Bone Conduction Transducers: output force dependency on load condition

The aim of this investigation is to determine the output force variability of the new bone conduction transducer, called balanced electromagnetic separation transducer (BEST), and of the conventional transducer from Radioear B71. The variability of the output force is dependent on different load conditions. The loads used were the mechanical impedance of the skull of thirty test persons and the mechanical impedance of the artificial mastoid (AM) 4930 from Brüel & Kjær. The hypothesis is that BEST has less variability since it has inherent damping of the spring elements, while B71’s damping takes place through the skin and the soft tissue, i.e. the damping is mostly patient dependent. The output force of the transducers were determined in three different ways: 1. Transducer modelled as a four pole equivalent. 2. Transducer modelled as an impedance analogy lumped parameter model. 3. Direct measurement using the AM (not completed in this study). Models of the transducers were determined from frequency response measurements simply called variables in this report. As it was expected the variation of the output force calculated with the mechanical impedance of the test persons was less for BEST than for B71. For BEST, the standard deviation of the force is approximately +2 to −3 dB at 316 Hz while for B71 the deviation at 200 to 500 Hz is up to +4 to −10 dB. Both transducers show greater load dependency at higher frequencies. BEST has a standard deviation range from +0.8/− 1 dB to +3/− 5 dB in the frequency region from 0.8 to 3 kHz. B71 shows a standard deviation in force of +4 to −6 dB from 1.2 to 10 kHz. The results also show that, though the mechanical impedances of the test objects and the impedance of the artificial mastoid are very different (about 7 dB relative to 1 Ns/m between the mean and AM) the output force does not differ as much as the impedances. Comparing the output forces with BEST and the two impedances (mean human skull and AM), it yields lower force magnitude when the force is calculated with the mean value of the impedance of the test objects (up to 10 dB rel 1 N/V for the frequencies from 1.3 to 10 kHz). B71 shows also lower force magnitude with the impedance of the test persons than with the impedance of AM but beginning at a lower frequency than BEST, (from 6 up to 10 dB rel 1 N/V for 0.4 to 10 kHz).