English: A diagram showing the definition of bandwidth (B) for a bandpass filter. f₀ is the centre frequency, f_H is the higher cut-off frequency, and f_L is the lower cut-off frequency. The 0 dB level is the level of the peak of the bandpass response, which is not necessarily located at the centre frequency. Also the centre frequency is located at either the arithmetic or geometric mean of the upper and lower cut-offs depending on context and conventions.