510 Sound Waves*P17.36Assume you are 1 m away from your lawnmower and receiving 100 dB sound from it. The intensityof this sound is given by 100101012dBdBWm2=−logI; I=−1022. If the lawnmowerradiates as a point source, its sound power is given by Ir=℘42π.℘==−4110012622m WmW2af.Now let your neighbor have an identical lawnmower 20 m away. You receive from it sound withintensity I==×−012625 1025..W420m2af. The total sound intensity impinging on you is102 5 101 002 5 10252−−−+×=×222... So its level is1010025 1010100 01212dBdBlog..×=−−.If the smallest noticeable difference is between 100 dB and 101 dB, this cannot be heard as achange from 100 dB.Section 17.4The Doppler EffectP17.37
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