Bone Conduction: Sound Through Your Skull
We are able to perceive sound thanks to our brain decoding all the various frequencies collected through our inner ear. This is not just done with the ear but also directly through our skull. This is now being utilised through technology like google glass. This is thanks to mathematical and computational models by researchers to understand the whole process -- bone conduction -- works. Along with technologies such as Google Glass, it is looking to be utilised in medical equipment, their paper was published this week in Nature Communications.
At the heart of the inner ear is the cochlea, this is an elastic structure known as the basilar membrane. This is numerous hair cells connected to a massive array of nerve cells. These hairs get excited when sound waves move the fluid around them. Depending which nerves get stimulated depends what sounds we perceive.
Typically the basilar reacts to stimulation from the eardrum and the sound waves that simulate that. This research also looks at the bone surrounding the inner ear, called cochlear bone. This is where bone conduction research has been focused.
In their new paper, Tatjana Tchumatchenko from the Max Planck Institute for Brain Research in Frankfurt and Tobias Reichenbach from Imperial College London, explained how bone conduction works with the help of fluid dynamics calculations.
Their model has provided new insights into bone conduction but helping to explain how the basilar membrane is able to interpret different sounds depending where they happen on the membrane. Higher frequencies excite a smaller band then low frequencies, which excite the whole membrane as an example.
The research was able to determine how two sounds with slightly different frequencies that arrived at the same time, can overlap and excite the same regions. This means it should be very hard for our brains to tell that there are distinctions between very similar frequencies. In this case the combination of tones are produced in the inner ear because of its snail shell structure.
“In our study, we have shown that the combination tones can leave the inner ear in the form of a fast wave along the bone surface, and not, as previously assumed, by a wave along the basilar membrane,” Tchumatchenko said in a press release.
The new model also shows how both vibrations of the cochlear bone and vibrations of the air in the ear canal can generate waves along the basilar membrane. “Our results provide an elegant explanation for this long-known but poorly understood observation," Reichenbach said in the release.
Several technologies, like Google Glass and some hearing aids, have already begun using bone conduction to assist with hearing. The new model scientists a better understanding of the interactions between fluid compartments in the cochlea and the cochlear bone's dynamics, and could help to improve technology even further.
Source: Reichenbach T, Tchumatchenko T.A cochlear-bone wave can yield a hearing sensation as well as otoacoustic emission. Nature Communications. 2014.