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Janice Lynn Bennett - 81422.PDF Page 11 <br />Blasting Impacts Assessment for the Proposed GCC Rio Grande, Inc Quarry in Pueblo County, Colorado <br />bration curve from RI 8507 <br />3.3 Blast Noise (Air- Overpressure) <br />The term "Blast noise" is a misnomer because much of the noise energy occurs at frequencies <br />below the hearing threshold of humans (16 to 20 Hz). Therefore, the proper term for blast - induced <br />noise is air - overpressure. As its name implies, air - overpressure is a measure of the transient <br />pressure changes. These pulsating pressure changes, above and below ambient atmospheric <br />pressure, are caused by acoustical waves traveling through the medium at the speed of sound. The <br />speed of sound varies in different materials, depending on the density of the medium. For <br />instance, pressure waves travel at the speed of 4,920 ft /s (1,500 m /s) in water, whereas, in air they <br />travel at only 1,100 ft /s (335 m /s) because air 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 occur <br />in one second of time. Blast noise occurs at a broad range of frequencies and the highest- energy <br />blast noise usually occurs at frequencies below that of human hearing ( <20 Hz). Because of this, <br />instruments that measure this "impulsive" blast noise or "airblast" are capable of recording at very <br />low frequencies, typically down to 2 Hz. <br />REVEY Associates, Inc <br />Page 8 July 2002 <br />