I'm looking for a long-lasting and energy-efficient speaker for a robot. I don't even care if it sounds shitty. In fact, sounding shitty would be a plus, since it would save me some effort of adding a distortion effect to a voice to sound more robotic.

I've thought about using a piezo powered by a signal matching its resonance to produce the most noise for the input power, then pulse modulate it. I'm sure it'd be efficient and sound shitty, but I'm not sure if that could produce enough tonal range to make speech sound intelligible.