Vibroacoustic behaviour of sandwich structures with spatially distributed resonators: Fid-Bau Portal

Vibroacoustic behaviour of sandwich structures with spatially distributed resonators

Hettler, Steffen

Lightweight constructions are based on multi-layer structures. Typical lightweight structures are also commonly known as sandwich structures which can be designed to have satisfactory sound insulation at high frequencies but do not perform well at low frequencies. The objective of this thesis is to investigate the implementation of internal resonators to improve the sound insulation of sandwich constructions at low frequencies with least possible mass increase. The research is carried out using double panel lightweight partitions. The benefits of double panel partitions are combined with the frequency dependent effect of distributed internal resonators. As internal resonant system, for this investigation internal vibration absorbers are implemented into the cavity of a double panel partition. With the focus on low frequencies, the interaction of the internal vibration absorbers with the modal behaviour of the panels is of particular interest. Based on the modal expansion method beam and plate models are derived. These analytical models allow the calculation of the sound transmission loss for both normal and oblique sound incidence. A tuning approach is derived using a simplified mass-spring model. This approach enabled the design of internal vibration absorbers for specific acoustical conditions. A significant increase in the sound reduction index at low frequencies was achieved by tuning the effect of the internal vibration absorbers to the structural resonance of the host double panel structure. The increase in the sound reduction index is experimentally verified in a semi-anechoic chamber. For this purpose, the influence of the internal vibration absorbers on the vibroacoustic characteristic of a double panel structure is determined by comparing the measured radiated sound power of the host structure with and without absorbers. This study successfully applied internal resonators to finite lightweight double panel structures for increasing the sound reduction at the MAMresonance. Since modal formulations allowed the investigation of mass efficient resonator distributions, the frequency dependent increase in the sound reduction was achieved without a significant mass penalty.