A New Bone Conductor with Improved Performance
Authors: Eric Burwood (NAL); George Raicevich (NAL); Harvey Dillon (NAL); Carit Laursen (Ortofon)
Acknowledgement: This project was a collaboration between the National Acoustic Laboratories (NAL),
a division of Australian Hearing and Ortofon, Denmark.
Introduction:
Aim:
The most commonly used bone conductor in Australia is based on a
design that dates back several decades; however, this device suffers
from limitations of user comfort and speech distortion.
To design a bone vibrator motor that can be used in a binaural bone
conductor hearing aid with improved distortion and comfort; and without
excessive decrease in performance; and that is safer than current
models.
Background Information:
Bone conduction hearing aids have not been comfortable devices to wear and anecdotal reports indicate that continuous excessive pressure on the
skin can produce skin ulceration. However, for people with middle-ear disorders, including those with active effusion, transmission of sound to the
cochlea via bone conduction rather than air conduction via the ear canal can provide more effective stimulation, unaffected by fluctuation in
conductive loss, and avoid exacerbating active middle ear infection. In Australia, the predominant need is for Indigenous people, especially children,
because of their high incidence of middle ear infection, often with chronic perforation of the eardrum. Hearing aids that enable children with chronic
middle ear disease to hear well in school are an important step to improve educational outcomes for these children, and long-term their living
conditions and health.
Method:
Produce design specifications.
Investigate the limitations of performance and comfort for existing bone conductor transducers.
Locate a commercial partner to design and manufacture the bone conductor.
Investigate different designs.
Investigate how the type and size of battery and headband force affects the output of a bone conductor.
Results:
Investigations at NAL found that the most used bone
conductor was fitted with a headband pressure sufficient to
close small capillaries in the skin (Ref 1). This caused
discomfort to the user and long term use may damage the
skin. Headband force must be selected to allow the
maximum perceived signal level. The area of the footplate
must be selected to provide safe operating pressure on the
head resulting in a comfortable bone conductor to wear.
Bone Conductor Performance
120
Ortofon BC
675 Zinc Air Cell
100
Commercial BC
675 Zinc Air Cell
80
40
1000
10000
Frequency (Hertz)
Display shows the bone conductor output for the new
Ortofon device and for the existing BC461device when
powered by a size 675 zinc air battery; Reference
Equivalent Threshold Force Level (RETFL) (Ref 3) and
speech (Ref 4) at 65 dB SPL with a 6 dB per octave filter
between 250 and 2000 Hertz.
Force (dB re 1
micro -Newton)
Speech 65dB SPL
-6dB/Oct 250 to
2000 Hz
RETFL
60
20
100
A commercial partner, Ortofon, was
located in Denmark that had design
and manufacturing experience in
small precision electromechanical
transducers.
BEST Bone conductor output
for different batteries
140
120
Alkaline Cell
Zinc Air 675
Zinc Air 13
100
80
60
40
100
Settle on BEST bone conductor
design by Bo Håkansson (Ref 2).
Ortofon optimised motor design to
meet performance requirements.
Volt-Ampere Power for Different
Cells with wire at 30 ºC
1000.0
Volt-Ampere
Power (mVA)
140
Force Level (dB re 1 uN)
•
•
•
•
•
100.0
10.0
1.0
Alkaline
675 Zinc Air
13 Zinc Air
0.1
1000
10000
Frequency (Hertz)
0
50
100
150
Nominal Wire Diameter (μm)
The button size zinc air batteries can limit the force output of the bone
conductor for some wire sizes used in a transducer.
Conclusion:
The outcome of this project will be a bone conductor that is comfortable to wear, cosmetically
acceptable, has low distortion, enables binaural fittings, and has an effective output for practical
use. The new bone conductor motor will enable children with chronic middle ear disease to hear
better in school.
Reference 1: Raicevich G, Burwood E, Dillon H, 2008. Taking the pressure off bone conduction hearing aid users. The Australian and New Zealand Journal of Audiology, Vol. 30 No.2, pp 113-118.
Reference 2: Håkansson B, 2003. The balanced electromagnetic separation transducer: A new bone conduction transducer. J.Acoust. Soc. Am. 113(2), pp 818-825.
Reference 3: ISO 389-3:1994, Acoustics - Reference zero for the calibration of audiometric equipment; Part 3: Reference equivalent threshold force levels for pure tones and bone vibrators.
Reference 4: Byrne D, Dillon H, Tran K, et.al., 1994. An international comparison of long-term average speech spectra. J. Acoust. Soc. Am. 96(4), pp 2108-2120.