The coupled nature of vibrational energy flow in fluid filled piping systems makes its control and subsequent reduction a difficult problem. This work experimentally explores the potential of different active, passive and active/passive control methodologies for control of vibrational power flow in fluid filled pipes. Circumferential modal decomposition and measurements of vibrational power carried by individual wave types were carried out experimentally. The importance of dominant structural bending waves and the need to eliminate them in order to obtain meaningful experimental results has been demonstrated. The effectiveness of the rubber isolator in reducing structural waves has been demonstrated. Improved performance of the quarter wavelength tube and Helmholtz resonator was obtained on implementation of the rubber isolator on the experimental rig. Active control experiments using the side-branch actuator and 1/3 piezoelectric composite yielded significant dB reductions revealing their potential for practical applications. A combined active/passive approach was also implemented as part of this work. This approach yielded promising results, which proved that combining advantages of both active and passive approaches was a feasible alternative. / Master of Science

Zugl.: Berlin, Techn. Univ., Diss., 2009

Abstract The adaptive Helmholtz resonator for decreasing harmonic pulsating pressure in a low-pressure hydraulic system is presented in this study. Adaptivity is executed by both open loop and closed loop controls so that continuous -20dB attenuation of the peak-to-peak value of the amplitude of pressure is reached. The study begins by a theory review including the theory of effective bulk modulus, sound velocity measurements, adjustable dynamic vibration absorbers and control methods of dynamic vibration absorbers. The main target of the paper, the Helmholtz resonator, is presented in its own chapter, albeit it is noted to be one application of the dynamic vibration absorber. The review is completed by the analytical model of the hydraulic pipe with a T-filter or Helmholtz resonator. After the theory review, the test equipment and its characteristics are presented. The main parts of the test equipment were a main pipe and an adjustable Helmholtz resonator, which were both developed for this study. Certain properties of the hydraulic oils used were determined experimentally to increase understanding of the system. The experimental section includes sound velocity measurements, sonic bulk modulus definitions, measurements of the resonant frequencies of the adaptive Helmholtz resonator in the test equipment, and tests of the open loop and closed loop control of the resonator. Control is verified to maintain -20dB attenuating of pressure pulsations in the system. The presented Helmholtz resonator and controls are available for installation into a hydraulic system to damp out harmonic vibrations at low frequency. For example, the roll in the size press of a paper machine might become excited to vibrate at its resonance frequency after the paper wad has washed through the nip. In that case, tuned Helmholtz resonators in the hydraulic cylinders of the size press would damp out the pulsating pressures, and if the resonators are adaptive, as presented in this study, they can operate in a wider frequency range.

No description available.

The work presented in this thesis can be divided into two main subjects. First, lightweight designs for acoustic devices such as Helmholtz resonators and loudspeakers used for noise control in rocket payload compartments are developed. Second, active control using a hybrid control system (with structural and acoustic actuators) was tested experimentally. Due to the weight limitations for this application, Helmholtz resonators and loudspeakers are re-designed in order to reduce the device weight as much as possible while maintaining performance. For Helmholtz resonators, this is done by modeling the resonator for different structural shapes, wall materials and wall thicknesses using a finite element analysis software. The final design is then compared to the rigid resonators and is shown to perform effectively. These designs are then successfully applied to the full-scale fairing at Boeing facilities. In order to design a lightweight loudspeaker, a comparative approach was used. A standard 12' loudspeaker is taken as the reference loudspeaker and weight reduction solutions are applied to it while maintaining performance. The loudspeaker is characterized using mechanical, electrical and acoustical theories, and an optimization process is applied in order to minimize a defined cost function, which was taken as the total sound pressure output over a targeted frequency range per mass of the actuator. The results are used to build a lightweight loudspeaker together with a lightweight box, and the new designs are tested for comparison with the reference loudspeaker and shown to increase performance by 1.7 dB over 60-200 Hz band while reducing the mass by 78%. The second part of this thesis investigates the performance of a hybrid active control treatment featuring distributed vibration absorbers (DAVAs) and loudspeakers applied on a scale payload fairing. Several aspects such as causality, reference signals, and maximum controllable levels of this feedforward control scheme are the subjects of analyses. The results show that this active control approach can achieve significant amount of interior noise attenuation, and the total actuator weight required to control an external level of 138 dB can be reduced to 9.2kg using lightweight loudspeakers. However, it is shown that the attenuation levels can still be improved further by actuator positioning that gives more effective coupling of the actuators with the structural and acoustic modes and by using multiple references for the control system. / Master of Science

Zugl.: Aachen, Techn. Hochsch., Diss., 2007.

