In recent years, various novel concepts of efficient low-frequency acoustic absorbers have been proposed. The present paper investigates in detail the non-linear acoustic response of resonators coupled to plates with thin slits. A laser-cut thin plate is positioned inside the neck of a Helmholtz resonator. Various shapes of slits are considered, including a cantilever beam. This system produces a strong acoustic response in the vicinity of the resonance frequency of the beam, which can be adjusted to target low frequencies. However, this acoustic response exhibits a significant dependence on the incident sound level. In this paper, these systems are mechanically and acoustically modelled and characterised under normal incidence in order to describe and predict the non-linearities observed. To this end, a generalised expression for the resistance and reactance of such thin structures is proposed and validated against experimental data.

Despite a significant number of new structures in the past few years, MEMS loudspeaker still are not competitive in terms of performance compared to non-MEMS loudspeakers for free field applications. For industrial perspectives, a high sound pressure level on a wide frequency band is required, as well as a low total harmonic distortion. To widen the frequency range of MEMS loudspeakers, we propose to separate the actuating element from the radiating one, in order to separate design constraints to reach an optimal structure. In this paper, the lumped element model of the loudspeaker in presented, as well as the innovative manufacturing process. Finally, the computed frequency response is compared to the measured one. At the resonance, pressures as high as 110dBSPL at 1kHz and at 10mm are reported for an active surface of 36mm2, which is above the known state of the art for a loudspeaker with similar dimensions. Also, the flatness of the radiated sound pressure in a wide frequency range is closer to the ideal frequency response of loudspeakers than other MEMS loudspeakers, due to the piston mode of the moving rigid plate of the loudspeaker. The total harmonic distortion, mainly due to the nonlinearity of the piezoelectric transduction, is below 5% for reasonable sound pressure levels in the usable frequency band. The use of digital signal processing and of a dedicated packaging will allow our loudspeaker to advantageously replace the main or secondary one in smartphones.

This paper presents the simulation results of a piezoelectric MEMS loudspeaker. The micro-fabricated device is composed of two wafers, the first one features two bending piezoelectric actuators with large displacements, and the second one features a mobile rigid plate surrounded by a small interstice providing a thermoviscous layer hence avoiding an acoustic short circuit between its front and back sides. Performances of the loudspeaker are evaluated using a lumped element model and a finite element model to estimate the pressure generated by the loudspeaker in free field and in an ear-occluded coupler. Size variants of the design and their models are presented. The current design allows a radiated pressure of 80 dBSPL from 500 Hz at 10 cm and at 30 V-pp for a 64 mm(2) footprint in free field, and more than 130 dBSPL in a IEC 60318-4 coupler with the same actuation voltage, which is above the known performances of the state of the art for MEMS loudspeakers with such dimensions.

There has been an increased demand for traceable calibrations at infrasonic frequencies in support of geophysical monitoring applications, an example being the Comprehensive Nuclear-Test-Ban Treaty Organization, which provides global international coverage for a nuclear testing ban, and requires the International Monitoring System. In this paper, a new laser pistonphone design is presented with the objective of establishing primary standards for sound pressure at very low frequencies down to 10 mHz. The piston is a modified accessorized loudspeaker driver whose diameter is equal to the diameter of the front pistonphone cavity. The volume velocity of the piston is measured through a laser interferometer and the current version was designed to have an upper frequency limit of 20 Hz, to overlap with the closed coupler reciprocity method of calibration. Particular attention has been given to the sealing to avoid pressure leakage loss. The dimensions of the front cavity were designed to allow the calibration of a large variety of sensors, including microphones, barometers, manometers and microbarometers. Examples of calibrations for several sensors are presented and also an uncertainty budget for the Brüel & Kjær type 4160 laboratory standard microphones, commonly used for primary calibrations. Finally, the metrological performance of the laser pistonphone is demonstrated by comparing the calibration results with those obtained with alternative methods.

This paper presents the simulation results of a piezoelectric MEMS loudspeaker. The micro-fabricated device is composed of two wafers, the first one featuring two bending piezoelectric actuators with large displacements, and the other one featuring a mobile rigid plate surrounded by a small interstice providing a thermoviscous layer hence avoiding an acoustic short-cut between its front and back sides. The current design allows a radiated pressure of 80 dBSPL from 1 kHz at 10 cm and at 30 Volts for a 64 mm2 footprint, which is above the known performances of the state of the art for MEMS loudspeakers with such dimensions.

