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[AUDITORY] SV: AC/BC: Why would bone conduction be poorer than air conduction?

Becky and others,


This is an interesting subject and I also have a few thoughts on this. As Jont eluded it is difficult to answer your precise issues unless we have more information, but we can break down the problem in a few specific issues that can shed some light on the problem.


I cannot say that I see these great differences of 30 dB but a difference of 20 dB is not too uncommon. First, the threshold is estimated by the normal HW procedure (I guess), which has an inherent variability of at least 5 dB. This makes the uncertainty for the two thresholds around 10 dB. Next, the 0 dB HL for AC and BC is determined as the average from a great number of normal hearing subjects, and only the group mean is expected to have the same AC and BC thresholds, not the individual. This spread would probably lead to 10-15 dB possible difference and these two factors alone almost account for the difference you reported.


However, if your question is why there is a individual spread in the AC and BC thresholds, leading to this uncertainty, that is a different question and require more insight into sound transmission by AC and BC. Here, size, geometry and material properties do affect the transmission. One issue that can be influencing the BC perception to a great extent is that the primary excitation of the inner ear is direct transmission, i.e. not through the outer and middle ear. The vibration in the bone around the inner ear are multi dimensional (3xtranslation, 3xrotation, etc). These adds in magnitude and phase and in model simulations it can be shown that due to constructive and destructive summation there is a large spread (20 dB) of the basilar membrane excitation, that is frequency dependent. This could in part explain the larger variability seen with BC threshold estimations. Things as skin in between the transducer and the skull is insignificant at low frequencies (as 500 Hz) but have a huge effect at 3-4 kHz where the resonances of the transducer housing interacts with the mechanical impedance of the skin covered bone. We have not seen any correlation between skiull bone size, thickness, mass etc on the skull vibration transmission and it seem unlikely that there is a simple relationship between those and hearing thresholds, at least not in the adults.


Sorry for the lengthy response but I got a bit warmed up.





Från: AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxxxxxxxxx] För Becky Lewis
Skickat: den 18 februari 2016 01:06
Till: AUDITORY@xxxxxxxxxxxxxxx
Ämne: AC/BC: Why would bone conduction be poorer than air conduction?


Hello all,


In general, when we hear using bone conduction (BC), we should expect to hear the same or better than when we hear using air conduction (AC) due to the physical properties of the ear. With poorer BC thresholds, generally the culprit that is offered in clinic is poor bone oscillator placement. However, there are patients who demonstrate BC thresholds that are up to 30dB poorer than AC thresholds at 0.5kHz in particular, which placement would not account for alone. Other frequencies do not produce this same effect. Additionally, movement of the oscillator can result in no change in this AC/BC difference.


Aside from bone oscillator placement, are there other reasons that could produce a BC threshold at 0.5kHz that is 20-30dB worse than AC threshold? I've started to consider variability in bone density, force of the oscillator on the temporal bone (Toll et al., 2011), the differences in properly calibrated oscillators... I am open to any thoughts or research articles recommended by this group to assist my finding an answer to this question.


Thank you in advance for your assistance!


Wishing you all the best,

Becky Lewis

Rebecca Lewis, PhC

Doctorate of Clinical Audiology (AuD) Student

Doctorate of Philosophy (PhD) Candidate
Speech and Hearing Sciences, University of Washington - Seattle

Expected Graduation Date: 6/30/2016