The following picture is of bone conduction headphones, the brand name is : AUDIOBONE FIT
This headphone does not use the longitudinal waves in the air. The electrical signal is converted to vibrations which conduct around the cochlea, via the temporal bone.
You can really feel the sound conducting through your bones when using these headphones.
However, in my opinion … there is not enough sound pressure, and the bass sound is not so strong. Also, the richness of sound is limited when using bone conduction headphones.
I did put high-end headphones over the bone conduction headphones to listen to the music. I felt a sense of depth in sound. I could even imagine the size of the stage, the position of the singer, and was able to feel like the music was just like a real concert through the headphones.
However, the sound was accurate in overlapping with the vibrations.
Note1:If the bone conduction headphones were to be made for vocalization, it would have to conduct around the styloid process.
The stylopharyngeus muscle and the stylohyoid muscle attach to the styloid process to suspend the larynx.
The direct conduction would make these muscle move easily, and I believe it would result in a positive effect for the voice.
I used FOSTEX TH-900 for test,and this headphone is really able to duplicate the natural sounds.