The ear can receive the stimulus of sound, which is actually vibrations. The visible ear in humans is called the auricle, or external ear. Sound waves can move through air and the outer ear collects sound pressure waves. Next, these waves are amplified through the middle ear and the medium that conducts these waves changes from air to a liquid. In the ear canal, the waves pass to the tympanic membrane or eardrum. From here, they pass to the inner ear, which is filled with liquid as well as epithelial tissue that has hair cells. These can project into the fluid and when sound waves move through the fluid, the hair cells fire a neurotransmitter. The sound waves become a nerve impulse. The auditory nerve will conduct this impulse to the hearing portion of the cerebrum which will actually "hear" the sound. The human ear can hear sounds between 20Hz and 20kHz.