https://phys.org/news/2019-01-mechanism-ear-exquisite-sensitivity.html

Both the ear's sensitivity and its selectivity—its ability to distinguish different frequencies of sound—depend crucially on the behavior of a minuscule gelatinous structure in the inner ear called the tectorial membrane, which Freeman and his students have been studying for more than a decade. Now, they have found that the way the gel membrane gives our hearing its extreme sensitivity has to do with the size, stiffness, and distribution of nanoscale pores in that membrane, and the way those nanopores control the movement of water within the gel.

The tectorial membrane lies atop the tiny hairs that line the inner ear, or cochlea. These sensory receptors are arranged in tufts that are each sensitive to different frequencies of sound, in a progression along the length of the tightly curled structure. The fact that the tips of those hairs are embedded in the tectorial membrane means its behavior strongly affects the way those hairs respond to sound.