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Blasting Impacts and Rock Slope Stability Assessment for Cottonwood Quarry in Grand County, CO <br />3.3 Blast Noise (Air -Overpressure) <br />The term "Blast noise" is a misleading because the largest component of blast -induced noise <br />occurs at frequencies below the threshold -of -hearing for humans (16 to 20 Hz). Hence, the <br />common industry term for blast -induced noise is "air -overpressure"_ As its name implies, air- <br />overpressure <br />iroverpressure is a measure of the transient pressure changes. These low -intensity pulsating <br />pressure changes, above and below ambient atmospheric pressure, are manifested in the form of <br />acoustical waves traveling through the air. The speed of sound varies in different materials, <br />depending on the density of the medium. For instance, pressure waves travel at the speed of <br />4,920 ft/s (1,500 m/s) in water, whereas, in air they travel at only 1,100 ft/s (335 m/s) because air <br />has a lower density. <br />When calculating maximum overpressure values, the absolute value of the greatest pressure <br />change is used — regardless of whether it is a positive or negative change. The frequency of the <br />overpressure (noise) is determined by measuring how many up-and-down pressure changes <br />occur in one second of time. Blast noise occurs at a broad range of frequencies and the highest - <br />energy blast noise usually occurs at frequencies below that of human hearing (<20 Hz). <br />3A 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 />As demonstrated by the example in Figure 3.4, a linear peak noise of 120 -OL equates to only <br />112 -dB -C and 85 -dB -A. Note that differences for individual blasts will vary depending on their <br />unique frequency -intensity spectrums. Since full -range recording of blast -induced noise can only <br />be done with linear scale instruments, it is imperative that all compliance specifications be <br />expressed in linear scale (dBL). <br />REVEY Associates, Inc, Page 7 April 2004 <br />