Diagram of flow across a semilunar valve. 1. Laminar flow through the proximal vessel. 2. The transient reduction in diameter as the blood flows across the valve results in increase in velocity due to the Bernoulli principle. 3. The high velocity results in post valvular turbulence. Signal dropout occurs in this region due to the disorganised flow. 4. Eventually at a distance from the valve, laminar flow resumes. When there is regurgitation the turbulence may also be seen in a subvalvar position.