High Impedance Mismatch Research Articles

Ultrasonic monitoring of hardening concrete

Monitoring results of velocities and attenuations of compressional and transverse ultrasonic waves in hardening concrete using short pulse through-transmission are presented. A fluid concrete is a highly attenuating heterogeneous medium, composed of several phases and granular classes. Different attenuation mechanisms are considered in order to account for experimental results: high impedance mismatch between air layers and solid-fluid clusters at early ages, Energy loss by viscous dissipation and wave scattering by heterogeneities (Rayleigh scattering at low frequencies, single or multi diffusion scatterings at intermediate and high frequencies) during the beginning of hardening. At early ages, the medium may be described as composed of a succession of large slabs of fresh concrete separated by very thin slabs of air. Attenuation is then computed from the effective transmission coefficient of this medium. This model is in good agreement with experiments. During the beginning of hardening, the medium may be seen as composed of big particles immersed in an effective medium formed by the ensemble of the particles of lesser granular sizes and water. Then, the Rayleigh scattering model at low frequencies and the multiple diffusion model predictions at intermediate frequencies are in good agreement with experiments.

Comparison of Estimation Techniques for Vibro-Acoustic Transfer Path Analysis

Vibro-acoustic Transfer Path Analysis (TPA) is a tool to evaluate the contribution of different energy propagation paths between a source and a receiver, linked to each other by a number of connections. TPA is typically used to quantify and rank the relative importance of these paths in a given frequency band, determining the most significant one to the receiver. Basically, two quantities have to be determined for TPA: the operational forces at each transfer path and the Frequency Response Functions (FRF) of these paths. The FRF are obtained either experimentally or analytically, and the influence of the mechanical impedance of the source can be taken into account or not. The operational forces can be directly obtained from measurements using force transducers or indirectly estimated from auxiliary response measurements. Two methods to obtain the operational forces indirectly – the Complex Stiffness Method (CSM) and the Matrix Inversion Method (MIM) – associated with two possible configurations to determine the FRF – including and excluding the source impedance – are presented and discussed in this paper. The effect of weak and strong coupling among the paths is also commented considering the techniques previously presented. The main conclusion is that, with the source removed, CSM gives more accurate results. On the other hand, with the source present, MIM is preferable. In the latter case, CSM should be used only if there is a high impedance mismatch between the source and the receiver. Both methods are not affected by a higher or lower degree of coupling among the transfer paths.

A focused two-dimensional air-coupled ultrasonic array for non-contact generation

Air-coupled ultrasonic sources are relatively inefficient because the high impedance mismatch at the air/solid boundary means that most of the input energy (in air) is reflected at this boundary. The objective of this research is to increase efficiency—specifically an increase in ultrasonic signal amplitude—by designing and building a focused, 2D-array of electrostatic transducers (individual diameters of 38 mm). The operating frequency of this array is in the range of 50–100 kHz; this range is selected for civil infrastructure applications. Numerical simulations are used to design an array by modeling the pressure field in air, and then optimizing an array consisting of 20 transducers to create a line-source. An array is then built (following this design) and the emitted pressure field (in air) of the as built array is measured with a microphone and compared to the pressure field predicted by the numerical model. Finally, the as built focused array is used as an ultrasonic source, and its robustness is verified by comparing the numerical simulation of a transient line-load on an elastic half-space with (completely non-contact) experimentally measured values. There is excellent agreement between these two representations, which confirms the possibility of developing a completely non-contact, scanning ultrasonic system in the 50–100 kHz range.

Low Q electrically small linear and elliptical polarized spherical dipole antennas

Electrically small antennas are generally presumed to exhibit high impedance mismatch (high VSWR), low efficiency, high quality factor (Q); and, therefore, narrow operating bandwidth. For an electric or magnetic dipole antenna, there is a fundamental lower bound for the quality factor that is determined as a function of the antenna's occupied physical volume. In this paper, the quality factor of a resonant, electrically small electric dipole is minimized by allowing the antenna geometry to utilize the occupied spherical volume to the greatest extent possible. A self-resonant, electrically small electric dipole antenna is presented that exhibits an impedance near 50 Ohms, an efficiency in excess of 95% and a quality factor that is within 1.5 times the fundamental lower bound at a value of ka less than 0.27. Through an arrangement of the antenna's wire geometry, the electrically small dipole's polarization is converted from linear to elliptical (with an axial ratio of 3 dB), resulting in a further reduction in the quality factor. The elliptically polarized, electrically small antenna exhibits an impedance near 50 Ohms, an efficiency in excess of 95% and it has an omnidirectional, figure-eight radiation pattern.

