System for measuring a speed of sound in a liquid (22, 32, 42) contained in a vessel (21, 31, 41) or in a gaseous medium contained in the same vessel above the surface of the liquid, the system comprising • a first acoustic transmitter (27, 37, 47, 57) mounted on one side of the liquid surface for transmitting a first acoustic signal (29, 39, 49, 59) into the liquid (22, 32, 42) or into the gaseous medium to travel inside a first travelling plane, • a first acoustic receiver (25, 35, 45, 55) mounted on the same side of the liquid surface as the first transmitter for receiving the first acoustic signal, • at least one electronic control and data processing unit for controlling operation of the transmitter (27, 37, 47, 57) and of the receiver (25, 35, 45, 55) and for determining the speed of sound from a time of flight of the first acoustic signal, characterized in that • the first transmitter (27, 37, 47) or a second transmitter is further arranged to emit a second acoustic signal (20, 30, 40, 50) in form of an acoustic wave into the wall of the vessel (21, 31, 41, 51) to travel inside the wall of the vessel along a perimeter of the first travelling plane until it is received by the first acoustic receiver (25, 35, 45, 55) or by a second acoustic receiver which is placed at a predetermined distance from the first or second transmitter, respectively; • the at least one electronic control and data processing unit is arranged to ∘ obtain a first time of flight of the first acoustic signal and a second time of flight of the second acoustic signal by measuring the period of time between emission and reception of the respective signal; ∘ obtain a speed of sound in the vessel wall from a data memory, wherein the speed of sound in the vessel wall may be a predetermined value or a measured value; ∘ determine the perimeter of the first travelling plane from the speed of sound in the vessel wall and from the second time of flight, ∘ determine the length of the travelling path of the first acoustic signal (29, 39, 49, 59) from the perimeter of the first travelling plane and from an information on the geometric shape of the first travelling plane obtained from the data memory, ∘ determine the speed of sound in the liquid or in the gaseous medium from the length of the travelling path of the first acoustic signal (29, 39, 49, 59) and from the first time of flight. • a first acoustic transmitter (27, 37, 47, 57) mounted on one side of the liquid surface for transmitting a first acoustic signal (29, 39, 49, 59) into the liquid (22, 32, 42) or into the gaseous medium to travel inside a first travelling plane, • a first acoustic receiver (25, 35, 45, 55) mounted on the same side of the liquid surface as the first transmitter for receiving the first acoustic signal, • at least one electronic control and data processing unit for controlling operation of the transmitter (27, 37, 47, 57) and of the receiver (25, 35, 45, 55) and for determining the speed of sound from a time of flight of the first acoustic signal, • the first transmitter (27, 37, 47) or a second transmitter is further arranged to emit a second acoustic signal (20, 30, 40, 50) in form of an acoustic wave into the wall of the vessel (21, 31, 41, 51) to travel inside the wall of the vessel along a perimeter of the first travelling plane until it is received by the first acoustic receiver (25, 35, 45, 55) or by a second acoustic receiver which is placed at a predetermined distance from the first or second transmitter, respectively; • the at least one electronic control and data processing unit is arranged to ∘ obtain a first time of flight of the first acoustic signal and a second time of flight of the second acoustic signal by measuring the period of time between emission and reception of the respective signal; ∘ obtain a speed of sound in the vessel wall from a data memory, wherein the speed of sound in the vessel wall may be a predetermined value or a measured value; ∘ determine the perimeter of the first travelling plane from the speed of sound in the vessel wall and from the second time of flight, ∘ determine the length of the travelling path of the first acoustic signal (29, 39, 49, 59) from the perimeter of the first travelling plane and from an information on the geometric shape of the first travelling plane obtained from the data memory, ∘ determine the speed of sound in the liquid or in the gaseous medium from the length of the travelling path of the first acoustic signal (29, 39, 49, 59) and from the first time of flight. ∘ obtain a first time of flight of the first acoustic signal and a second time of flight of the second acoustic signal by measuring the period of time between emission and reception of the respective signal; ∘ obtain a speed of sound in the vessel wall from a data memory, wherein the speed of sound in the vessel wall may be a predetermined value or a measured value; ∘ determine the perimeter of the first travelling plane from the speed of sound in the vessel wall and from the second time of flight, ∘ determine the length of the travelling path of the first acoustic signal (29, 39, 49, 59) from the perimeter of the first travelling plane and from an information on the geometric shape of the first travelling plane obtained from the data memory, ∘ determine the speed of sound in the liquid or in the gaseous medium from the length of the travelling path of the first acoustic signal (29, 39, 49, 59) and from the first time of flight.