Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 430

Q = Directivity factor.
= 2 for one reflecting plane.
= 4 for two reflecting planes.
= 8 for three reflecting planes.

15.7.5.11 Noise Level at a New Distance from the Noise Source

SPL = SPL 1 + 20 log ( d 1 )/( d 2 )

where
SPL = Sound pressure level at new distance ( d 2 ).
SPL 1 = Sound pressure level at d 1 .
d n = Distance from source.

15.7.5.12 Calculating Daily Noise Dose
The following formula combines the effects of different sound pressure levels and allowable expo-
sure times:

Daily noise dose = ++++
++++

CC C …

…

C

1 2 3 n

TT T T

1 2 3 n

where
C i = Number of hours exposed at given SPL i .
T i = Number of hours exposed at given SPL i .

15.7.5.13 OSHA Permissible Noise Levels

T SPL = 8/2 ( SPL -90)/5

where
T SPL = Time in hours at given SPL.
SPL = Sound pressure level (dBA).

15.7.5.14 Converting Noise Dose Measurements to 8-Hour Equivalent TWA

T WA eq = 90 + 16.61 log ( D )/100

where
T WA eq = Eight-hour equivalent TWA (dBA).
D = Noise dosimeter reading (%).

15.7.5.15 Noise Reduction in Duct System

NR = 12.6 P α 1.4 / A

where:
NR = Noise reduction (dB/ft).
P = Perimeter of duct (in.).
α = Absorption coefficient of the lining material at frequency of interest.
A = Cross-sectional area of duct (in. 2 ).

15.7.5.16 Sound Intensity Level
Sound intensity level is the power passing through a unit area as the sound power radiates in free
space.

L 1 = 10 log I / I o