Adaptive membrane-type acoustic metamaterial with autonomous tuning to incident sound fields

Membrane-type acoustic metamaterials (MAM) consist of a thin pre-stressed membrane with masses attached to the membrane. Even though these metamaterials are very lightweight and thin, they exhibit narrow frequency bands (so-called anti-resonances) in the low frequency range with very high sound insulation values - much higher than a homogeneous wall with the same mass. This makes MAM a very promising solution for noise control applications with strict constraints on the mass and size of noise control treatments. The narrow bandwidth of the anti-resonances, however, is a considerable inhibiting factor for the application of MAM to noise sources with changing frequencies of tones (e.g. generated by rotating machines). The objective of this project is to demonstrate for the first time an adaptive MAM which is capable of autonomously tuning itself to an incident sound field with a spectrum containing both broadband and tonal components. In the project, an adaptive control algorithm will be developed in conjunction with actuation methods and sensors integrated on the MAM to achieve improved broadband and tonal noise reduction, compared to the passive MAM. In particular, the adaptive MAM will be capable of identifying deviations between a tone in the noise source spectrum and the anti-resonance frequency of the MAM and it will adapt itself within milliseconds to adapt the anti-resonance frequency to this tone. The sensitivity of the adaptive MAM with respect to non-linearities as well as changes of the dynamic properties of the MAM (e.g. due to temperature changes) will be investigated. The adaptive control algorithm will be developed to be robust with respect to these disturbances, thus enhancing the applicability of the adjustable MAM to more practical noise control applications. As a final result of this project, an experimental demonstrator of an adjustable MAM will be built and tested for different noise source configurations.