Sound intensity is defined as:

• A. The amount of sound power per unit area; total sound put out in all directions
• B. The frequency of the sound
• C. The energy per cycle of a wave
• D. The speed of sound in air

Answer: (A)

Sound intensity is the amount of sound power passing through a unit area, i.e., the rate at which acoustic energy crosses a surface. If a source emits total sound power P uniformly in all directions, the intensity at distance r in a free field is I = P/(4πr^2), showing how intensity falls off with distance. Intensity can also be expressed in terms of wave properties, such as I = p^2/(ρc) for a plane wave, where p is sound pressure, ρ is air density, and c is the speed of sound. The other options describe frequency (pitch), energy per cycle, or propagation speed, none of which define intensity.