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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.

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
Measured Sound Pressure Level with SLM in dB
Estimated Sound Power Level (dB) at distance from source

At 5 ft

At 10 ft

At 15 ft

At 20 ft

At 30 ft
80
82
84
86
88
90
92
94
96
98
100
95
101
104
107
110
97
103
106
109
112
99
105
108
111
114
101
107
110
113
116
103
109
112
115
118
105
111
114
117
120
107
113
116
119
122
109
115
118
121
124
111
117
120
123
126
113
119
122
125
128
115
121
124
127
130

 

Table 2: Calculating the Noise Perimeter Zone from the sound power
Estimated Sound Power Level (dB)
80 dB at distance from source
Noise Perimeter Zone for Target Control Levels (dB)
85 dB at distance from source
90 dB at distance from source
90
3 ft
2 ft
1 ft
95
5 ft
3 ft
2 ft
100
9 ft
5 ft
3 ft
105
16 ft
9 ft
5 ft
110
29 ft
16 ft
9 ft
115
52 ft
29 ft
16 ft
120
92 ft
52 ft
29 ft
125
164 ft
92 ft
52 ft
130
292 ft
164 ft
92 ft

 

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