Impedance matching function of middle ear:
Impedance matching is one of the important functions of middle ear. The middle ear transfers the incoming vibration from the comparatively large, low impedance tympanic membrane to the much smaller, high impedance oval window.
Middle ear is an efficient impedance transformer. This will convert low pressure, high displacement vibrations into high pressure of the air into, low displacement vibrations suitable for driving cochlear fluids.
The impedance of cochlear fluids is approximately equal to that of sea water (i.e. 1.5 x 106 N.sec/m3 ). Because of this high impedance of cochlear fluids only 0.1 % of incident energy would be transmitted.
Two processes are involved in the impedance matching mechanism of middle ear. They are:
1. The area of the tympanic membrane is larger than that of the stapes foot plate in the cochlea. The forces collected over the ear drum are concentrated over a smaller area, thus increasing the pressure over oval window. The pressure is increased by the ratio of these two areas i.e. 18.75 times.
2. The second process is the lever action of the middle ear bones. The arm of the incus is shorter than that of the malleus, and this produces a lever action that increases the force and decreases the velocity at the stapes. Since the malleus is 2.1 times longer than the incus, the lever action multiplies the force by 2.1 times.
Figure showing impedance matching mechanisms of middle ear
Due to this impedance matching effects of the middle ear about 50% of the incident energy is transmitted to the cochlea. But for this function of middle ear only 3% of the energy will be transmitted to the cochlea.
Experiments have proved that transmission reached the peak around 1 kHz. Below and above this frequency the transmission was less effective. The drop in transmission at low frequencies is probably due to elastic stiffness of the various components of the middle ear. The most important deciding factor for dampening transmission of low frequency sounds in humans is the annular ligament. Another factor is the air in the middle ear cavity. This also dampens transmission of low frequency sounds. As the ear drum moves in the air in the middle ear cavity is compressed reducing the movement of the ear drum. If the middle ear cavity is vented to the atmosphere, by insertion of a grommet this effect disappears improving transmission of low frequency sounds.
The drop in transmission of high frequencies is affected by different factors:
1. Above the frequency of 2 kHz the motion of the ear drum is broken into separate zones, and as the frequency is increased the effective vibrating area of the ear drum is progressively reduced, thus causing a reduction in sound transmission.
2. At high frequencies a relative motion develops between the malleus and incus. This is due to the mass effect of the ossicles. This is known as the mass effect.
Impedance matching is one of the important functions of middle ear. The middle ear transfers the incoming vibration from the comparatively large, low impedance tympanic membrane to the much smaller, high impedance oval window.
Middle ear is an efficient impedance transformer. This will convert low pressure, high displacement vibrations into high pressure of the air into, low displacement vibrations suitable for driving cochlear fluids.
The impedance of cochlear fluids is approximately equal to that of sea water (i.e. 1.5 x 106 N.sec/m3 ). Because of this high impedance of cochlear fluids only 0.1 % of incident energy would be transmitted.
Two processes are involved in the impedance matching mechanism of middle ear. They are:
1. The area of the tympanic membrane is larger than that of the stapes foot plate in the cochlea. The forces collected over the ear drum are concentrated over a smaller area, thus increasing the pressure over oval window. The pressure is increased by the ratio of these two areas i.e. 18.75 times.
2. The second process is the lever action of the middle ear bones. The arm of the incus is shorter than that of the malleus, and this produces a lever action that increases the force and decreases the velocity at the stapes. Since the malleus is 2.1 times longer than the incus, the lever action multiplies the force by 2.1 times.
Figure showing impedance matching mechanisms of middle ear
Due to this impedance matching effects of the middle ear about 50% of the incident energy is transmitted to the cochlea. But for this function of middle ear only 3% of the energy will be transmitted to the cochlea.
Experiments have proved that transmission reached the peak around 1 kHz. Below and above this frequency the transmission was less effective. The drop in transmission at low frequencies is probably due to elastic stiffness of the various components of the middle ear. The most important deciding factor for dampening transmission of low frequency sounds in humans is the annular ligament. Another factor is the air in the middle ear cavity. This also dampens transmission of low frequency sounds. As the ear drum moves in the air in the middle ear cavity is compressed reducing the movement of the ear drum. If the middle ear cavity is vented to the atmosphere, by insertion of a grommet this effect disappears improving transmission of low frequency sounds.
The drop in transmission of high frequencies is affected by different factors:
1. Above the frequency of 2 kHz the motion of the ear drum is broken into separate zones, and as the frequency is increased the effective vibrating area of the ear drum is progressively reduced, thus causing a reduction in sound transmission.
2. At high frequencies a relative motion develops between the malleus and incus. This is due to the mass effect of the ossicles. This is known as the mass effect.
Last modified: Sunday, 11 May 2008, 11:18 AM