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Figure 4. Top row: comparison of the anterior (yellow) vs. posterior (blue) eardrum areas divided by the malleus (red) for human (A), cat (B), guinea pig (C), and chinchilla (D). The anterior-to-posterior area ratios are 1.6, 1.9, 1.1, and 1.0, respectively. Bottom row: comparison of malleus cross sections from micro computed tomography imaging, with round cross sections for human and cat and I beam-like cross sections for guinea pig and chinchilla. Adapted from Puria and Steele, 2010, with permission.
The circumferential collagen fibers may be needed to strengthen the eardrum and maintain its curvature. In addition, modeling studies have shown that the cir- cumferential fibers are needed for good low-frequency transmission (Fay et al., 2006). The output of the ear- drum comes together at the malleus handle, which in our sailboat analogy is like the mast. But although boat masts are often designed to have rounded cross sec- tions, like the human and cat malleus handles, this is not always the case.
Eardrum Symmetry and Malleus Shape
In the human and cat, the malleus divides the eardrum into unequalsections(Figure4,AandB,respectively).What’s more, the human malleus has a circular, mostly solid cross section (Figure 4A), and the cat malleus has an ellipti- cal, fluid-filled cross section (Figure 4B), which makes it lighter than if it were all bone. Both of these rounded cross sections appear to be well-suited to rocking or twisting motions with respect to the long axis of the malleus handle. Such motions could be promoted by the larger area on one
Figure 5. A: rocking motion of the fused malleus–incus complex in chinchilla with respect to an anterior–posterior axis (Axis 1). B: rocking motion of the human malleus–incus complex about Axis 1 dominates at low frequencies. At higher frequencies, eardrum asymmetry could cause rotation of the malleus about Axis 2 to minimize the moment of inertia, while the flexible joint between the malleus and incus could still allow the incus to rotate with respect to Axis 1. C: for the mouse, Axis 1 dominates for low frequencies, whereas Axis 2 is thought to dominate for high frequencies. A and B adapted from Puria and Steele, 2010, with permission; C generated by Hamid Motallebzadeh of the OtoBiomechanics Group at EPL.
Fall 2020 • Acoustics Today 31