What is the function of the middle ear?
The middle ear amplifies the sound waves that arrive at the eardrum and make it vibrate. This is necessary because the sensory cells in the inner ear are embedded in fluid and sound is less strongly perceived in fluid (the effect is known when you submerge in the bathtub).
How is the gain achieved? The eardrum is much larger than the corresponding window to the inner ear - so the sound pressure increases. The leverage of the small ossicles additionally increases the pressure. In addition, two muscles regulate the tension of the eardrum and the power of transmitting the signal from the stapes plate (the last auditory ossicle) to the oval window of the inner ear.
Function of the inner ear
In the inner ear, the sound waves run up the coils of the cochlea. Depending on the frequency, they activate the sensory cells of the embedded "hearing tube" at the beginning (at high tones) or at the end (at low frequencies). Each sound is decomposed and gives a typical sensory stimulus, which is passed on to the brain.
It is important that the sensory cells for high frequencies are excited at the beginning, then the sound waves do not continue; conversely, the sound waves of the low frequencies activate the sensory cells for high frequencies a little before they continue - so they become much more stressed in the long run and wear off faster.
Control of the sense of balance
As far as the function of hearing - the sense of balance is controlled by the sensory cells of Sacculus, Utriculus and those of the semicircular canals. The tips of the sensory cells protrude into a jelly mass, on which lie small stones for weighting, the otoliths. This whole construction floats in liquid.
If there is a body movement, the liquid first moves, then the jelly mass delays with the sensory cells. How strong and fast the jelly mass moves away from its resting position depends on the extent of movement - the tips of the sensory cells are bent, this is reported as a signal to the brain. The brain uses this information and the signals from the eyes and muscles to determine the position of our body in space - whether we are sitting, lying, falling or turning.