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This paper extends the simplified airport noise model Rapid Aviation Noise Evaluator (RANE) [Torija et al., Journal of the Acoustical Society of America, Vol. 141, No. 2, 2017, pp. 981–995], adding capability of including fully nonisotropic noise sources. This extended tool, RANE v2, is developed as a part of multidisciplinary acoustic assessment of novel aircraft, in order to produce ground contours around airports and helipads. Version 2 extends the capability of RANE to accommodate predictions of future air vehicles implementing propulsion systems solution with inherent directional properties. The model uses three-dimensional noise emission surfaces around a series of discretized segments that represent the aircraft flightpath. The main inputs are the sources’ sound power level, the distance from the flightpath at which a level is observed, and the source three-dimensional directivity. The directivity function may take analytical or numerical form, allowing for experimental data inputs. This paper demonstrates the use of spherical harmonics as a form of directivity function with a closed-form analytical solution for calculating the noise exposure contours. Results and comparison against the Federal Aviation Administration’s Aviation Environmental Design Tool module for helicopter community noise indicate that exposure contour coordinates can be estimated for high and low noise exposure levels. The incorporation of source directivity allows for the assessment of lateral attenuation, engine installation effects, and transition operations (for vertical to horizontal flight and vice versa) via the assumption of individual source directivities and, therefore, complex noise surfaces. As a consequence of the analytical nature of the model, low computational requirements allow for fast exploration of the design space and parametric studies, with minimal input requirements. The capabilities of RANE v2 are demonstrated by predicting noise footprints for three helicopters, each of different size, performance, and directivity characteristics.

In this study, the simulation and experiment of a thermoacoustic Stirling heat engine (TASHE) with a phase-adjuster are demonstrated. The phase-adjuster is used as a tool to tune the acoustic field of the TASHE in order to maintain the high performance of the system. The core components of the TASHE comprising an ambient heat exchanger, regenerator, hot heat exchanger, and feedback pipe, are located in the torus section. Besides, there is a resonator pipe, connecting to the torus, which acts as compliance to maintain the system stability, and the phase-adjuster is installed at the end of the resonator. Firstly, the TASHE is modeled by DeltaEC to search for the optimal configurations of the prototype. Due to the variations of acoustic load or operating conditions from the design criteria, the engine absolutely cannot provide maximum output power. The proposed phase-adjuster could keep its high efficiency by re-matching the frequency. In the experiments, the self-excited and steady-state temperatures of the TASHE are around 755 K and 670 K, respectively. Here, this TASHE can provide an acoustic power of up to 40 W, and an energy conversion efficiency of 12.03%. In comparison, there is a reasonably good agreement between the measured and DeltaEC simulated results. This can reflect on the preciseness of the proposed model. In the case of the TASHE operated under the off-design condition, these scenarios certainly drop its efficiency. Consequently, the role of the phase-adjuster in improving its performance by tuning the acoustic field is presented here.

Holographic acoustic field has shown great potential for non-contact robotic manipulations of millimeter or sub-millimeter size objects to effectively deliver acoustic power. The latest technology for generating dynamic holographic acoustic field is through phased transducer array, where relative phases of emitted acoustic waves from transducers are independently controlled to modulate the acoustic interference field. While the forward kinematics of a phased array based robotic manipulation system is simple and straightforward, the inverse kinematics (i.e., the mapping from a given holographic acoustic field to array phases for control purpose), however, is mathematically non-linear and unsolvable, presenting challenges in developing wider applications of holographic acoustic field for robotic manipulation. Considering, thus far, there are still no effective solutions reported, the authors put intensive efforts to solve this problem using a machine learning approach, which we refer to as AcousNet. Experimental results demonstrate the effectiveness of the proposed method for dynamic holographic acoustic field generation from phased transducer array.

This article presents a cross-domain model that defines and sets the acoustic signal generation process due to the fast dose deposition induced by a proton beam penetrating an energy absorber. The proposed model reduces the complex iono-acoustic energy transformation process to a simple impulse-response of a linear-time-invariant (LTI) system and is validated by comparing LTI output simulation results (in both time and frequency domain) with experimental acoustic signals emitted by a physical proton beam at 20 MeV and 70-120-ns pulse time width. Thanks to the intrinsic simplicity of the system, it is possible to predict and estimate the effective acoustic power at the sensor and the resultant beam range measurement precision. More importantly, the information coming from LTI simulations provides well-defined methodologies that allow to increase acoustic signal amplitude from +3 up to +9 dB and to improve the measurement precision of the beam range localization (up to +/ - 0.3 mm versus +/ - 0.6 mm state of the art for 200-MeV energy and 75-mGy total dose).

