A method of calculating a time difference is disclosed. The method includes receiving a first ultrasonic signal (r21) from a first transducer (UT1) and receiving a second ultrasonic signal (r12) from a second ultrasonic transducer (UT2). The first and second ultrasonic signals are sampled to produce respective first and second sampled ultrasonic signals (502). Points having a value greater than a first threshold are selected from the first and second sampled ultrasonic signals (510). A difference in travel time between the first and second ultrasonic signals is calculated (512) in response to the selected points.