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# Long description

This figure consists of four graphs of gain against frequency, representing four types of filter. In each case, the gain is either at 1 or at 0, depending on the frequency. Areas where the gain is 1 are labelled ‘passband’, while areas where the gain is 0 are labelled ‘stop band’. The transitions between 1 and 0 (or vice versa) are vertical, and labelled ‘cut-off frequency’.

Part (a) is a low-pass filter. There is a single cut-off frequency, f subscript c. At frequencies below the cut-off, the gain is 1 (passband). At frequencies above the cut-off, the gain is 0 (stop band).

Part (b) is a high-pass filter. There is a single cut-off frequency, f subscript c. At frequencies below the cut-off, the gain is 0 (stop band). At frequencies above the cut-off, the gain is 1 (passband).

Part (c) is a band-pass filter. There are two cut-off frequencies, f subscript c1 and f subscript c2. At frequencies below f subscript c1 and above f subscript c2, the gain is 0 (stop band). At frequencies between f subscript c1 and f subscript c2, the gain is 1 (passband).

Part (d) is a band-stop filter. There are two cut-off frequencies, f subscript c1 and f subscript c2. At frequencies below f subscript c1 and above f subscript c2, the gain is 1 (passband). At frequencies between f subscript c1 and f subscript c2, the gain is 0 (stop band). Because the stop band is a narrow region between two passbands, the stop band is also known as the notch in this type of filter.