Question

A 90dB sound wave strikes an eardrum whose area is 5.0×10^-5m^2. How much energy is absorbed by the eardrum per second

Answer 1

-- 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.)