I read about experiments that modeled blackholes as a liquid passing through a lavel valve at supersonic speed, the point were the water went supersonic was like the event horizon of a blackhole for sound - the sound could go in but not out. They were looking for sighns of Hawkings radiation which in this case would be faint sound coming out of the holes. They said that "phonons" and "virtual phonons" would play the parts of photons and virtual photons. What is a phonon? Thanks.