The aerodynamic noise is the most important component of the acoustic problem of a control valve, since it is generated by the pressure waves produced by the fluid turbulence or by other fluodynamic phenomena connected with supersonic waves (“impact cells”). Cavitation and mechanical vibrations are in comparison just potential noise sources, because it is possible to avoid them (at least theoretically), while it is not possible to control a fluid flow rate without generating turbulence. For this reason the noise is almost ever negligible in case of non cavitating liquids, where the velocity is low, while it is sensible for gas at subsonic conditions and very loud under critical flow condition, where velocity and turbulence become very high. The aerodynamic noise of conventional valves has not a characteristic acoustic spectrum which can be easily identified, since it has high volumes in a wide range of frequencies between 1000 and 8000 Hz, with prevailing peaks between 2000 and 6000 Hz. Higher frequencies are generated by valves provided with low noise trims, where realized with many small flows arranged in parallel.
PARCOL (1996) NOISE MANUAL