3-2. Sound Pressure level In A Room.
pressure level LpD2R2 is wanted; and REL SPLDIRl
and REL SPLD2R2 (in dB) are read from the
The sound pressure levels at a given distance or
ordinate (vertical axis) of figure 3-1 for the specific
the sound power levels for individual equipment
items can often be obtained from equipment sup-
SPL change when only the Room Constant is
pliers. Appendix C also provides sound level and
changed and there is no change of distance (i.e.,
power level estimates for general classes of me-
the equipment distance remains constant), the
chanical equipment. Once the characteristics of
the sound source has been determined, then the
same distance value for D1 and D2 is used and the
sound level at any location within an enclosed
equation is solved. For estimating SPL change
space can be estimated. In an outdoor "free field"
when only the distance is changed and there is no
(no reflecting surfaces except the ground), the
change in Room Constant (i.e., the equipment
sound pressure level (SPL) decreases at a rate of 6
remains in the same room, with no change in
dB for each doubling of distance from the source.
absorption), the same value of Room Constant for
In an indoor situation, however, all the enclosing
R1 and R2 is used and the equation is solved. For a
surfaces of a room confine the sound energy so
complete analysis, the calculations must be carried
that they cannot spread out indefinitely and be-
out for each octave frequency band.
come dissipated with distance. As sound waves
c. Simplified table for SPL correction for dis-
bounce around within the room, there is a build-up
tance and room constant. Table 3-1 represents a
o f sound level because the sound energy is
simplification of figure 3-1 for a special condition
"trapped" inside the room and escapes slowly.
of distance and room constant. Much of the collec-
a. Effect of distance and absorption. The reduc-
tion of equipment sound data in appendix C is
tion of sound pressure level indoors, as one moves
given in terms of SPL at a normalized distance of
across the room away from the sound source, is
3 feet and a normalized room constant of approxi-
dependent on the surface areas of the room, the
amount of sound absorption material on those
from those normalized 3-foot SPLs to some greater
areas, the distances to those areas, and the dis-
distance for a variety of different Room Constants.
tance from the source. All of this is expressed
Table 3-1 must not be used in converting sound
quantitatively by the curves of figure 3-1. Figure
power level (PWL) data to sound pressure level
3-1 offers a means of estimating the amount of
(see equation 3-2 and table 3-2).
SPL reduction for a piece of mechanical equipment
d. SPL in a room when PWL is known. The
(or any other type of sound source> in a room, as
one moves away from some relatively close-in
second major use of figure 3-1 is in determining
distance to any other distance in the room, pro-
the SPL in a room when the sound power level of
vided the sound absorptive properties of the room
the source is known. Equation 3-2 provides this.
(Room Constant) is known. Conversely figure 3-1
Lp D , R = Lw + REL SPLD,R
(eq 3-2)
also provides a means of estimating the sound
where LpD,R is the SPL to be determined at
reduction in a room, from a given source, if the
distance D in the room of Room Constant R, Lw
distance is constant and the amount of absorptive
the sound power level of the source (in dB re
treatment is increased.
10-12W) and REL SPLD,R is read from the ordinate
b. General application of figure 3-1. Figure 3-1
of figure 3-1 for the point of intersection of the D
may be used for estimating SPL change from any
and R values specified. In most uses, the value of
given condition of Room Constant and distance to
REL SPLD,R will be negative, so this amounts to a
any other wanted condition of Room Constant and
subtraction function. Hence, the signs must be
distance. This can be expressed by equation 3-1:
followed carefully. The calculation is repeated for
each octave band.
3-1
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