An equalizer allows the sound in specified frequency bands to be amplified or reduced, in order to adjust the quality and character of the sound.
There are different types of equalizer for various uses, such as the parametric equalizers that are controlled using the knobs built into each mixer channel, or the graphic equalizers that allow multiple frequency bands (such as 7, 15, or 31 bands) to be adjusted using sliders.
In general, the most commonly used equalizers are the parametric equalizers equipped on each channel of the mixer. Rarely are the sounds of microphones and instruments that are input to the mixer perfect for delivery as-is to the venue. When mixing music that involves many instruments, some parts may inevitably be difficult to pick out. In this situation, adjusting only volume and panning is not sufficient, and equalizers can be used to adjust each frequency band to make the best characteristics of each instrument stand out.
A parametric equalizer allows control over several separate frequency bands. When the high-end needs to be boosted or the low-end needs to be cut, operate the knobs for the relevant bands by turning them clockwise to boost the frequencies, or anti-clockwise to cut the frequencies. This offers an intuitive means of control, and is found on many mixers. Some different types include a two-band type (high and low), a three-band type (high, mid, and low), and a four-band type (high, high-mid, lo-mid, and low). The parametric equalizers on some mixers allow the frequencies of each band to be adjusted.
A three-band equalizer system, which divides the frequency bands into LOW (low range), MID (middle range), and HIGH (high range).
High-pass filters (HPF) are filters used to only allow through frequencies that are higher than the one that is set.
Although it isn't really called an "equalizer", the high-pass filter shares similar characteristics with equalizers, in that it reduces the frequencies below the range that is set. By reducing low-range sound, high-pass filters can be useful in cutting out popping noises due to vocal breathing, or eliminating unwanted bass drum sounds that may be picked up by a microphone when recording a hi-hat. A HPF can also be effectively used on some string, brass and woodwind instruments.