If the engine room is free to radiate sound from all

system for neighborhood acceptance (para 33c of

four of its walls, and if all four walls are of similar

the N&V manual).

construction, the area A in equation 33 should be

the total area of all four walls, and the radiated

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sound is assumed to be transmitted uniformly in all

A lightweight prefabricated garage-like structure

directions. If only one wall is radiating the sound

might be considered as a simple enclosure for a

toward the general direction of the neighbor posi-

small on-base power plant. The transmission loss of

tion, it may be assumed that the sound is trans-

such a structure might be inadequate, however,

mitted uniformly over a horizontal angle that is

and the enclosure would not serve its intended pur-

120 wide, centered at a line that is perpendicular

pose. A calculation procedure is given here for

to the wall under consideration. This procedure

evaluating this situation.

will give a calculated estimate of the SPL at a

neighbor position fr sound transmitted through a

the use of the "room acoustics" material in para-

solid wall whose TL and area are known. Of

graph 53 of the N&V manual and the source data

course, if a lightweight wall does not have suffi-

in paragraphs 27 and 28 of this manual and in

cient TL to meet the need, a heavier wall should be

chapter 7 of the N&V manual, it is possible to cal-

selected.

side an.

engine room along the wall that radiates

noise to

the outdoors. The sound pressure level

solid wall is readily adaptable to a wall containing a

L

door or window or other surface or opening having

equation

54 in the N&V manual. The N&V equa-

a TL different from that of the wall. It is necessary

tion 54

is repeated here:

to calculate the effective TLC of the composite wall

and to use TLC in the procedure above. The TLC of

the composite wall may be determined from one of

This equation is modified to become equation 31

the methods given in paragraph 5-4e of the N&V

below for the case of the sound pressure level out-

manual.

Constant of the "receiving room") becomes infinite.

opening in an outside wall may be required to per-

mit ventilation of the room or to supply air to an

tity 10 log 1/4 is 6 dB. Thus, equation 31 is:

engine. Noise escaping through that opening might

(3-1)

L

be disturbing to the neighbors. The sound power

level LW of the escaping noise can be calculated

The sound power level LW radiated by this wall is

with the material given in paragraph 722 in the

(from eq. 7-18 in the N&V manual)

N&V manual, and the SPL at the neighbor position

(3-2)

estimated from the tables 63 or 64 distance

2

where A is the area of the radiating wall, in ft.

terms of the N&V manual. If excessive amounts of

Equation 33 combines equations 31 and 3-2:

noise escape through the opening, a dissipative

muffler should be installed in the opening (para

(3-3)

3-4).

This equation must be used carefully. For a large-

area wall with a low TL in the low-frequency re-

gion, it is possible for equation 33 to yield a calcu-

the roof of that building. For a building with a

lated value of sound power level radiated by the

practically flat roof and a 2- to 5-ft.-high parapet

wall that exceeds the sound power level of the

around the edge of thereof, the noise radiated from

source inside the room. This would be unrealistic

the roof has a significant upward directivity effect.

and incorrect. Therefore, when equation 33 is

This results in a lower amount of sound radiated

used, it is necessary to know or to estimate the

horizontally from the roof surface. There are no

PWL of the indoor sound source (or sources) and

measured field data for the directivity effect of

not allow the LW of equation 33 to exceed that

roof-radiated sound, but a reasonable estimate of

value in any octave band. When the PWL of the

this effect is given in table 31. Without a parapet

radiating wall is known, the SPL at any distance of

around the roof, slightly larger amounts of sound

interest can be calculated from equation 61 or ta-

are radiated horizontally; and a sloping room radi-

bles 63 or 64 of the N&V manual. The directivity

ates still higher amounts of sound horizontally.

of the sound radiated from the wall is also a factor.

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