We’re working on some energy harvesting projects – and we learned about this amazing project – a radio powered only by the sound of the voice of the person talking at the transmit end.
This was able to transmit 150 miles with NO batteries at all. And no ‘super caps’ either. Just the transient power of his voice. Michael Rainey provides a schematic here: Schematic of voice powered transmitter, El Silbo.
The trick for using a speaker as a microphone to absorb sound wave power is to get a good step-up transformer. The speaker has current, but little voltage, even into a high impedance. Of course, you can get more power output using the tin can approach, demonstrated in this video: