Measurement of Vibration Resulting from Non-contact Ultrasound Radiation Force

In modal testing, the most common excitation method is a transducer in mechanical contact with the object under test. While this method is effective, there are delicate structures where it is desirable to excite vibrations without physical contact. The ultrasound radiation force provides a noncontact excitation method resulting from the nonlinear interaction of sound waves scattering from an object. The incident ultrasound consists of sine waves with frequencies of f(1) and f(2); the resulting radiation force has a component at the difference frequency f(1)-f(2). By combining the difference frequency radiation force with a scanning vibrometer, previous studies have demonstrated completely non-contact measurements of resonance frequencies and operating deflection shapes of structures ranging from microcantilevers to classical guitars. In the current study, a 19.6 by 8.1 by 0.37 mm clamped-free brass cantilever, with a resonance frequency of 610 Hz, was excited using the radiation force from a focused ultrasound transducer. By mounting the transducer on a computer-controlled translation stage, it enabled measurements of the edge-spread function for the transducer; measuring this distribution is an important first step towards quantifying the applied radiation force.