Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 145

The fleshy appendage on the side of the head (the pinna) is not as well developed in humans as in some animals. Its function is to focus sound into the ear canal. It helps us to localize the source of sound, particularly in the vertical direction, and is more effective at higher frequencies. The ear canal is about 25 mm long and ends at the tympanic membrane (eardrum) which is under tension and has the thickness of at sheet of paper.

The eardrum is connected to the malleus, the first of the three small bones known as the auditory ossicles (see Figure 4.2). The middle ear air cavity is connected to the back of the mouth by the Eustachian tube. The smallest of the ossicles, the stapes, which is about half the size of a grain of rice and the smallest bone in the human body, is connected to a small oval window in the cochlea. The cochlea consists of spiral fluid‐filled cavities inside the bone of the skull. The cochlea is comprised of a passageway which makes two and one half turns rather like a snail shell. Connected to the cochlea are the semicircular canals which are the balance mechanism and unrelated to hearing.

Figure 4.2 Tympanic membrane (eardrum) and three auditory ossicles.

The passageway of the cochlea is separated into a lower and upper gallery (scala tympani and vestibuli, respectively) by a membranous duct (Figure 4.3). The upper and lower galleries are connected together only at the apex. Figure 4.4 is a schematic representation showing the cochlea “unrolled.”

Figure 4.3 Section through the cochlea and details of the organ of Corti.

Figure 4.4 Cochlea “unwrapped” to show working of the ear schematically.