A method of determining a velocity of air flowing in an air flow direction through an opening. The method includes locating first and second transducer assemblies in the opening, to position the first and second transducer assemblies spaced apart by a predetermined distance along a substantially straight line of sight defining a non-zero angle in a horizontal plane between the line of sight and the air flow direction. A controller is operatively connected with the first and second transducer assemblies respectively, via a four-wire interface in which two of the wires are for power transmission and two of the wires are for transmitting signals. The transducer assemblies are time-synchronized. First and second electronic signal pulses are transmitted between the first and second transducer assemblies. Based on the transit times, the non-zero angle, and the predetermined distance, the velocity of the air flowing in the air flow direction is determined.