| Popis: |
Applications of noise isolation/absorption in air and ground vehicles, where the sound content is a product of the rotating engines, require the efficiency of the absorbing material at the low-frequency range between 50 and 250 Hz. However, conventional sandwich panel-type of solutions, which can be modeled as ‘m-k-m’ systems, perform poorly in this band of frequencies. Sound Transmission Loss (STL) of a panel at the low-frequency range is stiffness and resonance controlled, while frequencies above the fundamental are dominated by the mass law. The stiffness- controlled region depends primarily on the elastic stiffness of the mounting and the bending stiffness of the panel transforming the issue to a classic vibration isolation/absorption problem. However, since the STL curve of a finite panel has a first major reduction at the resonant frequency, conventional sandwich panels demonstrate a certain inefficiency in this range. Therefore, a design for an acoustic isolation/absorption concept, targeted at low frequencies is proposed in the form of a panel mounted on bearings based on the KDamper concept, resulting to a much improved behavior of the STL curve around the resonant frequency. The KDamper, which can be considered as an extension of the traditional Tuned Mass Damper (TMD), is a novel passive vibration isolation and damping concept, based essentially on the optimal combination of appropriate stiffness elements, including a negative stiffness element. The latter is realized via pre- stressed disc (Belleville) springs, arranged in appropriate geometrical configurations. The KDamper supplements the inertial forces of the added TMD mass with the stiffness force of the negative stiffness element. Among others, this can provide significant comparative advantages, especially in the very low frequency range, such as better damping characteristics without the need of heavy additional masses. Following an impedance-based approach for the calculation of the STL of a panel, a comparison between three kinds of mounts (“m-c-k”, “TMD”, “KDamper”) highlights the effects and advantages of the proposed concept. Furthermore, considering a 1 m\(^2\) rectangular gypsum board, the appropriate parameters of the KDamper mounts are calculated for a preliminary examination. Moreover, the STL of the board is examined. The flexibility of the concept suggests that it can be incorporated as the unit cell in a lattice forming an elastic/acoustic meta-material, while it can be used in many more possible applications such as acoustic doors and in room acoustic treatment in general. |
