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• • . <br /> Review of Blasting Operations at Robinson Brick Company's Siloam Clay Mine <br /> 2.3 Blast Noise Impacts and Controls <br /> This section reviews blast noise monitoring technology, discusses impacts on neighboring people: <br /> and structures, and presents recommended blast noise controls. <br /> 2.3.1 Blast Noise (Airblast) Monitoring Scales: <br /> Blasting creates noise at a broad range of frequencies. The highest intensity blast noise usually <br /> occurs at frequencies below that of human hearing (<20 Hz). Because of this, instruments that <br /> measure impulsive blast noise or "airblast" are capable of recording at very low frequencies, <br /> typically down to 2 Hz. When measurements include low frequency noise with a flat response, <br /> they are called "linear scale" measurements. Linear scale airblast measurements are typically <br /> expressed as dB-L. Regular acoustical noise measurements taken for the purpose of monitoring <br /> compliance with industrial sound-pressure-level standards almost always use weighted scales that <br /> discriminate ac;ainst low frequency noise. Therefore, for a similar noise source, A-weighted and <br /> C-weighted scales will usually record significantly lower levels of noise. As demonstrated by the <br /> example in Figure 4.8, a linear peak noise of 120 dB-L equates to only 112 dB-C and 85 dB-A. <br /> Since full-range recording of blast-induced noise can only be done with linear scale instruments, it <br /> is imperative that compliance specifications for blast measurements also be expressed in linear <br /> scale (dB-L). <br /> G <br /> 0 ' 7CA <br /> Q FU <br /> G <br /> , is ,m ,Gaa ,c.oao so <br /> i,,..nar,"11 h 64461 <br /> Weighting Scales <br /> e na <br /> ea <br /> Io yl� <br /> T� <br /> rn <br /> Time Histories <br /> ro <br /> ea <br /> 0 .`GC a9G GM � am Iwo <br /> E,tcuncv I.+I <br /> Alter Siskind and Summers,(1974) Fourier Frequency Spectra <br /> Figure 2.5--Effects of Weighted Filtering on Airblast Records <br /> GEOT'EK& Associates, Inc. Page 14 January 99 <br />