[填空题]
At a rock concert, a dB cl51; yxfupbfs wid/c: z v;4+meter reg f8+uwaf;fd3v3yo sc -istered 130 dB when placed 2.8 m in front of a loudspeaker o-f 3afvw+odys 3ufc; 8n 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
