A filter is the simplest form of EQ section possessing only the parameters of cut-off frequency and slope.
The cut-off frequency is the frequency at which the response has already dropped by -3 dB.
The slope is the rate at which the level drops above the cut-off frequency, measured in dB/octave. It is easy to design filters with slopes of 6, 12, 18 and 24 dB/octave.
- A low-pass filter allows low frequencies through and reduces the level of high frequencies.
- A high-pass reduces the level of low frequencies.
- A band-pass filter reduces the level of low and high frequencies.
- A notch filter filters out a narrow band of frequencies.