A head shadow or acoustic shadow is a region of reduced amplitude of a sound because it is obstructed by the head. Sound may have to travel through and around the head in order to reach an ear. The obstruction caused by the head can account for a significant attenuation (reduced amplitude) of overall intensity as well as cause a filtering effect. The filtering effects of head shadowing are an essential element of sound localisation—the brain weighs the relative amplitude, timbre, and phase of a sound heard by the two ears and uses the difference to interpret directional information.