A bandpass filter, also known as BPF, is a type of filter circuit that unlike low pass and high pass filters passes frequencies within a certain range and attenuates frequencies outside that range. Bandpass is an adjective that describes a type of filter or filtering process. It differs from passband, which refers to the actual portion of the affected spectrum. Hence, one might say “A dual bandpass filter has two passbands.”

A bandpass signal is a signal containing a band of frequencies not adjacent to zero frequency, such as a signal that comes out of a bandpass filter. An ideal bandpass filter would have a completely flat passband (e.g. with no gain/attenuation throughout) and would completely attenuate all frequencies outside the passband. Additionally, the transition out of the passband would have brick walls characteristics.

Here are some standard parameter settings for bandpass filters:


Frequency Band


Sets the frequency band unaffected by the filter. In other words, all frequencies outside the specified frequency band will be attenuated. Once a frequency goes beyond the specified frequency band limits, its amplitude will gradually decrease till it has no amplification (the signal will be very weak, or non-existent)


Q Factor


A band-pass filter can be characterized by its Q factor. The Q-factor is the reciprocal of the fractional bandwidth. A high-Q filter will have a narrow passband and a low-Q filter will have a wide passband. These are narrow-band and wide-band filters.




It sets the degree of amplification applied to the audio signal. Furthermore, it increases or decreases the resulting sound volume. The amplification will affect frequencies in both the pass and attenuation bands.




Sets the frequency attenuation rate. This control changes the transition between the affected and non-affected frequencies. Higher values correspond to a sharper transition.


To sum up


In reality, no bandpass filter is ideal. The filter does not attenuate all frequencies outside the desired frequency range completely. In particular, there is a region just outside the intended passband which attenuates frequencies but does not reject them. This is a filter roll-off. It is usually expressed in dB of attenuation per octave or decade of frequency. Generally, the design of a filter seeks to make the roll-off as narrow as possible, thus allowing the filter to perform as close as possible to its intended design.


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