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Distributed Active Foils for Noise Reduction

Exploitable results

The DAFNOR-project aims at the study and validation of active foils as acoustic and/or structural actuator in active noise control applications (ANC) and active structural acoustic control (ASAC). Three types of actuator/sensor panels equipped with PVDF active foils have been studied, built and tested. The first panel is an acoustic actuator, consisting of two flexible PVDF sheets bonded to a honeycomb carrier structure. The second actuator is a smart foam, in which a thin PVDF layer is integrated in a passive foam. In this actuator the passive absorption characteristics of the foam material are combined with the active control of sound transmission or absorption at low frequencies. The third panel is a structural actuator/sensor, for which a specially shaped PVDF sheet is glued on each side of an aluminium plate. One sheet is used as a volume velocity sensor, while the other is a volume velocity actuator. The two PVDF sheets form a collocated sensor/actuator pair controlling the volume velocity of the plate. A second prototype of this actuator was built, it is a clamped beam equipped with two PVDF layers at each side. A detailed study was made of the working principle of a distributed collocated actuator/sensor pair, validated with measurements on both prototypes. For these panels, numerical models have been derived to simulate the behaviour and to optimise the design of the active panels. Different control strategies for the harmonic and broadband control of sound transmission through a panel partition with one of the active panels have been compared. A fourth actuator that was tested in the project is the EMFi film, supplied by VTT, which is a flat electrostatic loudspeaker panel. Tests in laboratory conditions show that all four actuators are capable of increasing the sound transmission loss through a panel partition, and to control the sound absorption. Finally, the control strategies and active panels were demonstrated on two industrial relevant test set-ups: the active control of sound in a car enclosure and the active control of sound transmission loss through a machine enclosure. These demonstrations prove that the control strategies and panels equipped with active foils can be used in 'real-life' applications. Conclusions The main objective of the project is realised, that is the proof that active foils are effective in the active control of sound and vibration. Based on PVDF material, an acoustic actuator was designed and tested. The final demonstration with this actuator showed that the sound transmission loss through a panel partition could be increased considerably. The smart foam actuator proved that a hybrid design is effective for the absorption of sound. The passive characteristics of the foam material will absorb the sound at high frequencies, while the active system will improve the sound absorption at low frequencies. The final prototype of the structural acoustic actuator proved that a collocated distributed actuator/sensor pair can be built for the active control of volume velocity of a panel partition, and simulations proved that this is an efficient approach to minimise the broadband sound transmission through a panel. The EMFi panel, not developed in this project, was used successfully in experiments on active control of sound and is most appreciated for its response that is quite flat in a broad frequency range.