People
Ing. Karina Šimonová, Ph.D.

2024, IEEE Sensors Journal, 24 (16), p. 25486-25492), ISSN 1530-437X

This study investigates the directivity of electrostatic (condenser) microphones featuring a circular membrane and a back-plate segmented into four parts. We analyze the relationship between the direction of arrival of incoming sound pressure waves and the non-axisymmetrical displacement field of the membrane using both analytical and numerical models. Since the output voltage of an electrostatic microphone is proportional to the mean value of the membrane displacement over the fixed electrode surface, the membrane displacement field influenced by the acoustic pressure field inside the transducer must be accurately calculated. By subtracting the output voltage signals from two opposite electrodes, we obtain two directional outputs with bidirectional directivity patterns perpendicular to each other. To validate our theoretical findings, we conduct measurements on an experimental transducer specimen. The experimental results, obtained using a scanning laser vibrometer to analyze the membrane displacement field and directivity measurements performed in an anechoic chamber, confirm our theoretical predictions. Bidirectional patterns at both outputs, which are perpendicular to each other, are observed in the frequency range corresponding to the pass band of the transducer, except at very low frequencies where phase problems occur.

2023

Wireless sensor networks have become popular in the domain of environmental noise monitoring in recent years. However, low-cost and high quality microphones are needed when there are numerous sensor nodes. Theoretical models of miniaturized low-cost electroacoustic transducers and experimental verification of their performance in noise sensors is, therefore, of interest. This thesis deals with analytical and semi-analytical approaches describing the behaviour of the square clamped plate. The plate is used as a moving electrode of a miniaturized condenser microphone coupled with an acoustic pressure field inside a thin layer of thermo-viscous fluid trapped between the square plate and the backing electrode of the electrostatic transducer. The analytically calculated pressure sensitivity of the receiver is compared to the numerically (FEM) calculated value. The classical simple lumped-element model of an electret microphone, which is used in a previous version of the sensor network developed by the Laboratory of Special Projects UBTI FD CTU in Prague, is also described. Several experimental results measured on both prototype versions of the sensor, previous and recent, are presented including, the normalized transfer function of the electroacoustic path and the dynamic range of the present sensor, along with the temperature dependence of the new sensor.

2023, Forum Acusticum 2023, Madrid, European Acoustics Association), ISBN 978-88-88942-67-4, ISSN 2221-3767

Miniaturized electroacoustic transducers whose moving electrodes are perforated for technological reasons have appeared in the literature recently. Since the perforation influences strongly the frequency response of the microphones, particularly at lower frequency range due to the acoustic short circuit, the precise modeling of such devices is of high interest. This contribution presents the analytical modeling aspects and theoretical results using the porosity approach. The effects of viscous and thermal boundary layers and the coupling of the displacement field of the moving electrodes in form of perforated plate (flexible clamped at all boundaries or rigid elastically supported) with the acoustic field inside the device are taken into account. The approximated analytical results are compared to the reference numerical (FEM) ones. The benefits and drawbacks of the analytical method are discussed.

2023, Micromachines, 14, ISSN 2072-666X

Microfabricated electroacoustic transducers with perforated moving plates used as microphones or acoustic sources have appeared in the literature in recent years. However, optimization of the parameters of such transducers for use in the audio frequency range requires high-precision theoretical modeling. The main objective of the paper is to provide such an analytical model of a miniature transducer with a moving electrode in the form of a perforated plate (rigid elastically supported or elastic clamped at all boundaries) loaded by an air gap surrounded by a small cavity. The formulation for the acoustic pressure field inside the air gap enables expression of the coupling of this field to the displacement field of the moving plate and to the incident acoustic pressure through the holes in the plate. The damping effects of the thermal and viscous boundary layers originating inside the air gap, the cavity, and the holes in the moving plate are also taken into account. The analytical results, namely, the acoustic pressure sensitivity of the transducer used as a microphone, are presented and compared to the numerical (FEM) results.

2020, 2020 Smart City Symposium Prague, New York, IEEE Press), p. 1-6), ISBN 978-1-7281-6821-0

The article describes experience received from one-year period of the pilot project of the air quality sensor network operation. The sensor network consists of 13 units deployed in city Litomerice. The unit - AirTracker was developed in UBTI CTU in Prague with motivation of price reduction allowing mass deployment of the network. Architecture of units and sensor network is described in the article. During the entire one-year pilot operation following values were measured: concentration of dust particles and VOC in the air and level of acoustic noise. The weather station was used to identify the characteristics of air pollution diffusion. The analysis of all data is described there along with the expectation of public authorities and the explanation of local conditions and constraints.

