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I <br /> Finally , the third characteristic of sound is duration , or how long <br /> it lasts . <br /> Duration of Sound <br /> One of the characteristics of a sound is duration how long it <br /> lasts . You hear an airplane approaching ,pproaehing , its sound reaches amaximum as <br /> FP it flies overhead , and its sound diminishes and dies has the plane <br /> disappears . The change in sound pressure level can be 'charted as a <br /> hill—shaped curve that clearly illustrates the duration of si)und . Often , <br /> when examining airport noise , we are concerned with durations defined as <br /> the amount of time the sound pressure level remains within 10 dB of the <br /> maximum sound pressure level during the flyby. Duration its illustrated <br /> in Fig. 6. <br /> 120 • <br /> 110 -. <br /> tMAX.LEVEL <br /> -13 Lo <br /> -s 100 MAX.LEVEL <br /> - <br /> minus 10dB <br /> 90 'r DURATION <br /> 1 - <br /> 80 <br /> TIME <br /> FIG. 6. ILLUSTRATION OF DURATION OF SOUND, <br /> 1 <br /> 15 <br /> .,... m-ctsvncr.., . .... _. .. <br />