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Two methods for determination of the effective wavenumber of Gaussian beams in absolute gravimeters

Křen P., Pálinkáš V.

Metrology and Measurement Systems

2018

Vol. 25, nr 4

701--713

This paper presents two methods for evaluation of the effective wavenumber of nearly-Gaussian beams in laser interferometers that can be used for determination of a so called diffraction correction in absolute gravimeters. The first method, that can be simply used in situ, is an empirical procedure based on the evaluation of the variability of g measurements against the amount of light limited by an iris diaphragm and transmitted to a photodetector. However, precision of this method depends on the beam quality similarly as in the case of the conventional method based on measurement of a beam width. The second method, that is more complex, is based on beam profiling in various distances and on calculation of the effective wavenumber using the second spatial derivative of a non-ideal beam field envelope. The measurement results achieved by both methods are presented on an example of two absolute gravimeters and the determined diffraction corrections are compared with the results obtained by measurements of beam width. Agreement of methods within about 1 μGal have been obtained with average diffraction corrections slightly exceeding +2 μGal for three FG5(X) gravimeter configurations.

Diffraction correction of frequency response for loudspeaker in rectangular baffle

Dobrucki A.

Archives of Acoustics

2006

Vol. 31, No. 4

537-542

The problem of diffraction correction of frequency response of loudspeaker in finite baffle was considered by H. Olson [2] in thirties of XX century. He found experimentally the frequency responses of corrections caused by interference of direct and diffracted waves. He predicted necessity of eccentric placing of loudspeaker in the baffle in order to avoid a deep dip in frequency response for frequency higher as twice as for lower limiting frequency of the baffle. However, the problem was never solved theoretically except the simplified case of loudspeaker centrally placed in circular baffle. In the paper a theory of diffraction on the baffle edge of the wave radiated by a point source eccentrically placed in the rectangular baffle is presented. The results of calculations of the diffraction corrections for various configurations of the source and the baffle are presented as well. The statistical dependence between irregularity of the frequency response of the diffraction correction above the lower limiting frequency of the baffle and standard deviation of the diffraction path is shown. The conclusions can be useful for design process of the baffles.