How does a person hear sound?
The ear is the organ used to detect sound in humans and some animals. The ear consists of three major sections: the outer ear, middle ear, and inner ear. The outer ear, the external section of the ear, consists of a cartilage flap called the pinna. The pinna’s size and shape forms a transformer to match the impedance of the sound wave in air to that at the end of the ear canal by gradually funneling the wave’s sound energy into the ear. To hear more sound, people can increase the size of the pinna by cupping their hand around the back of the pinna—in effect, increasing its size and funneling capabilities.
Once the sound has entered the ear canal, it moves toward the eardrum, where the longitudinal waves cause the eardrum to move in and out depending upon the frequency and amplitude of the wave. The middle ear includes the eardrum, the hammer, anvil, and stirrup, the three smallest bones in the human body, and the oval window on the inner ear. The eardrum is 17 times larger than the oval window, and this difference in area makes the inner ear act like a hydraulic machine, increasing the changes in pressure on the eardrum to that on the oval window at least seventeen-fold. The three bones link the eardrum to the oval window. They act like a lever system to further increase pressure on the oval window. The mechanical advantage of the lever system varies with frequency, peaking at about 5 in the 1 to 2 kilohertz frequency range. Thus the middle ear is like a complex machine that can amplify the pressure on the eardrum by a factor approaching 100 as it transfers the energy of the sound wave to the inner ear. Muscles connected to the eardrum and the three bones can react to very loud sounds and reduce the sensitivity of the ear, thus protecting it from damage.
The inner ear is a series of tubes and passageways in the bony skull. It consists of the cochlea, a spiral-shaped tube that changes the longitudinal sound wave into an electrical signal on the nerves connecting the ear to the brain. The inner ear also has three semicircular canals that sense the body’s motions and give rise to a sense of balance. The oval window is at one end of the cochlea. Sound waves transmitted through the middle ear to the oval window cause a traveling wave in the fluid of the cochlea. This wave in one of the three tubes within the cochlea, the Organ of Corti, causes hair cells, called cilia, to tilt back and forth. The tilting causes chemicals to pass through channels in the nerve, creating electrical impulses that travel along the auditory nerve to the brain for analysis. The further the hair cells are from the oval window, the lower the sound frequency to which they are sensitive. Thus different nerves are excited by different frequencies, allowing the ear to distinguish the sounds’ frequencies.