How the ear drum works

ossicles.jpgA computer simulation of the eardrum developed by researchers at Standford University’s Departments of Mechanical Engineering and Otolaryngology shows that the shape optimizes the transmission of sound waves from the outer to the inner ear, by transferring more force to the ossicles.

The ear is wonderful feat of natural engineering. Structurally, it is divided into the outer, middle, and inner ear. The outer ear, or pinna, is perfectly shaped to channel sound waves along the ear canal towards the middle ear, which consists of the tympanic membrane (the tympanum, or eardrum), and three bones called the ossicles (left). The ossicles are the smallest bones in the human body, which are now known to have evolved from the gills of an ancient fish.

Each ossicle is named for the way it is shaped. The hammer (or malleus, in Latin) is attached to the inside surface of the eardrum, transmits vibrational energy from the eardrum to the anvil (incus) and the stirrup (stapes). The latter is attached to the oval window of the cochlea, a spiral-shaped, fluid-filled structure in the inner ear. Thus, vibrations of the eardrum are transmitted to the cochlea, causing movements in the viscous fluid within it. These movements are detected by hair cells, which convert them into electrical impulses that are then sent to the brain along the auditory nerve.

The simulation below (from another study), illustrates the biomechanics of the ossicles; the eardrum is coloured white, the ossicles red and green, and the cochlea blue.

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The structure and function of the eardrum has been studied extensively. The eardrum has an asymmetrical conical structure, is placed at a steep angle with respect to the ear canal, and has radially and circumferentially organized collagen fibres within it. How this is related to its function was, however, unclear.

The Stanford team, led by Charles Steele, a professor of mechanics and computation, set out to answer several questions: Why is the mammalian eardrum conically shaped? Why is the eardrum set at such a steep angle? And what is the significance of the highly organized array of collagen fibres?

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The mathematical model developed by the team, and the resultant computer simulation (above) shows that the shape of the eardrum transmits more force to the ossicles than a flat membrane, especially at higher frequencies. The angle of the eardrum with respect to the ear canal gives the eardrum a larger area with respect to the canal, increasing transmission to the cochlea. Finally, because of the asymmetry of the eardrum, and the organization of the collagen fibres inside it, high frequency sounds produce a large number of discordant (or mistuned) resonances in the eardrum. The mistuned resonances increase the ear’s sensitivity to high frequency sounds. They are added together where the malleus is attached to the eardrum, and sound waves of all frequencies are then smoothly transferred along the ossicles to the inner ear.

The results are published in advance on the website of the Proceedings of the National Academy of Sciences. They could find direct application in the design of microphones and loudspeakers.