-- The reference level for sound wave intensity is the threshold of human hearing, defined as 10⁻¹² W/m² .
-- This question is talking about a 90dB sound. So its intensity is 10⁻³ W/m² .
-- The question tells us the size of the area that this sound is hitting. So the power absorbed by that area is ...
Power = (intensity) x (area)
Power = (10⁻³ W/m²) x (5 x 10⁻⁵ m²)
Power = (10⁻³ x 5 x 10⁻⁵) W
Power = 5 x 10⁻⁸ Watts
(That's 5 x 10⁻⁸ Joules of energy per second.)
