Institute of Fundamental Technological Research

Measurement of the shear elastic constant of soft and highly damping tissue of high Poisson ratio is quite a challenging task. It is proposed to evaluate shear wave velocity and damping of tissue by measuring the shear skimming bulk waves using one interdigitated interdigital transducer on a piezoelectric layer, such as polyvinylidene fluoride, applied to the surface of the small tissue sample.

Interdigital transducer, surface acoustic waves, electrostatics, mixed boundary-value problem

Planar systems of electrodes arranged on both sides of dielectric piezoelectric layer are applied in numerous transducers. They are capable of electronic beam-steering of generated wave both in azimuth and elevation. The wave-beam control is achieved by addressable driving of two-dimensional transducer through proper voltage supply of electrodes on opposite surfaces of the layer. In this paper a semi-analytical method of analysis of the considered transducer is proposed, which is a generalization of the well-known BIS-expansion method. It was earlier exploited with great success in the theory of interdigital transducers of surface acoustic waves, theory of elastic wave scattering by cracks and certain advanced electrostatic problems. The corresponding nontrivial electrostatic problem is formulated and solved numerically

beamforming, transducer array, BIS-expansion

Systems of planar electrodes arranged on dielectric or piezoelectric layers are applied in numerous sensors and transducers. In this paper electrostatics of such electrode systems is presented and exploited in the analysis of distributed piezoelectric transducer dedicated to surface elastometry of biological tissues characterized by large Poisson modulus. The fundamental Matlab code for analyzing complex planar multiperiodic electrode systems is also presented.

interdigital transducer, surface elastometry, BIS-expansion

The mode-expansion method is widely applied in waveguide analysis. Particular applications usually include only a few modes to obtain explicit analytical results for simple waveguides. Here, we apply the method to the evaluation of the dynamic properties of elastic strips; to obtain useful results for thin strips, tens of plate modes must be involved in the expansion, representing the vibration shape of the strip. Although this increases the numerical complexity of the analysis, it is still easy for both numerical implementation and for physical interpretation of results. The final result we seek in this paper is the matrix of harmonic impedance of strips that provides dependence of displacements on both upper and lower sides of a strip of rectangular cross-section on arbitrary traction applied to these sides, where both the traction and displacements are expressed in the spatial Fourier series (other sides of strips are assumed to be stress-free). Detailed discussion of numerical issues is provided for isotropic strips and example applications are presented for strips interacting with surface waves or with other strips arranged in a „woodpile” super-lattice.

Strips, Vectors, Harmonic analysis, Shape, Acoustics, Accuracy, Stress

Directional excitation and detection of sound in the periodic baffle system are analyzed using the method developed earlier in electrostatics. The phenomenon is exploited in phased array transducers applied in ultrasonography, nondestructive evaluation and testing. The nice feature of the applied method is that the matrices of the resulting system of the equations are relatively easy for computations and that the results satisfy exactly the energy conservation law. Illustrative numerical examples present the far-field wave-beam steering generated in a baffle system that may be considered as a model of a linear ultrasonic transducer array.

Phased array transducers, Bragg scattering, BIS-expansion

The BIS-expansion method is widely applied in analysis of SAW devices. Its generalization is presented for two planar periodic systems of perfectly conducting strips arranged perpendicularly on both sides of a dielectric layer. The generalized method can be applied in the evaluation of capacitances of strips on printed circuits boards and certain microwave devices, but primarily it may help in evaluation of 2-D piezoelectric sensors and actuators, with row and column addressing their elements, and also piezoelectric bulk wave resonators.

Electrostatics, Piezoelectric devices, Surface acoustic wave devices, Dielectrics, Capacitance, Printed circuits, Microwave devices, Microwave theory and techniques, Capacitive sensors, Microwave sensors

Mixed boundary-value problem for periodic baffles in acoustic medium is solved with help of the method developed earlier in electrostatics. The nice feature of the method is that the resulting matrices are relatively easy for computations and that the results satisfy exactly the energy conservation law. Illustrative numerical examples present the wave-beam steering (in the far-field) in a baffle system that may be considered as a model of one-dimensional ultrasonic transducer array.

