Fibrous absorbers, such as glass wool, are widely applied as thermo-acoustic insulation. The lightweightness and good fire-smoke-toxicity properties of glass wool makes this material a well proven insulation material, e.g. in aircraft cabins. However, since the sound absorption of fibrous materials is governed by viscous losses, their sound insulation performance is reduced at low frequencies. Acoustic metamaterials, on the other hand, have emerged in the recent years as new sound insulation materials with particular efficiency in the low-frequency regime. Due to the dispersive properties of most acoustic metamaterials, however, this efficiency typically is limited to relatively narrow frequency bands. Therefore, it seems to be reasonable to combine the strengths of both types of sound insulation materials in order to achieve good sound insulation characteristics at low and high frequencies with low additional weight and/or installation space. In this contribution, first results from investigations of how the low-frequency acoustic performance of glass wool can be improved by adding acoustic metamaterials are presented. The investigated concept employs a thin plate-type acoustic metamaterial (PAM), which exhibits tunable anti-resonance frequencies with transmission loss values much larger than the corresponding mass law. The PAM is combined with lightweight aircraft grade glass wool and attached to a plate in order to improve the sound transmission loss of the plate. An analytical model is used to predict the acoustic performance of this design. Finally, the concept is validated using a 1.2 m² test sample, which is experimentally characterized using sound intensity measurements in a laboratory.