Diplomski rad sadrži detaljni izvod izraza za akustičnu snagu, popračen skicama i slikama, za primjer ploče jednostavno oslonjene na svim krajevima. Akustička snaga prikazana je grafom kao funkcija frekvencije. Snaga je dobivena koristeći izvedene izraze koji su implementirani u računalski program, a riješeni su numeričkim metodama u programskom jeziku Python. Prikazana je razlika rezultata s obzirom na drugačije ulazne parametre, tj. različite dimenzije ploče, različite materijale ploče te različite pozicije sile., The thesis contains a detailed derivation of the expression for acoustic power, accompanied by sketches and pictures, for the example of a plate simply supported at all ends. Acoustic power is graphed as a function of frequency. The power was obtained using derived expressions that were implemented in a computer program, and were solved by numerical methods in the Python programming language. The difference in results with regard to different input parameters, i.e. different plate dimensions, different plate materials, and different force positions, is shown.

Spectral Doppler ultrasound velocimetry involves systematic analysis of the spectrum of frequencies that constitute the Doppler signal. This chapter presents a general perspective on Doppler signal anlyses and describes the spectral Doppler ultrasound devices commercially available for clinical use. They include continuous-wave (CW) Doppler, pulsed-wave (PW) Doppler and duplex Doppler devices. Within the realm of obstetric usage, the application needs are diverse and require various choices of equipment. For example, fetal Doppler echocardiography requires advanced duplex ultrasound instrumentation, which combines the capabilities of high-resolution two-dimensional imaging with the PW Doppler mode and an acoustic power output appropriate for fetal application. For umbilical arterial hemodynamic assessment, simpler, substantially less expensive CW Doppler equipment with a spectral analyzer may be sufficient. It is essential therefore that one develop a basic understanding of the implementation of Doppler ultrasound technology.

A new type series inertance tube pulse tube refrigerator is introduced. It includes series cold heads. Each cold head's pulse tube connects an inertance tube as phase shifter. The acoustic power of the first pulse tube is partially used by the first inertance tube for phase shifting, while the remaining is used for generating additional cooling power. It is high efficiency with multi refrigeration temperature. A two-dimensional axisymmetric model was introduced for simulation by the commercial CFD software ANSYS Fluent. The numerical results show that it has high potential for getting higher efficiency. A protype series inertance tube pulse tube refrigerator was made and tested. Its efficiency is higher than the ordinary inertance tube pulse tube refrigerator at higher temperature range. At 232K, the efficiency is increased by 10%. The results show that it is valuable for further development.

This paper studies the vibration and noise analysis of flux-modulation double-stator electrical-excitation synchronous machine (FM-DSEESM) in both no-load mode and on-load mode by using the mode superposition method. First, the theoretical excitations of vibration and noise, namely the radial electromagnetic force harmonics, are deduced and classified by their sources. Second, the finite element analyses (FEA) of these radial electromagnetic force harmonics are analyzed via Fourier decomposition and compared to their theoretical results for validation. Third, to perform the vibration and noise analysis, the modal test is conducted, and the modal FEA model is established based on the tested modal results. Also, the radial electromagnetic forces are coupled with the modal FEA model to conduct the vibration and noise analysis, and the responses of sound power levels in A-weighting are calculated. By using the modal participating factors, the main contributing modes are determined in the vibration and noise responses.

