Detection of Ultrasonic Lamb Waves in Paper Using an Optical MEMS Microphone
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Laser ultrasonics has been used to measure the bending stiffness of paper products by measuring the dispersion of ultrasonic plate waves. In laser ultrasonics, ultrasound can be generated by absorption of pulsed laser spot while detection can be carried out by Laser Doppler Interferometry. The research presented in this paper describes a new method to detect ultrasonic plate waves using a recently developed acoustic transducer, more specifically an optical Micro ElectroMechanical System (MEMS) microphone with broadband capability. The MEMS device operates as a non-contact proximity probe placed less than ¼ a millimeter away from the plate. The signals are detected with a capacitive micromachined ultrasonic transducer (cMUT) in which the back electrode of the capacitive transducer on a transparent substrate is shaped as an optical diffraction grating. The displacement of the transducer membrane is determined using an optical interferometer. By applying voltage to deflect the membrane electrostatically, the detection sensitivity is kept at an optimum level. The main purpose of the research presented herein was to test this MEMSs ability to detect ultrasonic waves propagating through paper, to increase the signal-to-noise ratio (SNR), and to calibrate the device in order to quantify the limitations on sensitivity in the context of the detection of ultrasound in paper. Similar tests were conducted for comparison with a modified Mach-Zehnder Interferometer, a more traditional method used for laser ultrasonic detection, and its results are presented in this paper.