The pinna influences localization because its depressions and ridges filter the high-frequency aspects of the signal in a way that depends on the direction of the sound. Pinna effects are particularly important when one must localize sounds while listening with only one ear, because monaural hearing precludes the use of the interaural differences available during binaural hearing, and when the sound source is in the medial plane of the head, where interaural differences are minimized because the sound source is equidistant from both ears. The tympanic membrane is located at the end of the ear canal rather than flush with the surface of the skull. Sounds reaching the eardrum are thus affected by the acoustic characteristics of the ear canal. The right ear is the near ear when the sound comes from the right, and it is the far ear when the sound comes from the left.