A group of students camping during the summer are caught in an afternoon thunderstorm. One of the students has brought along, by a stroke pf good luck, a sound-level meter. A thunder clap registers an intensity level of 100dB on the meter. The sound arrived 4 s after the lightning flash was seen. Assuming that the source of thunder clap produced spherical waves, calculate acoustic power produced.

Answer should be 2.3x10^5 W, I'm just not sure how to get it