Assessing and Mitigating Noise Impacts
Author:
These noise guidelines include an excellent primer.
Table A
APPROXIMATE ADDITION OF SOUND LEVELS
| Difference Between Two Sound Levels | Add to the Higher of the Two Sound Levels |
| ≤1 dB | 3 dB |
| 2-3 dB | 2 dB |
| 4-9 dB | 1 dB |
| ≥10 dB | 0 dB |
(United States Environmental Protection Agency, Protective Noise Levels, Condensed Version of EPA Levels document, EPA 550/9-79-100, November 1978, Office of Noise Abatement & Control, Washington, D.C.)
“[T]wo proximal noise sources that are 70 dBA each do not have a combined noise level of 140 dBA. In this case the combined noise level is 73 dBA.”
“SPL increases approaching 10 dB result in a perceived doubling of SPL. … An increase of 10 dB(A) deserves consideration of avoidance and mitigation measures in most cases. … In general, the EPA’s “Protective Noise Levels” guidance found that ambient noise levels # 55 dB(A) L(dn) was sufficient to protect public health and welfare and, in most cases, did not create an annoyance (EPA 550/9-79-100, November 1978). In non-industrial settings the SPL should probably not exceed ambient noise by more than 6 dB(A) at the receptor. An increase of 6 dB(A) may cause complaints.”
[Note that L(dn), the day-night average sound level, is generally calculated with adding 10 dB to values measured at night, i.e., an L(dn) of 55 dB(A) implies an average night level of 45 db(A).]
Table B
HUMAN REACTION TO INCREASES IN SOUND PRESSURE LEVEL
| Increase in Sound Pressure (dB) | Human Reaction |
| <5 | Unnoticed to tolerable |
| 5-10 | Intrusive |
| 10-15 | Very noticeable |
| 15-20 | Objectionable |
| >20 | Very objectionable to intolerable |
(Down, C.G. and Stocks, J.; Environmental Impact of Mining. Applied Science Publishers Ltd., ISBN 0853347166, 1978)
Table C
PROJECTED NOISE LEVELS
| Noise Source | Measurement | 1,000 feet | 2,000 feet | 3,000 feet |
| Primary and secondary crusher | 89 dB(A) at 100 ft | 69.0 dB(A) | 63.0 dB(A) | 59.5 dB(A) |
| Hitachi 501 shovel loading | 92 dB(A) at 50 ft | 66.0 dB(A) | 60.0 dB(A) | 56.5 dB(A) |
| Euclid R-50 pit truck loaded | 90 dB(A) at 50 ft | 64.0 dB(A) | 58.0 dB(A) | 54.4 dB(A) |
| Caterpillar 988 loader | 80 dB(A) at 300 ft | 69.5 dB(A) | 63.5 dB(A) | 60.0 dB(A) |
(Barksdale, R.D., editor, 1991. The Aggregate Handbook: National Stone Association [Washington, DC], 1.V)
“At distances greater than 50 feet from a sound source, every doubling of the distance produces a 6 dB reduction in the sound.” [The reduction may be only 3 dB per doubled distance for low-frequency sound and for a "line" source.]
[Most manufacturers do not publish the noise levels of their machines, with the following exceptions:
Clipper 2.5 MW Wind Turbine -- SPL @8 m/s [~40% capacity]: 104 db(A), according to IEC 61400-11
Nordex N90/2500 — at 95% red. nominal output: 104.5 dB(A) [LS model]/107 dB(A) [HS model]]
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