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In this paper a frequency-correction factor is proposed for Bethe's small-aperture coupling relation for a transverse diaphragm in a waveguide. Experimental data on many shapes and sizes of apertures have shown this factor to be highly accurate up to and somewhat above the resonant frequency of each aperture. Also included are approximate formulas for the resonant Q and the resonant length of an aperture, and for the effect of wall thickness.
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S.B. Cohn (1952) studied this question.
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