I suppose in theory any pendulum could be modulated by transducer to vary its lenght as it swings back and forth to create gravity waves with a signal superimposed on the regular vibration because there will be a corresponding (minute) alteration in the Earth's own gravity field as the mass oscillates back and forth.

I can just see the headlines: Andromeda rocks to Earth Gravity FM !

__________________
By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.