Spatial evolution of pulses of low-energy phonons in liquid 4He

Abstract The system of equations that describes the temporal and spatial evolution of low-energy phonons (l-phonons) is obtained for the cases of plane and axial symmetry. From the solutions of these equations, we discuss the time-resolved measurements of the angular distribution of l-phonons. These show a rather unusual mesa shape with a flat top in the direction normal to the heater. We consider that the flat top is due to the creation of high-energy phonons (h-phonons).