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The Eardrum

5 amazing facts you might not have known

The eardrum is a thin, slightly concave disc of skin located at the end of the ear canal, and is known in the medical world as "tympanic membrane" or "myringa". It’s function is to capture vibrations in the air and transfer them to the inner ear (the cochlea), giving us a perception of sound. This simple transfer of energy has been the source of intense scientific interest since the late 1800’s, as scientists have been investigating how such a small piece of skin can capture such a broad range of sounds at great sensitivity. In this article we’ll look at the fascinating anatomy of the eardrum and unveil some of its clever designs.

1. It’s slightly concave


Most eardrums are fairly thin (averaging 0.1mm) and about the size of your little finger nail (diameter of 8-10mm). It also points inward like a cone, and until recently this concave shape was thought to be insignificant. Recent research however, has shown that a cone-shaped eardrum can transfer more force through to the inner ear than a flat one. This is particularly true for high pitched sounds, which humans need to hear to understand consonants in speech [1].

An image by Henry Gray (1918) Anatomy of the Human Body. This cross section of the ear shows the eardrum, labeled as Tympanic membrane, in the center of the illustration and cone shaped. 

2. Maximizing space


The average human ear canal has a diameter of around 7mm [2][3], so how can the eardrum be larger at 8-10mm? You’ll notice in most diagrams that the eardrum is titled forwards, which allows for greater surface area than if it was completely vertical, thus improving the level of sound transmission.
 

Another image by Henry Gray (1918) Anatomy of the Human Body. This shows a close up cross section of the middle ear, and the oblique angle of the eardrum.

3. Stiffness


Interestingly, the eardrum does not have a uniform level of tension across it’s surface, it has two distinct segments. The larger segment is called the "pars tensa", and as the name suggests is fairly taut like the skin of a drum. The smaller upper segment is known as the "pars flaccida", and is flacid or floppy in comparison. Researchers are still looking at the role of the pars flaccida, however current theories around it’s function are:

1.      The pars flaccida may help the ear drum pick up more low frequencies and therefore increase our sensitivity to a wider range of sounds [4].


2.      The pars flaccida may negate some of the effects of being congested, keeping our hearing acute for longer until the problem resolves [5].

Another image by Henry Gray (1918) Anatomy of the Human Body. This shows us a view from behind the eardrum looking outward. Both the "tense" and "flaccid" portions of the eardrum are labeled. 

4. Equal tempering

The challenge that engineers face when designing headphones, speakers or microphones is how to achieve a smooth frequency response. In other words, making sure the system responds equally to sounds across the spectrum, not giving bias towards a certain pitch.

It’s long been known that the eardrum can pick up a vast range of sounds from very low (20Hz) to very high (20,000Hz), however it is only recently that scientists have understood how it can do this smoothly and efficiently.

The eardrum has 3 distinctive layers to its structure:
 
1.      The outer layer is called epithelium, which is a form of skin found in many places around the body and is fairly thin.


2.      The middle layer is known as the fibrous layer, and is what gives the ear drum it’s strength and rigidity.


3.      The inner layer is a mucous lining which is also found coating the entire middle ear cavity.

The middle layer contains collagen fibres which are arranged in a very specific pattern. One set of fibres stretch out from the center of the drum radially, and the other circumferentially. The structure and arrangement gives the ear drum strength, and allows it to vibrate more evenly when it’s picking up low, medium or high pitched sounds [6].

 

5. Repair and restore

If the eardrum sustains damage and becomes perforated (from a cotton bud going too deep for example), then it can usually repair itself. The healing process takes up to 2 months, as long as further trauma is avoided and the ear is kept free from infection [7][8].

If the perforation does not heal, an ENT surgeon can close it by performing a minor operation known as “myringoplasty”. A small incision is made to lift up the ear drum, and cartilage or fascia (muscle lining) is grafted over the perforation and allowed to heal.

 

An endoscopic image of an eardrum with a small perforation.

Conclusions

The properties of the human eardrum are only just starting to be uncovered, thanks to modern imaging and acoustic technology. Advancements in our understanding of how this small part of the ear works, has helped engineers improve microphone and speaker design, lead to advances in ear surgery, and is helping hearing aid manufacturers take the next step to truly implantable hearing aids. With research ongoing we will certainly be keeping our eye on this rapidly changing field of Audiology. 

References

1. Fay, J.P., Puria, S. and Steele, C.R., 2006. The discordant eardrum. Proceedings of the National Academy of Sciences, 103(52), pp.19743-19748.

2. Faddis, B. T. (2008). "Structural and functional anatomy of the outer and middle ear". In W. Clark & K. Ohlemiller (Eds.), Anatomy and physiology of hearing for audiologists (pp. 93–108). Thomson Delmar Learning.

3. Staab, W., 2020. Ear Canal Dimensions | Wayne Staab, Phd | Hearinghealthmatters.Org/Waynesworld/. [online] Wayne's World. Available at: <https://hearinghealthmatters.org/waynesworld/2014/ear-canal-dimensions/> [Accessed 25 June 2020].

4. Rosowski, J.J. and Lee, C.Y., 2002. The effect of immobilizing the gerbil’s pars flaccida on the middle-ear’s response to static pressure. Hearing research, 174(1-2), pp.183-195.

5. Stenfors, L.E., Salen, B. and Winblad, B., 1979. The role of the pars flaccida in the mechanics of the middle ear. Acta Oto-Laryngologica, 88(1-6), pp.395-400.

6. Stenfeldt K, Johansson C, Hellström S. The Collagen Structure of the Tympanic Membrane: Collagen Types I, II, and III in the Healthy Tympanic Membrane, During Healing of a Perforation, and During Infection. Arch Otolaryngol Head Neck Surg. 2006;132(3):293–298. doi:10.1001/archotol.132.3.293.

7. Orji, F.T. and Agu, C.C., 2008. Determinants of spontaneous healing in traumatic perforations of the tympanic membrane. Clinical Otolaryngology, 33(5), pp.420-426.

8. Department of Otolaryngology Head and Neck Surgery. 2020. Eardrum (Tympanic Membrane) Perforation. [online] Available at: <https://www.entcolumbia.org/health-library/eardrum-tympanic-membrane-perforation> [Accessed 25 June 2020].

 

Banner image by: By Michael Hawke MD - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40796102

 

Tympanic membrane perforation image: By Michael Hawke MD - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=40804258

This article was written by Conor Boland, Durham Hearing Specialists ©2020.

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