The quantity of sound energy travelling per second typically across a unit area at a point of the medium is the intensity of a sound wave at that location.

The intensity of a sound, or the energy communicated by the sound wave near the listener’s eardrum, determines its loudness. The sensitivity of the listener’s hearing also affects the volume. As a result, the loudness of a particular intensity of sound may vary from listener to listener. Furthermore, even to the same listener, two sounds of the same intensity but different frequency may have differing loudness. This is due to the fact that the sensitivity of the ears varies depending on the frequency. As a result, loudness is subjective, but intensity is objective