508_Physics ProblemsTechnical Physics

508_Physics ProblemsTechnical Physics - 510 *P17.36 Sound...

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510 Sound Waves *P17.36 Assume you are 1 m away from your lawnmower and receiving 100 dB sound from it. The intensity of this sound is given by 100 10 10 12 dB dB Wm 2 = log I ; I = 10 2 2 . If the lawnmower radiates as a point source, its sound power is given by I r = 4 2 π . ℘= = 41 1 0 0 1 2 6 2 2 m W m W 2 a f . Now let your neighbor have an identical lawnmower 20 m away. You receive from it sound with intensity I == × 0126 25 10 2 5 . . W 42 0 m 2 a f . The total sound intensity impinging on you is 10 2 5 10 1 002 5 10 25 2 −− = × 22 2 . . . So its level is 10 10025 10 10 100 01 2 12 dB dB log . . × = . If the smallest noticeable difference is between 100 dB and 101 dB, this cannot be heard as a change from 100 dB. Section 17.4 The Doppler Effect P17.37
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