Application of the focused ultrasonic transducer for remote measurements of ultrasonic fields

Abstract

Measurements of the ultrasonic fields are an essential part of ultrasonic inspections in various scientific and industrial applications. The measured ultrasonic fields provide information about the properties of their sources, e.g., inner structure, defects, delaminations etc.
When there is no need to measure the whole ultrasonic field at the selected spatial zone, the measurement of the field point-by-point can be done using hydrophones, e.g. ultrasonic transducers of a small diameter. The field is measured at the point where the hydrophone is located. The use of hydrophones for the ultrasonic field measurements is complicated due to the need to place these transducers at the exact points of the field, that distorts the field, and also the limited spatial resolution, which is defined by the size of the transducer. The high costs for the producing of transducers, which diameter is equal 1-2mm, and the limited bandwidth also make the use of such transducers unacceptable.
For this purpose it is possible to use focused ultrasonic transducers. Such transducers are used in ultrasonic microscopy and subsurface measurements.
In this paper the possibility to use the focused transducers for remote measurements of the radiated or backscattered ultrasonic fields is analysed. This gives possibility to obtain the ultrasonic signals without placing the transducer at the exact point of the ultrasonic field. The bandwidth and spatial resolution or the size of the focal point can be chosen by selecting the transducer with desirable parameters like diameter, focal distance and frequency.
One of the problems is the estimation of the accuracy or what is the difference between the true ultrasonic field and the ultrasonic field measured remotely using focused transducers. There are no transducers, which would radiate the standard ultrasonic field, so there is no practical instrumentality to compare the measured ultrasonic field with a true field. The theoretical modelling is proposed for the calculations of the equivalent of the real field. The calculated real field is tagged as a true field. The comparison of the measured ultrasonic field is proposed to be done in order to estimate the accuracy of the measurements of the ultrasonic fields using focused ultrasonic transducers. Different criterions are proposed for the estimation of the true and measured ultrasonic fields.
The method of the remote measurements of the ultrasonic fields using focused transducers is proposed, also the estimation of the measured field is proposed using the theoretically calculated true ultrasonic field.