What is the expression for the sound intensity I in terms of the source power P and the area A over which the power spreads?

What is the expression for the sound intensity I in terms of the source power P and the area A over which the power spreads?

• A. I = P*A
• B. I = A/P
• C. I = P/A
• D. I = P^2/A^2

Answer: (C)

Sound intensity is the power per unit area—the rate at which energy passes through a given area. If a source emits power P and that power spreads over an area A, the intensity is I = P/A. This makes intuitive sense: more area means the same amount of power is spread thinner, so the intensity goes down in proportion to 1/A. In free space, a point source radiates over a spherical area A = 4πr^2, giving I = P/(4πr^2). The units are watts per square meter. Expressions that multiply by area or square the power don’t match the idea of energy crossing area, which is why they aren’t correct.

Sound intensity is the power per unit area—the rate at which energy passes through a given area. If a source emits power P and that power spreads over an area A, the intensity is I = P/A. This makes intuitive sense: more area means the same amount of power is spread thinner, so the intensity goes down in proportion to 1/A. In free space, a point source radiates over a spherical area A = 4πr^2, giving I = P/(4πr^2). The units are watts per square meter. Expressions that multiply by area or square the power don’t match the idea of energy crossing area, which is why they aren’t correct.