(A) SOUND WAVES CAUSE                              (B) SOUND WAVES CAUSE

      AN INCREASE IN THE                                       A RISE IN PRESSURE

      FLUID PRESSURE AS                                        UPSTREAM OF THE

      IT APPROACHES                                               THE VALVE.

      CYLINDER.

- AS SHOWN IN OPEN CHANNEL FLOWS, WHETHER THE UPSTREAM FLOW CONDITIONS CAN BE INFLUENCED BY THE PRESENCE OF AN OBSTACLE, DOES OF COURSE DEPEND ON WHETHER THE SPEED OF THE SOUND WAVES IS GREATER THAN THE FLUID VELOCITY.

- THE SPEED OF SOUND IS THEREFORE AN IMPORTANT FLOW PARAMETER.