Abstract: The majority of underwater acoustic systems use narrow band sounding signals. Consequently, there is extensive knowledge on how the relevant spatial filters should be analysed and designed. The difficulty begins when the sounding signal has a broad spectrum. Applying the results of narrow band signal analysis to broadband signals leads to serious errors. The same is true for spatial filtration methods when applied to narrow band signals. The article presents a general method for analysing spatial filters designed for broadband signals. It also gives an assessment of the errors caused by applying spatial filtration methods designed for narrow band signals.