The aim of this work is to develop a prototype of a modular heat pump with focusing on noise reduction by separating the two main noise sources of a heat pump: the compressor and the fan. The first part of this master thesis work consisted of developing two modules, the compressor and the fan modules from a monobloc BOSCH heat pump. The compressor, the refrigerant circuit, the inverter, the expansion valves and the electronic board were installed into the compressor module while the evaporator and the fan were installed in the fan module. Different rapid prototyping technologies were used in to order to obtain the needed components, such as, laser cut, hot wire cut and metal bending. As a result, a functional modular heat pump was developed presented as a solution that allows the end user to manage the preferred location to install both modules and consequently the two main noise sources. The second part included the development of a second sound barrier around the compressor module. Hence, an acoustic enclosure was built. The development of the enclosure was based in three acoustic principles: high acoustic impedance gradient between materials, sound leakage insulation and absence of structure-borne noise. Consequently, a study was conducted about the best material to use taking into account several high density materials. The material chosen was WPC (Wood plastic composite), a composite of rice husk and PVC (Polyvinyl chloride) scraps. In addition, an air gap between the enclosure and the compressor module was designed in order to avoid structure-borne noise. Sound measurements were performed to the compressor module with and without the acoustic enclosure around it. The compressor module solely, presented values of sound power of 50,8dB and 53,3dB in night and max day modes, respectively, correspondingly a reduction of 6,7dB and 10,7dB when compared with the initial monobloc unit. However, the most significant result was obtained with the WPC enclosure, which presented values of 35,2dB and 35,8dB in night and max day modes, respectively. These values demonstrate an excellent behavior of the developed enclosure with regard to sound reduction since they are 22,3dB and 28,2dB lower than the values of the initial monobloc unit. On the other hand, the developed solution presented some tonality problems which may increase the level of sound annoyance. Lastly, a user experience was realised what proved to be very useful since it provided valuable information about market chances, preferred places to install the modules, positive and negative aspects and improvement suggestions. O objetivo deste trabalho é desenvolver um protótipo de uma bomba de calor modular com foco na redução de ruído através da separação das duas principais fontes de ruído: o compressor e a ventoinha. A primeira parte deste trabalho de dissertação de mestrado consistiu no desenvolvimento de dois módulos, o módulo do compressor e o modulo do ventilador, partindo de uma bomba de calor BOSCH do tipo monobloco. O compressor, o circuito refrigerante, o inversor, as válvulas de expansão e a placa eletrónica foram instalados no módulo do compressor enquanto que o evaporador e o ventilador foram instalados no módulo do ventilador. Diferentes tecnologias de prototipagem rápida foram utilizadas para obter os componentes necessários, como corte a laser, corte com fio quente e quinagem de metal. Como resultado, uma bomba de calor modular e funcional foi desenvolvida e apresentada como uma solução que permite ao utilizador final gerir o local preferido para instalar ambos os módulos e, consequentemente, as duas principais fontes de ruído. A segunda parte contempla o desenvolvimento de uma segunda barreira de som em torno do módulo do compressor. Assim, foi construído um invólucro acústico. O desenvolvimento deste baseou-se em três princípios acústicos: gradiente de alta impedância acústica entre materiais, isolamento de fugas de ondas sonoras e ausência de ruído transmitido por componentes rígidos (structure-borne noise). Consequentemente, foi realizado um estudo sobre qual o melhor material a utilizar tendo em conta diversos materiais de alta densidade. O material escolhido foi o WPC (Composto Plástico-Madeira), um composto de casca de arroz e restos de PVC (Policloreto de vinila). Além disso, um espaço de ar entre o invólucro e o módulo do compressor foi projetado a propagação de ruído proveniente de componentes rígidos (structure-borne noise). Foram realizadas medições de som ao módulo do compressor com e sem o invólucro acústico. O módulo do compressor apresentou valores de potência sonora de 50,8dB e 53,3dB para os modos noturno e máximo diurno, respectivamente. O que significou uma redução de 6,7dB e 10,7dB quando comparado com a unidade monobloco inicial. No entanto, o resultado mais significativo foi com a utilização do invólucro em WPC que apresentou valores de 35,2dB e 35,8dB para os modos noturno e máximo diurno, respectivamente. Estes valores demonstraram um excelente comportamento do invólucro desenvolvido no que diz respeito à atenuação de ruído, uma vez que apresenta valores 22,3dB e 28,2dB menores que os valores da unidade monobloco inicial. Por outro lado, a solução desenvolvida apresentou alguns problemas de tonalidade o que poderá significar um aumento do nível de incómodo sonoro. Por fim, foi realizada uma experiência de utilizador que provou ser batante útil, pois forneceu informações valiosas sobre oportunidades de mercado, locais preferenciais para instalação dos módulos, aspectos positivos e negativos e sugestões de melhorias. Mestrado em Engenharia Mecânica

This study performs two recent methods on the nonlinear long-short wave interaction system (NLSWIS) to obtain novel formulas of solitary solutions representing an optical field that does not alter by multiplication a sensitive balance between the different terms impacts in the medium. Elastic medium is defined as the medium that can adjust in the figure due to a deforming strength and come back shape to its original form when the force eliminates. The wave is produced by vibrations that are a consequence of acoustic power. Also known as the sound wave, or acoustic wave. For examples of elastic mediums are air, water and so on. The improved $$\left( \frac{G'}{G}\right)$$ expansion method and novel $$\left( \frac{G'}{G}\right)$$ expansion method are implemented, which are considered as two general methods in this field. Applying these methods to many nonlinear partial differential equations gives diverse and different forms and configuration solutions that allow users of those two methods a large number of solutions that provide broader scope to use.

