Equalizers are ubiquitous in audio equipment, be it a simple tone control on a stereo, a hi-fi multi-band graphic equalizer, or a parametric equalizer band in a guitar pedal. Although based on the same principles, equalizers serve different purposes depending on their context. In reproduction systems, equalizers serve to cancel any undesired filtering effect due to the involved equipment (loud speakers, amplifiers, etc.) or room acoustics. Musicians use equalizers as a means for shaping their sound, i.e., as part of a musical expression. Equalizers are built from linear time-invariant (as long as you do not touch the controls) filters. There are two kinds of equalizers: graphic and parametric. In graphic equalizers, a desired frequency response (i.e., attenuation and amplification of certain frequencies) is achieved by changing the gain at a set of fixed center frequencies. In parametric equalizers, the desired frequency response is obtained using a number of bands, each of which can be controlled by a center frequency, a bandwidth, and a gain (or other similar parameters). In the following, we will cover the design of parametric equalizers although the principles can also be adapted for graphic equalizers.