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[AUDITORY] AC/BC: Why would bone conduction be poorer than air conduction?
Dear Becky, Dear All,
Using a vibrator placed on the mastoid and an accelerometer placed on the
forehead, my colleague John West and I made bone conduction frequency
response measurements, a long while ago, with eight normal adults. A
classic B&K continuous sweep sine wave source was used. The results
whilst being individually consistent were markedly variant from person to
person. Trough to peak differences greater than 10dB were found in the
range ~200Hz to ~2kHz for some of the subjects.
We partly monitor our speech production on the basis of bone conduction
and our aim was to gain an initial understanding of the possibility that
this pathway might contribute to some aspects of anomalous speech
acquisition in young children. In the present context, however, it
may be that Becky?s observations are also in part explicable by person to
person acoustic skull response differences.
All god wishes,
At 08:52 19/02/2016, Shahnaz, Navid wrote:
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.
From: AUDITORY - Research in
Auditory Perception [AUDITORY@xxxxxxxxxxxxxxx] on behalf of Stefan
Sent: Thursday, February 18, 2016 11:37 PM
Subject: SV: AC/BC: Why would bone conduction be poorer than air
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
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
Ämne: AC/BC: Why would bone conduction be poorer than air
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
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
Speech and Hearing Sciences, University of Washington - Seattle
Expected Graduation Date: 6/30/2016
name="Acoustic method for calibration of audiometric bone vibrators
Content-Description: Acoustic method for calibration of audiometric
vibrators Margolis 2012.pdf
filename="Acoustic method for calibration of audiometric bone
Margolis 2012.pdf"; size=1000235;
creation-date="Fri, 19 Feb 2016 08:51:24 GMT";
modification-date="Fri, 19 Feb 2016 08:51:24 GMT"