Band-Pass Filter

The term Band-Pass Filter (BPF) describes the filtering process. BPF refers to a type of filter circuit that passes frequencies within a specific range. In addition, it weakens signals that fall out of that range. In relation to this, a bandpass signal is a one that involves a band of frequencies not joined to a zero frequency. This signal usually comes out of a Band-Pass Filter. A perfect Band-Pass Filter would have a totally flat passband that would mitigate all frequencies that fall out of said passband. As a result, the transition would be completely solid.

Band-Pass Filter: a Description


The Bass-Pass Filters have standard settings regarding frequency band, quality factors, gain, and slope. To understand this, let’s start from the beginning: frequency. First, the frequency band is capable of setting a band unaffected by the filter. Then, it mitigates the frequencies that fall outside of a specific spectrum. Afterwards, once it reaches its limit, the amplitude tends to decrease until the signal turns very weak or disappears. Since the Q-factor is reciprocal to the fractional bandwidth, a high-quality filter will have a narrow passband (and a low-quality filter will have a wide passband).


Another standard-setting regarding the Band-pass Filter is gain. Gain sets the amplification degree applied to the audio signal and has a direct impact over the frequencies of both the pass and the attenuation bands. Also, it tends to rise or reduce the sound volume.


Slope is the third and last standard setting in this list. It sets the frequency attenuation rate. By doing this, the transition between the affected and the untouched frequencies varies. In this understanding, a higher value translates into a more precise transition.

Band-Pass Filter: Conclusions

It’s impossible to find a perfect BPF. In this same vein, it is impossible to mitigate all the frequencies that fall outside the desired range because there is an area outside the intended passband that is capable of weakening frequencies, but can never reject them. Do not lose sight of the fact that this is a filter roll-off expressed in dB of attenuation per octave or decade of frequency. The BPF makes the roll-off as tight as possible.