at a rock concert. a dB meter registered 130dB when placed 2.8 m infront of a lo

Question

at a rock concert. a dB meter registered 130dB when placed 2.8 m infront of a loudspeaker on the stage ( a) what was power output ofthe speaker, assuming uniform spherical spreading of thesound and neglecting absorption in the air? (b) how far away wouldthe sound level be a somewhat reasonable 90 dB? at a rock concert. a dB meter registered 130dB when placed 2.8 m infront of a loudspeaker on the stage ( a) what was power output ofthe speaker, assuming uniform spherical spreading of thesound and neglecting absorption in the air? (b) how far away wouldthe sound level be a somewhat reasonable 90 dB?

Explanation / Answer

Intensity level of sound = 130 dB distance from the speaker r = 2.8m                                     = 10 log ( I1 / I0 )                                   130 = 10 log ( I1 / 10-12 )                                   13 = log ( I1 / 10-12 )                                   ( I1 / 10-12 ) = 1013                                        I1 = 10W /m2           Power o/p of the speaker P = I *A                                                     = 10W /m2 *4(2.8)               ( since spherical spreading Area of sphere is 4r2 )                                                    = 9.85*102W b) If the intensity level is  90dB         90 = 10 log (I2 / I0 )          90 = log( I2 / 10-12 )           I2 = 10-3 W /m2      Power o/p is P = I *A                       9.85*102W =  10-3 W /m2* 4 r2              calculate the above to get solution r = .......... m                          

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