Here's my situation - I have a centrifugal compressor blowoff line that is creating too much noise. I am being told that the velocity through the pipe may be too high. Values as follows - 3" globe valve increasing to 4" diameter sch 40 pipe 17.5 feet long, increase to 8" sch 40 before silencer. 350 psi before valve blowing off to atmosphere, ICFM capcity of compressor is 7400 but blowoff will never see that due to modulation. How can I calculate the acceptable velocity of 4" pipe (or any size pipe) blowing off to atmosphere w/o shock wave noise? and is that considered "sonic velocity"? I have a customer very upset about the noise level and I am stumped!!