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Chapter 26 RELATIONSHIP OF NOISE LEVEL TO TEMPERATURE PROFILE EarIY Mornlny Mitl M\/ornlny Lele Morning Early Aaemoon - CIeYO. TryO C> t lap OI InrerNen u moupe m bWre b p,e..nl Q O A. e. C. D. Temperslure A. sign end low temperature Inrenlone Present -sound Inlensilication refulla B. Loner inrenlOn perllsllY disappean - sound intenellY near source diminnnltl- tocufing al Inver nnpo G. Surface lemperalure eaceeda nigM1esl upper elr Nmpenlure -sound not relurnad IP surface oy relrecllon p. Temparelure Inrenlom wiped out -upward rcfraclan copses minimum sound lanais a1 ell dnlmco the colder months because oC the higher u7nd speed that prevails during this time of the year. In the warmer months wind speeds are normally lower and help to prevent the formation of temperature inversions by promoting mixing in the lower atmosphere. Figure 26-P shows a composite of raw path diagrams calculated (rum radiosonde data obtained from Kennedy Airport in New York Citv. The ray paths were calculated at intervals of two degrees elevation. Southerly winds caused downward refraction and produced high-noise level conditions in the northwest, north, and northeast directions and caused upward refraction and produced low-noise level conditions in the southwest, south, and southeast directions. Normal sound levels would occur in the easterlyand westerly directions as isovelocity con- ditions yielded straight ray paths. Calculations such as these are ex- pensive and rely on having access to nearby radiosonde data. Con- stantly changing weather conditions and high cost make this ap- proach impractical for most blasting operations. In critical operations the use of a pilot charge orlow-frequency sound generator to calibrate the atmosphere may be preferable. Favorable atmospheric conditions for blasting: ~' • Clear to partly cloudy skies with fleecy clouds, light winds and a steadily increasing surface air temperature from daybreak to shot time. Blast time should be delayed to midmorning to allow eazly morning temperature inversion, if any, to be eliminated. Unfavorable atmospheric conditions for blasting: • Foggy, hazy, or smoky days with little or no wind. Condition typical of temperature inversion and high air-pollution index. • During strong winds accompanying passage of a cold front. • During periods of the day when surface temperature is falling. 444 Vibration and Air Blast Overpressure Air Blasi Effect dBL psi 181 3.0 Convenltonel structures severely damaged. 171 1.0 Most windows break. 161 0.3 151 0.1 Some windows break. 141 0.03 Some large plate glass windows may break. USBM interim limit of allowable air blast 136 dBL 131 0.01 dBL = 20 log F where Pp is the reference pressure of 0.0002 mlerober. The L signifies linear Irequency response. rrgure co-u. a summary of the resuitc of many Stutliea on the effect of shock waves /mm chemical explosions, nuclear explosions end supersonic elrerafl on ordinary structures. • Too early in the morningor after sunset on clear days with very light winds. • Cloudy days with a low ceiling, especially when there is little or no wind. Air Blast Damage Levels. Many studies have been made on the effects of shock waves From chemical explosions, nuclear explosions, and supersonic aircraft on structures. These studies have established that the most critical parts of ordinary structures to blast waves are window panes. As a consequence, most damage criteria are presently based on window pane breakage. A summary of the results of these studies is given in Figure 26-Q. Because air blast data from blasting operations frequently show considerable variation, the preferred ap- proach is to take the steps recommended below to minimize the source level, so that safe levels will be maintained even under adverse at- mospheric conditions that cannot be reliably detected or avoided. Methods of controlling air blast: • Avoid [he use of unconfined explosives. 1. Bury detonating cord one foot or more. 2. Use ]ow•]oad detonating cord and bury a few inches or more if necessary. 3. Do not mudcap in populated areas unless absolutely necessazy. 445 ~~ •