A peak EQ raises or lowers a hill or a valley around a center frequency. It is also called a bell EQ, or referred to as part of a parametric EQ. By combining frequency, gain, and Q (bandwidth), you adjust presence, boxiness, harshness, and resonance. It covers everything from broad tone shaping to narrow problem solving. Deciding the purpose of the move first keeps you from touching more of the spectrum than you need to.
Three basic controls
The center frequency sets where you work, gain sets how much you raise or lower, and Q sets how wide the effect is. A low Q works broadly and gently, while a high Q works narrowly and intensively. The same gain sounds very different depending on Q, so check the three separately.
Boosting
Use it to add definition to vocal consonants, snap to a snare, or presence to a guitar. Boosting also raises the level, though, making the part more likely to collide with other instruments. Raise only the band you need, and match output levels when you compare.
Cutting
Use it to reduce boxiness, harshness, or a band that clashes with another instrument. For a narrow resonance, a setting close to a notch works; for broad muddiness, a low Q is a candidate. Cutting too much takes away the character of the sound, so judge in the context of the song.
How it differs from shelf and notch
A shelf EQ works on a wide region at the end of the spectrum, a notch on a very narrow region, and a peak EQ around a center frequency. Rather than treating everything with one of them, choose the shape that fits your purpose.
The order of operations
First listen for the problem or the content you want, then set the frequency, choose the Q, and finally move the gain as little as possible. The spectrum display is only an aid. You can review the basics of EQ in the EQ article.
