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Focus wave mode pulses from cylindrical apertures

University of British Columbia

Previous works have shown that it is possible to create a pulsed beam of acoustic energy which maintains amplitude and beam width better than either conventional single frequency or wide-band beams of comparable frequency content. The reconstruction technique to date uses Helmholtz' approximation on a planar array composed of individually addressable elements. In this work, a new array geometry is studied. Simulated focused scalar fields representing approximations to the Focus Wave Modes (FWM) are generated from a finite cylindrical aperture. The driving functions are derived using Huygens' representation for an infinite cylindrical surface. Performance of this new launching scheme is compared to the planar array launch of FWMs and Gaussian pulses. Through the course of simulations, we illustrate how the array spacing or discretization and the surface truncation affect the reconstruction quality and performance. The influence of array size and shape defined by the ratio of length over radius on the spreading and attenuation of the radiated beam is studied. The beam width, amplitude and sidelobe intensity are compared to those obtained from a planar array. It is shown that the cylinder length can be truncated to a certain value, Z[sub pc1], without affecting the pulse reconstruction at the origin. A shorter array can also be used with some loss in reconstruction quality. The generated localized wave from the cylindrical aperture is also compared to the Gaussian pulse. It is shown that a sufficient cylinder length will generate a field with better quality close to the array and similar performance and quality for further distances than that synthesized by a planar array of the same radius. Finally we use an approximation of the source functions to solve the problem caused by space-time inseparability of localized waves. We show that it is possible to find a simple relation between the various functions that drive the array elements. Without affecting significantly the radiated field, this method reduces dramatically the number of independent generators that are necessary to launch the pulse. The pulse can be generated from a cylindrical aperture using a single generator—while for a planar array of the same radius, more than 4000 independent generators would be required to obtain a similar beam quality.

Thesis/Dissertation

application/pdf

2009-02-10

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http://hdl.handle.net/2429/4423

Electrical and Computer Engineering

University of British Columbia

UBCV

DSpace