Techniques for Implementing Near-Field Acoustical Holography

Near-field acoustic holography (NAH) has fundamentally changed noise diagnostics in that it enables one to get all acoustic quantities such as the acoustic pressure, particle velocity, acoustic intensity, sound power, normal surface velocity and structural wave number information simply by taking the acoustic pressure measurements in the near field of a target source surface. The insight into the acoustic characteristics of a sound source that one can get from NAH cannot be matched by any conventional methods. This article describes commonly used ways to implement NAH for reconstructing acoustic quantities in 3D space. The original implementation of NAH is through a Fourier transform that is suitable for a surface containing a level of constant coordinates in a source-free region. To extend NAH to arbitrary geometry, the Helmholtz integral theory is employed and implemented through the boundary-element method (BEM). An alternative is the Helmholtz equation least-square method (HELS) that offers an approximate rather than an exact solution to the acoustic field generated by an arbitrary source. Other methods are developed to visualize an acoustic field radiated from a source in motion or that from a source subject to an impulsive excitation. The efforts of able researchers have made NAH an ever more powerful tool to gain an insight into the characteristics of sound generation and propagation in 3D space.