Phase changes of ultrasonic bulk waves through focusing measured using a noncontact ultrasonic method

Abstract

Focusing an ultrasonic surface or bulk wave to a point can maximise the amount of ultrasonic energy at a point and increase the sensitivity of a measurement. Using noncontact ultrasonic methods we focus an ultrasonic wave pulse on or in a sample. Focusing can be achieved by using a shaped source or by the surface profile of the sample itself.Experiments and modelling confirm the known result that once the wave pulse passes through the focal point, it will undergo a phase shift. Using non-contact ultrasonic methods on a sample with spherical or cylindrical geometry, one can exploit the surface profile of the sample to focus an ultrasonic wave, as there is no refraction to consider at the sample surface under the transducer.

Simulation of wave propagation inside a human eye: acoustic eye model (AEM)

“…The evolution of the shape of the wave, which transforms from a half-cycle, monopolar rarefaction pulse to a full single-cycle, rarefaction-compression wave when it passes through the focus, is indicative of the Gouy phase shift effect which was also observed by other authors in the case of focusing of surface and bulk ultrasonic waves in various configurations [23][24][25][26][27][28][29]. The Gouy phase shift is a well-known nπ/2 axial phase shift experienced by any converging wave when it passes through a focus [30].…”

The effect of EMAT coil geometry on the Rayleigh wave frequency behaviour

“…Consideration must also be given to any effects due to the generation coil, or the geometric curvature of the focused EMATs. There is no theoretical reason for this to affect the frequency content, but it could affect the signal phase [24].…”

“…This shows there is some discrepancy between the focused and unfocused simulations, with a slight tendency towards lower frequencies in the unfocused simulations. Discrepancies are likely due to the spreading of the unfocused wavefront as opposed to the coherent wavefront seen at the focal point of the focused design, leading to variations in phase as the wave reaches each wire in the coil [24]. In both cases the peak is seen at around 2 MHz for the x velocity data, suggesting the generation is closer to the delta spike model shown in section 5.1, while the z displacement data is closer to the lower peaks expected from the square wave model.…”