1 

A model for flowinduced noise of Helmholtz resonatorlike cavities
This paper presents a single prediction model for the noise generated by a turbulent boundry layer flow grazing on the opening of a Helmholtzresonator like cavity. The prediction model is validated by comparison with an experimental study. The measured spectra inside the cavity are correctly predicted by the model.

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2 

Numerical modeling of alumpedacoustic source actuated by a piezoelectric stack device that is driven by a switching amplifier
This paper studies an acoustic source with a relatively small thickness and high bending stiffness. Piezoelectric actuators are used to drive the moving component of the acoustic source. In the current study, a lumped model is used to represent the acoustic source that is excited by a piezoelectric stacked actuator. The equivalent electrical circuit of the lumped acoustic source can directly be connected to the electrical circuit of a switching amplifier. Various methods are used to estimate the radiation impedance of the acoustic source. The effectiveness of these methods is investigated when they are used in combination with the equivalent electrical circuit of the lumped acoustic source, the actuator and the amplifier. Finally, result of a numerical finite element simulation is compared with the results of the fullylumped equivalent electrical circuit. Copyright © by International Institute of Acoustics & Vibration. All rights reserved.

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3 

An acoustic radiator with integrated cavity and active control of surface vibration
This paper presents a method to realize an acoustic source for low frequencies with relatively small thickness. A honeycomb plate structure which is open on one side combines the radiating surface and the major part of the air cavity. The vibration of the plate is controlled with a decentralized feedback controller. The fundamental resonance caused by the mass of the plate and the compliance of the air cavity are controlled, as well as the firstorder bending mode, while avoiding possible instabilities due to the fluidstructure interaction. The smooth and well defined frequency response enables robust feedforward control for further response equalization. The influence of different actuation principles on the overall system efficiency is compared.

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4 

Modelling propagation of sound emitted by miniairguns
A modelling approach is described in which timedomain underwateracoustic source functions are generated using a model that predicts the motion of airbubbles generated by miniairguns operated at various pressures. These timeseries are Fouriertransformed to produce frequencydomain descriptions of the source. These are combined with complex transfer functions describing the propagation of sound in shallowwater waveguide geometries so that source spectra can be converted to received spectra by a simple multiplication process for specified source and receiver positions. Received spectra can be inverseFourier transformed to produce timeseries of received signals. Calculations are performed to yield three acoustic variables: one pressure and two particle velocities (in the radial and vertical directions). Timeseries are shown for a test environment in which total acoustic fields are made up of contributions from waterborne and bottominteracting paths. © 2018 80th EAGE Conference and Exhibition 2018 Workshop Programme.

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5 

The acoustic source strength of highenergy blast waves: combining measurements and a nonlinear model
In the densely populated area of the Netherlands, the objective of the Netherlands Ministry of Defence is to find an optimal balance between military training and the impact on the surrounding civilian community. A special case concerns large weapons, such as artillery or demolitions, which create highenergy blast waves. These waves have a low frequency content, typically between 15 and 125 Hz, and can propagate over large distances. As a result it is a relative important cause for annoyance. The challenge is to determine accurately the acoustic source strength. This source is then used in a dedicated model for military training facilities, to calculate rating sound levels around the facility for different training situations and to calculate the effect of measures. This model uses a linear sound propagation and an equivalent linear source strength. The source strength is measured at a large distance, where the sound propagates linearly. As a consequence the ground and the meteorology have an important effect, and one has to correct for it. A more efficient approach has been tested, where the sound pressure measurements have been performed close to the source, at typically less than 10 meters distance. The linear source strength is then calculated by applying a nonlinear propagation model. The results are compared to the conventional measurement method. Another advantage of applying the nonlinear model, and the nonlinear source strength, is that the effect of mitigation measures close to the source can be determined.

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6 

Direct measurements of acoustic damping and sound amplification in corrugated pipes with flow
The flowinduced pulsations in corrugated pipes result from a feedback loop between an acoustic resonator and the noise amplification at each shear layer in the axisymmetric cavities forming the corrugations. The quality factor of the resonator is determined by the reflection coefficients at the ends of the corrugated pipe and by the damping in the pipe. In this work, the damping of acoustic waves in a set of smooth and corrugated pipes is measured by a direct method. For these measurements, the tested pipes are placed between two measuring pipes equipped with flushmounted pressure transducers to allow reconstruction of the acoustic waves with the twomicrophone method. Loudspeakers are used to generate acoustic waves, and anechoic terminations allow near reflectionfree conditions. The tests are done in air without flow and with flow velocities up to 60 m/s. The results for the corrugated pipes allow to investigate the influence of the corrugations on the damping. Two cases are considered: Without flow, the viscothermal damping is increased for the corrugated pipe compared to a smooth pipe of same diameter. When there is a flow in the pipe, the damping results depend strongly on the flow velocity. At certain frequencies which depend on the flow velocity, the damping increases or decreases. The regions of increase or decrease are shown to be at constant ranges of Strouhal numbers. The decrease of the damping can in some cases be such that acoustic waves are amplified through the corrugated pipe. This corresponds to the acoustic source behavior of the shear layers. The measured amplification is compared to the computed source term due to the axisymmetric cavities.

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7 

Numerical modeling of electricalmechanicalacoustical behavior of a lumped acoustic source driven by a piezoelectric stack actuator
The present work describes the electrical, mechanical and acoustical behavior of a thin honeycomb structure as an acoustic source. The acoustic source has to operate in the low frequency, quasistatic regime and is driven by a piezoelectric stack actuator. In addition, a twoway energy flow between the actuator and a connected amplifier is investigated. In particular, the effectiveness of energy recovery from the reactive components of the acoustic source is evaluated to improve the overall radiation efficiency. A lumped model is used to represent the acoustic source that is excited by a stacked piezoelectric element. The required power supply and resulting radiation efficiency are evaluated when a conventional nalogue amplifier is used. The result is compared to the case in which some parts of the stored power are recovered and sent back to the connected switching amplifier. The study reveals 20% increase in the radiation efficiency and more than 80% decrease in the amount of required input power through recovering the reactive power in the system.

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