Why bone conduction can ruin a train journey

Ever been in the quiet coach of a train and been annoyed by others? Now German railways are experimenting with a new way of ruining a peaceful train journey. If you fall asleep and rest your head on the window, an advert is beamed straight into your skull via bone conduction.

Normally, you hear most sound via your ear canals (the holes in the side of your head). But vibrations of the skull can also be picked up by the inner ear. This is why a recording of your own voice sounds odd. When you talk normally you are hearing a combination of the sound that vibrates through the skull and the waves that go into the air, around the side of your head and into your ear canals. A recording of your own voice lacks sound passing through the skull via bone conduction.
Bone conduction has been attracting a lot of attention recently in audio, maybe because Google Glasses use it. Bone conduction headphones have the advantage of not blocking the ear canal so other sounds around you can be heard, alerting you to danger and allowing you to chat to friends. You can buy bone conduction headphones to go into your mp3 player, like Panasonic’s soon to be released RP-BTGS10 headphones. Some users of these types of headphones have complained about a lack of bass, however. One pair I tried couldn’t produce enough volume to be heard above traffic when I walked about Salford.
If you’re travelling on one of these experimental trains, I suggest you take a pillow along. Some squidgy material between yourself and the window will isolate you from the vibrations, and prevent you being disturbed by the adverts.