Andrzej Dobrucki

Theory of piezoelectric axisymmetric bimorph

Abstract A theory of a piezoelectric axisymmetric bimorph is presented in this paper. Bimorphs are often used as electroacoustic transducers, but their use is much wider. A piezoelectic bimorph consists of two or more layers which are placed asymmetrically to the middle surface of the structure. When voltage is supplied to the bimorph, a bending moment is produced which causes transversal deflections of the structure. Average elastic parameters are calculated. Equations for the calculation of bending moments and stretching forces produced by voltage are derived. When the bimorph is a shell of revolution with any shape of meridian, the derived equations can be solved by numerical methods. The finite-element method (FEM) is applied to solve this problem. The bimorph can also be used as a sensor. A theory of such a sensor is also presented. The results obtained by means of the described theory and numerical methods have been verified experimentally or by comparing them with results obtained analytically for a simple structure. It has been proved theoretically that the electric signal produced by a circular transducer clamped on the outer rim of a piezoelectric disc is equal to zero.

Thermodynamic processes in an RF pulsed excited CO2 slab-waveguide laser

A pulsed excitation of the laser plasma in gas lasers creates an acoustic wave in the laser reservoir. It changes thermodynamic parameters of the laser plasma in the laser cavity like pressure, and temperature, as well, and consequently it changes the density of the laser plasma, or, in other words, the refractive index of the laser medium. Tuning laser frequency during the pulse developing is observed as a result. The measurements of the pressure, temperature, and refractive index changes in an RF pulsed excited CO2 slab-waveguide laser are purposes of the work. The pressure changes are measured with calibrated microphones situated close to the laser plasma. The temperature changes are calculated via measured refractive index characteristics, and simple formulas linking the refractive index with the gas density. The picture of the acoustic wave propagation in the laser cavity is presented. The obtained results give the picture of the laser plasma behavior during the pulsed excitation. It leads to a single frequency pulsed laser operation design.

Reduction of numerical head model for calculation of Head-Related Transfer Function

Measuring the Head-Related Transfer Function (HRTF) is an efficient method in taking the influence of human body on sound spectrum into consideration. The data base used in reproduction of the sound source position is built using the measurement results. The base is individual for each human, which makes it impossible to make a versatile base for all listeners. In this paper a numerical model of artificial head and its reduction is presented. The model allows to determine the value of HRTF without the measurements making. A method which is often used to determine the acoustical field parameters is the boundary element method, which was used to calculate the values of HRTF in this work.

Pulsed injection of the feedback signal to a cw and pulsed RF excited CO2 slab-waveguide laser

The experiment is performed on a three-mirror laser resonator. The injection is arranged in two ways: by a feedback from the movable third mirror, and by chopping the running back laser beam when the third mirror is stable.

Comment on “Investigation of sound waves generated by the Hall effect in electrolytes” [J. Acoust. Soc. Am. 111 (5), 2087‐2096 (2002)] (L)

An improved model of magnetohydrodynamic transducer is presented. According to this model, the equations for direct and inverse transduction effect have the same form and they fulfill the reciprocity principle. The general form of equation for transduction coefficient in magnetohydrodynamic transducers is given

Models of acoustic field in rooms

When designing premises that require good acoustics, their purpose needs to be known. It determines preparation of the acoustical project which includes acoustics related to sound wave propagation in a given area. For instance, in case of designing large concert halls, theatre or cinema halls, so-called modeling studies with using a numerical analysis might be of help in solving issues related to architectural acoustics. They allow to determine both what the most favourable shape of the interior is and which of its surfaces should absorb, reflect, disperse the sound or direct it in another place. We use specialized software to make good acoustic adaptations, which greatly facilitates us making decisions in the design phase.

Test signals used in electroacoustics and speech technology

The presentation consists of two parts. In the first one, the typical signals applied for testing of electro-acoustical devices are presented. These signals are e.g: harmonic signal, slowly and quickly tuned sinusoid, impulses, Gaussian noise, Maximum Length Sequence, Golays Complementary Sequences. The statistical and spectral properties of these signals are described. The methods of analysis of stationary and nonstationary signals are presented as well. In the second part, the signals used for the evaluation of quality of coded and transmitted speech are presented. For this aim, the natural or artificial speech signals are used. The test material can consist of units having semantic meaning as well as of nonsense syllables or words. For the Polish language, the Polish Standard provides a set of logatome lists. American ANSI norms include, among others, rhyme tests. The International Telecommunication Union (ITU-T) guidelines specify the requirements for the test signal base for telecommunications applications. This recommendation presents a set of test signals of various complexity with many typical speech parameters. These signals are intended for both subjective and objective evaluation of the quality of speech transmission.

Silent Speech Recognition by Surface Electromyography

For some time, new methods based on a different than acoustic signal analysis are used for speech recognition. The purpose of nonacoustic signals is to allow silent communi‐ cation. One of these methods based on the electromyography signal is generated by the human speech articulation system. This article presents a device for electromyographic (EMG) signal acquisition and the first measurements from its use.

Subjective and objective evaluation of sound quality of radio programs transmitted via Digital Audio Broadcast (DAB+) System

The work presents results of research on the sound quality of different radio programs transmitted via Digital Audio Broadcasting (DAB+). This assessment has been provided with a use of psychoacoustic model as well as standard listening tests, using an ACR (Absolute Category Rating) method of scaling, and CCR (Comparison Category Rating) method. Results have shown that sound quality gets worse when bit-stream is of the lowest values (48 kbit/s or 24 kbit/s). Application of the Spectral Band Replication processor significantly improves the perceived quality, which is satisfying for bit streams higher than 64 kbit/s, particularly for jazz and popular music. The assessment with ACR method (recommended for broadcast by International Telecommunication Union) showed better notes than CCR one. It means that recommended method is less critical. Also results obtained with psychoacoustic model are more similar to obtained with CCR method. The attributes of spatial impression change in different ways. The greatest distortion has been observed for the perspective and spaciousness of sound image, while the sound color as well as localization stability and accuracy of phantom sources remained almost the same.

System for High Speed Measurement of Head-Related Transfer Function

Recently surround-sound systems have become popular. The effect of “surrounding“ the listener in sound is achieved by employing acoustic phenomena which influence localizing the source of the sound. Similarly to stereophony system the time, volume and phase interrelations in signals coming from each sound source are taken into account. Additionally the influence of the acoustic system created by the pinna, head and torso on the frequency characteristic of the sound is taken into consideration. This influence is described by the Head-Related Transfer Function (HRTF). The knowledge of the human physical body characteristics’ influence on the perception of the sound source location in space is being more and more frequently applied to building sound systems.