We propose a method to evaluate the Total Harmonic Distortion generated by a cantilever-based PZT loudspeaker inside an IEC 60318-4 coupler. The model is validated using experimental data of a commercial loudspeaker. Using the time domain equations of the equivalent electrical circuit of the loudspeaker inside the coupler and a state space formulation, the acoustic pressure response is calculated and compared to the measurement of the manufacturer. Next, the stiffness, transduction and capacitance nonlinear functions are evaluated with a Double-Beam Laser Interferometer (DBLI) and a nanoindenter on test devices and on the commercial loudspeaker. By introducing the nonlinear functions into the model as amplitude-dependent parameters, the THD generated by the loudspeaker is calculated and compared to the value provided by the manufacturer. The good agreement between the measurement and the simulation could allow for a rather quick simulation of the performance of similarly designed loudspeakers at the early stage of the design, by only estimating the static linearity of the main nonlinearity sources.

The objective of the paper is to provide a suitable analytical approach to describe the behaviour of a square miniaturised (MEMS) receiving transducer made up of a square membrane having the same dimensions as the external dimensions of the transducer itself, loaded by both a thin square small sized fluid-gap and a peripheral cavity connected together and set rear the membrane. This device departs from the other previous square devices in that the peripheral cavity is rear the membrane at the periphery of the backing plate (backing electrode). This architecture (derived from the circular one suggested previously) enables to optimize the sensitivity of the transducer while retaining both a cartesian geometry and the smallest dimensions possible (surface area and thickness). The analytical approach accounting for the effects of the interior geometrical discontinuity on the displacement field of the membrane used here to describe such square transducers (electrostatic or piezoelectric) departs from previous ones in that it avoids multi-modal analysis which exhibits procedural difficulties due to the coupling of Dirichlet-like (membrane) and Neumann-like (fluid) eigenfunctions (emphasized here by presence of the geometrical discontinuity in the fluid-filled part of the device). Additionally, an approximate analytical solution appropriate to express the displacement field of the membrane and to estimate the sensitivity with a good accuracy in the lower frequency range is presented. FEM solutions are provided, against which the analytical results have been tested.

The model of electroacoustic MEMS transducer with a moving square shaped clamped plate loaded by a thin fluid gap and a peripheral cavity is presented herein. The behaviour of the transducer, namely the thermal and viscous boundary layers effects originating in the fluid gap between the moving electrode and the fixed one and the strong coupling between the moving electrode displacement and the acoustic pressure field in the fluid gap, have to be described correctly by the model. The modelling approach proposed herein involving the integral method for describing the acoustic pressure in the fluid gap requires an analytical expression of eigenfunctions of the square shaped clamped plate. Such an approximate expression in form of two-dimensional cosine series proposed recently suffers from slow convergence and inaccurate boundary conditions. The solution proposed herein is based on the series expansion over the system of functions satisfying exactly the boundary conditions, which leads to faster convergence, hence lowering computational costs. The proposed eigenfunctions are described and the difference from the previous approximations is discussed. Finally, the acoustic pressure sensitivity of the transducer is presented and compared to the results of a reference finite element model.

The use of planar micro-beams as moving parts of acoustic and electroacoustic devices has increased recently because of their geometrical simplicity, hence lowering fabrication costs. The precise modelling of such devices is then of interest. The miniaturized transducer proposed herein is composed of a planar rigid micro-beam attached at one end to a flat spring (the other end remaining free) surrounded by thin slits and loaded by a thin fluid layer (situated in the gap between the micro-beam and a reduced-size backplate) and a small cavity, both being placed behind the beam. Such a configuration reduces the overall size of the device (no need of an external cavity) and enables to adjust more parameters comparing to the case of the backplate of the same size as the one of the micro-beam. The thermoviscous damping effects originating in the fluid-filled parts of the device (slits, air-gap, and cavity) are taken into account. As a result of the model, the displacement of the micro-beam is calculated and compared with the reference finite element solution, the acoustic pressure sensitivity of the transducer is finally presented and discussed.

