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ed?ij3?.Z <br />III. NOISE SOURCES <br />Measurement of equipment noise in operation at the mine was accomplished as part of the 2004 study. <br />These measured noise data were supplemented with manufacturer's published data and E. D. L's noise <br />emission database. These data are used in Part IV to develop a theoretical acoustic model to project known <br />source noise over certain distances to determine project noise levels at selected locations. <br />A. GENERAL <br />The noise emissions from mining equipment can be quantified by engineering/scientific descriptors. The <br />basic descriptor relies on time, space, and power and once these are known noise levels, noise <br />footprints, noise contours and statistical distribution can be quantified. For the most part, these data are <br />determined by tests and occasionally by analysis. Equipment manufacturers conduct noise tests and <br />publish the data in their equipment specifications. Many times the data presents only the A-weighted <br />noise levels, which means that the noise reduction effects of berms, barrier sound walls, etc. can not be <br />accurately determined because the spectral content (frequency tone, pitch) of the noise sources was not <br />measured. Once the noise emission has been determined then the noise at any distance from the noise <br />source can be computed including an assessment of the mitigation, provided by berms, barriers, noise <br />walls and terrain. <br />Most of the noise emission data presented in this report has been normalized to a decibel level at 100 ft. <br />from the equipment (standard convention). These noise values can then be used to determine the noise <br />level at any other distance from the source. The calculations are based upon the inverse square law, <br />which is applicable to sound, light, electromagnetic radiation and other physical phenomena. The inverse <br />square law in its simplest form means that the energy from a source spreads out uniformly in a sphere or <br />hemisphere and decreases as the square of the distance. For noise sources that are on or very close to <br />the ground the hemispherical spreading is applicable. Noise expressed as decibels, decrease by 6 dB <br />every time the distance from the source is doubled, assuming that there is no wind, temperature <br />gradients, atmospheric turbulence, absorption of the sound by the atmosphere or physical barriers of <br />some type. For example, consider a piece of mining equipment that produces 80 dB(A) at 100 ft., then <br />the sound level at other distances would be as follows. <br />Distance - Ft. dB(A) <br />100 80 <br />200 74 <br />400 68 <br />800 62 <br />1600 56 <br />3200 50 <br />6400 44 <br />This reduction of noise with distance is also called geometric attenuation and is the basis or starting point <br />in determining the extent of the noise footprints or noise contours. <br />Rev # EDI Job No. C3616 <br />Page 9 of 17 <br />3925 S. Kalamath St., Englewood, Colorado 80110 • voice: 303-761-4367 • fax: 303-761-4379