Propagating phonons in liquid 4He

at T approximately=0.1 K is studied as a function of pressure input power and propagation distance. Using a superconducting tunnel junction detector it is found that a single injected pulse can form two propagating pulses at low pressures but only one at high pressures. It is shown that three-phonon scattering can explain this behaviour. At low pressures, one group of phonons propagates ballistically while another group consists of strongly interacting phonons which travel at the ultrasonic velocity.