The ongoing robotic revolution in oceanic science puts new requirements on sonar technology. Small platforms require compact multi-purpose transducers, with strict requirements on power consumption and heat dissipation. Introducing single crystal ferroelectrics as the active material of the transmitter can be one way of meeting the new requirements. The large electromechanical coupling coefficient of single crystals can enable an extension of the usable frequency band compared to conventional PZT. For the applications considered in this work, the usable frequency band is restricted by both the transmitted acoustic power and the reactive electrical power. Single crystals as the active materials can double the usable band, but the acoustic matching required for this can be difficult to obtain in practice. We investigated an air-backed, plane 1-3 composite transducer, matched to water by acoustic matching layers. For many applications, the diversity provided by a large usable frequency range is more important than a flat acoustic power response, and the transducer can be used far beyond the -3 dB limit. We defined the usable band by requiring maximum -12 dB ripple in transmitted acoustic power and maximum 50% reactive power. The matching layers were optimized to maximize the usable band according to this definition, in contrast to the conventional approach where matching layers are optimized for maximally flat response. Under the chosen definitions, our modelling showed that with a single crystal as the active material we could achieve 188% usable frequency band relative to the resonance frequency, compared to 121 % for a PZT.

With the development of the implantable neural recording system, how to power the device has already became an significant problem. Researchers have come up with several methods such as utilizing battery. However, these two early methods both have weakness which limit the further development of the implantable medical device (IMD). Thus, new methods emerge to solve these problems, which serve as ideal substitute for the battery, including the radio frequency (RF) radiation, inductive power transfer (IPT) and acoustic power transfer (APT). This paper mainly focuses on these three methods and discusses their advantages and the disadvantages.

International audience; In the absence of natural convection, efficient heat transfers rely on externally forced flows. Generating such flows with acoustic waves rather than mechanical fans would enable remote locations to be cooled using virtually infinite lifetime transducers. This outlook is reinforced by the recent discovery that standing acoustic waves drive streaming flows of much higher velocities if the background medium is inhomogeneous. This regime of streaming is investigated experimentally in a cavity filled with stably-stratified air in which horizontal sound waves are found to significantly enhance heat transfers. The additional heat flux scales as the square of the input acoustic power for low amplitude waves and increases with the air stratification. These two features qualitatively match theoretical predictions, although corrections possibly ascribed to gravity are observed.

An advanced duct microphone array analysis based on a user friendly iterative Bayesian Inverse Approach (iBIA) has been successfully applied to assess the modal content and the Sound Power Level of fan/outlet guide vanes (OGV) broadband noise. It is shown that iBIA provides considerable reduction of artefacts associated to array sidelobes leading to an improved dynamic range in the reconstructed mode distribution plots. The method also benefits from a data-dependent and fully automated regularisation procedure. Moreover, it allows to control the sparsity degree of the reconstructed modal content: thus different sparsity levels can be tailored for specific noise components, such as fan tones or broadband mode components. It also applies directly to the cross-spectral matrix of measurements, thus advantage can be taken of averaging the data over several snapshots whenever the assumption of stationarity and ergodicity holds. These features facilitate its application to industrially relevant configurations such as the UFFA fan test rig that was operated by AneCom AeroTest in order to deliver reference data for broadband noise modelling in the frame of the European Project TurboNoiseBB. The iBIA has been applied to perform a full azimuthal and radial mode detection as well as axial-wavenumber and azimuthal decompositions. Two configurations of fan/OGV-spacing along two working lines have been tested. The radiated noise has been measured by in-duct microphone arrays located at the intake and downstream of the fan module. Longer rotor-stator gaps are shown to reduce high frequency levels at approach conditions for both inlet and exhaust noise over a large frequency range. The benefit of long gaps tends to vanish for higher fan speeds and even leads to noise increase at high frequencies. Duct sound power estimates obtained from a full azimuthal and radial decomposition have been compared to estimates from both the wavenumber and the azimuthal decompositions, the latter two requiring further assumptions regarding the modal energy distribution. Based on this approach, the widely used equal energy per mode and equal energy density per mode assumptions have thus been evaluated on a large industrially relevant experimental data set.

