Bone conduction – from headphones to Bluetooth sunglasses

What is bone conduction?

Sound can travel in one of the two ways. The first more known method of air conduction is the way in which sound waves propagate through the middle ear to the inner ear where they are consequently sent via nerve signals to the brain. The second, is the less understood method of bone conduction, where sound can be conducted through our skulls via the temporal bone. This in turn allows sound vibrations to travel to the inner ear where they affect tiny hairs capable of detecting different sound frequencies. These subsequently send signals to the brain for us to decipher.

Both air and bone conduction are important modalities for us to recognize the sound around us. Bone conduction, in particular, is responsible for why we usually perceive a recording of our voice to sound higher in pitch that we expect. Our skulls conduct lower frequencies than sound transmitted via air and hence we consider our voices to sound fuller and richer when we speak.

How can we apply bone conduction to the real world?

Although we are coming to understand more about the principles of bone conduction, it has been seen in practice for centuries. Beethoven, for example, had a form of sensorineural deafness with damage to the inner ear. By placing a metal rod between his teeth connected to a piano, he was still able to listen to music and create masterpieces. In modern applications, the original Google Glass implemented the use of a ‘Bone Conduction Transducer’ which places a speaker in and near the temporal bone for sound wave propagation.

Bone Conduction Headphones

More recently, bone conduction headphones have popped up within the marketplace. AfterShokz is one such company, with military roots, who have introduced bone conduction technology to the mass market. The AfterShokz Aeropex wireless headphones are said to promote safety by allowing users to be spatially aware of their surroundings while also being comfortable to wear for long-term. The target demographic being runners, cyclists and those going for work.

However, in situations such as a train or bus commute, the high decibels negate the use of these types of headphones. Moreover, as bone conduction works better with lower frequencies, they may not be the most appropriate for those wishing to experience a full spectrum of sound.

Image: AfterShokz

Bone conduction, as aforementioned, plays a vital role in those with hearing impairments. Bone-anchored hearing systems (BAHA) are a two-part system with a titanium bone implant and outside sound processor. An ear nose throat (ENT) specialist can place an implant into the mastoid bone behind the pinna of the ear. While not the most natural looking aid, the device can be a blessing for a range of people including those with congenital ear malformations, single-sided deafness or those suffering from chronic ear infections.  

Bluetooth Sunglasses

Akin to Google’s original version of Google Glass, companies have also attempted to capitalize on the use of bone conduction speakers in wearable lens accessories. An example of this is well-known American audio brand Bose. Their version of Bluetooth sunglasses titled Bose Frame Audio Sunglasses combines a familiar wayfarer/round design with a bone conduction speaker to tempt the masses. The Bose Frame Audio Sunglasses is able to do this with modern capacities such as the ability to stream music from Spotify, Apple music and more.

Image: Bose Corporation

While the sunglasses allow users to stay aware of surroundings, inherent limitations of bone conduction devices include the ability to hear ambient sounds without any forms of isolation. Of course, if you are expecting a technically proficient audiophile sound – it would be best to look elsewhere. Perhaps their use case would be best serviced while listening to podcasts, audiobooks and sat nav directions while cycling or working out.

What does the future hold for bone conduction technologies?

The research surrounding bone conduction and its implications in hearing is just about getting started. While the technology may not suit the audiophile requiring the paramount from sound reproduction, it is rather similar (albeit more rudimentary) to the early origins of wireless audio technology. Perhaps more streamlined and newer technologies can recreate an inner symphony for the end-user while retaining the benefits of safety, spatial awareness and use in those with hearing impairments. For now, bone conduction carves out a sub-niche in the world of audio and looks like its here to stay.    

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