Flange-type filters are used to filter out unwanted frequencies from a signal. They can be used as an alternative to band pass or low pass filters. The flange type filter consists of two parallel plates which have holes drilled through them. These holes are arranged such that they form a series of resonant cavities. When a sinusoidal voltage is applied across the plates, it will resonate at certain frequencies determined by the size and spacing of the holes. This allows the user to select a particular frequency range to pass through the filter.