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It is shown in this paper that the conventional triangulation technique is not very reliable for locating the impact point even in isotropic plates when the sensors are placed close to the point of strike for two reasons: First, it is difficult to pinpoint the exact time of arrival of the signal and, second, the Lamb modes in a plate are dispersive. Dispersive signals attenuate differently at various frequencies and propagate with different speeds causing distortions in the received signals, and thus introduce error in the time of flight measurement. The triangulation technique assumes that wave speeds in all directions are the same, which is not true for anisotropic plates. Here an alternative approach based on an optimization scheme is proposed to locate the point of impact in isotropic and anisotropic plates. A formulation is presented for the general anisotropic case. Experiments are carried out with an aluminum plate by dropping balls on the plate and picking up acoustic signals at different locations. The impact points predicted by the conventional triangulation technique and the proposed modified method are compared for this isotropic plate. Then it is investigated how the prediction would change if the plate is assumed to have some anisotropy.

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A number of techniques are available for acoustic source localization in isotropic plates without knowing the material properties of the plate. However, for a highly anisotropic plate acoustic source localization requires some knowledge of the plate material properties or its group velocity profile. In absence of this information one requires a large number of sensors to predict the acoustic source point in the plate. All proposed techniques for acoustic source localization with a few sensors assume the straight line propagation of waves from the source to the receiving sensor with an average group velocity when the plate material properties are not known. However, this assumption is not true for an anisotropic plate. Although the currently available techniques work well for weakly anisotropic plates since the wave path does not deviate significantly from the straight line propagation they fail miserably for highly anisotropic plates.In this paper acoustic source is localized in an anisotropic plate when non-circular wave front is generated. Direction vectors of wave fronts are obtained from the Time-Difference-Of-Arrivals (TDOA) at three sensors placed in a cluster. Four such direction vectors are then utilized in geometric vector analysis to accurately obtain the acoustic source location. The proposed technique is illustrated on an orthotropic plate that generates rhombus shaped wave front. It should be noted that the proposed technique does not require wave propagation along a straight line from the source to the sensor. It also does not need the knowledge of the material properties of the plate.

Highlights► Age-related tissue stiffening has a profound effect on human morbidity and mortality. ► Scanning acoustic microscopy can quantify the localised stiffness of discrete tissue components in situ. ► Age-related aortic stiffening is both localised to collagen fibril-rich regions and associated with collagen fibrosis. ► Age-related structural remodelling events can be directly linked with in situ mechanical changes using scanning acoustic microscopy.

Among the methods for the determination of mechanical properties of living cells acoustic microscopy provides some extraordinary advantages. It is relatively fast, of excellent spatial resolution and of minimal invasiveness. Sound velocity is a measure of the stiffness or Young's modulus of the cell. Attenuation of cytoplasm is a measure of supramolecular interactions. These parameters are of crucial interest for studies of cell motility, volume regulations and to establish the functional role of the various elements of the cytoskeleton. Using a phase and amplitude sensitive modulation of a scanning acoustic microscope (Hillman et al., 1994, J. Alloys Compounds. 211/212:625-627) longitudinal wave speed, attenuation and thickness profile of a biological cell are obtained from the voltage versus frequency or V(f) curves. A series of pictures, for instance in the frequency range 980-1100 MHz with an increment of 20 MHz, allows the experimental generation of V(f) curves for each pixel while keeping the lens-specimen distance unchanged. Both amplitude and phase values of the V(f) curves are used for obtaining the cell properties and the cell thickness profile. The theoretical analysis shows that the thin liquid layer, between the cell and the substrate, has a strong influence on the reflection coefficient and should not be ignored during the analysis. Cell properties, cell profile and the thickness of the thin liquid layer are obtained from the V(f) curves by the simplex inversion algorithm. The main advantages of this new method are that imaging can be done near the focal plane, therefore an optimal signal to noise ratio is achieved, no interference with Rayleigh waves occurs, and the method requires only an approximate estimate of the material properties of the solid substratum where the cells are growing on.