The Middle Ear
The next question is how does the ear take this sound wave, which are simply vibrations of air particles, and get them past the air-fluid interface between the outer ear and inner ear? The middle ear has important structures that help amplify a sound wave that is being funneled in from the outer ear and transfer the energy to the inner ear. The tympanic membrane, which is also known as the eardrum sits between the ear canal and the middle ear. Behind the eardrum are the ossicles and tympanic muscles. Each of these bones and muscles play a role in how the sound wave is moved from the air-filled ear canal to the fluid-filled cochlea. The Eustachian tube that is connected to ...view middle of the document...
The footplate at the end of the stapes is then moved in and out of the oval window like a piston in an engine. This creates a wave in the inner ear fluid that is a replication of the sound wave.
Through this transfer function, the sound wave is amplified. If you compared the area of the eardrum to the area of the oval window, the eardrum is 18 times larger than the oval window. Because of this change in surface area, pressure is much greater, which is how amplification is done (How stuff works).
The Inner Ear
Now that the fluid in the cochlea has been set into motion, what happens next? The cochlea is by far the most complex part of the ear. The main role of the cochlea is to take the vibrations caused by the sound wave and converts them into electrical information for the brain to interrupt (how stuff works).
The structure of the cochlea is made up of three chambers, the scala vestibule, scala media and in the middle is the scala tympani. These three chamber coil up into the shape of a snail shell. Imagine these chambers rolled out, the best way to understand this is to picture a straw that is folded in half with the two tubes stacked on top of each other (scala vestibule on top, and scala media on the bottom) These two tubes meet together to create the basilar membrane, which is contained in the scala tympani. There are only two openings at the end of the cochlea. One, which we already mentioned is the oval window, which is found in the scala vestibuli. The other sits in the scala media and is called the round window. Both of these windows would sit at the end of the tubes that were stacked on top of each other. Now that’s a lot of big words that were mentioned, huh? Let’s now go into detail about what they do with a sound wave that is transferred from the middle ear.
The stape’s footplate at the oval window moves in and out when the eardrum is transferring energy into the inner ear. This transfer of energy pushes and pulls the basilar membrane and travels down the basilar membrane to the round window. The round window helps with displacement of the fluid when it’s moving. This movement of energy is similar to the keys of the piano. Imagine started at one end of the keyboard and pushing each of the keys in one motion all the way to the end. The basilar membrane structure is very interesting. All along the basilar membrane there are nerve fibers arranged in a particular order. At the beginning of the basilar membrane by the oval window, nerve fibers are short and stiff and as you move to the end of the basilar membrane, the fibers get taller and more flexible (how stuff works). The reason for gradual changes in nerve fibers has to do with frequencies. The way the basilar membrane is laid out is exactly like the keys of a piano. At the beginning of the basilar membrane are the high frequencies and at the end are the low frequencies. As a particular frequency sound wave travels along the basilar membrane, it triggers the fibers at a certain...