A system and a method for measuring a speed of sound in a liquid contained in a vessel or in a gaseous medium contained in the same vessel above the surface of the liquid. The method comprises: transmitting a first acoustic signal into the vessel to travel inside a first travelling plane. Emitting a second acoustic signal into the wall of the vessel to travel inside the wall of the vessel along a perimeter of the first travelling plane until it is received and obtaining a first time of flight of the first acoustic signal and a second time of flight of the second acoustic signal and obtaining a speed of sound in the vessel wall from a data memory. Determining the speed of sound in the liquid or in the gaseous medium from the length of the travelling path of the first acoustic signal and from the first time of flight.