13 June 2005 TV-holographic mapping of airborne sound fields for the design of parametric arrays
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Abstract
Parametric acoustic arrays are built to generate highly directional audio sound by nonlinear interaction of ultrasound. Arrays especially built for applications at high audio sound pressure use the most effective ultrasonic transducer for airborne sound, i.e., a piezoelectric (PZT) bimorph. Since the individual transducer elements are very small (<16 mm in diameter) several hundred of them have to be combined to reach the desired audio sound pressure level. For high performance it is a prerequisite that all transducers radiate in phase. However, fluctuations in their properties result in according fluctuations in their phases. The construction of such a device therefore requires a non-intrusive technique for monitoring amplitude and phase of a three-dimensional sound field without creating any nonlinearity. TV-holography or Electronic Speckle Pattern Interferometry (ESPI) in its time-averaging mode combined with reference wave modulation has been applied for this purpose. The recordings represent a two-dimensional projection of the sound field integrated along the viewing direction. The three-dimensional field is obtained from many such projections through the sound field at different angles in a tomographic setup. Inversion by filtered backprojection yields the three-dimensional sound amplitude and phase that can be utilized to optimize transducer operation. The performance of such a system is demonstrated in the development of an ultrasonic array at 38.5 kHz. It is shown how the generation of highly directive audio sound has been improved by guidance from the optical results. The highly directional source of audio sound finally produced is needed for an application in monument research where loose areas in historical murals have to be identified.
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Holger Joost, Holger Joost, Klaus D. Hinsch, Klaus D. Hinsch, Gerd Gulker, Gerd Gulker, } "TV-holographic mapping of airborne sound fields for the design of parametric arrays", Proc. SPIE 5856, Optical Measurement Systems for Industrial Inspection IV, (13 June 2005); doi: 10.1117/12.612573; https://doi.org/10.1117/12.612573
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