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The application of boundary element methods at NPL to near-field acoustic measurements on cylindrical surfaces.

Wright, L; Robinson, S P; Humphrey, V F*; Hayman, G (2005) The application of boundary element methods at NPL to near-field acoustic measurements on cylindrical surfaces. NPL Report. DQL-AC 011

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In this report, a description is given of work at the National Physical Laboratory to implement near-field techniques to characterise the far-field response of acoustic sources. The approach uses boundary-element methods to solve the integral form of the Helmholtz equation in order to predict the far-field response from discrete samples of the complex acoustic pressure measured on cylindrical surfaces. This has been done using a two-stage process, first deriving the required pressure gradient on the surface, and then calculating the desired far-field acoustic response. Both the Burton-Miller method and the CHIEF method have been implemented to overcome the uniqueness problem inherent in this method and a limited comparison made between them. To reduce computational size of the problem, use has been made of Fourier decomposition of the data from the scans. To speed up the computation, the NPL distributed computing network has been used to process the results for each of the Fourier coefficients, reducing the computational times by a factor of up to 20.
To undertake the near-field measurements, the NPL Open Tank facility was used. This is a state-of-the-art underwater acoustic test facility for undertaking high quality near-field scans. The facility benefits from a precision positioning system that operates under computer control to perform automated scans over designated surfaces in the near-field of an acoustic source. Measurements were made using a small probe hydrophone, with the amplitude and phase of the acoustic pressure recorded at discrete points on the cylindrical surface.
To exercise the method, measurements have been made of the field produced by a specially constructed directional source at frequencies of 13.9 kHz and 27.5 kHz. The results of the farfield predictions calculated from the near-field data have been validated by comparison with actual measurements made in the far-field, with agreement achieved of better than 1 dB over much of the acoustic beam-width. A discussion is presented of the influences on the accuracy of the method, and the possible causes of any discrepancies between predicted and measured results.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DQL-AC 011
Subjects: Acoustics
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3155

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