The bones of the middle ear
The eardrum is very thin, measures approximately 8-10 mm in diameter and is stretched by means of small muscles. The pressure from sound waves makes the eardrum vibrate.
The vibrations are transmitted further into the ear via three bones in the middle ear: the hammer (malleus), the anvil (incus) and the stirrup (stapes). These three bones form a kind of bridge, and the stirrup, which is the last bone that sounds reach, is connected to the oval window.
The oval window
What is the oval window? The oval window is a membrane covering the entrance to the cochlea in the inner ear. When the eardrum vibrates, the sound waves travel via the hammer and anvil to the stirrup and then on to the oval window.
When the sound waves are transmitted from the eardrum to the oval window, the middle ear is functioning as an acoustic transformer amplifying the sound waves before they move on into the inner ear. The pressure of the sound waves on the oval window is some 20 times higher than on the eardrum.
The pressure is increased due to the difference in size between the relatively large surface of the eardrum and the smaller surface of the oval window. The same principle applies when a person wearing a shoe with a sharp stiletto heel steps on your foot: The small surface of the heel causes much more pain than a flat shoe with a larger surface would.
The round window
The round window in the middle ear vibrates in opposite phase to vibrations entering the inner ear through the oval window. In doing so, it allows fluid in the cochlea to move.
The Eustachian tube
What is the Eustachian tube? The Eustachian tube is also found in the middle ear, and connects the ear with the rearmost part of the palate. The Eustachian tube’s function is to equalise the air pressure on both sides of the eardrum, ensuring that pressure does not build up in the ear. The tube opens when you swallow, thus equalising the air pressure inside and outside the ear.
In most cases the pressure is equalised automatically, but if this does not occur, it can be brought about by making an energetic swallowing action. The swallowing action will force the tube connecting the palate with the ear to open, thus equalising the pressure.
Built-up pressure in the ear may occur in situations where the pressure on the inside of the eardrum is different from that on the outside of the eardrum. If the pressure is not equalised, a pressure will build up on the eardrum, preventing it from vibrating properly. The limited vibration results in a slight reduction in hearing ability. A large difference in pressure will cause discomfort and even slight pain. Built-up pressure in the ear will often occur in situations where the pressure keeps changing, for example when flying or driving in mountainous areas.