Dr. Ing. Libor Husník

This article describes the experimental setup for estimating the moisture in wood pellets and wood chips by measuring the attenuation of the first longitudinal acoustic resonance mode that is excited in the cylindrical container. The resonance quality is related to losses of sound energy, which are brought about by several phenomena and moisture is one of them. The metrological traceability was carried out by the “loss on drying” method, supported by calibrated balance and a set of reference materials. Using the “loss on drying” method, the amount of moisture in wood pellets was measured in the ranges of 0 %mc to 10 %mc. For acoustic tests in the range of 0 % mc to 56 % mc, reference woodchip material was used. Experimental results show feasibility of concept of traceable acoustic moisture measurement based device.

This paper describes a Digital Acoustic Projector (DAP), which is a hybrid sound system combining both a digital loudspeaker and a parametric array to convert directly an n-bits digital electrical signal into sound. It consists of a set of ultrasonic transducers emitting a sinusoidal signal modulated by a sampled and quantized low-frequency signal. Self demodulation effect is used to demodulate the acoustic signal during its propagation in the air. As a proof of concept, a DAP is designed to convert a 4-bit digital electrical signal into sound. First experimental results are presented.

This article presents an experimental setup for the ultrasonic temperature measurement with a single acoustic transducer. The ultrasonic thermometer uses the simple impulse-echo method to determine the time of flight of short 75 kHz acoustic bursts travelling through a waveguide from which the velocity of sound and subsequently the temperature (of the environment surrounding the waveguide) is computed. The waveguide consists of 500 mm long stainless steel tube with an inner diameter of 22 mm, connected in series with a tube of the reduced diameter (10 mm, 171 mm long). The difference in cross sections of respective parts allows to determine sound velocity only in the segment of the thermometer which is fully immersed into the measured medium. The whole system is controlled by the NI LabVIEW application which uses the NI PXI system for both generating the signal for the electrostatic actuator and measuring the signal from the receiver part of the circuit. Implementation of the electrostatic actuator allows to use one acoustic transducer without the need of complex supporting electronics or time synchronisation between signals. All the signals representing the initial acoustic pulse and echoes from the waveguide are present in a single channel record. The temperature estimation is computed from basic signal parameters. The principle of the thermometer prototype operation and first calibration results in the temperature range of (-20 to 90) °C are shown. Results of calibration and subsequent interpolation applied on measured data show current limits of the thermometer.

The objective of the paper is to provide a simplified analytical approach to describe the behaviour of a cylindrical emitting transducer with a piezoelectric polymer (PVDF) membrane which is loaded by a thin air-gap connected to a small central toroidal cavity through small holes in a back-plate. This paper departs from a previous one in that the presence of a perforated back-plate separates the volume behind the membrane in two parts (the air-gap and the central cavity); it departs also in that the construction of the analytical solutions is not made in terms of eigenmodes of both the membrane and the air-gap (as usual) in order to avoid difficulties due to the coupling of Dirichlet and Neumann eigenfunctions (exact solutions are used, involving integral formulation for one of them).

This work aims at providing a theoretical description of an ultrasonic emitting transducer, which consists of a cylindrical Polyvinylidene fluoride (PVDF) membrane coupled to an air-filled back-cavity. Based on the fundamental equations of piezoelectricity and linear acoustics, and assuming axisymmetrical geometry, an analytical model which describes the coupling of the displacement field of the membrane with the pressure fields on both of its sides is presented. Solutions are obtained using modal expansion theory for the displacement field of the membrane and for the acoustic pressure field in the axial-direction of the back-cavity. This model permits to predict the resonances of the system, which are not taken into account in an earlier work [1]. A lumped elements network is then derived from the analytical solution, valid in the lowest frequency range, when keeping only one mode and when considering two levels of approximations. Comparison between measurement results and theoretical ones of the pressure radiated by two prototypes of emitter shows an overall good agreement.

Electroacoustic transducers based on the condenser principle have usually a planar back electrode which is a one piece entity. The previous work studied theoretically the possibility of making the back electrode partitioned. Such an arrangement can be used in a design of the so called digital loudspeaker, in which the sizes of the partitioned electrode are in the ratio of the powers of 2, but this is only a special case. The aim of this work is to study performance of a system modeling the electrostatic transducer with the partitioned back electrode by way of measurement on an experimental sample. Various combinations of signals are applied on the partitioned electrode and the transducer response are measured.

This paper deals with the design and implementation of the device aimed at navigation of visually impaired people. First part of the article presents a review of existing assistance devices for visually impaired, in the second part follows description of the system using low-cost input and output personal computer interfaces (joystick, video grabbers, NVIDIA 3D Vision) and description of design of novel acoustic transducers, focused for use in aids for the blind.

A partitioned electrode in an electrostatic transducer can present among others a possibility for making the transducer with the direct D/A conversion. Nevertheless, partitioned electrodes, the sizes of which are proportional to powers of 2 or terms of other convenient series, do not have the corresponding force action on the membrane. The reason is the membrane does not vibrate in a piston-like mode and electrode parts close to the membrane periphery do not excite membrane vibrations in the same way as the elements near the center. The aim of this work is to suggest the way of equalization of force contributions from different partitioned electrodes by varying their sizes. Principles presented here can also be used for other membrane-electrode arrangements.

