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Published Articles


The Volume 12, No 1, March 2007




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Practical Single MPP Absorber

Dah-You Maa


https://doi.org/10.20855/ijav.2007.12.1204


The construction and properties of microperforated panel (MPP) absorbers are discussed. MPP is a plate perforated with numerous sub-millimeter orifices forming a resonant sound absorber with a cavity behind. It has been shown that low values of the perforate constant k and the orifice diameter d are essential for the MPP to have high absorption over a wide frequency band. To find the exact absorption limits, one can assume for that k ! 1 as a start, because both specific resistance and wide frequency bands of absorption require k to be one or less. The orifice diameter d is chosen as 0.1 mm, so that the peak absorption coefficient (resonance absorption) is at 1000 Hz, and the higher sound frequencies may be included in the absorption region. Is it possible for a single MPP to cover the whole absorption region required in practice? The half-absorption limit and 0.5 absorption coefficient limit were used as criteria for comparison with different load resistances. MPP absorbers designed for use typically absorb sound over approximately two octaves, and the new absorbers with low values of k and d are found to be better for r ! 1 (relative acoustical resistance equal to the characteristic impedance "c in air), over about 2.5 octaves. This increases to around 3 octaves for r ! 2 or 3. In addition, the MPP in a reverberant sound field will absorb over a wider frequency ranges due to increased high frequency absorption. If the increase in range is enough, the absorption region will join forces with the secondary absorption regions, due to the multi-resonance property of the resonance structure, to form a continuous absorption region. The design of a single MPP absorber is given and its realisation is discussed.


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Identification and Active Feedback-Feedforward Control of Rotor

Kari M. J. Tammi


https://doi.org/10.20855/ijav.2007.12.1205


This experimental work presented demonstrates the use of identification, feedback, and feedforward methods to control rotor vibrations. The experiments were performed on a rotor test rig having a 3-kg rotor supported by journal bearings; the first bending resonance of the rotor shaft was about 50 Hz. Identification was carried out with a method taking into account the disturbances due to rotation. The method, using a reference signal generated from speed measurement, was able to discard the forced vibrations due to the mass imbalance. The active control objective was to reduce the radial response at the rotor midpoint by an electromagnetic actuator located outside the bearing span of the rotor. The feedback system was a proportional-derivative-type controller, which increased the damping of the system. A feedforward control system was constructed to work together with the feedback controller. The control methods produced a significant decrease in the midpoint responses of the rotor at sub-critical speeds. For supercritical speeds, the decrease in the responses was more modest due to the restricted control authority. The stability of the feedforward controller was studied in order to explore the relationship between the system damping and the modelling accuracy required by the feedforward control system.


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Electromechanical Coupled Vibration for Double Coupled Micro Beams

Lizhong Xu and Xiaoli Jia


https://doi.org/10.20855/ijav.2007.12.1206


In this paper, the electromechanical coupled dynamic equations for the double coupled micro beams are presented. The linearisation of the dynamic equations is made. From the linear dynamic equations, natural frequencies and vibration modes of the double coupled micro beams are investigated. The forced responses of the double coupled micro beams to voltage excitation are derived. A number of useful results are given. These results are useful in the design and manufacture of the micro-electro-mechanical systems (MEMS) with multi-coupled micro beams.


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The Tuned Liquid Column Damper as a Cost-Effective Alternative for the Mechanical Damper in Civil Engineering Structures

Franz Ziegler


https://doi.org/10.20855/ijav.2007.12.1207


The geometric analogy between the classical tuned mechanical damper (TMD) and the tuned liquid column damper (TLCD) is worked out with emphasis on the modal tuning process. To extend the frequency range of application of the TLCD, the piping system is sealed to make use of the resulting gas-spring effect on the (relative) fluid flow. Applications of the TLCD with a sealed U-shaped piping system to tall buildings and slender bridges effectively reduce dominating horizontal vibrations, equally as well as an increase of the modal structural damping. A high-rise office tower under wind loads, a skeletal structure with base isolation against seismic loads, and a footbridge under the excitation of walking pedestrians illustrate the effectiveness of the TLCD when properly attached to these main structures. In the course of the cantilever method of bridge construction, critical states are encountered in windy situations. Simulations and laboratory model testing prove that a TLCD attached to the tip of the cantilevered bridge supplies sufficient damping and thus allows longer spans. To counteract effectively structural vertical vibrations, a novel design of a pipe-in-pipe TLCD is analysed, tuned with respect to the basic mode, and built in a single span steel bridge. Converting a TLCD into smaller units subjected to fine tuning yields an even more robust control. To reduce early peaks in the response to shock load, active control of either one, TMD or TLCD, is needed to render their effect hybrid. Such an active tuned liquid column damper (ATLCD) with a controlled gas supply from a standby high pressure vessel is briefly discussed.


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