Old Mobile Telephones
My dad used to work for the telephone company back when it was The Telephone Company. I got to see the insides of the phone building pretty often, and one day he showed me the equipment that handled mobile phone calls. It was just a couple of devices, about 4 feet tall I guess, and you could switch the sound on to make sure it was working or to debug it if it wasn't.
He thought it was funny that people would pay so much for a mobile phone and then mostly use it for mundane conversations. He switched it on and sure enough some guy was saying something like "Hey, honey, I'll be home in a little while, want me to pick up something at the grocery store?"
The rest of the telephone office was the coolest thing, because in those days they used huge (by today's standards) rotary relays to connect calls and the entire floor was filled with rows of racks 12 feet tall, full of relays chattering away. it's amazing to me nowadays to think that when you called someone local back then you had a direct conductive metal link to them. Nowadays when you call it's buffered on so many levels it's almost more lke real-time voice mail than like a phone call was back then.
Anyway, when you picked up your handset at home, your relay at the office would activate, and as you dialed your rotary dial, your relay would rotate too, making a clicking sound as it worked. I'm not sure exactly how connections were made, but the sound of thousands of people dialing was amazing, because you'd hear this huge 3-dimensional clicking coming from throughout the building. The clicking was completely random, but sometimes there would appear to be waves of clicks sweeping from one end of the building to the other. Then sometimes almost everything would stop and, you might hear some clicks off in the distance, and then there would be a shower of clicks all around.
I dream of making an art installation one day to simulate the effect.