We study the balance of outgoing and incoming power for acoustic wave scattering by a cascade of flat blades in uniform subsonic mean flow. In nonzero mean flow the Kutta condition at the trailing edge of the cascade plates leads to the production of vorticity in the form of unsteady vortex sheets attached at the trailing edges, which results in a hydrodynamic power flux and the exchange of energy between the unsteady vorticity and the acoustic field. Although the scattering problem itself has been subject to much previous research, a comprehensive study of the composition of the outgoing energy flux for cascade scattering has received little previous attention. As an application of our detailed study we show how the time-averaged energy balance can be used to explain certain symmetries of the field in the angle of incidence with respect to the cascade face when there is no mean flow and the effect of zero acoustic reflection at certain angles of incidence with mean flow. We also show that, depending on the angle of incidence of the acoustic wave and the velocity of the mean flow, the vortex sheets can either absorb or generate acoustic energy. This can, for certain parameter values, result in a net increase in outgoing sound power over the incident sound power, as we demonstrate on a number of numerical examples that are computed using the semi-analytical solution described in recent work by Maierhofer and Peake (J. Sound Vib., vol. 481 (2020), 115418).

This study investigates the problem of sound radiation from a thin vibrating annular plate. The plate is supported elastically at its circumference and embedded into the bottom of a circular cavity in a rigid plane. The selected model provides results covering the occurrence of sound radiation from the thin annular plates with any arbitrary boundary configurations. The cavity and the half-space are filled with air, and the air in the cavity plays the role of the compatibility layer. Therefore, the vibrations of the plate are coupled with that of air. Application of the compatibility layer allows the decomposition of the coupled problem of sound radiation to two problems: one for the vibration of the plate coupled with the cavity and the other for the sound radiation from the cavity to the half-space. Consequently, the plate is not coupled to the half-space directly. Therefore, the modal impedance coefficient of the plate is not required to solve the problem. The coefficients of the cavity are sufficient for the calculations, and they can be calculated significantly faster than in the case of performing a numerical integration while simultaneously satisfying the assumed accuracy by using radial polynomials. This also makes it possible to simplify numerical calculations. Finally, the total acoustic power, the acoustic pressure, and the vibration velocity have been obtained using the proposed approach and analyzed numerically. In the specific case when the cavity depth is reduced to zero, the nondimensionalized added virtual mass incremental as well as the modal impedance coefficients of the plate are calculated. Using the obtained results, the resonant frequencies can be accurately determined within a broad frequency band. The described method can be used for calculating the sound radiation for any arbitrary boundary configuration of the thin annular plates. In addition, the proposed approximation reduces the time required for the numerical calculations by roughly 2000 times compared to the time required for the numerical integration.

Air-coupled ultrasonic (ACU) testing has proven to be a valuable method for increasing the speed in non-destructive ultrasonic testing and the investigation of sensitive specimens. A major obstacle to implementing ACU methods is the significant signal power loss at the air–specimen and transducer–air interfaces. The loss between transducer and air can be eliminated by using recently developed fluidic transducers. These transducers use pressurized air and a natural flow instability to generate high sound power signals. Due to this self-excited flow instability, the individual pulses are dissimilar in length, amplitude, and phase. These amplitude and angle modulated pulses offer the great opportunity to further increase the signal-to-noise ratio with pulse compression methods. In practice, multi-input multi-output (MIMO) setups reduce the time required to scan the specimen surface, but demand high pulse discriminability. By applying envelope removal techniques to the individual pulses, the pulse discriminability is increased allowing only the remaining phase information to be targeted for analysis. Finally, semi-synthetic experiments are presented to verify the applicability of the envelope removal method and highlight the suitability of the fluidic transducer for MIMO setups.