Air-coupled ultrasonic testing of diffusion bonds

The diffusion bond between two steel plates can be ultrasonically evaluated, at normal incidence in an immersion experiment, by analyzing the frequency dependence of the echo reflected from the imperfect bond. The interfacial stiffness, derived from the echo amplitude, correlates well with the bond-strength. However, a non-contact method is desirable for applications where immersion or contact is not wanted or even dangerous for damaging the material. This above mentioned bond-echo technique would not work in the situation of air-coupling as the reflected echo becomes then too weak due to the high impedance mismatch at the air–solid interface. Therefore we propose a theoretical method based on the study of two neighbouring resonance frequencies of the diffusion bonded plate–plate structure. In this way the physical signal sensitive to the adhesion status is not the (too weak) echo reflected from the bond, but the resonance frequency of the whole plate–plate system, and this frequency is detectable as working at resonance ensures high enough signal levels. It was shown that the odd resonance is as well sensitive to the plate thickness as to the interfacial bond parameter, whereas the even resonance feels only the plate thickness. On the basis of a theoretical formula, it is possible to extract, from a single point measurement, out of these two resonance frequencies both the plate thickness and the interfacial stiffness. In this way bond information is separated from geometrical information. Finally it is shown that thickness differences between the plates did not affect the reliability of the bonding-strength predictions.

FEM simulations of horn loudspeakers and their experimental verification

Horn loudspeakers are in common use for sound reinforcement of large rooms because of their high efficiency. In the design of horn loudspeakers, special care has to be taken on the horn’s geometry and its interaction with the properties of the driver. In the present work, the finite-element analysis of these coupled acoustical–mechanical systems is compared with classical analytical and finite-difference methods. All simulations are compared with experimental data, obtained from two different horn loudspeakers: First, a folded bass cabinet transducer and, second, a midrange horn loudspeaker. The bass cabinet is 50×54×104 cm in size; the midrange horn measures 40×44×44 cm. Both cabinets are made from wood; therefore, the horn’s geometry can only be approximated to the ideal exponential characteristics. In general, the effect of finite horn length, resulting in reflections at the mouth due to mismatch in acoustical impedance, has to be considered. The bass horn, especially, has to be simulated very precisely, since this loudspeaker shows higher impedance mismatch. The finite-element simulations carried out with ANSYS show excellent agreement with the measurements. The sound-pressure level generated was investigated as a function of frequency and angle of radiation.

Particulate suspensions as ultrasonic contrast agents for liver and spleen.

Ultrasonic backscatter and attenuation coefficients of a medium can be increased by the addition of solid, micron-size inhomogeneities. A potentially useful agent for ultrasonic contrast of liver images has been identified. Iodipamide ethyl ester (IDE) particles can be produced in the form of dense, relatively incompressible solids with high impedance mismatch to water. The chemical, biochemical, and pharmacologic properties of the small, uniform diameter IDE particles permit safe intravenous injection followed by rapid accumulation of reticuloendothelial (RE) cells of the liver and spleen, and later elimination from these organs. Since the particles are phagocytized by RE cells, present in normal liver but not in tumors and many lesions, the selective enhancement of ultrasonic backscatter should improve detectability of lesions that are hypoechoic or isoechoic compared with surrounding tissue. The mechanisms of particle-ultrasound interaction may be described by relative motion attenuation, and scattering from a cloud of dense, incompressible spheres for the case of IDE particles in agar. Thus, values of attenuation and backscatter can be controlled by choice of ultrasound frequency and particle concentration and size. When the particles are accumulated in rat and rabbit livers, additional mechanisms induce attenuation and backscatter in excess of that predicted by IDE in agar. This preliminary work demonstrates that solid, biocompatible particles may be useful as an ultrasonic contrast agent.

A particulate contrast agent with potential for ultrasound imaging of liver

Ultrasonic backscatter and attenuation coefficients of a medium can be increased by the addition of solid, micron sized inhomogeneities. A potentially useful agent for ultrasonic contrast of liver images has been identified. Iodipamide ethyl ester (IDE) particles can be produced in the form of dense, relatively incompressible solids with high impedance mismatch to water. The chemical, biomechanical, and pharmacological properties of the small, uniform diameter IDE particles permit safe intravenous injection followed by rapid accumulation by reticuloendothelial (RE) cells of the liver and spleen, and later elimination from these organs. Since the particles are phagocytized by RE cells, present in normal liver but not in tumors and many lesions, the selective enhancement of ultrasonic backscatter should improve detectability of lesions which are hypo- or iso-echoic compared to surrounding tissue. The mechanisms of particle-ultrasound interaction may be described by relative motion attenuation, and scattering from a cloud of dense, incompressible spheres for the case of IDE particles in agar. Thus, values of attenuation and backscatter can be controlled by choice of ultrasound frequency and particle concentration and size. When the particles are accumulated in rat livers, additional mechanisms induce attenuation and backscatter in excess of that predicted by IDE in agar. This preliminary work demonstrates that solid, biocompatible particles may be useful as an ultrasonic contrast agent.