The organ of Corti, located in the cochlear duct, is the auditory organ. It contains about 20,000 hearing receptor cells and many supporting cells. These receptor cells are called hair cells. The organ of Corti sits on the basilar membrane, a flexible, fibrous structure on the floor of the cochlear duct. As a pressure wave travels through the cochlear duct it causes the basilar membrane to vibrate. The basilar membrane is narrow and stiff at the base of the cochlea (like the strings of a harp or piano used in playing the high notes), where it resonates in response to high-frequency sound waves. The basilar membrane is wide and less stiff near the apex of the cochlea (like the strings of a harp or piano for the low notes), where it resonates in response to lower-frequency pressure waves. This resulting vibration causes the organ of Corti to vibrate, which is sensed by hair cells. Depending on the volume of the sound, either a few hairs move, as in the case of a soft sound, or many hairs move, as in the case of the loud sound.