Design of tuned lightweight metamaterials for broadband sound transmission and sound absorption

Over the past decades, tightening ecologic and economic requirements have urged industry to introduce lightweight design. Lightweight structures typically combine low mass with high stiffness, which impairs their noise and vibration insulation performance. This conflicts with rising customer expectations and increasingly stringent noise exposure regulations. Classical solutions to improve the vibro-acoustic performance usually rely on adding mass or volume, conflicting with lightweight design requirements. In the search for novel solutions that can comply with these conflicting requirements, vibro-acoustic locally resonant metamaterials have recently emerged and shown promising potential. By introducing resonators on a sub-wavelength scale to a flexible host structure, a stop band can be created, which is a frequency range in which no free wave propagation is possible. By creating resonance-based stop band behaviour for the bending waves in flexible partitions, a targeted frequency range of strong noise and vibration attenuation can be achieved. The sub-wavelength nature of locally resonant metamaterials can enable lighter and thinner vibro-acoustic solutions, also able to target the hard-to-address low frequency range.

While the potential of vibro-acoustic locally resonant metamaterials has been demonstrated, the transition towards engineering solutions still requires more accurate and robust vibro-acoustic performance predictions. Damping is inherently present in locally resonant metamaterial realisations and affects their vibro-acoustic attenuation performance. Whereas damping tends to reduce the peak attenuation, it has also shown potential to broaden the frequency range of attenuation. However, damping is usually omitted in the modelling of locally resonant metamaterials, hampering accurate vibro-acoustic performance predictions. Moreover, while their predominantly narrowband performance makes locally resonant metamaterials especially suitable for vibro-acoustic problems in targeted frequency ranges, achieving broadband vibro-acoustic performance is highly desired. In view of attaining broadband performance, the potential role of damping needs to be further clarified.

The main goal of this dissertation is to assess the impact of damping on the vibro-acoustic performance of locally resonant metamaterial plates, in order to gain physical insight in the effects of damping, to obtain more accurate vibro-acoustic performance predictions and to investigate the broadening of the frequency range of attenuation.

After investigating the translation from bending wave stop band behaviour to acoustic insulation improvements, the impact of damping on the vibration attenuation of locally resonant metamaterial plates has been assessed by means of dispersion curve analysis. The vibration attenuation in and around the stop band is mainly governed by the resonator damping, reducing the peak attenuation inside the stop band and increasing the attenuation in a broadening frequency range around the stop band. Damping in the host structure mainly increases the vibration attenuation outside the stop band. The damping influenced vibration attenuation predictions have been experimentally validated by means of dispersion curve measurements.

The impact of damping on the sound transmission loss has next been analysed for infinite periodic and finite locally resonant metamaterial plates. The sound transmission loss in and around the stop band is mainly governed by the resonator damping, reducing the peak attenuation and improving the possible low-frequency coincidence zone after the stop band. The resonant sound transmission due to structural modes outside the stop band is reduced in a broadening frequency range around the stop band. Damping in the host structure mainly reduces the resonant sound transmission outside the stop band. The damping influenced sound transmission loss predictions have been experimentally validated by means of insertion loss and sound transmission loss measurements.

Eventually, in view of improving and broadening the acoustic insulation performance, the potential of a locally resonant metamaterial double panel has been analysed and experimentally investigated.