The problem of determining the pressure in a circular cylindrical cavity of radiusbconnected to a circular input waveguide of radiusais formulated in terms of an integral equation. Values of the reflection coefficient in the guide and of the pressure in the cavity are given in terms of the frequency and of the dimensionless ratioa/b. The problem of the cavity with an input and an output waveguide is also analyzed, and values of the transmission coefficient of this filter are obtained.