Bandstop Resonator Research Articles

Evaluation of piezocomposites for ultrasonic transducer applications influence of the unit cell dimensions and the properties of constituents on the performance of 2-2 piezocomposites

A theoretical model on piezoceramic polymer composites with laminar periodic structure is presented. A salient feature of this model is that it can treat explicitly how the unit cell dimensions and other material properties influence the performance of an ultrasonic transducer made of 2-2 piezocomposites. The model predicts that there exist a series of modes associated with the periodic structure of a composite, which is beyond the stop-band edge resonance prediction. One of the main concerns in designing a composite transducer is how the surface vibration profile changes with frequency and how this is influenced by the aspect ratio of the ceramic plate. It was predicted that as long as the thickness resonance is below the first lateral mode frequency, there is always a frequency f/sub 1/ which is near the thickness resonance and at which the polymer and ceramic vibrate in unison. The effect of aspect ratio is to change the position of f/sub 1/ with respect to the thickness resonance frequency and the bandwidth in which polymer and ceramic have nearly the same vibration amplitude and phase. It is also predicted that, when operated in a fluid medium such as water, there will be a resonance mode which has a frequency determined by the velocity of the fluid medium and the unit cell length d and is associated with the oscillation of the fluid. The behavior of a composite plate as an acoustic transmitter and receiver and the influence of the aspect ratio of the ceramic plate on them are also investigated.

Three-dimensional metallodielectric photonic crystals exhibiting resonant infrared stop bands

Using standard microelectronic techniques, we have fabricated arrays of infrared metallodielectric photonic crystals (IR MDPCs) on silicon substrates. The metallic “atoms” are located on a three-dimensional (100)-oriented face-centered-cubic lattice. Resonant stop-band characteristics have been measured with rejection levels of up to 20 dB and widths of up to 83% of the center frequency. We demonstrate structures with stop bands across the midinfrared wavelength range from 2 to 12 μm. Angular studies of the photonic stop bands show an insensitivity to incident angle for some of the structures. The IR MDPC results are compared with measurements made on microwave-scale MDPC structures to help in understanding the infrared results.

Theoretical modeling of resonant modes of composite ultrasonic transducers

Although a great deal of effort has been devoted to the modeling of composite piezoelectric materials, most of the earlier works are based on the assumption that the structure of the composite relative to the wavelength is very fine. Such approximation cannot address the complete dynamic behavior of composites. In order to understand the overall characteristics of composite ultrasonic transducers, a dynamic model was developed, in which the acoustic waves propagating in 2-2 composites along the thickness direction were analyzed by solving the coupled elastic equations of the constituent phases. By neglecting the boundary conditions of the free surfaces and simply taking the resonator thickness as half a wavelength, the resonant modes of the composite transducers as functions of aspect ratio of the ceramic plate elements and volume fraction of ceramic phase can be calculated from this model. The theoretical dispersion curves for the thickness mode and the lateral periodical mode agree with the experimental results. The vibration distribution in the ceramic and polymer phases at the resonant frequency as a function of the composite thickness as well as the volume fraction of the ceramic phase are obtained, and through the discussion of the vibration field the variation rule of the resonant frequency is well explained. For the resonant frequency the results of the isostrain model, the stopband resonance model, and the T-matrix model are consistent with the predictions made by this model under the special condition of very fine structure. >

Investigation of millimeter-wave scattering from frequency selective surfaces

A comparative numerical and experimental analysis of scattering from dielectric-backed frequency-selective surfaces in W-band (75-110 GHz) was carried out. The examples studied include metal (aluminium), resistive (bismuth), and bismuth-loaded I-pole or linearized Jerusalem cross arrays on fused silica, all of which exhibit a band-stop resonance in W-band as a general feature. The arrays were fabricated using standard photolithographic techniques. The numerical analysis involves the solution of an electric field integral equation using subdomain rooftop basis and testing functions within the framework of the Galerkin testing procedure. The lossy nature of the materials was fully accounted for. A comparative analysis of doubly stacked aluminium I-pole arrays was also performed. The numerical analysis exploits a variant of the cascade method in that the immediately adjacent dielectric layers are included in the construction of the scattering matrix for the frequency selective surface. This allows the higher-order evanescent Floquet modes to decay sufficiently at the dielectric boundaries so they can be ignored in the scattering matrix. >

Localisation d'Anderson des ondes dans les réseaux acoustiques unidimensionnels aléatoires

We review some papers on the Anderson localization phenomenon in an acoustical context. Using Furstenberg's theorem, we point out general properties of random 1D-lattices described by chains of two-port networks. We distinguish between variations of the propagation constant and variations of the characteristic impedance. We then study the example of sound tubes with resonant, closed branched pipes of random sizes; we discuss analogies between this example and other macroscopic or microscopic media. Experiments and numerical simulations allow us to exhibit plainly the Anderson localization phenomenon, and to compare the effects of disorder with classical reactive effects of dissipative effects. In particular we show how disorder causes an enlargement of the «resonance stop bands» by disorder. Finally, we discuss the behaviour of «regular super-lattices», obtained by assembling identical sets of several random cells Proprietes generales de reseaux unidimensionnels aleatoires decrits par des chaines de quadripoles a partir du theoreme de Furstenberg en distinguant variations de la constante de propagation et variations de l'impedance caracteristique. Etude de tuyaux sonores munis de cheminees laterales fermees resonnantes de dimensions aleatoires

On the Theory of Self-Resonant Grids

An approximate theory is developed to predict the frequency response of a self-resonant grid. The grid is comprised of capacitive and inductive elements and exhibits a band-stop resonance. The analysis is based upon the derivation, from physical considerations, of an equivalent circuit representation of the grid structure. Predicted results compare well with measured data.