Why Do Bone Conduction Hearing Aids Have an Extended Frequency Range?

Question

Answer

The simple answer is, because they can. It is easy to consider Bone Conduction and Air Conduction transmission of sound as being equivalent from a measurement perspective. Air Conduction transmission occurs in the acoustic domain and if one reviews specification of output of an amplifying device via the acoustic domain, the reference is 20 dA Pa and this is referred to as dB SPL. In the case of a Bone Conduction transmission, the energy is transferred as vibration to the skull. The reference standard here is one micro Newton.

Coupling of bone conduction transducers can be via a transcutaneous pathway (through the skin) or via direct bone conduction at a point source (Like an implanted Baha fixture and skin protruding abutment). This direct bone conduction pathway allows for more efficient transmission of vibrational energy to the cochlea via a transcranial pathway. The frequency response and resonance peaks for a Bone Conduction transducer look very different from that of an acoustic receiver transmitting energy via the Air Conduction path. It is important to remember that a Bone Conduction transducer is an electromechanical device providing a vibrational output to the skull.

In Bone Conduction, less force output is needed to send an equivalently loud sound to the cochlea than via the air conduction pathway (electroacoustic). The net result is patient reported sound quality that is less likely for distortion due to the efficiency of the vibrational transmission. Depending on the design of the vibrator in the Bone Conduction system, wider bandwidths can be created with little potential for distortion. One may reference the work of:

Lenhardt ML, Skellett R, Wang P, Clarke A. Human ultrasonic speech perception. Science, 5 July 1991 VOL 253 no 5015, pp 82-85 for a look at the potential for high frequency and ultrahigh frequency Bone Conduction transmission of sound.

George Cire is the Clinical Manager for Bone Anchored Solutions at Cochlear Americas in Englewood, CO. Dr. Cire received his Au.D from A. T . Still University, School of Health Sciences in Mesa, AZ. Dr. Cire has been working in the Audiology field for 29 years.