A new passive treatment to reduce sound transmission into payload fairing at low frequency is investigated. This new solution is composed of optimally damped vibration absorbers (DVA) and optimally damped Helmholtz resonators (HR). A fully coupled structural-acoustic model of a composite cylinder excited by an external plane wave is developed as a first approximation of the system. A modal expansion method is used to describe the behavior of the cylindrical shell and the acoustic cavity; the noise reduction devices are modeled as surface impedances. All the elements are then fully coupled using an impedance matching method. This model is then refined using the digitized mode shapes and natural frequencies obtained from a fairing finite element model. For both models, the noise transmission mechanisms are highlighted and the noise reduction mechanisms are explained. Procedures to design the structural and acoustic absorbers based on single degree of freedom system are modified for the multi-mode framework. The optimization of the overall treatment parameters namely location, tuning frequency, and damping of each device is also investigated using genetic algorithm. Noise reduction of up to 9dB from 50Hz to 160Hz using 4% of the cylinder mass for the DVA and 5% of the cavity volume for the HR can be achieved. The robustness of the treatment performance to changes in the excitation, system and devices characteristics is also addressed. The model is validated by experiments done outdoors on a 10-foot long, 8-foot diameter composite cylinder. The excitation level reached 136dB at the cylinder surface comparable to real launch acoustic environment. With HRs representing 2% of the cylinder volume, the noise transmission from 50Hz to160Hz is reduced by 3dB and the addition of DVAs representing 6.5% of the cylinder mass enhances this performance to 4.3dB. Using the fairing model, a HR+DVA treatment is designed under flight constraints and is implemented in a real Boeing fairing. The treatment is composed of 220 HRs and 60 DVAs representing 1.1% and 2.5% of the fairing volume and mass respectively. Noise reduction of 3.2dB from 30Hz to 90Hz is obtained experimentally. As a natural extension, a new type of adaptive Helmholtz resonator is developed. A tuning law commonly used to track single frequency disturbance is newly applied to track modes driven by broadband excitation. This tuning law only requires information local to the resonator simplifying greatly its implementation in a fairing where it can adapt to shifts in acoustic natural frequencies caused by varying payload fills. A time domain model of adaptive resonators coupled to a cylinder is developed. Simulations demonstrate that multiple adaptive HRs lead to broadband noise reductions similar to the ones obtained with genetic optimization. Experiments conducted on the cylinder confirmed the ability of adaptive HRs to converge to a near optimal solution in a frequency band including multiple resonances. / Ph. D.

The purpose of this report was to investigate how an intake for a battery-powered leaf blowershould be designed to reduce sound emissions. A reduced sound emission improves both theworking environment for the user but also reduces disturbances to the surroundings. The workbegan with a market analysis of different leaf blowers’ intakes. Other industries and differenttypes of noise reduction systems were also investigated. The market analysis stated that animplementation of Helmholtz resonators in an intake has great potential to reduce the tonalsound that a battery-powered leaf blower emits. As the sound image of a battery-powered leafblower is very tonal, the idea with resonators was to reduce the tonal spikes to achieve a lowersound level. If the work could prove that the resonators provide a high effect in the area aroundthe tonal sound spikes, then with some optimization, the resonators could be adapted and lowerthe sound equally at these frequencies. The work then progressed with testing whetherresonators work effectively. The result of the test showed that resonators can dampenfrequencies with high power, which led to the work continuing with a Concept Generation. The concept generation resulted in four different concepts that all had different placements andimplementations of resonators in the intake. These concepts were tested again and comparedto a reference that did not have any sound dampening features. From this test, two conceptsperformed at a high level, which were Back Cone and Big and Large, and these were furtherdeveloped. Further development produced four different variants of Big and Large. These werevariants with different numbers of resonators. Back Cone was produced in three differentvariants with different sizes of resonators. The purpose of these variants was to investigate howthe size and number of resonators affect the sound dampening ability. These variants weretested according to ISO-22868:2021. The final test revealed that four prototypes achieve therequired value for sound minimization. However, several external parameters emerged that affected the test results. One parameter was likely a floor reflex between the leaf blower and themicrophone directly behind the intake. The influence of the external parameters leads to thecredibility of the test results being unclear and thus these prototypes cannot be approved. During the same test, a speaker was used as the sound source instead of the fan. In this test, itappears the same four prototypes achieve a sound-minimizing effect that achieves the requiredeffect in the same range as in the tests with a fan as a sound source. Even in this test, there wereseveral external parameters that affected the result and therefore the prototypes could not beapproved with certainty according to the requirements specification. The project can be summed up by stating that resonators have great potential to reduce thetonal sound that a battery-powered leaf blower generates. The project has also shown howdifferent placements of resonators affect the sound dampening effect. During test two, a prototype emerged that can be approved according to the requirement specification as itperformed 17,5 dB maximum sound dampening effect right above the tonal frequency and sixdB at the right frequency. The project cannot determine with certainty if the variants of intakeused in the final tests meet the requirement specification due to the external parameters thataffected the results. However, the project can state that the resonators have achieved an effectin all tests. The maximum effect achieved was analysed in test one and was 27 dB soundminimization. The project ends by proposing further work such as investigating how an intakewith resonators should be designed to be manufactured in mass production. The influence ofthe thickness of the intakes surface boundary layer on the effect of the resonator should also beexamined, as well as the distance of the resonator to the fan and the outer edge of the intakeshould be examined together with the several other points presented in the chapter Discussions.

