Any vibrating source can set up sound waves in air. The greater the energy of vibration the greater the amount of energy carried by the sound wave. So, we can see that the amount of energy carried by a sound wave depends on the amount of energy with which the source vibrates.

The amount of energy (E) transferred per unit time (t) per unit Area (A) perpendicular to the direction of motion is called the intensity (I) of the wave.

Since energy per unit time is power, intensity can also be defined as power per unit area.

The SI unit of intensity is watt per meter square (W/m²).

Key terms

• Intensity is a measure of the rate of sound energy (power) per unit area.