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Impedance mismatch is when there's a difference in acoustic impedance between two mediums. When this happens sound is reflected at the boundary. There's a mismatch between air and the water in your inner ear. If it wasn't for a clever lever mechanism in your ear, most of the sound would be reflected back out at the boundary and our sense of hearing would be terrible! Two of the three bones in your middle ear act as a lever giving mechanical advantage to your eardrum.

The Ultrasound gel video is here:    • Ultrasound gel does amazing things to...  

Credits:
1) 3D ear animation - National Institute on Deafness and Other Communication Disorders, part of the National Institutes of Health https://en.wikipedia.org/wiki/File:Jo...
2) 2D ear diagram - Lars Chittka; Axel Brockmann https://commons.wikimedia.org/wiki/Fi...

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