Laser-assisted Vibroacoustic Holography

Establishing the root cause of noise in vibrating structures is critical in reducing or eliminating the noise. To date, numerous methods, both measurement- and reconstruction-based techniques, have been developed to aid noise reduction in vibrating structures. The notable difference between measurement and reconstruction methods is that the former only offers absolute values of certain physical quantities at specific locations. The measured data are typically isolated, insufficient, and uncorrelated. As a result, it is difficult to establish an effective noise reduction strategy. In contrast, the latter provides comprehensive and correlated vibroacoustic quantities both on source surfaces and in 3D space, but only under certain conditions. Over the past decades, significant efforts have been made to improve the feasibility and robustness of the existing noise reduction methods.

Particularly, the reconstruction technique based on Fourier transform-based near-field acoustic holography (NAH) has drawn attention owing to its ability to reconstruct vibroacoustic quantities. Unfortunately, this approach suffers from many shortcomings such as the requirement that source surface be planar, spherical, or cylindrical, etc. which severely limits its practical applications. Efforts to overcome these challenges have taken shape in recent years. For instance, the development of statistically optimized NAH (SONAH) has successfully removed some of the restrictions. However, its reconstruction is limited to acoustic quantities, not structural vibrations. Moreover, SONAH requires a source-free environment that is non-existent in practice. The boundary element method (BEM)-based NAH enables one to tackle an arbitrarily shaped object, but requires a significant amount of measurements and computations in the mid-to-high frequency regimes, making it impractical for engineering applications.

Recently, laser-assisted vibroacoustic holography has been developed. The underlying principle of this technology is Helmholtz equation least squares (HELS)-based HAH. This new technology is a promising tool to reconstruct all vibroacoustic quantities with higher computation efficiency based on limited measurements on the surface a complex structure and in 3D space. Specifically, it enables one to see operational deflection shapes of a vibrating structure, structural resonances, where sound sources are located, how sound waves are emitting from vibrating structures and how they transmit through a structural surface, etc. With this comprehensive knowledge, engineers will be able to establish the optimal noise and vibration mitigation strategies in an easy-to-understand manner.

Antonio Figueroa (PhD candidate), Mr. Michael Telenko Jr. from Shiloh Industries together with Dr. Lingguang Chen and led by Dr. Sean F. Wu (University Distinguished Professor at Wayne State University) have published the theory and applications of laser-assisted vibroacoustic holography in the research journals, Journal of Theoretical and Computational Acoustics¹ and Journal of Sound and Vibration.²

In this approach, the input data comprised both acoustic pressures measured by a small microphone array in the field and normal surface velocities measured by a laser vibrometer at discrete points on the source surface. The modified HELS method was used to reconstruct the distribution of the normal surface velocity on the entire source surface and the results are compared to the benchmark data. The acoustic power was also reconstructed, and results compared to those of measurements. In addition, a semi-empirical formula was derived to depict the dimensionless structural damping ratio, and reconstructed results were interrogated and validated experimentally.

The results demonstrated that the dimensionless structural damping ratio for metals such as steel is not constant but rather frequency dependent. The proposed semi-empirical formula enabled one to predict a dimensionless damping ratio spectrum over the frequency range of 0 – 10,000 Hz. The reconstructed normal velocity and acoustic pressure distributions were smooth and more accurate than those produced by the existing methods. Furthermore, it allows one to utilize non-uniform measurement approaches; higher-density measurements on surfaces accessible to accelerometers and laser vibrometers, and lower density measurements on surfaces inaccessible to measurement instruments. This provides a high flexibility in measurements for engineering applications.

In summary, laser-assisted vibroacoustic holography enables one to determine all vibroacoustic quantities including dimensionless structural damping ratio spectrum. The method is feasible and convenient for analyzing vibroacoustic responses of arbitrarily shaped vibrating structures. The results demonstrated the advantages of using input data comprising both field acoustic pressure and normal surface velocity, making this technology much superior to all previous versions. In a statement to Advances in Engineering, University distinguished Professor Sean F. Wu said that laser-assisted vibroacoustic holography is suitable and applicable to most structure-borne sound radiation and transmission, and provides a direct and easy-to-understand way to analyze structural vibration and sound radiation, leading to the most cost-effective noise and vibration mitigation strategies.

Laser-assisted vibroacoustic holography has been successfully used to analyze and reduce noise emission and sound transmission through the front dash panel of a full-size F-150 Pick-Up Truck.¹ In fact, it is suited for diagnosis, analysis, and reduction of noise emission from any complex vibrating structure.