[填空题]
At a rock concert, a dB metel(j( xxvtd 5x1r registered 130 dB when pla3 r7+nbd 52rwnya 8btrced 2.8 m in front of a loudspeakerw2 ad+nr3b8by5 7rnt r on the stage.
(a) What was the power output of the speaker, assuming uniform spherical spreading of the sound and neglecting absorption in the air?$\approx$ $ \times10^{2 }$W(“Round to one decimal place)
(b) How far away would the sound level be a somewhat reasonable 90 dB? $ \times10^{2}$m
