Physiology* of the ear : the outer ear and the medium ear

Outer ear

Auricle : the first part of the ear reacts to sounds. It directs the sound waves inside the ear in the manner of a funnel. Without it, the sound waves would enter the auditory canal by following a more direct trajectory. This trajectory would be difficult to follow for the sound waves and “not very economical” because of the losses, which would hinder the hearing and the understanding of the sounds. In order for sound waves to penetrate the ear with minimal loss, the resistance must not be too high. The auricle reduces the pressure difference between the outside and the inside of the ear. In a way, it plays the role of an intermediary by facilitating the transition to sound waves. A larger quantity of sound waves enter the ear canal.

Auditory canal : after passing through the auricle, the sound waves travel two to three centimeters from the ear canal before coming to the eardrum. Its function is to transmit the sounds of the auricle to the eardrum. To protect the eardrum, the auditory canal is slightly curved, preventing insects, for example, from reaching the eardrum. Cerumen presents in the ear canal also plays a protective role by retaining dust, dirt, insects and other undesirable elements. It is also a natural hearing aid that automatically amplifies the weakest and least piercing sounds of the human voice. The ear thus compensates for the weaknesses of the human voice and facilitates the hearing and understanding of a current conversation.

 

Medium ear

It transmits the sounds from the outer ear to the inner ear.

Ear drum : it is very sensitive to the pressure of the sound waves that make it vibrate.

The bones of the middle ear: the vibrations are transmitted in the ear via the three bones of the middle ear: the hammer, the anvil and the stirrup. These three bones together form a kind of bridge, and the stirrup, the last bone of the chain, is connected to the oval window. The oval window is a membrane that lines the entrance of the cochlea located in the inner ear. When the ear drum vibrates, the sound waves are transmitted to the stirrup via the hammer and the anvil before being amplified by the oval window. When sound waves are transmitted from the ear drum to the oval window, the middle ear plays the role of an acoustic transformer that amplifies the sound waves before transmitting them to the inner ear. The pressure exerted by the sound waves on the oval window is about 20 times greater than that exerted on the ear drum. The increase in pressure is due to the difference between the relatively large dimensions of the ear drum and the smaller dimensions of the oval window. One can observe this same principle when a person wearing shoes with stiletto heels walks on your foot. The very fine heel is much worse than a wide sole!

The Eustachian tube is connected to the back of the palate. Its function is to equalize the air pressure on both sides of the ear drum and thus prevents an accumulation of pressure in the ear. The tube opens to swallowing and thus equalizes the pressure inside and outside the ear. Swallowing causes the opening of the horn that connects the palate to the ear, thus equalizing the pressure. An accumulation of pressure in the ear can be observed when the pressure behind the ear drum is different from the pressure exerted on the outside of the eardrum. When this pressure is not equalized, it accumulates at the level of the eardrum and prevents it from vibrating properly. The decrease in the vibration of the eardrum slightly diminishes the auditory acuity. Pressure often accumulate in the ear when pressure changes, especially in airplanes or cars, in mountainous areas.

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