Operating principle of systems with the direct acoustic D/A conversion, which are sometimes called digital loudspeakers, brings new features to the field of transducer design. There are many design possibilities to these systems, using different transduction principles and spatial arrangement of constituting parts. This paper deals with the single-acting condenser transducer, suitable for micromachining applications, in which the membrane is driven by a partitioned back electrode. While in conventional transducers the electric force between the back electrode and the membrane is evenly distributed, in digital transducers it is no longer the case. Consequences to membrane vibrations for some cases of excitation by various distributions of forces representing given binary combinations from the dynamic level are presented.

One of the possibilities to design a transducer with the direct digital-to-analog conversion, sometimes called a digital loudspeaker, is the miniature condenser transducer manufactured on a silicon chip. Only recently this micro technology has been made available commercially, which opens further application possibilities. The proposed article is aimed at the study in which the back electrode of the electrostatic transducer is partitioned into sections having total areas proportional to powers of 2. Since the electrostatic force acting on the membrane is affected by the distribution of bit groups, which cannot be even, said electrostatic force will not be a linear function of the signal voltage. Correction coefficients for some arrangements are searched for.

The article deals with subjective evaluation of compression artifacts for various coding systems. Quality of compressing systems depends on how much these artifacts are audible. Since coding schemes evolve rapidly, advanced coding technologies may treat different critical material differently in comparison with their predecessors. The objective of this paper is to compare several sound compressing systems on the basis of different types of critical sound materials. Data show that some coding systems process certain types of sound signals better. We can infer that sound coding systems are "specialized" in certain types of sound material, e.g. speech, transient signals, etc.

Transducers with the direct D/A conversion, sometimes called digital transducers, either loudspeakers or earphones, are searching their ways into being. There have been several attempts to design such devices, but none of them left research laboratories and made its way to commercial use yet. Most of them use "classical" electroacoustic transduction principles, i.e. electrodynamic or electrostatic. In this article the possibility to use opto-acoustic transduction principle is explored. First, the principles of physical phenomena used in this transducer are revised. Then, some construction details in light of their usage in the digital earphone are described.

For measurement and analysis of multi-channel electro-acoustical systems we used logarithmically swept signals. This method uses driving the system under test by a test signal, recording of its response to the test signal, and digital analysis of obtained results. A so called "inversion filter" is derived from the driving signal. By convolution of the system's response with this inversion filter we can obtain an impulse response of the system under test. This method enables analysis of tested systems with nonlinear distortion. When analyzing multimedia audio systems it is possible to analyze channels individually or their mutual influencing. In this project we dealt with the analysis of individual components of the electro-acoustical chain and monitoring of their performance, including compression sound systems.

Zavádění digitálních technologií do přenosových a záznamových systémů probíhá již mnoho desítek let. Tento trend samozřejmě nevyhnul ani zvukové technice a v současné době jsou k dispozici všechny součásti elektroakustického řetězce v digitální formě. Všechny, až na periferie, tedy elektroakustické měniče, mikrofon a reproduktor. V následujícím článku bude podán současný stav na poli tzv. digitálního reproduktoru, tedy elektroakustického měniče, který je buzen paralelním tokem digitálních bitových signálů a který neobsahuje elektrický D/A převodník. Budou představena možná principielní uspořádání, problémy, které jednotlivá řešení přinášejí a úvahu o budoucnosti těchto systémů.

This article addresses another aspect of transducers with the direct digital to analog conversion, sometimes called digital loudspeakers, which is the radiation impedance. In a transducer array embodying such a system, every value of the acoustic pressure from its dynamic range is radiated by a different number of elementary transducers i.e. different total surface of membranes, driven by the PCM signal. Since the critical frequency depends mainly on the total surface of membranes, an interesting phenomenon appears, i.e. every sound pressure level is radiated with a different radiation impedance. As a result, different levels may be radiated differently. In the article radiation impedances for some types of arrangments of transducer fields embodying digital transducers are presented.

8th International Student Conference on Electrical Engineering POSTER 2004

This paper deals with methods for subjective evaluation of signals compressed by various types of sound codecs.

The project consisted of three main parts, entitled Digital Audio Signal Processing with Respect to Their Perception, Measurement and Analysis of Audio Systems Using MLS and Swept Signals, and finally Processing of Low Level Audio Signals Recorded in Noisy Environment.

Article deals with the problem of signals used for driving transducers with the direct digital-to-analog conversion. Analysis of transducers in the field arrangements acting as signal filters is given.

Article deals with problems of transfer function of electroacoustic transducers used in transducer fields and their effect on resulting signal.

Článek se zabývá vlivem rozložení měničů napájených bitovými toky paralelního digitálního signálu v poli měničů, které tvoří digitální reproduktor. Je podána analýza několika typů rozložení měničů.

The paper describes a measurement on two cadaverous temporal bones. The Human Ear Canal Transfer function was determined and results are discussed.