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VIBRATECH® <br />Another way to express air overpressure is in pounds per square inch (Ibs/inz). Using dB can be <br />very confusing, however using psi can alleviate much of the confusion because psi is not <br />logarithmic but rather linear. PSI can be converted to and from dB. By converting dB to psi <br />direct compazison is then possible: i.e., 0.04 psi is twice as high as 0.02 psi. In contrast, <br />doubling 120 dB results in 126.02 dB, showing that comparisons of this sort should never be <br />made in terms of dB. <br />Conversion formula: <br />dBL = 20 log P/Po <br />where: P =the air overpressure measured in psi <br />Po =the reference pressure of 2.9 x 10 '9 or (0.0000000029 psi), <br />Equivalent to 0.0002 microbazs <br />MECHANIStLIS of AIRBLAST OVERPRESSURE <br />Air overpressures are developed from several parts of the blasting mechanisms. Two factors are <br />different aspects of the way explosive gases escape. Expanding gases can either escape through <br />natural cracks, fissures, and joints existing before the blast or through the cracks and openings, <br />which are developed by the blast. Another way in which the gases escape is through the <br />boreholes if the stemming is ejected. Another mechanism is the small vertical movement of the <br />ground surface from the traveling seismic waves, which generate very small air pulses. This <br />factor is directly related to the total volume of the blast and the total displacement of that <br />volume. For some large blasts, especially in cast blasting, this factor can be a matter of interest. <br />When a lazge amount of material is set into motion this can generate an effect that is similar to a <br />transient wind. An example would be the winds, which are generated by snow avalanches. <br />Pressure pulses are developed and travel through the rock and continue outward into the <br />atmosphere, even when the blast is completely confined. These pulses are audible to humans at <br />high frequencies. An example of this is hitting a wall with a hammer, which generates a <br />vibration in the wall, which then extends into the air and travels to the human ear. <br />The two most important blast design parameters, which affect the intensity of airblast <br />' overpressures, are the maximum pounds per delay and the depth of burial of the chazge. The <br />depth of burial is not only a primary factor in determining the intensity of the air pressures <br />developed, but it also has an effect on the frequency and duration of the pulses. <br />AIRBLAST VELOCITY <br />' Ground waves travel much faster than airblast overpressure velocities. Airblast waves typically <br />travel at a speed of 1,000 to 1,100 ft/sec. The ground waves will have passed before the airblast <br />overpressure arrives. The time between the vibration and the air overpressure will depend upon <br />the duration of the blast and the distance from the blast. Temperature, wind velocity and <br />direction will affect the velocity of the airblast. It is not uncommon for persons to not react to <br />nor detect the ground vibration, but will react adversely to the arrival of the airblast and the <br />' 4 <br />