Bone conduction, is it something different from the normal phenomena of listening through the ears. I checked on some literature . Is it so that Bone conduction has very less to do with the ears and more to do with vibrations of the bones in the skull. Maybe I am mistaken . Please anyone can present a simple picture here and maybe some text to refer . Thanks !
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Prologue: Bone conducted signals are considered to be evoked by the perilymph fluid moving back and forth out of the cerebrospinal cavity, via the cochlear aqueduct, towards the scala tympani. And not by mechanical deformation of the cochlea. “the fact that the bone conduction phenomenon is actually the result of the push-pull movement of the perilymph fluid instead of the presumed deformation of the bony structures.” some text to refer: Applying physics makes auditory sense.
I am affiliated with this book. I am the co-author of the Appendices.
Year Article Author(s) Source 2010 1 Applying physics makes auditory sense : a new paradigm in hearing Abstract | Full-text Heerens, W.C., Ru, J.A. de Medicine (2010), pp: 1-74
pp: 13: Objections against the existing bone conduction signal transfer hypothesis: The common model for the transfer of a bone conducted signal is based on the vibration of the temporal bone. However, both the material and the construction of this bony envelope are extremely rigid, while the cochlear shape of the cavity excludes the possibility that parts of this cavity can function as the hypothesized resonators. Therefore, sound stimuli cannot be transferred in such a manner. Certainly not the weak vibration signals with pressure variations of approximately 20 mPa, equal to a pressure variation of 0.2 x 10^-6 atmosphere that are generated by a normal sound stimulus of 60 dB SPL. Hence ‘bone conduction’ must have a cause other than the deformation of the rigid petrous bone that forms the cochlear bony envelope.
The physiological structure data:
• The walls of the cochlear envelope are extremely rigid. Hardly compliant to not compliant at all. So bone conduction based on deformation of that envelope is not possible.
I assume that it is clear, without further explanation that the petrous bone can be regarded as a non-deformable body when confronted with the extremely weak impact of acoustic signals. We see that for a completely normal 60 dB SPL sound pressure signal, clearly showing that the vibrations of bone in the cochlea offer only extremely small, and therefore completely negligible contributions. It means that we are talking about deformations which is therefore completely negligible as a possible mechanism for signal transfer. I dare say, no waving arm statements. Contributions that all result in deformations comparable to an incredibly small fraction, are all leading to naught, one cannot escape the conclusion that there must be another cause for the phenomenon of bone conduction.
Bone conducted signals are considered to be evoked by the perilymph fluid moving back and forth out of the cerebrospinal cavity, via the cochlear aqueduct, towards the scala tympani. And not by mechanical deformation of the cochlea.
pp: 47: It has been established that the rigid cochlear envelope is hardly deformable. This leads to the realistic hypothesis that the ‘bone conducted’ sound signals evoke stimuli by means of a similar process to those of the airborne sound stimuli, namely by means of a push-pull movement of the perilymph, in this case, out of the cerebrospinal cavity via the cochlear aqueduct.
pp: 19-21: Bone conduction: As the petrous bone is the toughest bone in the human body, it is practically impossible for deformation to occur through the vibrating movements that are caused by acoustic pressure changes. Therefore, there must be another means for this signal transfer. A bone conduction audiogram – apart from a reduced transfer of especially the higher frequencies – is not fundamentally different from a pure airborne tone threshold audiogram. It follows that the perilymph movement within the cochlea must somehow also generate the bone conducted signal transfer. And indeed there is such a possibility. The scala tympani is directly connected with the cerebrospinal cavity by means of the cochlear aqueduct, which has its opening in the vicinity of the round window. The cochlear aqueduct enables the perilymph to move backwards and forwards between the cranial cavity and the cochlear channels (scala tympani and scala vestibuli). The perilymph in this cranial cavity is exposed to the alternating pressures caused by the sound induced vibrations of the shell shaped cranial bones that are located in the front and the back of the skull. As a consequence of Newton’s fundamental law of motion we know that when a larger amount of fluid inside the skull must be brought into motion, the effect of these stimuli will be smaller for the higher frequency contributions than for those of the airborne stimuli for similar frequencies. Furthermore, the stapes introduces the airborne signal that results in a backwards and forwards motion of perilymph, in opposite phase to the back and forth motion of perilymph along the basilar membrane, which is evoked by bone conduction. This results in a slight reduction of the airborne signal, which actually is the stronger of the two stimuli.
pp: 19-21: The alternating movement of perilymph between the cerebrospinal cavity and the scala tympani via the cochlear aqueduct will also be partially directed towards the round window. This splitting up into two directions of the perilymph movement creates a reduced perilymph velocity in front of the basilar membrane, and finally results in a reduction of the bone conduction signal. The rate of reduction depends on the mobility ratio between the oval window and the round window. A further decrease in round window mobility, related to the mobility of the oval window, results in a higher perilymph movement in front of the basilar membrane, which evokes a higher bone conduction signal, while an increase would result in a lower one.
pp: 19-21: This hypothesis that bone conduction consists entirely of the push-pull movement of perilymph from the cerebrospinal cavity via the cochlear aqueduct, and is evoked by the vibration of the shell shaped bones of the skull, while the rigid temporal bone does not deform at all, is just one logical step further than the functional possibilities indicated by others.
Sound travels faster through solid material than air. It also travels more efficiently, the sound does not diminish as quickly over long distances as the molecules in the air do not collide as often and can cause the sound waves to decompose over time and distance.
This is accentuated when the solid through which the sound is travelling is long in one dimension. This is why old westerns have scouts listening to at railroad tracks to see if a train is coming. Sound travels along the rails much further than we can hear through the air.
Bone conduction uses this principle - you can touch a walking stick to your jaw bone and put the stick onto a wall and hear sound from the otherside of the wall more efficiently. This cool hearing aid uses the jawbone and skull as a microphone to pick up sound waves. I'm not advocating the product, but it demonstrates the idea - as microphones get smaller and smaller with shrinking hearing aids, it might make sense to use a different pick up for the sound.
Note that the bones of the middle ear use bone conduction to get the sound into the cochlea where the nerves actually pick up sound. This serves to isolate the delicate inner workings of the ear from the outside.