Spatial distribution of acoustic radiation force modal excitation from focused ultrasonic transducers in air

Recent studies have utilized the acoustic radiation force for non-contact modal excitation of structures in air. When two ultrasonic frequencies, for example, f1 = 610 kHz and f2 = 600 kHz, are incident on an object, the acoustic radiation force produces a driving force at a difference frequency f1-f2 = 10kHz. The current study compared the spatial distribution of driving force from a pair of co-focused transducers emitting f1 and f2, to a single focused transducer emitting an amplitude modulated signal of both f1 and f2. The difference frequency ranged from 400 Hz to 80 kHz. Ultrasonic transducers, with focal spot diameters of ~2 mm mounted on translation stages, could be directed at a 100kHz PCB-378C01 microphone or a 19.8 x 6.8 x 0.37 mm clamped-free brass cantilever monitored by a Polytec PSV-400 vibrometer. When mixing of frequencies f1 and f2 was solely due to the acoustic radiation force, the driving force was localized to a region a few mm in diameter. However, in other cases, very broad spatial distributions of difference frequency excitation were measured; this indicated non-acoustic radiation force mixing of f1 and f2, such as within the transducer. The practical implications for non-contact modal excitation using acoustic radiation force will be discussed.