The external ear The external ear is composed of the visible part of the ear (the auricle) and the external auditory canal that leads to the eardrum. The human auricle is made up mainly of cartilage and skin. This makes it very soft. The deep central part of the auricle is called the auricular cavity, which leads to the opening of the external auditory canal. The outer ear is the channel through which most of the sound is transmitted, but does not have the function of sensing sound. The human outer ear only directs sound and gives it a degree of enhancement, so we need to be in a good position to align with the direction in which the sound is coming from. Most animals have muscles in the outer ear, so it makes more sense that they can usually hold their ears up. The human outer ear also has these muscles but cannot actually hold the ear up. The outer ear causes an enhancement or amplification of sound by 10-15 dB in the frequency range of approximately 1.5 kHz to 7 kHz, due to resonance. The frequency of the external auditory canal is approximately 2.5 kHz, and the resonance frequency of the auricle is closer to 5 kHz. Middle Ear Anatomically, the middle ear is more complex, beginning with the tympanic membrane at the end of the ear canal. The tympanic membrane is cone-shaped and translucent towards the inner ear and separates the middle ear cavity or tympanic cavity from the ear canal. The tympanic membrane consists of two layers of fibers; one layer radiates outward from the center of the tympanic membrane and its outer layer consists of circular fibers. The main part of the middle ear cavity lies between the tympanic membrane and a bony wall (capsule) that is filled with air and communicates with the pharynx through the eustachian tube. The middle ear has three small auditory bones: the hammer bone, the anvil bone, and the stapes bone, also collectively known as the auditory chain, which connect the tympanic membrane to the inner ear and transmit the vibrations produced by the tympanic membrane by sound to the inner ear. Middle Ear Muscle The middle ear has two small muscles: the tympanic tensor muscle and the stapedius muscle. The tympanic tensor muscle is connected to the hamate stalk (the tympanic membrane is attached to the hamate stalk). The stapedius muscle is attached to the wall of the tympanic cavity. These small muscles have the following two functions: ． middle ear muscle is an important part of the suspension system of the auditory chain . The The middle ear muscle also plays a protective role, as its contraction reduces the sound pressure transmission through the auditory chain, allowing people to be less exposed to loud sounds. The eustachian tube, which connects the tympanic cavity to the pharyngeal cavity, is open when coughing or swallowing. It regulates the pressure in the tympanic cavity and maintains a balance between the pressure in the tympanic cavity and the external atmospheric pressure. Role of the middle ear The leverage of the auditory chain and the difference in size between the tympanic membrane (60 mm2) and the oval window (3 mm2) help to increase the air pressure in the external auditory canal, so that this pressure can drive the denser inner ear fluid. This boosts gain by approximately 30 dB. Inner Ear The inner ear (labyrinth) is the most complex part of the ear. It is located just behind the middle ear and is a cavity made of rock-like bone filled with fluid. The rock-like bones act as a protection. The inner ear is very important because it converts vibrating sound waves into nerve impulses. From an anatomical point of view, the inner ear is composed of three main parts: the vestibule, the semicircular canal and the cochlea. In its outer wall there are two windows, one called the oval window and connected to the pedicle of the stapes and the other called the round window. The vestibule is connected to the semicircular canal and the cochlea. The semicircular canal is involved in the understanding of sound, and it is also an important part of the balance organ. The cochlea is a progressively smaller diameter tube that spirals and spins to a point similar to the shell of a snail. In humans, the cochlea has two and a half turns. The cochlea is divided into three parts by the basilar membrane and Reissner’s membrane: the vestibular, middle and tympanic steps. The vestibular and tympanic steps contain ectolymphatic fluid, while the middle step contains endolymphatic fluid. The cortical apparatus is located on the surface of the basilar membrane of the middle stage and is topped by a capping membrane that runs parallel to the base. The inner side of the cortical apparatus is lined with hair cells, and the other side has three layers of outer hair cells, the cilia of which are in contact with the capping membrane. The hair cells and nerve fibers are held in position by the supporting cells. When the basilar membrane is stimulated by stapedial movements, it produces traveling wave movements. The different motions of the basilar membrane and the lid membrane result in shear stress movements of the hair cell cilia and stimulation of the nerve fibers. The following is a detailed look at how the human ear perceives sound: Our starting point is any directional sound source in the environment around the human ear. Changes in air pressure transmitted to the ear cause the eardrum to vibrate, and the handle of the hammer bone, which is connected to the eardrum, also vibrates, and transmits the vibrations to the anvil and stapes, whose pedicle is connected to the oval window, which transmits the vibrations to the fluid (lymphatic fluid) in the vagus. Since the fluid is not compressible and the circular window is flexible, when the oval window is pushed forward the circular window moves in the opposite direction and the basilar membrane moves as a result, causing shear stress movements of the hair cell cilia and stimulating the nerve fibers. Tone perception The top of the basilar membrane perceives low frequencies and the bottom perceives high frequencies. Different hair cells are stimulated to sense different frequencies. Because of the complex structure of the basement membrane, not all vibrations reach the entirety of the basement membrane. In fact, the traveling waves travel along the membrane from the window to the top of the cochlea like waves traveling across the surface of a pond. The amplitude gradually increases to a maximum and then decreases sharply. The place where the maximum is generated depends on the frequency of the sound. Loudness perception The loudness perception seems to be related to the number of stimulated hair cells. We found that the sound is immediately transmitted to the basilar membrane of the ear and eventually to the brain. The result is that the human ear is able to accurately distinguish sounds that last for a rapid and very short period of time, such as the sound of people talking.