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Refracto-vibrometry is a technique that uses a laser Doppler vibrometer to measure acoustic pressure fields. The vibrometer laser is directed through a medium towards a stationary retroreflective surface. Acoustic waves (density variations) for which the wavefronts pass through the laser, as the beam travels from the vibrometer to the retroreflector and back, cause variations in the integrated optical path length. This results in a time-varying modulation of the laser signal returning to the vibrometer, enabling optical detection of the acoustic wavefronts. In the current experiment, a Polytec PSV-400 scanning laser Doppler vibrometer, sampled at 100 MHz, monitored the waves emitted by a 1 MHz Panametrics V303 ultrasound transducer immersed in a water tank. The time-varying signal detected by the vibrometer at numerous scan points was used to generate videos of the time evolution of acoustic wavefronts; these videos will be presented. Refracto-vibrometry was also used for optical measurements of the time of flight of ultrasonic waves through different materials, including samples of lead and fabricated bone. This enabled determination of wave propagation speeds. The wave speeds obtained with optical detection using refracto-vibrometry were in agreement with measurements using a conventional ultrasonic transducer to detect the wavefronts. 

In modal testing, a common excitation method is a transducer in mechanical contact with the object under test. However, for some structures it is desirable to excite vibrations without physical contact. One promising excitation technique is the acoustic radiation force. However, a challenge in using this technique is that the acoustic radiation force is spread out over a finite-diameter focal region. We describe a method to directly measure the spatial distribution of this force. An ultrasound transducer emitted sine waves with frequencies of, for example f1 = 600.610 kHz and f2 = 600 kHz; the resulting radiation force had a component at the difference frequency f1-f2 = 610 Hz. A MicroAcoustic Instruments BAT6 ultrasound transducer was focused to an approximately 2 mm diameter spot on a 19.6 by 8.1 by 0.37 mm clamped-free brass cantilever with a 610 Hz fundamental frequency. A vibrometer measured the response as this focus spot traversed the edge of the cantilever. This enabled determination of the distribution of the acoustic radiation force being delivered by the transducer. This may be helpful in future studies that involve modeling the force applied to a structure using the acoustic radiation force.