Noise is generated by mass flow rate and velocity. Certain valves have greater acoustical efficiency (the ability to turn energy into noise) than others. That being said, a control ball valve will make a lot more noise than a globe control valve. Liquid flows: If the inlet pressure is 200 psi and there is a 20 or 30 psi drop, it is unlikely to make much noise, but if the inlet pressure is 30 psi and the valve discharges to atmospheric pressure, the flow is almost guaranteed to be cavitating. Cavitation can be very noisy, even from relatively low inlet pressures. Someone mentioned flashing in an earlier post: In my experience flashing is generates a low rumble but is not opressively loud. Cavitation generates a sharp crackling noise and can be deafening. Gas flows: General rule of thumb is if the (absolute) outlet pressure is less than about half the (absolute) inlet pressure, the flow will be shocking sonic in the vena contracta and the valve will be noisy. This is the noise of a jet engine on afterburner and the decibel numbers can go into three digits easily. Compressible noise control valve trims split the flow up into small streams and run the flow through many stages so the pressure ratio in each stage remains less than the critical drop. What this REALLY does is to control velocity so the flow never goes sonic. A muffler works very much the same way. Back to the original question: a 30 psi drop through a ball or butterfly valve discharging to atmosphere can be really, really noisy.

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