Simpson 886-2, Operator'S Manual, Page: 13 / 24

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*Table 3-1. Permissible Noise Exposure Limits
Duration per day in hours Sound level, dB(A) SLOW response
32 80
16 85
8 90
6 92
4 95
3 97
2 100
1-1/2 102
1 105
1/2 110
1/4 or less 115
*From Federal Register, Vol. 46, No. 162, August 21, 1981.
3.3 Sound Fields
3.3.1 Free Field
A free field contains no reflecting objects. Ideally it prevails around a point sound
source located in free space. However, it can be considered to exist whenever
the sound level being measured is caused mainly by sound waves coming di-
rectly from the sound source. This can occur even if there are reflecting objects,
if measurements are made close to the sound source.
3.3.2 Reverberant Field
A reverberant field contains reflecting objects and the sound level being mea-
sured is caused mainly by reflections of the sound source. This usually occurs at
a moderate distance from the sound source.
3.3.3 Semi-Reverberant Field
This field is encountered most commonly and exists whenever the sound level
being measured contains a significant amount of both reflected and direct sound
waves.
3.4 Microphone Orientation
a. When making measurements in a free or semi-reverberant field, orient the
microphone so that the angle of incidence, formed by incoming sound waves
and axis of the microphone, is equal to the random incidence angle of the
microphone. For the 886-2 this angle is 70° (refer to Figure 3-2).
b. When making measurements in a reverberant field, orientation of the micro-
phone is not critical. However, it is recommended, where possible, that the
operator use the same orientation (70°) as for the free or semi-reverberant
field.
c. Vertical orientation of the microphone is not critical except where it is in line
with a second sound source or a high-amplitude reflection of a sound source.
It usually is invalidated by a second sound source, or by a nearby reflecting
surface.
d. When evaluating a particular sound source, avoid holding the microphone
in such a way that the reading is invalidated by a second sound source, or by
the proximity of a nearby reflecting surface.