2020, Journal of Sound and Vibration, 473, ISSN 0022-460X

The paper is mainly concerned with the analytical approach of the behaviour of a two-dimensional miniaturized MEMS transducer, namely a rectangular or square clamped plate loaded by a fluid-gap (squeeze film), surrounded by a small cavity (reservoir), and excited by an incident acoustic field (assume to be uniform on the plate). Until now, the problem has not been analytically solved owing to the geometry of the device in conjunction with the nature of the diaphragm (elastic plate) and its boundary conditions (zero deflection and zero normal slope along all edges); namely analytical eigenfunctions do not exist for the clamped plate. On the other hand, the analytical approach classically used to express the acoustic field in the fluid-gap relies on a modal expansion which does not match correctly with both the displacement field of the diaphragm and the boundary conditions at the entrance of the reservoir. Then, two particular questions arise: how to derive analytically the modal behaviour of the loaded clamped plate, and what analytical approach for the acoustic field in the fluid gap is convenient to describe its coupling with the displacement field of the plate? The aim of the paper is both to provide basically an exact analytical approach and to handle a numerical implementation (FEM) against which the analytical results are tested.

2020, Forum Acusticum 2020, Societé Francaise d'Acoustique), p. 2539-2542)

Several realizations of electroacoustic MEMS transducers where the moving electrode is perforated for technological reasons have been published recently. Since the presence of the holes changes the vibration characteristics of the plate (in terms of eigenmodes) and the coupling between the incident acoustic pressure and the pressure inside the transducer through these holes influences significantly the behaviour of the transducer, a precise modelling of such devices is of interest. The modelling approach proposed herein employs an approximated expression of the eigenfunctions of the perforated clamped plate in form of two-dimensional series expansion over the system of functions related to the solution for one-dimensional beam clamped at both ends, the coefficients of the series being calculated from the numerical solution for the eigenfunctions of the perforated clamped plate. The convergence issues and the dependence of the approximation error on the number of terms in the series are discussed. An integral formulation providing the acoustic pressure field inside the air gap between the moving electrode and the fixed one is then proposed. It takes into account the strong coupling between the acoustic pressure field in the air gap and the displacement field of the plate, the thermal and viscous boundary layer effects and the acoustic short circuit between both sides of the perforated plate through the holes.

2019

Wireless sensor networks have become popular in the domain of environmental noise monitoring in recent years, however low-cost and high quality microphones are needed in case of high number of sensor nodes. Theoretical models of miniaturized low-cost electroacoustic transducers and experimental verication of their performance in noise sensors is therefore of interest. This report deals with analytical and semi-analytical approaches describing the behaviour of the square clamped plate used as a moving electrode of miniaturized condenser microphone, coupled with acoustic pressure eld inside a thin layer of thermo-viscous fluid trapped between the square plate and backing electrode of the electrostatic transducer. The analytically calculated pressure sensitivity of the receiver is compared to the numerically (FEM) calculated one. The classical simple lumped-element model of electret microphone, which is used in a sensor network developed by UBTI CTU, is also described and several experimental results measured on the sensor are presented.

2019, Proceedings of the 23rd International Congress on Acoustics, integrating 4th EAA Euroregio 2019, Dresden, Deutsche Gesellschaft für Akustik), p. 7361-7368), ISBN 978-3-939296-15-7, ISSN 2226-7808

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.

2018, Euronoise 2018, Madrid, European Acoustics Association), p. 337-340), ISSN 2226-5147

Electroacoustic MEMS transducers are involved in a variety of applications such as consumer devices, but their use increases today also in the domain of wireless noise monitoring systems. Since the high quality (and low price in case of high number of sensor nodes) is needed, the precise modelling of such devices leading to the new solutions is of interest. The transducer, whose model is described herein, consists of a square plate as a moving electrode loaded by a thin fluid gap and a rectangular peripheral cavity, the square shape of the transducer being advantageous from the point of view of microfabrication because of its geometrical simplicity. The analytical approach relies on the integral formulation for the acoustic pressure field in the thin fluid gap (the dissipative effects of the thermal and viscous boundary layers being taken into account) coupled with the displacement field of the plate expressed as a sum of eigenmodes. Special attention is paid herein to the expression of the eigenfunctions of the plate, which are searched for in an approximated analytical form of two-dimensional Fourier series, the coefficients of the series being derived from a simple numerical solution for the plate without the loading acoustic elements. The acoustic pressure sensitivity of the transducer, used as an acoustic receiver, is obtained from the analytical model of the transducer, the comparison with the numerical results provided by a finite element model (a reference against which the analytical results are tested) is presented and discussed.

2017, Akustické listy, 23 (1-4), p. 9-17), ISSN 1212-4702

Properties of the low-cost electret microphones can differ significantly among individual items due to the production inaccuracies. This paper presents experimental estimates of some parameters of the equivalent circuit of a low-cost electret microphone which cannot be measured directly, namely the tension of the diaphragm and the equivalent polarization voltage generated by the electret layer. Analysis of the impact of some equivalent circuit parameter variations on the sensitivity of the measured samples has been also carried out (particularly of those that can vary significantly due to the low-cost production). The results show the key role of the equivalent polarization voltage differences, while the differences of the other parameters such as the air gap thickness and the tension of the diaphragm have been found to be much less important.