Electromagnetic bone-conduction transducers

Variable reluctance electromagnetic transducers are most commonly used in clinical audiology. These transducers consist of an armature with a permanent magnet, which is screwed to the top of a plastic casing and suspended above a yoke, with a small air-filled gap separating the two components. An electrical input signal passing through coils of wire wound around the armature generates a dynamic magnetic field that interacts with the static magnetic field of the armature to create a magnetic flux across the gap between the armature and the yoke. The vibration of the mass of the armature created by the magnetic force is propagated to the casing, thus creating the vibratory stimulus that is applied to the skull.

6.1.1.1 Radioear B-71 and B-72 transducers

Recommendations for audiometric bone-conduction vibrators from the International Electrotechnical Commission (IEC) and the American National Standards Institute (ANSI) are that the transducer should have a plane, circular contact tip with an area of 1.75 cm² and that it should be coupled to the mastoid process with a force of 5.4 N (IEC 373 (1971); ANSI 3.13 (1987)). Radioear (New Eagle, PA) developed two electromagnetic models that meet these specifications; the B-71 and the B-72. Both are encapsulated in a rectangular plastic casing with a circular protruding portion that is pressed against the skull with the appropriate force level by a steel sprung headband (Radioear model P-3333).

The Radioear B-71 weighs approximately 19.9 g without the headband attached and has a frequency response characterised by three resonant peaks at 0.45, 1.5, and 3.8 kHz, in order of decreasing amplitude (Richards & Frank, 1982). The frequency response curve drops off steeply below 0.25 kHz and above 4 kHz (Dirks & Kamm, 1975; Richards & Frank, 1982), severely limiting output at lower and higher frequencies. The output of the B-71 is also limited by the high levels of total harmonic distortion at low-frequencies (Arlinger, Kylen, &

Hellqvist, 1978; Dirks & Kamm, 1975; Dolan & Morris, 1990; Parving & Elberling, 1982).

The poor low-frequency response of the Radioear B-71 is improved upon with the B-72 bone-vibrator, although the output in the high-frequencies is further restricted. The B-72 has an added dynamic mass, which increases its weight to 48 g and lowers the resonant peaks to approximately 0.25, 1.25, and 3.7 kHz (Dirks & Kamm, 1975; Richards & Frank, 1982).

Electroacoustic advantages of the increased mass of the B-72 device are higher output levels and reduced harmonic distortion at 0.25 kHz (Dirks & Kamm, 1975). Unfortunately, the downside of the larger size is an increase in the amount of airborne sound leakage, or acoustic radiation, resulting from the vibration of the larger case at higher test frequencies (Bell, Goodsell, & Thornton, 1980; Frank & Crandell, 1986; Frank & Holmes, 1981). The high degree of acoustic radiation suggests that the B-72 is unsuitable for audiometric testing at frequencies of 2 kHz and above (Frank & Crandell, 1986; Frank & Holmes, 1981).

6.1.1.2 Präcitronic KH-70 transducer

A third electromagnetic transducer with a plane circular contact area of approximately 1.75 cm² is the Präcitronic KH-70 (Dresden, Germany), which is approximately 2.5 times the length and five times the weight of the B-71. The KH-70 bone vibrator has a frequency

response characterised by one major resonance at 0.2 kHz and a gradual decline in output beyond the resonant frequency up to 14 kHz, above which there is a precipitous drop in the output (Frank & Ragland, 1987; Richter & Frank, 1985). Although the dynamic range becomes more limited as the test frequency approaches 16 kHz, the frequency response of the KH-70 enables it to be used for testing bone-conduction thresholds up to and including 16 kHz (Hallmo, Sundby, & Mair, 1991; Richter & Frank, 1985). The encapsulation of the vibrator mechanism of the transducer in a rubber housing effectively reduces the levels of airborne sound leaking from the bone-conductor, ensuring high-frequency bone-conduction thresholds are not artificially enhanced by acoustic radiation (Frank & Crandell, 1986).

Despite the significant electroacoustic advantages of the KH-70 bone vibrator, its suitability for clinical use is limited by its large, heavy, and cumbersome design, which reportedly makes stable placement on the mastoid without touching the pinna or hair very difficult (Frank & Ragland, 1987; Hallmo & Mair, 1996; Mair & Hallmo, 1994). This is a particular concern when measuring hearing thresholds following middle ear surgery performed via a retroauricular approach. In these situations any bone-conductor must be placed with care to avoid disrupting the wound and causing pain; a task that can be difficult even with a B-71 bone-conduction vibrator. Hallmo and Mair (1996) also note that correct retroauricular placement of the KH-70 was particularly difficult when defects were present in the mastoid cortex postoperatively. They suggest that this issue was the cause of the change in bone-conduction thresholds documented after surgery, rather than any cochlear trauma. Certainly the inability to reliably make this distinction limits the usefulness of the transducer clinically.

Despite difficulty with correct placement, evidence suggests that the test-retest reliability of thresholds measured at 0.25 – 16 kHz with the KH-70 transducer is satisfactory for clinical applications, at least with non-surgical patients (Frank & Ragland, 1987; Hallmo et al., 1991).

6.1.1.3 Balanced electromagnetic separation transducer

A recently developed bone-conduction vibrator, the new balanced electromagnetic separation transducer (BEST) offers significantly improved performance at low test-frequencies.

Håkansson (2003) reported that a transducer based on the BEST principle offered a good frequency response and low distortion through a design that incorporated a second, opposing air gap to counterbalance static forces. Electroacoustic assessment of a new version of the BEST transducer adapted for serial production, the Radioear B81, confirmed that maximum

test levels can be significantly increased below 1.5 kHz using this technology, compared to the B71 (Jansson, Hakansson, Johannsen, & Tengstrand, 2014). However, the output above 4 kHz is not increased beyond that of the B-71 (Håkansson, 2003; Jansson et al., 2014), therefore it is still not possible to test thresholds in the EHF range using a BEST device.