Physiology of Hearing

Introduction
The physiology of our hearing mechanism can conveniently be divided into three topics:

1 The outer ear (auricle or pinna) and ear canal
2 The middle ear
3 The inner ear

The Auricle and Ear Canal.
Each hole in the side of the skull leads into an ear canal. The ear canal is an irregular cylinder with an average diameter of less than 0.8 mm and about 2.5 cm long.

The ear canal (figure 1) is open at the outer end which is surrounded by the pinna (or auricle). The pinna plays an important spacial focusing role in hearing. The canal then narrows slightly and widens towards its inner end, which is sealed off by the eardrum.

Thus the canal is a shaped tube enclosing a resonating column of air – with the combination of open and closed ends. This makes it rather like an organ pipe.
THE EAR CANAL. [Rigden, 1960]

The ear canal supports (resonates or enhances) sound vibrations best at the frequencies which the human ears hear most sharply. This resonance amplifies the variations of air pressure that make up sound waves, placing a peak pressure directly at the eardrum.

For frequencies between approximately 2 KHz and 5.5 KHz, the sound pressure level at the eardrum is approximately 10 times the pressure of the sound at the auricle.

The Eardrum – interface between outer and middle ear.
Airborne sound waves reach only as far as the eardrum. Here they are converted into mechanical vibrations in the solid materials of the middle ear.

Sounds (air pressure waves) first set up sympathetic vibrations in the taunt membrane of the eardrum, just as they do in the diaphragm of some types of microphone. The eardrum passes these vibrations on to the middle ear structure.
The Middle Ear Ossicular Chain – Malleus (Hammer) , Incus (Anvil) , and Stapes (Stirrup).

The middle ear contains three small bones known as the Malleus, Incus, and Stapes. (Fig. 2). These bones form a system of levers which are linked together and driven by the eardrum. Malleus pushing Incus, Incus pushing Stapes.
The Malleus, Incus, and Stapes.
Working together as a lever system, the bones amplify the force of sound vibrations.
The inner end of the lever moves through a shorter distance but exerts a greater force than the outer end.

In combination the bones double or triple the force of the vibrations at the eardrum.

The muscles of the middle ear modify the performance of this lever system as an amplifying unit. They act as safety devices to protect the ear against excessively large vibrations from very loud sounds – a sort of automatic volume control.

Although these tiny muscles – the smallest in the human body – at both ends of the ossicular chain can be moved voluntarily, their normal contractions are reflex triggered when sound exceeds a certain level. As the noise level rises, one set of muscles tightens to restrict the movement of the malleus thus weakening the vibrations transmitted within the middle ear. At the same time the stapes muscle contracts to pull the stapes away from the oval window so that less vibration is passed along to the very sensitive inner ear.

The Oval Window – Interface Between Middle & Inner Ear.
The stirrup passes the vibrations to the “oval window” which is a membrane covering an opening in the bony case of the cochlea.

The size of the oval window compared to that of the eardrum (15 to 30 times smaller) produces the critical amplification needed to match impedances between sound waves in the air and in the cochlear fluid. Apart from the amplification of the bone lever system of the middle ear, this concentration of force amplifies the incoming vibrations of sound about 15 to 30 times.

Summary of the Amplifications from Outer to Inner Ear.
Within the 4 cm or so occupied by the outer and middle ears, three distinct physical principles operate to magnify weak vibrations in air so that they can establish pressure waves in a liquid:

1 The organ pipe resonance of the ear canal may increase the air pressure fore 10 times.
2 The mechanical advantages of the bone lever system may nearly triple it.
3 The pinpointing arrangement of the eardrum and the oval window may provide another thirty fold increase.

The result of these three mechanisms may be an amplification of a sound wave by more than 800 times before it sets the liquid of the inner ear in motion.

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