Microphones and Speakers
We use microphones all over the place, and most people have one or more on them at any point in time. Most work on a fairly simple concept, using 2 plates. One of them is much thinner than the other, and acts as the diaphragm, the part that moves as a result of sound. The other one is thicker, and works to make the 2 plates into a capacitor. The sound waves change the distance between the two plates slightly, and therefore changes the capacitance of the system. These changes in capacitance are measured and turned into sound via speakers. Speakers work on a principal that is similar but opposite. Instead of measuring, the diaphragm is moved by varying electric fields in a coil around a magnet. By charging the coil with the right amount of electricity at the right time, it allows for sound waves that mimic what the microphone recorded to be produced. This is a very analog system, meaning it isn't controlled by a system of 1's and 0's being interpreted by a processor, but rather the strength of the charge resulting from sound into the mic. Obviously this can be converted back and forth from digital, but the speaker will always be a very analog type of technology.