The current paper presents the broad-range sensitivity analysis and uncertainty quantification for the performance of a thermoacoustically-driven pulse tube cryocooler to make it robustly perform using a low-grade heat source. Accordingly, the onset and steady-periodic operations of the cryocooler are numerically simulated using Rott’s one-dimensional equations. The effects of increasing hot/cold core’s number, engine’s phase-shifter length, and mean pressure on the cooling features and hot-source temperature are investigated. In this regard, a looped-branched, octa-engine, four-stage cryocooler is proposed that operates with a steady temperature difference of 132 K at a no-load cold temperature of 77 K. In contrast to previous findings, the results show that increasing the number of stages or the charge pressure does not necessarily improve thermodynamic performance. Moreover, the uncertainty in the operation of a quad and octa-engine, four-stage cryocooler caused by the geometric and material characteristics is estimated using the Morris method. It is demonstrated that the overall operation is more sensitive to the hot-core dimensions than to the cold-core dimensions. Besides the hot-core dimensions, gas properties and solid thermal conductivity strongly influence the transient and steady-periodic features of the cryocooler, respectively. Furthermore, doubling the hot cores in the four-stage system increases the uncertainty of the acoustic power up to five times, leading to more uncertainty in the cooling features. Drawing on the results of this paper, a multi-stage cryocooler can be designed to optimize the overall performance and reliability.

Looped travelling-wave thermoacoustic electric generator (LTTAEG) can transform thermal energy to electric power using thermoacoustic effect. Based on our previous effort on multi-stage LTTAEG with side-branched and dual-opposed linear alternators (LAs) as acoustic-to-electric transducers, for a higher output electric power and thermal-to-electric efficiency, a 4-stage LTTAEG is studied numerically and experimentally in this paper. In the simulation, the performance of 4-stage looped thermoacoustic heat engine is investigated based on linear thermoacoustic theory while the performance of dual-opposed LA is studied based on a novel network model. A new coupling algorithm between the 4-stage looped TAHE and dual-opposed LA is also proposed. In experiment, with 6 MPa pressurized helium as working gas, 650 °C and 25 °C heating and cooling temperatures. A highest electric power of 6.1 kW with thermal-to-electric efficiency of 15.86% and a highest thermal-to-electric efficiency of 19.64% with electric power of 4.37 kW is obtained when the electric capacitance connected to the LA is 17 μF and the electric resistance is 40 Ω and 75 Ω, respectively. Due to the high oscillating pressure in the system, some nonlinear phenomena have caused the simulation results deviated from the experimental results, especially in acoustic power and electric power, which should be investigated and considered in the future study.

In this work, acoustic metamaterials based on the concept of extended tube are investigated. The metastructure consists of an array of units, each featuring a thin tube protruding from a perforation into a backing cavity to form a Helmholtz resonator. First, an impedance model for a single-layer extended tube structure is developed using the equivalent circuit method. For the tube in which the viscothermal effect can not be ignored, an equivalent fluid model is used to characterize its acoustic property and end corrections are introduced to take into account the flow distortion effect at the two ends. Then, two cost-effective samples are fabricated and tested using the impedance tube where good accuracy between the prediction and measurement is found. The structural simplicity and low-frequency absorption property allow the extended tube acoustic metamaterials to be utilized in scenarios where installation space for acoustic absorption materials is constrained. As an example, we demonstrate its use on a commercial kitchen hood. Standard acoustic power measurement shows its superior noise reduction performance over conventional porous materials of the same thickness.

The noise from a double-column multi-gap reactor is very loud and should be controlled. Based on different gap configurations, the gap configuration design schemes of the double-column multi-gap reactor to noise reduction are proposed. According to the positions of the gaps, the 20 gaps of the actual reactor are simplified into five gaps. Then, 31 reactor finite element simulation models are constructed respectively according to the gap position or length. A three-dimensional (3-d) electromagnetic–mechanical weak-coupled model is used to study the average surface vibration velocity of each reactor core simulation model with different gap configurations. The 3-d anisotropy of magnetisation curves (BH curves) and magnetostriction curves of laminated Grain-oriented steel plates are considered. Some gap configuration design schemes are proposed on the basis of the results of simulations. Two double-column multi-gap reactor models are used for comparative testing. The test results show that the gap configuration design schemes can reduce the sound power level of the reactor by about 6.836 dB. Finally, gap configuration design schemes for noise reduction of a double-column 20-gap reactor are proposed.

