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Blasting Impacts and Rock Slope Stability Assessment for Cottonwood Quarry in Cwdnd 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 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 fl/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 3A, a linear peak noise of 120-dBL 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 />