We examine the tides in the Kitikmeot Sea using year-long time-series from moored instrumentation in Dease Strait, and a 3D barotropic numerical tidal model of the region. The in-situ data show strong tidal damping during wintertime seasonal sea ice cover, with a 50-60% reduction in M2 and K1 tidal elevation and 65% reduction in M2 and K1 tidal velocities at the sea ice maximum. We hypothesize the damping largely occurs in Victoria Strait, the eastern gateway of the Kitikmeot Sea, where tidal-induced ridging causes thick, rough ice to accumulate over its shallow sill. Using the numerical model, FVCOM, we independently vary sea ice friction and sea ice thickness, and show that the observed wintertime tidal damping likely requires both very rough ice and a partial sea ice blockage in the sill region. Analysis of the model shows different dynamics and dissipation of the dominant M2 and K1 tides. Both M2 and K1 tides are dominated by the Atlantic tides entering through Victoria Strait. Arctic tides, entering from the west, have a minor, but significant, contribution to the M2 tide. Overall, the K1 tide, after 19% dissipation in Victoria Strait and 24% in adjoining bays, propagates far into the region and behaves as a Helmholtz resonator in Dease Strait and Coronation Gulf. In contrast, 92% of the M2 tidal energy does not reach Dease Strait because, in addition to dissipation in Victoria Strait (29%), it is significantly diverted into adjoining bays and around an amphidrome in eastern Queen Maud Gulf. The K1 tide, with double the wavelength of the M2 tide, is less diverted. / Graduate / 2020-07-22

L’objectif de ce travail est le développement de dispositifs de contrôle de la diffusion des ondes acoustiques à l’aide de surfaces hétérogènes, pour des applications à l’acoustique urbaine. Pour remplir cet objectif, deux méthodes sont employées. La première à l’aide d’une métasurface, la direction d’une onde réfléchie est contrôlée. La seconde étude concerne l’influence d’un traitement inhomogène aux parois d’un guide ouvert sur les effets conjoints ou compétitifs d’absorption, de confinement et de rayonnement de l’onde. Nous montrons expérimentalement la possibilité de dissimuler un objet disposé sur un mur pour une onde acoustique audible. Pour y parvenir, une métasurface composée de différents résonateurs de Helmholtz est conçue et est réalisée de façon à être la plus fine possible. Ces travaux sont réalisés dans le domaine fréquentiel mais également dans le domaine temporel, ce qui permet de mettre en avant le caractère large bande de la métasurface. Il est démontré numériquement et expérimentalement que la direction des ondes réfléchies peut être contrôlée. Enfin la dernière partie est consacrée à l’influence d’une paroi hétérogène sur la propagation d’une onde acoustique à l’intérieur d’une rue. Une rue pouvant être assimilée à un guide d’onde ouvert engendre donc des modes de propagation complexes, dus aux pertes par rayonnement. La présence d’un matériau poreux sur les parois d’un guide vient perturber fortement la localisation spatiale des modes, ce qui les rend plus ou moins fuyants. / The aim of this thesis is to develop a scheme for controling the propagation of acoustic waves using heteregenous surfaces. Its results can be applied in the field of urban acoustic. The thesis is composed of two sections, each of them employing a different method. The first section focuses on controling the direction of a reflected wave, using a metasurface. The second concentrates on the influence of an inconsistent treatment to the side of an open waveguide on the wave joint and competitive effects of absorption, confinement and radiation. Part one provides experimental evidence that it is possible to conceal an object placed on a wall from an audible acoustic wave. To prove it, the thinest possible metasurface was constructed with Helmholtz resonators. The experimental results were compared to a numerical study realized with finite elements. This work was made in both temporal and frequency domains, allowing to point out the wide frequency characteristics of the metasurface. The numerical and experimental results show that the direction of a reflected wave can, indeed, be controled. Part two analyse the impact of a heterogeneous wall on the spreadinf of an acoustic wave in a street. Due to radiation losses, the street produces complex ways of propagation. The presence of a porous material on a waveguide‘ side deeply disrupt the spatial location of these waves, making them more or less fleeting. In particular, depending on the position of the material in the street, certain waves will be more confined to the inside of the street, radiating less towards the open external environment. They are consequently, less cushioned.