In telecommunications, optics, and acoustics, passband is the portion of spectrum, between limiting frequencies (or, in the optical regime, limiting wavelengths), that is transmitted with minimum relative loss or maximum relative gain by a filtering device. For example, radio receivers generally include a tunable band-pass filter with a passband that is wide enough to accomodate the bandwidth of a single station.

Passbands are found in many systems outside of telecommunications. For example, most traditional musical instruments are tunable sonic band-pass filters with narrow passbands. Woodwind instruments such as the flute and penny whistle are good examples: the flute is stimulated by broad-band sonic noise at the mouthpiece but resonates only in a narrow passband around the fingered note. Overblowing a flute (that is, playing higher notes with the same fingering as a lower note) is possible because the flute has multiple passbands for any given fingering: the note that emerges is dependent on both the fingering and the spectrum of wind noise at the mouthpiece.

In general, there is an inverse relationship between the width of a filter's passband and the time required for the filter to respond to new inputs: broad passbands yield faster response. This is a consequence of the mathematics of Fourier analysis.

Note 1: The limiting frequencies are defined as those at which the relative intensity or power decreases to a specified fraction of the maximum intensity or power. This decrease in power is often specified to be the half-power points, i.e., 3 dB below the maximum power.

Note 2: The difference between the limiting frequencies is called the bandwidth, and is expressed in hertz (in the optical regime, in nanometers or micrometers).

Note 3: The related term "bandpass" is an adjective that describes a type of filter or filtering process; it is frequently confused with "passband", which refers to the actual portion of affected spectrum. The two words are both compound words that follow the English rules of formation: the primary meaning is the latter part of the compound, while the modifier is the first part. Hence, one may correctly say 'A dual bandpass filter has two passbands'.

Source: original from Federal Standard 1037C and from MIL-STD-188