Nearfield acoustical holographic techniques used to visualize radiated sound fields

Abstract

The term signal processing is customarily used in reference to the analysis of data obtained as a function of time; similarly the term image processing refers to the enhancement of two-dimensional spatial data. For wave fields, such as optical or acoustical fields, the wave equation couples the space and time variables and permits an extremely powerful signal processing technique referred to here as generalized holography. In this technique temporal data are measured over a two-dimensional spatial surface, and the data are processed to reconstruct and visualize the entire three-dimensional wave field. The power of this technique arises from the enormous expansion of information which occurs when going from the two-dimensional measurement to the three-dimensional reconstruction. Although the theory of generalized holography is exact, conventional optical and acoustical holography suffers from serious intrinsic limitations, such as the spatial resolution being limited to the radiated wavelength. At The Pennsylvania State University we are developing a new acoustic generalized holographic technique, called Nearfield Holography, which overcomes the limitations of conventional holography. In this technique a brief measurement with a microphone array can be used to visualize the surface motion of a complex vibrating structure, to reconstruct the entire three-dimensional sound pressure field, particle velocity field, and vector intensity field, and to map the flow of acoustic energy from the sources to the farfield. With this system one may pinpoint (within centimeters) acoustic energy producing sources on vibrators with small structural features (such as musical instruments) even when the radiated wavelength is several meters. In this talk our most recent Nearfield Holography equipment and processing algorithms will be described, and computer graphic reconstructions of vibrating surfaces (moving in real-time) and radiated sound fields will be shown. [Nearfield holography research is supported by the Office of Naval Research (Physics Program) and the National Aeronautics and Space Administration.]