With the ACARE directives on aircraft noise reductions for 2050, aircraft turbofan engines acoustic liners will have to overcome contradictory goals: to guarantee an efficiency at low frequencies while reducing treated surface area and liners thickness. The purpose of this paper is to present some concept reviews in the development of transducers-based acoustic liners. The use of transducers enables the coupling of several physical domains: acoustics, mechanics and electronics. As shown in former work, this can lead to highly efficient devices enabling low frequency absorption with thin liners with or without flow. The present work aims to better understand the key phenomena governing acoustic absorption and frequency range available for a given liner thickness and several transductions such as electrodynamic, electrostatic and piezoelectric. The effects of adding an electronic shunt circuit and using several transduction principles on the absorption level and bandwidth are discussed. In order to study several types of transductions, Finite Element Method and Lumped Element Method modelisations have both been developped but only the second one is used in this paper. The transductions are compared considering absorption efficiency due to the shapes of the devices on one hand and to the possibilities offered by the connection to an electrical impedance in terms of absorption enhancement and/or frequency shift on the other hand. Some perspectives are given in terms of structures optimization, electrical shunt impedances choices and expected absorption efficiency.

This paper presents an accurate characterisation based on an experimental set-up and mathematical models of a dynamic pressure infrasound generator. The Commissariat a l'energie atomique (CEA) has developed microbarometers to measure infrasound waves in the atmosphere. To characterise its sensors and validate their requirements, an infrasound generator has been designed, which covers a frequency range from 4.0 10(-4) Hz to 300 Hz. This pressure generator still needs accurate characterisation as there is no standard reference in the infrasound frequency range for such sensor calibration. The research focused on 17 parameters that affect the behaviour of this infrasound pressure generator. Two analytical models of the sound pressure in the cylindrical cavity of the calibrator are presented. An experimental characterisation was also performed to adjust the model parameters with genetic algorithms to the measurements, and the results of the comparison between the measurements and the models are discussed. This study highlights the influence of the thermodynamic transition from isothermal to adiabatic transformation and the influence of viscoelastic non-linearities of a loudspeaker membrane. It aims to characterise the amplitude and phase responses of the generator in order to develop an improved infrasound calibration device for microbarometers with a similar technology. (C) 2019 The Authors. Published by Elsevier Ltd.

Demand for calibration at infrasonic frequencies has emerged in response to earth monitoring problems. The primary standard for sound pressure is defined through the reciprocity calibration method specified in the International Electrotechnical Commission (IEC) Standard 61094-2:2009. This method is based on the use of closed couplers and is routinely applied by the National Metrology Institutes for a large frequency range; however, infrasonic frequencies below 2 Hz have not been explored until recently. The acoustic transfer admittance of the coupler, including the heat conduction effects of the fluid, must be modelled precisely to obtain accurate microphone sensitivity. IEC 61094-2:2009 provides two standardised solutions for the correction of heat conduction. However, researchers have noted significant deviations between these corrections at low frequencies in plane wave couplers, indicating that one or both techniques incorrectly calculate the influence of heat conduction. In this paper, the limitations of the standardised formulations at infrasonic frequencies are identified and two alternative solutions are proposed. An experiment is also reported, which highlights the discussed limitations of the standardised formulations for acoustic transfer admittance, while also demonstrating the validity of the proposed alternative formulations at frequencies down to 0.04 Hz.

Precise modeling of 1D acoustic devices (passive or active, miniaturized or not) containing a planar beam loaded by a fluid gap and cavities is of interest in a variety of applications (transducers, acoustic filters, metamaterials, etc.). An analytical approach presented herein enables to describe the vibration of the planar elastically supported rigid beam of rectangular cross-section surrounded by very thin slits and loaded by the fluid gap, which is divided in three parts of different thicknesses (the central part being the thinner one), the thermoviscous losses originating in the fluid being taken into account. Comparing to the case of the fluid gap of uniform thickness, such a geometry provides more parameters which can be adjusted in order to achieve the required behavior (resonant or damped, etc.). The analytically calculated beam displacement is presented and compared to the numerical solution provided by finite element method (a reference against which the analytical results are tested).

Reduced-size backing electrode microphones have been developed recently to achieve an easier match to specified response requirements. Such a development has used both analytical and numerical multi-physics modelings to validate the architecture efficiency. This microphone is composed of a membrane covering an annular cavity surrounding a central backing electrode. This simplified structure leads however to a higher sensitivity and an larger bandwidth, as shown in previous publications. A new modeling, based on lumped elements modeling, has been developed, in order to provide an easy a quick design tool for choosing the microphone parameters, these ones being fitted more precisely with a FEM modeling. These tools have leaded to the development of prototypes which have been characterized. The comparison of the measured data to the ones computed with the different models is presented. It shows a pretty good agreement between the several modelings and the experimental data, and it enlightens the need to take into account parasitic capacitance effects.