• Sounds are transmitted to the ear by vibrations in the air but also by vibration of the bones in your skull. This is called bone conduction. 

• Historically, Bone-conduction devices had been tested since the 16th century. The first practical one was the 1879 Rhodes Audio phone and Electric bone- conduction hearing aids appeared in 1923.

• Whales living in the ocean have ears inside their bodies where seawater and hydraulic pressures have no effect.
Whales use the lower jawbone to catch vibrations that convey through water and transmit them to their ears.

• Beethoven became hard of hearing late in life, but he kept to write the great music continuously.
Beethoven held a baton between his teeth and pressed it tightly against his piano. He listened by transmitting the piano sound vibrations from his teeth to his hearing organs by way of his skull.