This paper presents a comprehensive study to determine the structural damping and vibroacoustic responses of an arbitrarily shaped planar structure subject to non-contact acoustic excitations under free boundary conditions using a modified HELS (Helmholtz equation least squares) method. The input data consist of the normal surface velocities measured by a laser vibrometer at a discrete number of points on the source surface, and the acoustic pressures measured by a small array of microphones in the field. The normal surface velocity distribution over the entire surface of the plate is then reconstructed by the HELS method and compared to benchmark data. Similarly, the reconstructed acoustic power level spectra are compared to those measured by the array of microphones. The reconstructed vibroacoustic quantities using the modified HELS method are interrogated, and the method's accuracy evaluated. Specifically, a limited number of normal velocity data points on the surface of the target structure are taken as input to the HELS formulations. The reconstructed velocity distributions on the entire surface of the structure with much higher density even for areas of the target structure with little or no input data were compared to the benchmark results. It is worth noticing that no other vibroacoustic technologies are available that allow for complete reconstruction of the normal surface velocity distributions based on limited input data. The dimensionless damping ratio of the structure is also determined. Results indicate that the dimensionless damping ratio for metals, for instance steel, is frequency dependent rather than a constant. Moreover, the empirical formulation developed in this study enables one to get the dimensionless damping ratios continuously over the frequency range from 0 to 10,00 0 Hz. (C) 2020 Elsevier Ltd. All rights reserved.

This paper presents a new method known as laser-assisted, modified Helmholtz Equation Least Squares method-based reconstruction to determine vibro-acoustic quantities of an arbitrarily shaped vibrating structure subject to a noncontact acoustic excitation. Unlike the traditional near-field acoustical holography (NAH) that relies on the acoustic pressures measured in the near-field, this new methodology enables one to collect the input data at a remote distance by using a laser vibrometer, making measurement setup much easier than all previous NAH approaches. Most importantly, by measuring the normal components of the surface velocities at the accessible areas, one can acquire near-field information of a vibrating structure directly, rather than indirectly through reconstruction based on the acoustic pressures measured in the near field. To ensure the accuracy in reconstruction of all the vibro-acoustic characteristics of a structure, the acoustic pressures at a few points in space are also measured. By combining these partial input data, one can acquire a complete picture of the vibro-acoustic field of an arbitrarily shaped vibrating structure, which includes operation deflection shapes, distributions of the acoustic pressures and the normal component of the acoustic intensities on a source surface, and the time-averaged acoustic power of the structure. When the input power is specified, one may determine the frequency response functions, the natural modes, the structural damping ratios of the structure, sound transmission loss, sound transmission path, etc. Examples of using this technology to acquire an in-depth understanding of the vibro-acoustic characteristics of an automobile front dash panel are presented.