A New Bone Conductor with Improved PerformanceAuthors: 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: 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.

Aim: 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.

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.

A commercial partner, Ortofon, was located in Denmark that had design and manufacturing experience in small precision electromechanical transducers.

The button size zinc air batteries can limit the force output of the bone conductor for some wire sizes used in a transducer.

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.

Settle on BEST bone conductor design by Bo Håkansson (Ref 2). Ortofon optimised motor design to meet performance requirements.

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.

BEST Bone conductor output for different batteries

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Bone Conductor Performance

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Commercial BC675 Zinc Air Cell

Speech 65dB SPL-6dB/Oct 250 to2000 HzRETFL Volt-Ampere Power for Different

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