the IC to drive the transducer. (Typical hearing aids run on a 1.4 V zinc-air button cell.) Even worse, IC power consumption exceeded 60 mW—more than 60 times the power consumption of an ordinary hear-ing aid. And the amplitude of the sound generated by the piezoelectric transducers in the low-frequency range was small, that is, the output was insucient. is led me to conclude fairly early that development of a commercial product using piezoelec-tric transducers wasn’t feasible.”us, early in his eorts, Iwakura was forced to embrace a paradigm shi. He looked for ways other than the piezoelec-tric method to drive the transducer. At this point, he tapped into his experience in developing balanced armature type (BA type/electromagnetic drive method) receivers immediately aer joining Rion and decided to proceed using an electromag-netic transducer. He received a grant from NICT (National Institute of Information and Communications Technology) to develop this electromagnetic transducer, giving him a fresh start. But things didn’t proceed smoothly.“I created a transducer prototype with an electromagnetic structure and achieved higher output than the piezoelectric trans-ducer. But the electromagnetic transducer was extremely delicate, vulnerable to impact. It wouldn’t have withstood real-world use. Simply dropping it would aect the magnetic armature. So—another major obstacle.”Curiously, Iwakura says, he never considered giving up. His retirement in 2011, nearly two years aer the request to create a pro-totype, worked to his advantage. Released from his managerial responsibilities, he found himself in an environment and mind-set that allowed him to devote his time and eorts to his project. e company wasn’t prepared to mobilize major resources for the project at this point. Iwakura spent his days by himself, working through trial and error. To his colleagues, he may have appeared to be ghting a solitary battle with no nish line in sight.Piece Together Correct Logic So at the Goal will Come in Sighten came a turning point. Iwakura had continued creating one prototype aer another and presented them within his department. One day, in search of a solu-tion, someone provided a major clue.“Dr. Munehiro Date had become an advisor at Rion aer his retirement from RIKEN. He’d oen provided advice. Once, he sug-gested I should provide air gaps on not one but both sides of the armature. at was the biggest hint. A little while back, I’d patented a structure resembling what he’d suggested, although the movement wasn’t the same. But I had a feeling this was a breakthrough. e hint led to nding the correct sequence of logic for creating a novel structure for the BA-S-type transducer.”Another name for the term armature is “movable iron piece.” In a conventional BA-type structure, the armature is placed between two magnets with air gaps in between. It’s a cantilever structure, with the armature xed to the yoke. In this case, the armature has to be magnetic. It’s produced by annealing inside a high temperature, hydrogen atmosphere furnace to achieve its magnetic properties, so the armature ends up having lower spring character-istics than common spring materials. In contrast, the new BA-S structure uses four magnets to create BA structures on both the right and le sides. A at plate-shaped armature is positioned between them, with sucient air gaps secured. Four springs are placed between the armature and the yoke. e springs provide the force that returns the displaced armature. is novel structure dramatically improved impact resistance and made it possible to make a transducer with much more freedom in design, one that could be made to any dimensions.“I was quite impressed with the resulting prototype. at was in 2013. At that point, Bone conduction hearing aids are worn using a headband and must be held tightly in place, cre-ating signicant physical discomfort.First, a mold of the ear shape is taken, followed by a 3D scan, to perform 3D modeling of an ear chip on the computer. The ear chips are ulti-mately made to t individual ear shapes using a 3D printer. This manufacturing process was de-veloped through joint efforts between Watanuki and the production department.The actual BA-S type electromechanical converter adopted for the cartilage conduction hearing aid is this size.This is the new structure of the electromechanical converter devel-oped for the cartilage conduction hearing aid (conventional type to the left, new BA-S type structure to the right). The most notable fea-ture of the new structure is the ad-equate air gaps at both right and left ends of the armature. The mode of vibration for the new structure is translational.Conventional typeComparison of electromagnetic structures for conventional BA type and new type(schematic illustration)Examples of actual structures (mode of vibration: pendulum for conventional type and translational for new type)YokeThe new structure (BA-S)ArmatureMagnetCoilSpringDirection of magnetizationKeisuke WatanukiEngineering Manager of Group 1, Component Technology Development Department, Technical Development Center. He spent many years designing hearing aid hous-ings, applying his knowledge of mechani-cal engineering. In developing the cartilage conduction hearing aid, he dedicated him-self to designing a special device for testing hearing aid characteristics.4
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