Many concepts for acoustic meta materials rely on additive manufacturing techniques. Depending on the production process and material of choice, different levels of precision and repeatability can be achieved. In addition, different materials have different mechanical properties, many of which are frequency dependent and cannot easily be measured directly. In this contribution the authors have designed different resonator elements, which have been manufactured utilizing Fused Filament Fabrication with ABSplus and PLA, as well as PolyJet Fabrication with VeroWhitePlus. All structures are computed in FEA to obtain the calculated Eigenfrequencies and mode shapes, with the respective literature values for each material. Furthermore, the dynamic behavior of multiple instances of each structure is measured utilizing a 3D-Laser-Scanning Vibrometer under shaker excitation, to obtain the actual Eigenfrequencies and mode shapes. The results are then analyzed in regards to variance between different print instances, and in regards to accordance between measured and calculated results. Based on previous work and this analysis the parameters of the FEA models are updated to improve the result quality.
Vibroacoustic metamaterials (VAMM) have the potential to reduce unwanted noise components in a very targeted and narrow-band frequency range. Many VAMM concepts are based on mechanical resonators that act as vibration absorbers in their natural frequency and thus absorb energy that would otherwise be radiated in the form of airborne sound and perceived as noise. Often, during the design phase, it is not yet possible to adequately assess which surfaces will be acoustically problematic and in which frequency ranges disturbing noise components are going to be. In addition, many VAMM concepts can only be manufactured using additive manufacturing (AM) processes, due to their complex geometry. However, AM parts often have strongly anisotropic material behavior, depending on the manufacturing process, which makes a prediction of the vibroacoustic behavior even more difficult. Direct integration into casing structures during the design phase is therefore not practical and economically feasible in most cases. This paper therefore investigates the extent to which retrofitted resonators can be effectively used in existing casing structures. For this purpose, FDM-printed bending resonators made of ABS and PLA are used, which have already been measured with regard to their natural frequencies in a previous publication. Different variants are attached to a demonstrator housing and the surface vibration velocity is measured using a 3D laser scanning vibrometer, and compared with the basic variant without resonators. The radiated airborne sound is also measured. Furthermore, a comparison is made with a dynamic FEM simulation in order to be able to evaluate its prediction quality, in particular with regard to the additively manufactured resonators.
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