Selective excitation using phase shifted ultrasound radiation force from focused transducers in air

In recent years, it has been demonstrated that the ultrasound radiation force can be used as a noncontact technique for modal excitation. A novel capability of this method is the ability to perform selective excitation. A phase shift is imparted between the modulation signals emitted from a pair of ultrasound transducers focused at different positions on an object. When there is no phase difference between the radiation force produced by the transducer pair, they push in unison on the object, which induces symmetric or transverse eigenstates. When there is a 180° phase shift between the radiation force from the transducers, they will excite antisymmetric or torsional eigenstates. We describe the first demonstrations of this selective excitation using high-frequency focused ultrasound transducers.