The applications of Acoustic Black Holes to attenuation of Coach/Bus Frame Structure Vibrations

Abstract An acoustic black hole (ABH) is a structure whose cross-sectional thickness decreases exponentially to zero, and the phase velocity and group velocity of wave propagation in those structure acoustic black holes decrease as the thickness decreases until they are zero. ABH produce some very magical physical phenomena. For example, because the wave velocity decreases to zero, the wave can never reach the center of the ABH, so the wave propagation in the acoustic black hole has neither refraction nor reflection; that is to say the vibration wave will not go through the acoustic black hole. As a result, the wave energy is concentrated in the center of the ABH. The arrangements of those acoustic black holes in the structures could change the directions of the vibrational wave propagation in the structures at NVH engineers’ wills. These amazing phenomena could play a magical role in reducing structural vibration, structural noise radiation and structure-borne noise. Basic structures for automotive body, aircraft fuselage and ship keel are elements of beams and trusses. This paper will present the vibration attenuations of basic 1D beams, 2D trusses and 3D block to show the effectiveness of ABH application to structural vibration attenuations. The advantages The method we used here is the attachable ABH. That is: we made the independent ABH parts which could be attached to any places we optimized in the structures, which are the most effective areas to attenuate the vibrations. A typical attachable ABH is composed of several 2D ABH stacked up in series. The test specimen is a rectangular frame composed of four beams. The number of the stacked ABH will be determined by experiments. The rule of thumb is the more of ABH the better attenuation. The places we selected to put the attachable ABH are nodes of the resonance frequency modes. The Vibration attenuations of coach/bus structure were conducted four subframes of coach/bus: a 1D hollow squared beam commonly used in coach/bus chassis and frames, a 2D rectangular frame and a 3D truss frame of a coach/bus subframes hosting a coach/bus suspension and a 12-meter coach chassis frame. All of the test results show huge vibration reductions：not only at low frequency vibration amplitude but also at the RMS value over broad band frequency bands. For the 12 meter coach chassis frame. The ABH attached at rear end of the frame and the seat fix bolt area reduce the vibration about 3dB. The advantages of the ABH method proposed in this paper are: the attachable ABH can be manufactured independently; nothing to do with any structures; the attachable ABH can be placed in any areas that deem to have effects on vibration mitigation; the effective area selections are the pivot points of the specific resonant frequency; it can be used to mitigate specific resonant frequencies or dominant frequencies of a structure which may have large vibrational amplitude which causes customer dissatisfaction; the attachable ABH could be used to attenuate the frame vibration at the design and development stages or fix the specific vibration problems even after the products are in the market.