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S E C T I O N 17. 3 • Intensity of Periodic Sound Waves 521 machines in this example is to be doubled, how many ma- Answer Using the rule of thumb, a doubling of loud- 10 log ! I 2 I 1 " - 2 * - 1 # 10 dB #10 log ! I 2 I 0 " * 10 log ! I 1 I 0 " # Thus, ten machines must be operating to double the I 2 # 10I 1 log ! I 2 I 1 " # 1 Loudness and Frequency The discussion of sound level in decibels relates to a physical measurement of the Of course, we don’t have meters in our bodies that can read out numerical values of our reactions to stimuli. We have to “calibrate” our reactions somehow by comparing dif- * 12 W/m 2 , corresponding to an in- tensity level of 0 dB. In reality, this value is the threshold only for a sound of frequency ≠ 0 dB). By using test subjects, the human response to sound has been studied, and the re- sults are shown in Figure 17.6 (the white area), along with the approximate frequency Infrasonic frequencies Sonic frequencies Ultrasonic frequencies Large rocket engine Jet engine (10 m away) Rifle Thunder overhead Rock concert Underwater communication (Sonar) Car horn Motorcycle School cafeteria Urban traffic Shout Conversation Birds Bats Whispered speech Threshold for hearing Sound level (dB) β 1 10 100 1 000 10 000 100 000 Frequency f (Hz) 220 200 180 160 140 120 100 80 60 40 20 0 Threshold for pain Figure 17.6 Approximate frequency and sound level ranges of various sources and that of normal human hearing, shown by the white area. From R. L. Reese, University Physics, Pacific Grove, Brooks/Cole, 2000. |