An equalizer (EQ) adjusts the level of selected frequency ranges. It is used to fix problems, make instruments fit together and shape tone.
Frequency ranges
These bands are rough guides, and different sources describe them with slightly different boundaries.
| Range | Approximate frequencies | Typical character |
|---|---|---|
| Sub | 20 to 60 Hz | felt more than heard, rumble |
| Bass | 60 to 250 Hz | weight, fullness |
| Low mids | 250 to 500 Hz | warmth, can become muddy |
| Mids | 500 Hz to 2 kHz | body, the core of many instruments |
| Upper mids | 2 to 6 kHz | presence, clarity, edge |
| Highs and air | 6 to 20 kHz | brightness, sparkle, openness |
The ear is most sensitive in the upper mids, so small changes there are very noticeable.
Filter types
- High-pass filter: removes content below a cutoff frequency. Often used to clear rumble and low-frequency buildup from sources with no useful low end.
- Low-pass filter: removes content above a cutoff. Used to soften brightness or push a sound further back.
- