Up until now, there was no indication of how Google's Project Glass would provide sound to the user. After all, if you're going to make calls using the AR specs, you need to hear what the other person has to say (unless it's a good scolding of course – that calls for a dose of quick muting). Will the user wear earbuds connected to the specs in order to receive sound? Will there be external speakers just loud enough for the user to hear?
According to a patent filed by Google, the company proposes using bone conduction to deliver sounds. This will be done using elements embedded within the Project Glass specs that will vibrate the frames. In turn, that vibration will reach the user's skull and pass the sound down into the user's inner ear.
Google calls this method "indirect" because the specs don't directly vibrate your bones. Instead, the frames act as a transport mechanism which actually brushes up against your nose, your ears, your cheek bones, your temples and more. Where the vibrations will actually enter the skull is unknown at this point, but this could possibly be a setting within Project Glass, allowing the user to pick the best audio "reception" point.
The patent, called "Wearable Computing Device with Indirect Bone-Conduction Speaker", was actually filed on October 10, 2011, but made public on January 24, 2013. The abstract states that the specs will include at least one vibration transducer that is configured to vibrate at least one portion of the specs based on the audio signal. Does that mean a stereo reception will require an element on both sides of the head?
Bone-conducting headphones have been around on the consumer market for quite a while. For example, Outi introduced a pair back in 2007 that resembled a pair of clip-on earrings. Sound was reverberated through the earlobes, passing tunes through the user's cartiledge instead of the air. And during CES 2013, Panasonic introduced a Bluetooth pair that delivered sound through the user's temples.
Wikipedia defines bone conduction as "the conduction of sound to the inner ear through the bones of the skull." Our skulls conduct lower frequencies better than air, and that's why our voice sounds different when it's recorded and played back.