A biquad is, technical speaking, a second-order recursive digital filter. It is used as the “building block” for more complex digital filters, such as steep crossover filters. A single biquad can be used to implement:

  • A single parametric EQ peak or notch filter

  • A single shelving filter

  • A single band of a graphic EQ

  • A 12 dB/octave crossover filter. (A 24 dB/octave slope uses two biquads, a 48 dB/octave slope uses four biquads.)