‘The dentaphone: a new scientific invention, which enables the deaf to hear by the sound-vibrations conveyed through the medium of the teeth, and the deaf and dumb to hear and learn to speak’
The above literature was published in 1879 by American Dentaphone in Cincinnati and their promotional literature on this device can be found here. This hearing aid relies on a completely different mechanism from the ear trumpet, namely bone conduction. A thin sheet of hardened rubber in a fan like structure was secured at the handle end by the user’s hand. With the apex of the fan held under the upper teeth and the fan flexed to form a diaphragm, sound waves could be transmitted through the maxilla and mastoid bones to the inner ear apparatus. The fan could be folded in three, pamphlet like to make it more portable. The device was patented in 1880 by Thomas Graydon (see pictures modified to landscape format). It was an effective hearing device but met with limited success because of its impracticality.
Whilst the claim is of a “new scientific invention”, knowledge of the benefits of bone conduction have been around for some time. This article shows a copper engraving from 1648 (the frontispiece of “Philosophus” by John Bulwer) of a deaf person listening to music by biting on the scroll of a played viola.