Bone conduction technology. To anyone hearing about it for the first time, the idea sounds either cool or totally bizarre. We’ve put together answers to the most frequently asked questions about bone conduction tech, along with a lot of information that no one asked for, just because it’s interesting.
What is it?
The basic explanation of bone conduction technology is that there is a device which sends vibrations through the bones in your skull, directly to your inner ear.
The path that normal, air conducted sound takes is through the open ear canal to the eardrum. The eardrum converts that sound to vibrations. The vibrations are sent via the ossicles behind the eardrum to the inner ear, or cochlea, which is filled with liquid.
The liquid moves and stimulates the hair cells in the cochlea. Those hair cells send nerve impulses through the auditory nerve to the brain, which interprets those signals as sound.
(You can read a more detailed version of this explanation here.)
The path looks a little different with bone conduction. Sound travels through cranial bones directly to the cochlea, completely bypassing the eardrum and ossicles.
Although the technology has been more widely implemented in recent years, the idea of bone conduction is an old one.
Due to timing, there’s some confusion about who first described the concept of bone conduction. In 1550, Girolamo Cardano, in his book “De Subtilitate,” described a way to hear sound by holding a rod between your teeth. The vibrations of sound traveled from the rod through the teeth and into the inner ear.
In an article published in “The Laryngoscope” in 1937, Dr. Noble H. Kelley credits Cardano’s student, Phillipus Ingrassia, with first describing bone conduction in his writings published posthumously in 1603.
In the late sixteenth century, Hieronymus Capivacci was the first to use bone conduction in the field of otology, as a method of diagnosing the source of deafness in his patients.
He would sit the patient down and place a long metal rod with one end between their teeth and the other end touching the string of a musical instrument. If the patient could hear the instrument, then Capivacci knew the eardrum was the issue. If they couldn’t hear it, then the auditory nerve was the source.
A brilliant pioneer of otology, Guichard Duverney was the first to say that the vibrations arrived in the inner ear through the cranial bones, and not through the Eustachian tube (which connects the middle ear to part of the throat). However, Schellhammer was the first to experiment and actually prove it in 1684.
Fun fact: baleen whales use bone conduction as their primary hearing mechanism.
What is it used for?
If you google “bone conduction” you’ll see Beethoven mentioned frequently in the results. Early in the nineteenth century, when he began to lose his hearing, Beethoven would hold a rod between his teeth and rest the other end on his piano so that he could still hear and compose.
Apart from Capivacci’s method of diagnosis and Beethoven’s unconventional composing, bone conduction was not applied to any practical use cases until 1879, when Richard Rhodes was issued the first patent for a hearing aid that used bone conduction.
You can even find the specs for his design through the US Patent Office—it’s not at all like what you’re picturing.
Speaking of hearing aids, you might have already heard of BAHAs, or bone-anchored hearing aids. A BAHA involves implanting a titanium screw in the mastoid bone of the skull, and then connecting a transducer to the screw.
Anders Tjellström successfully implanted the first BAHA in 1977. You can read a lot more about bone conduction in hearing aids here.
Bone conduction technology can be used by people with certain types of hearing loss. As long as the cochlea is fully functional, bone conduction tech can be used to hear.
In 2013, the biggest news in tech was the announcement that Google Glass was available for a limited number of “Glass Explorers,” before the smart glasses were available to the public in 2014. That first edition of the smart glasses came with bone conduction technology.
The final use of bone conduction technology is headsets. In the past, the usual reason for using one was for safety reasons during exercise.
You can find a variety of headsets for runners, cyclists, swimmers, and workout enthusiasts with an emphasis on safety. Since a bone conduction headset leaves your ears open, it’s safer to use when running outdoors because you can stay fully aware of your surroundings.
That’s why Sentien uses bone conduction technology in our Sentien Audio headset. It’s one of several features that make it possible to wear the audio interface headset for the entire day, without ever taking it off. Since a bone conduction headset leaves your ears open, you don’t have to keep taking something out of your ears every time you want to be aware of your surroundings.
The purpose of Sentien Audio is to connect you to your smartphone in the most efficient and seamless way. Part of that seamless experience is not having to think about the intuitive interface that you’re interacting with.
Most bone conduction headsets have the same basic design. The transducers, which convert sound to vibrations, rest on the temporal bone just in front of the ear on both sides of the head.
To get the best sound quality, those transducers have to maintain contact with the skin. The backframe wraps around the back of the head and presses the transducers into the skin. The more fitted the backframe, the more comfortable the headset will be.
Apart from safe exercise and creating an all-day audio interface, bone conduction headsets have also been used for augmented reality. Augmented reality blends the physical world with the digital, usually by layering visual, auditory, or even haptic elements on top of reality.
One excellent example of this was a concert performed at the National Maritime Museum in London. The entire audience was given bone conduction headsets which played a secondary layer of sound during the entire performance.
Bone conduction headsets have also been used to counteract tinnitus. You can find individual testimony from people who used bone conduction to produce masking sounds to manage their tinnitus, but research performed on this has been somewhat inconclusive.
From the results of this study in particular, you can see that implanting a BAHA helped 80% of the participants go from slight tinnitus severity to mild or absent.
This study tested the effects of bone conducted ultrasound (sound at a frequency greater than 24khz) to see if it could get rid of tinnitus completely. They concluded that they could produce longer periods of residual inhibition (“continuous reduction or disappearance of tinnitus”) with bone-conducted ultrasound.
You can also read the results of this fascinating study which attempted to discover if bone conduction tech could be used by soldiers impaired with tinnitus or impaired hearing to communicate in the field, rather than sending the soldiers home.
Is it safe?
Although some sites might claim that using a bone conduction headset is safer because it won’t damage the eardrum, this claim is misleading. Listening to music via bone conduction has the same risks of hearing loss as listening via air conduction. If the music is too loud, it can cause damage to the cochlea.
Is bone conduction somehow causing harm to the inner ear or to cranial bones? There is no research proving that it can cause harm. If you want to read a conclusive research paper on all things related to bone conduction, we recommend this one from the US Army Research Laboratory.
How’s the sound quality?
Here’s probably the most frequently asked question about bone conduction. Bone conducts lower-frequency sounds better than it does higher-frequency sounds. That’s one reason why your voice sounds different to you when you hear a recording of yourself. We hear our own voices partly through bone conduction.
True audiophiles might argue against listening to music through bone conduction, but the reality is that high-quality headsets like Sentien Audio will reproduce sound in the full frequency range of average human hearing: from 20hz to 20,000hz. The sound quality is actually surprising to people who try out a bone conduction headset for the first time.
The technology has come a long way since Beethoven stuck a rod in his mouth and hammered away at his piano.
Want to try bone conduction for yourself? Join the waitlist for Sentien Audio today.
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