Curve squeal is a highly disturbing tonal sound generated by rail vehicles like trains, metros or trams, when negotiating a sharp curve. The probability that squeal occurs increases with reduced curve radius of the track. Curve squeal noise is attributed to self-excited vibrations caused by stick/slip behaviour due to lateral creepage of the wheel tyre on the top of the rail. With respect to the large number of rolling stock units and the long lifetime of vehicles, there is an urgent need for a cheap and simple retrofitting measure to reduce curve squeal. Therefore, main objective of this paper is to investigate the potential to reduce curve squeal by means of active control in the form of dither in an efficient and robust way. Dither control has been applied in the field of mechanical engineering for systems including non-linear components. There it has been shown to suppress self-excited oscillations very efficiently. The control is an open-loop control. It consists in adding a forced vibration to the vibrational system. A time-domain model has been applied to investigate the mechanisms behind self-excited vibrations leading to curve squeal at the squealing noise rig at Chalmers University of Technology. The analysis showed, that in the presence of constant friction, the coupling between lateral and vertical direction is the driving mechanism for building up self-excited vibrations. Based on this insight, the potential of dither has been investigated. For the case considered here dither has the potential to reduce the overall kinetic energy on the wheel by more than 10 dB and on the rail by more than 20 dB. Further optimisation of dither forces with respect to the radiated sound power might increase this potential.

To improve the acoustic radiation performance of the sandwich transducer, a perforated plate is added to the radiation end of the transducer. The improved effect of the addition of a perforated plate on acoustic radiation performance of the transducer is studied. First, based on electromechanical equivalence principle, when a perforated plate is attached to a piston vibrator, the expressions of the vibration velocity of radiation surfaces, directivity index (DI) and radiated sound power are derived. Thus, the influences of the perforated plate on the sound radiation performance of the piston vibrator are analyzed. Then, the perforated plate is attached to the radiation surface of a sandwich transducer, and the electromechanical equivalent circuit of the transducer is established. Thus, the input impedance at the radiation end, vibration velocity and sound pressure level (SPL) of the transducer are derived. Further, the SPL of the transducer is calculated and the influences of the perforated plate on the transducer acoustic radiation performance with or without a matching layer are studied. And, the influences of the perforated plate on acoustic radiation performance of different operating frequency transducers are analyzed. Finally, the influences of the perforated plate structural parameters on the SPL of transducer are studied. The results show that the perforated plate with appropriate parameters could further improve the impedance matching between the input impedance and the radiation impedance at the radiation end of the transducer, thus the radiated SPL could be improved maximally. The research method is expected to applied in high-power transducer design.

As liquefied natural gas (LNG) is becoming a more and more vital role in the global energy market, the enormous amount of cold energy released during its regasification process has attracted worldwide attention. A three-stage traveling-wave thermoacoustic power generator (TWTAG) was proposed utilizing cold energy and low-grade waste heat simultaneously. Most importantly, a simplified structure was applied instead of the traditional structure with thermal buffer tubes (TBTs) and ambient heat exchangers (AHXs). Numerical results show that the system has superior performance together with a much more compact structure. In order to find the best location of the simplified system for acoustic power extraction, three different load-coupling modes were investigated separately. The three-stage TWTAG operating between 110 and 373 K is able to generate 11.4 kW and 10.8 kW electric power under hot-coupled and mid-coupled modes with 4 MPa helium gas as working gas, respectively. It is very enlightening that the cold-coupled mode can generate more than 15 kW acoustic power and shows the best performance. This work shows that thermoacoustic technologies have great potential in LNG cold energy and waste heat recovery.

Ship noise comprises mechanical noise, propeller noise, and hydrodynamic noise. The development of technology for reducing vibration and noise means that the noise of a ship’s propeller and that of its main and auxiliary engines are now controlled effectively, leaving the sea pipelines as the main source of noise. Therefore, it is very important to investigate how to measure the acoustic radiation at the nozzle of a pipeline. In this paper, a bench pipeline test system is established in a non-anechoic pool, and a method for measuring in the non-anechoic pool is proposed to solve the problem that when measuring directly in the pipeline, the results are distorted by the flow. By inserting the nozzle of the bench pipeline test system into the non-anechoic pool, the radiated sound power of the nozzle of the pipeline system is measured in different frequency bands by using a combination of near-field direct measurements and far-field spatial averages. The above measurement method is used to evaluate the effects of different noise sources in the pipeline system, given that they work independently of each other. The research results show that (i) this method can measure accurately the noise radiated at the nozzle of a pipeline system, (ii) in the pipeline system, the main sources of noise are the pump and the structural vibration that it generates, and (iii) the noise radiated at the nozzle of a pipeline system can be controlled effectively by replacing straight pipelines with hoses and reducing the number of pipeline components such as elbows.