The EasyQ is part of the Nortek Paradopp family and uses the Doppler effect. You hear the Doppler effect whenever a train passes by - the change in pitch you hear tells you how fast the train is moving. The EasyQ uses the Doppler effect to measure current velocity by transmitting a short pulse of sound, listening to its echo and measuring the change in pitch or frequency of the echo. There are many ways to measure the Doppler effect, each with its own advantages and drawbacks. Nortek implements a narrowband auto-covariance method because it has been established as robust, reliable and accurate. Sounds does not reflect from the water itself, but rather from particles suspended in the water. Long experience with Doppler current sensors tells us that the small particles the EasyQ sees move on average at the same speed as the water - the velocity it measures is the velocity of the water.