stopband
A band of frequencies, between specified limits, that a circuit, such as a filter or telephone circuit, does not transmit.
Note 1: Frequencies above the lower limit and below the upper limit are not transmitted, i.e., are not allowed to pass.
Note 2: The limiting frequencies are those at which the transmitted power level increases to a specified level, usually 3 dB below the maximum level, as the frequency is decreased or increased from that at which the transmitted power is a minimum.
Note 3: The difference between the limits is the stopband bandwidth, usually expressed in hertz.