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Janice Lynn Bennett - 81422.PDF Page 12 <br />Blasting Impacts Assessment for the Proposed GCC Rio Grande, Inc. Quarry to Pueblo County, Colorado <br />3.4 Air - Overpressure Measurement Scales <br />When measurements include low frequency noise (2 Hz and higher) with a flat response, they are <br />called "linear scale" measurements. Air - overpressure measurements are typically expressed in <br />decibels (dB) units and when the scale is linear, the unit designation is "dBL." Regular acoustical <br />noise measurements taken for the purpose of monitoring compliance with industrial sound - <br />pressure -level standards almost always use weighted scales that discriminate against low <br />frequency noise. Thus for a similar noise source, A- weighted and C- weighted scales will usually <br />record significantly lower levels of noise. Differences between decibel scale measurements for <br />individual blasts will vary depending on their unique frequency- intensity spectrums. Since full - <br />range recording of blast - induced noise can only be done with linear scale instruments, it is <br />imperative that all compliance specifications be expressed in linear scale (dBL). <br />The regulatory limit defined by USBM, as adopted in Colorado regulations, for air - overpressure <br />measured with 2 -Hz response seismographs is I33 -dBL. Damage to old or poorly glazed windows <br />does not occur until air - overpressure reaches about 150 dBL. More importantly, since the decibel <br />scale is a logarithmic ratio, the actual overpressure at 150 dB -L is 0.092 psi, versus 0.013 psi at <br />133 dB -L. Therefore, the actual pressure at the 133 -dBL limit, is over seven times (0.0917/0.0129) <br />lower than the threshold damage level at 150 -dBL. The relationships between actual overpressure <br />expressed in psi and decibel scale measurements are shown in the following Equations. NOTE: <br />Due to the logarithmic ratios used to decibel values, seemingly small changes in decibel readings <br />can equate to large changes in absolute overpressure (psi). <br />1 REVEY Associates, Inc <br />Equation 3.1 <br />Where dB = decibels, P = overpressure ( p s i ) , P = Threshold of Human Hearing Pressure (20 microPascals or 2.9 z 10' psi) <br />Page 9 July 2002 <br />