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Sound intensity is the average power emitted by a sound source per unit area perpendicular to the direction of propagation. It is measured in watts per square meter, the SI unit derived from power in watts and area in square meters. Intensity quantifies the loudness of a sound source based on how energetically the source vibrates.

When a sound source emits waves uniformly in all directions, the sound spreads over an increasingly larger spherical area. Since intensity is power divided by area, and the sphere's surface area increases with the square of the distance, intensity falls off as the square of the distance from the source.

Sound intensity is proportional to the square of the pressure amplitude creating oscillations in the medium. Since oscillation amplitude is proportional to pressure amplitude, and intensity increases with oscillation amplitude, the relationship between intensity and pressure amplitude is quadratic. This connection is explored in intensity and pressure of sound waves.

Sound sources oscillate hundreds or thousands of times per second, but the human ear perceives only the time average of these rapid oscillations. Using average power accounts for what humans actually hear rather than the fluctuating instantaneous values, making the intensity definition physically meaningful for perception.

A sound source's intrinsic loudness depends on how energetically it vibrates and the pressure amplitude it generates in the propagation medium. The intensity definition divides emitted energy by time and area to isolate the source's intrinsic loudness from factors like duration of vibration or size of the radiating surface.

Sound propagates through vibration of the medium's particles, so intensity increases with the pressure amplitude driving these oscillations. The greater the pressure amplitude, the larger the oscillation amplitude of medium constituents, and consequently the higher the sound intensity produced by the source.

Sound intensity is proportional to the square of the oscillation amplitude of the medium's constituent particles. This quadratic relationship means that doubling the oscillation amplitude increases intensity by a factor of four, making amplitude a critical factor in determining how loud a sound appears.