Sound field calculation, linear phased arrays, BIS expansion, far-field radiation pattern, beam-steering, transducer arrays, composite transducers, scattering, generation, fields

A procedure based on the Stroh formalism is presented that yields the approximation for the planar harmonic Green's matrix function for piezoelectric half-space that is valid in the vicinity of the surface, or pseudosurface, wave number. This approximation can be directly exploited in the spectral-theoretical analysis of surface acoustic wave devices. Otherwise, the known characterization of the surface wave supporting substrate can be immediately inferred from it, like the wave velocity, piezoelectric coupling coefficient, the wave damping in the case of leaky pseudosurface waves, and the corresponding surface displacement velocities.

Surface waves, Equations, Eigenvalues and eigenfunctions, Bismuth, Frequency, Electric potential, Stress, Transforms, Hafnium, Piezoelectric polarization

TIt is well known that spatial-temporal Green function characterizes sufficiently the elastic media. For the case of elastic interface waves which propagate at the interface between two, perfectly mechanically contacting piezoelectric half-spaces, this function, or its scalar counterpart that is the planar harmonic impedance, provides full characterization of electric properties observed at the interface, which can be applied in analysis of interdigital transducers embedded there, for instance. This impedance however is not easy for evaluation as a function of complex wave-number in the most interesting domain near the cut-off wave-number of bulk waves. Here, a perturbation analysis is presented exploiting the Stroh matrix formulation for piezoelectrics which yields the analytical approximation to the investigated electric impedance at the crystal planar cross-section.

surface acoustic waves, harmonic Green functions, waves in piezoelectrics

Interdigital transducers applied in surface acoustic wave devices have fingers (conducting strips) of varying lengths defining the transducer apodization. Most theoretical papers neglect finite fingers' length in evaluation of the electric field in the transducer plane and the transducer's capacitance. This letter presents the generalized Blotekjaer, Ingebrigtsen, and Skeie's expansion analysis that may help in filling this gap in the transducer theory.

Strips, Equations, Voltage, Electric potential, Green function, Anisotropic magnetoresistance

The spatial field distribution is the most frequent subject of standard electrostatic analysis. In this paper the system of several strips in external spatial-harmonic electric field, which causes the charge distribution on them, is considered. The solution is constructed as a linear combination of certain template functions, evaluated in spectral domain and satisfying the electric boundary conditions on the strips. The problem is analogous to wave scattering; this justifies the application of the wave-scattering terminology (i.e. incident wave for the external field and the corresponding ‘radiation condition’) in the considered nonstandard ‘electrostatic scattering’ problem. The strip total charge and the Bloch harmonics of the ‘scattered’ field are evaluated.

Planar electrodes, Interdigital transducers, EM scattering

Acoustic signals of small arms’ fire, the muzzle blast and the shock wave generated by a supersonic bullet in air are difficult to mask and can be exploited for localization of the hidden sniper. The problem is formulated for the curvilinear bullet path of unknown parameters (velocity and acceleration) in a system of nonlinear equations that can be solved numerically. A successful iterative solution is presented in a computed example based on technically feasible measurements of the shock wave arrival times at a system of nine omnidirectional microphones distributed in the protected area.

Sniper localization, Acoustic shock waves

Acoustic signals of small arm's fire, the muzzle blast and the shock wave generated by a supersonic bullet in air, are difficult to mask and can be exploited for localization of the hidden sniper. This paper presents the system of acoustic measurements, based on a number of both directional and omnidirectional microphones detecting the shock wave only, yielding exact solution for the sniper direction in spite of certain errors in the directional measurements. The system has a self-correcting ability concerning the sound directional measurements which contributes to the system technical feasibility. Auxiliary muzzle blast measurements would yield the sniper position.

shock waves, measurements

A spectral theory of single-phase unidirectional transducers of the surface acoustic waves is presented. It helps to evaluate the stop-band width where the directionality takes place, and allows one to optimize the transducer structure in order to obtain its symmetric frequency response, with maximum directionality at the stop-band center.

surface acoustic waves, interdigital transducers