Measurement of the acoustic intensity in the nearfield of a fluid-loaded, point-driven cylinder using nonplanar nearfield acoustical holography

Abstract

Nearfield acoustical holography (NAH) is confined to the reconstruction of acoustic fields in parallel planes with a hologram consisting of a measurement of the acoustic pressure over a plane boundary. Although ideal for planar sources, such as vibrating plates, NAH encounters problems in dealing with nonplanar sources. Therefore, we have modified the technique to apply specifically to a cylindrical geometry with a hologram comprising a cylindrical contour. The cylindrical hologram consists of the measurement of the amplitude and phase of the pressure field at 4096 points on this cylindrical contour. This contour is located close, and concentric with, a radiating cylindrical source (in this case a finite, point-driven cylindrical shell submerged in our underwater tank facility). We show how this hologram is processed in a rigorous manner to map the pressure, vector velocity, and vector intensity (real and reactive components) from the surface of the source into the farfield. The vector intensity maps are shown to be helpful, in many cases, in identifying the location of the point driver attached inside the shell. Both the active and reactive intensity fields will be shown. Since the vector velocity is also obtained from the cylindrical hologram, we can identify the radial mode shape of vibration from the radial velocity component at the shell surface, and along with the reconstructed surface pressure (which provides the fluid loading) we can also map the surface intensity. This latter quantity is used to compute the total power radiated by this mode shape. This new technique provides a comprehensive method to study radiation from cylinders.