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“Bone conduction is not a new discovery.
Ludwig Van Beethoven overcame deafness by biting a metal rod attached to his piano to hear his work.” source
articles
https://scholar.google.co.il/scholar?cites=10624021506786852239&as_sdt=2005&sciodt=0,5&hl=en
AliExpress sellers of bone conduction headphones/microphnes bluetooth headsets