The most impressive example I’ve personally observed is the alternative phone system used in US Navy ships. The primary phone system operates on ship’s power in the exact way that the phone systems on land have been operating for more than 100 years. It works well, but can fail in the most dire moment, when power is cut off as it tries to save lives or repair the damage caused by the course of. The solution is a phone powered by sound. It’s what it is, a phone which has no external power source , but is operated by sounds talking. It is done by attaching a superfine metal coil on the mouthpiece’s diaphragm. This allows that the wire to move while the diaphragm is deflected by the sound. The coil is placed within a magnetic field, which results in the production of tiny electrical current whenever the user speaks. The current causes motion in the speaker on the phone that is connected to it. The audio quality isn’t the best in quality, but I’ve heard it as good on a couple of landlines. It’s sufficient that it can save lives.