3. Estimation of the Dynamic Focused Ultrasound Radiation Force Generated by an Ultrasonic Transducer

Conventional excitation techniques such as modal impact hammer and shakers are commonly used in experimental modal testing. However, these excitation approaches require the excitation device to be in direct contact with test articles. It can result in distorted measurements, particularly for small structures, such as a MEMS cantilever and thumb nail size turbine blade. In addition, it is physically difficult or even impossible to apply these contact type excitations to some structures such as low stiffness structures or biological tissues. Moreover, these conventional excitations have limited bandwidth, usually less than 10 kHz, and thus are not applicable to extract information in higher frequency modes. Dynamic focused ultrasound radiation force has been recently used to excite structures with sizes ranging from micro to macro-scale and having a frequency bandwidth from tens of Hertz to up to 100 kHz. Therefore, it can potentially be used as an alternative, non-contact excitation method to these conventional contact excitation techniques for experimental modal analysis. Yet, this force remains to be quantified and calibrated in order to obtain the input-output relationship necessary to compute accurate frequency response functions of test structures. In this work a spherically focused ultrasound transducer (UT) is driven by double sideband suppressed carrier amplitude modulation (DSB-SC AM) signals with a scanning difference frequency and randomly varying carrier frequency. The radiated pressure field generated by the UT is experimentally measured employing a pressure microphone, which acts as a target object for the ultrasonic waves. Then, the recorded values are used to analytically evaluate the dynamic focused ultrasound radiation force. Results show that the measured radiation pressure and estimated force are characterized by a focal spot small enough to be compared to an impact hammer tip appropriate for future modal testing.