With Christmas nearly upon us, I decided to take a look into the science behind one of the coolest tech products available for the upcoming festive season, bone conduction headphones. These are not your traditional headphones. They aren’t buds that sit into your ears or headphones that cover them; in fact they take a position just in front of your ear and transmit music sound through your cheekbones.
What is bone conduction and how does it work?
Bone conduction is the conduction of sound to the inner ear through the bones of the skull. Many technologies are now using this phenomenon to transmit vibrations through the cheekbones. This clever method of audio transmission enables us to bypass the outer and middle ear, leaving the ear free to hear ambient noise.
Sound is usually thought of in terms of the propagation of sound waves through air. Our ears pick up the vibration of the air molecules and we hear frequencies within the range of twenty to twenty thousand hertz. Air is a good transmitter of sound. Think of a time when you’ve stuck your head underwater and all the background noise has completely faded away; air is a better transmitter than water. But how does it compare to solids? If you are a fan of old Hollywood films then you might remember how American Indian’s would crouch down and stick their ear close to the ground, later to pronounce what direction riders come from. Or equivalently if you ever lean your head against a train window, you hear far more sounds that you don’t pick up through air. Similarly, the bones in your skull which are solid transmit more frequencies than air to the cochlea, the hearing organ.
Why should you want one?
One of the coolest things about bone conduction is that even though it is now being used in lots of the latest audio technologies, it actually appears in our everyday lives. Have you ever been recorded on video and watched it again and thought, “I didn’t think I sounded like that?” In fact the reason why a person’s voice sounds different to themselves when on a recording is down to bone conduction. The human skull actually conducts lower frequencies better than air so people actually perceive their own voice through the vibrations in their skull, which leads them to infer that their own voice is lower and fuller than it actually appears to others. So when they hear a recording of their own voice, people often feel their voice is higher than expected.
Safety for cyclists
Bone conduction earphones have many practical applications. Firstly, there is the potential safety benefit they offer to cyclists and joggers, who often wear earphones on the road. Cyclists in particular, are in serious danger when they decide to isolate themselves from the noise of their surroundings by wearing earphones.
Wearing headphones while cycling is not currently illegal in Ireland, but there is a strong movement to ban it, especially in the United Kingdom. Last year, 90% of respondents to a BBC poll were in favour of banning cyclists wearing headphones, with Mayor of London, Boris Johnson, calling the habit “Absolutely nuts”.
Bone conduction headphones provide a safer solution for cyclists because they leave the outer ear free to hear traffic. One variant of bone conduction headphones aimed at cyclists is a product called “Safe & Sound” designed by Gemma Roper, a graduate of the Royal College of Art in London. The bone conduction modules clip onto the straps of a cycling helmet and press against the cheek bone, allowing you to hear your music playing but also hear traffic. Many cyclists, such as myself, currently employ the cheap alternative solution of putting one earphone in and leaving the other out. However, this is certainly not ideal because often I have to ramp up the volume extremely high to hear my music or just revert to putting both earphones in.
Uses in the military
Bone Conduction Technology is also being applied to the military. The British defense and security firm BAE Systems is currently developing an Army helmet also utilizing the technology for communication. This helmet will give soldiers the ability to more easily receive direct audio transmissions despite the cacophony of the battlefield. Mohammed Akhmad, BAE system’s principal scientist says “with this system the soldiers can safeguard their hearing with ear protectors whilst also receiving military voice communications, to enable them to perform their roles efficiently and safely”. The transducer for the helmet has been minimized to the size and weight of a fifty cent coin.
Talking windows on trains
In business there is also a new advertising concept that harnesses the bone conduction phenomenon to public transport. The “Talking Window” is a new advertising platform being tested by the German ad agency BBDO Germany and television broadcaster Sky Deutschland. They have developed a concept where passengers who lean their heads against glass train windows will feel like a voice inside their head is talking to them.
The prospect of this does not sit well with many people, however, questioning where advertising ends and harassment begins, picking on tired and bored commuters does not seem appropriate. The developers claim that the technology will not just be used for advertising; it can be used for music, entertainment, public service announcements and weather reports too. But maybe passengers just want some peace and quiet, or at least to choose what they are listening to.
Earplugs won’t actually block out the sound coming from this type of advertising; in fact, they would have the effect of enhancing it. This is because when an object such as an earplug fills the outer portion of the ear canal then the person experiences what is known as the “occlusion effect”. This is the booming echo your voice makes when you block your ears and speak at the same time. Your voice sounds different because you hear a combination of the air and bone-conducted signal of your voice. One simple way to avoid hearing the advertisements would be to place a dampening pillow in between your head and the window. The sound waves would not transmit through the pillow and thus not reach your head. Alternatively, you could just avoid putting your ear to the window in the first place!
Even if after reading this article, you are not craving one of those bone conducting earphones, you have to admit that they are absolutely awesome. It is amazing how science contributes to the creation of modern technology and how these new technologies transform our lives.