Anyone experienced bone conduction headphones / transducers ?

Hi.
Is the temporal bone the only one that works ? Would the forehead work ? would the occipital bone work ? How much force needs to have applied (like under a head strap?) What about loudness, can it be driven to discomfort ?
By itself, is the transducer intrinsically waterproof ? would it transmit audio underwater not as bone conducting, but as a speaker ?

----> https://www.gizmag.com/finis-neptune-bone-conduction-waterproof-audio-player/27606/pictures#2
(referring to this link not about the audio source but for the transducer itself)

Would you have any of these gadgets in unwanted/abandoned/failing state that I can purchase to teardown/test/evaluate ?

Your even use your teeth for best contact, otherwise Expect poor frequency response above 1kHz
Try chomping on an earbud