Determination of the structural response of lightweight structures by means of sound pressure measurements

From the dynamic response of a vibrating structure, it is possible to determine trustworthy insights about the acoustic and mechanical properties of that specific element. The structural wavenumber, propagating along a certain direction, can be determined from the dynamic response of the element, measured along a line of equally spaced points. Moreover, the radiation efficiency of the structure, for example, can be computed from the complex vibration velocity, measured over a grid of points on the surface of the element. Measurements of the vibrational filed are commonly performed by using accelerometers attached to the surface of the vibrating object. By using miniature transducers, it is possible to investigate a wide range of structures, even though non-contact transducers are sometime preferred, in order to avoid any small influence of added masses on the dynamic response of very lightweight elements. Optical methods have been widely used to overcome such issue, although they require very expensive equipment, such as a laser Doppler vibrometer. Near-field acoustic holography represents an alternative contactless approach, which allows the reconstruction of the structural dynamic response from the sound pressure measured with an array of microphones. In this study the possibility to accurately determine the structural response of a lightweight structure by measuring acoustic pressure in the vicinity of the vibrating surface has been investigated. The reliability of the results has been determined by comparing the structural response reconstructed from sound pressure measurement to the response directly measured by using accelerometers. Furthermore, the elastic properties of the structures, derived from their dynamic response, have also been compared.