17.3 Exercise 18

Sound is more effectively transmitted into a stethoscope by direct contact rather than through the air, and it is further intensified by being concentrated on the smaller area of the eardrum. It is reasonable to assume that sound is transmitted into a stethoscope 100 times as effectively compared with transmission though the air. What, then, is the gain in decibels produced by a stethoscope that has a sound gathering area of $15.0 \text{cm}^{2}$, and concentrates the sound onto two eardrums with a total area of $0.900 \text{cm}^{2}$ with an efficiency of $40.0 \%$?