Normal mode analysers for standing waves on liquids

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Abstract
This paper shows how it is possible to analyse standing waves on a fluid surface into normal modes by means of a measuring device. For waves with prescribed symmetry, the area of fluid in contact with strips of suitable geometry gives a signal proportional to the amplitude of a desired mode. When there are no special symmetry conditions the analyser takes the form of suitably shaped lozenges distributed above the fluid surface. The capacitance between the lozenges and the fluid surface is proportional to the volume between the surface and the analyser, which in turn depends on the amplitude of a given normal mode in the surface wave. The geometry of the desired mode determines the shape and distribution of lozenges in the analyser. It is noted that a volume analyser can also be used for exciting a desired set of normal modes on a fluid surface.