|MadSci Network: Engineering|
According to the Encyclopaedia Britannica, hypersound is defined to be sound waves of frequencies greater than 10 terahertz (10^13 hertz). So 7 gigahertz (7*10^9 hertz) sound waves would then be in the ultrasound range of frequencies. Ultrasound is produced by electromechanical tranducers. Tranducers are of two types, piezoelectric materials (such as quartz), or magnetostrictive materials (which are certain types of ferromagnetic materials). These tranducers can convert an electromagnetic wave into a sound wave, or vice versa. So to create a 7 gigahertz ultrasound wave using a quartz transducer, one would apply a 7 gigahertz electrical signal across the quartz crystal. And one must connect the quartz crystal to the medium in which you want the ultrasound to propagate. It is straightforward to generate such a 7 GHz electrical signal using readily available semiconductor devices. I do not believe that it is possible to make a motor with such a high RPM. Even in the megahertz range, signals are generated electronically rather than mechanically.
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