Google has used bone conduction for its Project Glass wearable computer, it's claimed, promising discrete notifications that only the wearer themselves can hear. The headset makes contact with the mastoid process, linked directly to the middle ear, insiders tell Geek, meaning any audio output - such as new messages, Google+ alerts, or other notifications - is piped in directly, completely inaudible to those around the Glass owner, and yet can still be perceived despite high background noise.
Bone conduction has been implemented on a number of wearable audio devices, from Bluetooth headsets - Jawbone's headsets use speech vibrations picked up through the upper cheek to perform noise cancellation, for instance - to stereo headphones. As well as cutting through loud background noise more efficiently, they also can help keep the user's ears open rather than plugging them up with earbuds.
That's particularly useful if you're using an AR device like Google Glass, which is intended to be worn semi-permanently. Google is yet to give any specific hardware details about audio from the headpiece - in fact, all specifications publicly shared to-date are subject to change, as Google tweaks the design ahead of the initial "Explorer" developer versions set to ship early in the new year - but it was assumed that a small speaker was embedded in the oversized arm-piece.
Such a speaker would have drawbacks, however. For instance, controlling volume would require repeated stabbing at buttons on the Glass device itself, unless automatic volume levels were implemented; that could lead to distractions for those around the wearer, if the volume was set too high. Meanwhile, some notifications might be private, or the audio could be a hands-free call, and discretion preferred.
Audio quality of bone conduction systems tends to be less audiophile-level than traditional headphones, but the technology's other advantages may well outweigh any shortcomings there. It's possible that the oblong pad in the image above - shared by Google back in May - is the bond conduction assembly.