Chapter 9 Optically Detecting Wavefronts and Wave Speeds in Water Using Refracto-Vibrometry Matthew T. Huber, Brent K. Hoffmeister, and Thomas M. Huber Abstract 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 PSV400 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. Keywords Acoustics • Vibrometry • Ultrasound • Refracto-vibrometry • Wave propagation 9.1 Introduction Ultrasound is used in a wide variety of applications ranging from the medical field to structural testing and dynamics. However, one of the challenges associated with the use of ultrasound is that there are limited non-invasive options available for observing traveling wavefronts and visualizing how they react to their surroundings. This means that systems often must be physically altered in order for one to have an understanding of what is going on in them. Refracto-vibrometry, [1–5] which involves using a laser Doppler vibrometer to optically sample the acoustic field, provides an alternative to the conventional ultrasound measurement methods traditionally performed by transducers. Much of the research performed with ultrasound takes place underwater because water functions as a coupling medium between the transducer and the object being analyzed. This means the amount of signal attenuated by the water is usually not significant. Ultrasonic transducers, for emission and detection of ultrasonic waves in water, are generally constructed using piezo-electric detectors. When acoustic waves strike the detector, an electrical signal is generated. Alternately, for measurements of acoustic regions with a resolution on the order of 1 mm, needle hydrophones are often used. To characterize sound fields with hydrophones, it is necessary to raster the position of the hydrophone to multiple locations and observe the acoustic field at each point. Refracto-vibrometry provides an alternative to the conventional methods of characterizing sound fields and analyzing ultrasound signals. The instrument utilized for this technique is a laser Doppler vibrometer [6]. A vibrometer is designed to measure the Doppler shift of a laser after it is reflected from a vibrating surface. However, in the Refracto-vibrometry method used for the current study, the vibrometer is directed at a motionless retroreflective surface and is used to detect the wavefront of the ultrasound pulse [2]. The acoustic waves are fluctuations of the density of the medium, in the current case, water. Because the index of refraction of water is related to its density, acoustical wave density variations also result in a variation of index of refraction. If an acoustic wave passes through the laser from the vibrometer, the index of refraction M.T. Huber • B.K. Hoffmeister Department of Physics, Rhodes College, 2000 North Parkway, Memphis, TN 38112, USA T.M. Huber ( ) Department of Physics, Gustavus Adolphus College, 800 College Avenue, Saint Peter, MN 56082, USA e-mail: huber@gac.edu © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_9 95
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