In terms of hearing, this means that we would perceive the sound at distance d 1 as louder than the sound at d 2 . Furthermore, as the diagram above illustrates, the perceived loudness doesn't just depend on the power of the sound source because as we move farther from the source, the perceived loudness of the sound diminishes. What we perceive as loudness, therefore, is measured in terms of power per unit area . We use the term intensity of a sound to describe the power per unit area that reaches our ears. Below is a summary of the relationship between energy, power, and intensity.
Energy is defined as the capacity to perform work.
Its units are the same as the units for work, namely Joules. The energy of a wave is proportional to the square of the amplitude of the wave. Power is the rate at which work is done, and is measured in units of Joules per second, which is given the name Watts. Thus, power is energy per time. Intensity is what we perceive as loudness, and it is the power per unit area of a sound.