Elimination of standing wave effects in ultrasound radiation force excitation in air using random carrier frequency packets

The ultrasound radiation force has been used for noncontact excitation of devices ranging from microcantilevers to acoustic guitars. For ultrasound radiation force excitation, one challenge is formation of standing waves between the ultrasound transducer and the device under test. Standing waves result in constructive/destructive interference causing significant variations in the intensity of the ultrasound field. The standing-wave induced intensity variations in the radiation force can result from minor changes in the transducer position, carrier frequency, or changes in the speed of sound due to changes in ambient temperature. The current study demonstrates that by randomly varying the ultrasound carrier frequency in packets, it is possible to eliminate the negative consequences resulting from the formation of standing waves. A converging ultrasound transducer with a central frequency of 550 kHz was focused onto a brass cantilever. The 267 Hz resonance was excited with the ultrasound radiation force with a carrier frequency that randomly varied between 525 kHz to 575 kHz in packets of 10 cycles. Because each packet had a different carrier frequency, the amplitude of standing wave artifacts was reduced by a factor of 20 compared to a constant frequency excitation of 550 kHz.