The invention provides a system and a method for measuring a signal propagation speed in a liquid contained in a vessel or in a gaseous medium contained in the same vessel above the surface of the liquid. A transmitter transmits a first signal in a first direction which is at an acute or right angle to a first reflective surface, wherein the first reflective surface reflects the first signal so that it travels in a second direction is received by a first acoustic or electromagnetic receiver. The transmitter transmits a second signal in a predetermined third direction which is at an acute angle to the first direction, where the first or a second reflective surface reflects the second signal so that it travels in a predetermined and angular fourth direction with respect to the first or second reflective surface and is received by the first or a second acoustic or electromagnetic receiver. The speed of sound is then determined under the assumption that both the first and the second signals travel at the same average speed.