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

Dear Becky,

I would be strongly inclined to think that the variabilities in BC with frequency and placement and other parameters you see in some way or another boil down to acoustic impedance mismatches. Vibrations from your BC oscillator somehow have to travel through skin then bone to the ear, and at each interface there is the potential for more or less of the sound energy to be reflected rather than transmitted, and this is likely to be frequency dependent. You say you see unexpectedly poor thresholds mostly at lowish frequencies. That sounds like frequency dependent impedance to me. The slower the vibration, the less the tiny little bone conductor has to make much of an impression on the much larger, heavier, hard, bony head, particularly when coupled through a layer of elastic skin. If you want to get a good intuitive feel for acoustic impedance, what it is, why it is important generally and also particularly for the question you are asking, I would recommend you try to get hold of a copy of Eric Heller's very fine book "Why you hear what you hear". One of the most accessible and clear introductions to physical acoustics I have come across.

All the best,


On 18 February 2016 at 00:05, Becky Lewis <becky103@xxxxxx> wrote:
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

Prof Jan Schnupp
University of Oxford
Dept. of Physiology, Anatomy and Genetics
Sherrington Building - Parks Road
Oxford OX1 3PT - UK