Noise is created by an object vibrating. The control valve components will tend to vibrate whenever they are subjected to high-velocity turbulent flow. Standard control valves will therefore tend to be noisy on high-pressure drop applications, particularly where flow rates are high since the low pressure experienced downstream of the seat ring (at the vena contracta) is accompanied by very high velocities reaching as high as the speed of sound. Special low noise valves are designed to drop pressure gradually so that velocities are controlled at low levels.
