Editing Middle ear (section)

==Function==
===Sound transfer===
[[File:MiddleEar Amplification.jpg|thumb|The middle ear matches mechanical impedance, like a lever.]]
Ordinarily, when sound waves in air strike liquid, most of the energy is reflected off the surface of the liquid.  The middle ear allows the [[impedance matching]] of sound traveling in air to acoustic waves traveling in a system of fluids and membranes in the inner ear.  This system should not be confused, however, with the propagation of sound as compression waves in liquid.

The [[acoustic impedance]] of air is about <math>Z_1 = 400 \;\mathrm{Pa\cdot s /m}</math>, while the impedance of cochlear fluids (<math>Z_2 = 1.5 \times 10^6 \;\mathrm{Pa\cdot s /m}</math>) is approximately equal to that of sea water. Because of this high impedance, only <math>\frac{2Z_1}{Z_1 + Z_2} = 0.05%</math> of incident energy could be directly transmitted from the air to cochlear fluids. 

The middle ear's impedance matching mechanism increases the efficiency of sound transmission. Two processes are involved: 

* Area Ratio: The area of the tympanic membrane is about 20 times larger than that of the stapes footplate in the cochlea. The forces collected over the eardrum are concentrated over a smaller area, thus increasing the pressure over the oval window.
* Lever: The malleus is 1.3 times longer than the incus.

Together, they amplify pressure by 26 times, or about 30 dB. The actual value is around 20 dB across 200 to 10000 Hz.<ref>{{Cite web |title=Journey into the world of hearing |url=http://www.cochlea.eu/en |access-date=2024-06-15 |website=www.cochlea.eu |language=en}}</ref><ref>{{Cite journal |last1=Hemilä |first1=Simo |last2=Nummela |first2=Sirpa |last3=Reuter |first3=Tom |date=1995-05-01 |title=What middle ear parameters tell about impedance matching and high frequency hearing |url=https://dx.doi.org/10.1016/0378-5955%2895%2900031-X |journal=Hearing Research |volume=85 |issue=1 |pages=31–44 |doi=10.1016/0378-5955(95)00031-X |pmid=7559177 |issn=0378-5955|url-access=subscription }}</ref>

The middle ear couples sound from air to the fluid via the [[oval window]], using the principle of "mechanical advantage" in the form of the "hydraulic principle" and the "lever principle".<ref>{{cite book |title=Biomedical Engineering Fundamentals |author=Joseph D. Bronzino |publisher=CRC Press |isbn=978-0-8493-2121-4 |year=2006 |url=https://books.google.com/books?id=C2ladV9aI8MC&q=middle-ear+lever+hydraulic&pg=PT84}}</ref>  The vibratory portion of the tympanic membrane (eardrum) is many times the surface area of the footplate of the [[stapes]] (the third ossicular bone which attaches to the oval window); furthermore, the shape of the articulated ossicular chain is a complex [[lever]], the long arm being the long process of the [[malleus]], the fulcrum being the body of the [[incus]], and the short arm being the lenticular process of the [[incus]].  The collected pressure of sound vibration that strikes the tympanic membrane is therefore concentrated down to this much smaller area of the footplate, increasing the force but reducing the velocity and displacement, and thereby coupling the acoustic energy.

The middle ear is able to dampen sound conduction substantially when faced with very loud sound, by noise-induced reflex contraction of the middle-ear muscles.