The analysis requires at least two wave gauges at different positions for determination of incident and reflected waves. The positioning of the wave gauges greatly affects the output of the analysis and for a two-gauge analysis singularities for frequencies representing an integer number of half wave lengths are produced:
(7.2)
where Dl12 is the internal gauge distance and L is the wave length at a given frequency. In practice, the governing equations of the analysis become illconditioned close to the singularity and Goda & Suzuki (1976) recommends that the following range of internal gauge distances should be avoided:
(7.3) 
The above relation is based on regular wave experiments. For irregular waves this means that only approximately 80% of the frequencies will be included in the analysis. Hence, the two-gauge method cannot be recommended for irregular waves.
For irregular waves more wave gauges should be used as it would generally eliminate (a part of) the singular frequencies and improve the measurement quality due to simple over-determination.
An additional wave gauge yields another two internal gauge distances Dl13 , Dl23 , of which the latter must be chosen such that the largest possible frequency range for which the bands around the singularities (equation give above) do not coincide for all internal gauge distances. Numerical investigations has resulted in a number of appropriate choices, the best ones being:
(7.4) 
Analysis results may be improved further by four or five-gauge analysis as additional gauges are placed with different internal distances.
