"Terrestrial mammals share to some degree consistently developed ear structures across species. Their ears consist of an outer ear, or pinna, as well as a middle ear, which the air cavity behind the eardrum that includes the stapes, anvil, and malleus, or hammer. Sound - a wave of pressure traveling through the air - causes the eardrum to vibrate. The structures in the middle ear transfer these vibrations to the fluid-filled inner ear. Within the inner ear is the cochlea, which contains hair cells - cells with protruding hairlike structure that determine the frequency range the listener will be sensitive to. Some groups of cells are more sensitive to low-frequency signals, while others specialize in the higher end of the spectrum. Hair cells serve as both detectors and amplifiers - the motion of the cells is converted into signals that are transmitted from nerve to nerve until the reach the brain and are processed into useful information."
Bernie Krause, The Great Animal Orchestra