It started as a inverse loudspeaker long time ago. My father worked at a turbine maufacturer late 1940´s and needed a tool for the mechanics to evaluate the machines status on site after overhaul. He and a companion managed to make a moving coil velocity transducer and a instrument combination. Our family have worked with machine vibrations since that time. Efforts resulted in patents at the time, find here a US I found the other day: patent
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