Get 70% off NordVPN's 3-years plan and get an extra 1-month subscription on top. Some lucky people will get an extra 1-year subscription on top. Go to https://nordvpn.com/steve and use the promo code steve at checkout. 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: https://www.youtube.com/watch?v=AzZ7DjS4ti4 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... You can buy my books here: https://stevemould.com/books You can support me on Patreon here: https://www.patreon.com/stevemould just like these amazing people: Nathan Williams Matthew Cocke Glenn Watson Mark Brouwer Joël van der Loo Alex Southwell Twitter: http://twitter.com/moulds Instagram: https://www.instagram.com/stevemouldscience/ Facebook: https://www.facebook.com/stevemouldscience/ Buy nerdy maths things: http://mathsgear.co.uk/