Is there a Sonic Boom in the Little Bang at RHIC?

The use of elliptic flow and correlation measurements as constraints to establish the transport properties of hot and dense QCD matter is discussed. Measured Two- and three-particle correlation functions give initial hints for a “sonic boom”. Elliptic flow measurements give the estimates cs∼0.35 and η/s∼0.1 for the sound speed and viscosity to entropy ratio.