High and Low Pass Filters

High Pass Filters and Low Pass Filters are the most type of audio filter. These filters begin to cut the amplitude of an input signal at a selected threshold frequency.

In the case of a high pass filter, all frequencies below a selected frequency will be progressively cut. Similarly, for a low pass filter, the frequencies above a selected threshold frequency will be progressively cut. These filters are sometimes called Cut-off Filters.

Standard Filter Types

High and Low Pass filters commonly have a gentle cut of -6 dB per octave, -12 dB per octave or a more aggressive -18 dB per octave.

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The circuits used to implement filters are not ideal. This means that the gain does not instantly change from 0 dB to -18 dB as the frequency crosses the threshold. The gain change is progressive. The quoted cut off frequency is the frequency at which the gain reduction is already 3dB below the input amplitude. This point is also called the 3dB Down Point.

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HighPass
High Pass Filter
LowPass
Low Pass Filter

Shelving Filters

This kind of filter applies an equal level of boost or cut to all frequencies beyond a user defined threshold frequency. Yet again the filter response is not 'ideal'

Band-Pass Filters

A Band-Pass Filter passes frequencies, in a chosen passband around a specified frequency. Frequencies beyond the band width are not allowed through the filter.  Essentially beyond the Q (bypass and) all frequencies are cut (attenuated).

A high-Q filter has a narrow passband while a low-Q filter has a wide passband. A high-Q filter is also known as a narrow-band band-pass filter. A low-Q filter is also known as a wide-band band-pass filter.

Stop-Band Filter

A Band-Stop Filter or Band-Rejection Filter, is the opposite of a Band-Pass Filter. Stop-Band Filters cut (attenuate) all frequencies, within a chosen Q (stopband), around a specified frequency.

As with a band-pass passband, a high-Q filter has a narrow stopband while a low-Q filter has a wide passband. A high-Q filter is also known as a narrow-band notch filter. A low-Q filter is also known as a wide-band notch filter.

Notch Filter

A Notch Filter is a narrow-band Band-Stop Filter. They are also called a “T-Notch Filter”, "Band Limit Filter", "Band-Elimination Filter", and "Band-Reject Filter".

The stopband is typically 1 to 2 decades wide i.e. the highest frequency cut is 10 to 100 times the lowest frequency cut, when used outside of audio applications. For audio applications, notch filters have high and low frequencies that can be just semitones apart.

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