As Stefan have nicely pointed out 0 dB HL reflects an average among a group of normal hearing adults and does not take into account individual differences. Beside individual variability, calibration can also be a major issue. ANSI provides the calibration bone Reference Equivalent Threshold Force Levels (RETFLs) as force thresholds with a reference of 1 micro-Newton. With so many variables, including temperature and type of artificial mastoid used (B & K or disk type from Larson and Davis or more recently an acoustic method which incorporate acoustic radiation from the bone vibrator -see Margolis attached), you will have to trust your service provider to derive the proper bone calibration levels for the mastoid utilized or you should calibrate your own device if you feel comfortable doing so and have proper calibration devices. Moreover, in order to test hearing down to 0 dB HL, the background noise should not exceed the Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms (ANSI S3.1-1999) especially for BC testing. In fact the most common discrepancy between AC and BC thresholds has been observed at 4 kHz, for BC thresholds being better than AC thresholds using HL values. There are two papers that have recently been published that have looked at BC thresholds distributions at different frequencies and false air bone gap at 4 KHz recently and I have attached the two for your reference. Hope this would shed some light into this matter.
Navid Shahnaz, Ph.D., Aud. (C)
School of Audiology & Speech Sciences
Faculty of Medicine
University of British Columbia
2177 Wesbrook Mall, Friedman Building
Vancouver, BC Canada V6T 1Z3
Tel. 604- 822-5953
From: AUDITORY - Research in Auditory Perception [AUDITORY@xxxxxxxxxxxxxxx] on behalf of Stefan Stenfelt [stefan.stenfelt@xxxxxx]
Sent: Thursday, February 18, 2016 11:37 PM
Subject: 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
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,
Rebecca Lewis, PhC
Doctorate of Clinical Audiology (AuD) Student
Doctorate of Philosophy (PhD) Candidate
Expected Graduation Date: 6/30/2016
Acoustic method for calibration of audiometric bone vibrators Margolis 2012.pdf
Description: Acoustic method for calibration of audiometric bone vibrators Margolis 2012.pdf
False Air-Bone Gaps.pdf
Description: False Air-Bone Gaps.pdf