Noise Perimeter Zones
Noise perimeter zones (NPZ) are another administrative control to limit exposure to noisy processes or equipment to as few workers as possible. NPZ are areas where noise levels of 90(85?) dB (A) or more are roped off and marked to keep out all workers who don't have to be there.
NPZ can be set up using a sound level meter to find the safe distance from the source (90 dB (A)) and the NPZ can be set up at that distance. Noise does not radiate from the source at the same level in all directions. Noise from machinery can be higher in one direction than another because the noise can also be either absorbed or reflected from surfaces it contacts, such as the ground or a wall. Therefore, measurements should be taken at several points in an area where people might be working. Once noise levels that are 90 dB (A) or more are determined, rope off this area as the Noise Perimeter Zone. Exclude all workers who do not need to be in that zone. All workers who need to work within the zone must wear hearing protection. The area can be marked "Noisy Area - Hearing Protection Required" in the same way that a "Hard Hat Area" is marked off.
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Of course, the entire site can be inside a Noise Perimeter Zone. Then everyone on site must wear hearing protection.
Another way for the employer to set up a NPZ is to measure the sound pressure level, using a sound level meter, at a distance from a noise source. Then measure the distance between the noise source and measurement point. Convert the sound pressure to the estimated sound power using the tables below and insert in second table to determine safe distances from the noise source.
Table 1: Converting sound pressure measurement to sound power
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Measured Sound Pressure Level with SLM in dB
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Table 2: Calculating the Noise Perimeter Zone from the sound power
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Estimated Sound Power Level (dB)
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80 dB at distance from source
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90
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3 ft
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2 ft
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1 ft
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95
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5 ft
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3 ft
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2 ft
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100
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9 ft
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5 ft
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3 ft
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105
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16 ft
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9 ft
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5 ft
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110
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29 ft
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16 ft
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9 ft
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115
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52 ft
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29 ft
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16 ft
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120
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92 ft
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52 ft
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29 ft
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125
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164 ft
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92 ft
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52 ft
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130
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292 ft
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164 ft
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92 ft
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