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Chapter 26 <br />- ~I,on, <br />•/ <br />raY. ~ Y <br />I \ <br />l ~ <br />I I <br />I <br />DL>ST S~rE <br />Verruca, DIOne .na~.nq uunO p~µopgi.m <br />Figure 26-L Propagetlon Irom en Isothermal condition (speetl is constant wlm altltutlel• <br />wave fi-ont shown in Figure 26-L, Under these conditions the decrease <br />in noise from the blast site would be given by the graph in Figure <br />26-H. <br />IF the weather conditions (temperature and wind velocity) are such <br />that a greater sound velocity in any direction occurs above the earth's <br />surface, a sound speed inversion exists. In this case the ra-y paths are <br />bent toward the earth's surface where the sound speed is less. Bending <br />of the ray paths is known as refraction and ray paths are always bent <br />toward regions of lower sound speed. A consequence of the refraction <br />process is that the wave fronts, which are spherical under isovelocity <br />conditions, become distorted and are not spherical. '- <br />When atemperature inversion exists, the temperature and sound <br />velocity increase with altitude. Under this condition the ray paths are <br />refracted back toward the earth's surface as illustrated in Figure <br />26-M. The noise level will decrease more slowly with distance than <br />under isovelocity conditions. An increase in the noise level by a <br />factor of two-three (six-10 decibels) is not uncommon under inversion <br />ronditions. <br />Conversely, when there is a decrease with altitude in temperature <br />and sound velocity, which is the stable and more frequent crondition <br />during the time of most blasting operations, the ray paths are re- <br />fracted away from the earth's surface as illustrated by Figure 26-N. <br />The noise level decreases rapidly with distance. <br />440 <br />Vibration and Atr Blast <br />o No,.. <br />® Lo.e ro,.. <br />SOVND SvEFD <br />,iny .aunE peen a i.ngnaW re <br />.ntl aunyee <br /> <br />uvn0 H Irm, <br />e ` p, <br />/ r/ <br />I <br />I <br />1 <br />aLGSi SITE <br />V.r,~[o~ Diane .nOV ~nq.ouM p gOOgo~~on a~C relpc~ an <br />Figure 26-M. Propagetlon from positive thermal gradient (speed Increases with altitude). <br />,. ^P~, . <br />+ eye <br />w.~.q qw~a ... - <br />m. ,oq;ay <br />IaC. p'.,,F IENann, o <br />wro Imo, <br />.oYq -- -~.e <br />eLesT si*[ <br />WmcW porN Wnµ wn0 OOpagg4m <br />Figure 26N. Propagation Irom negative thermal gradient (speed decreases with altltutle). <br />441 <br />• <br />• <br />al p~On. .b~~~q xV n. <br />Cee .aNf ~~iM1 0•.~on[. Irom plo.~ . ie <br />SDUND SPEED <br />Can y,unq .pec B lemgera~u.e <br />./n alb,uE. <br />D.. ..•a wm veq a nmu.o,... <br />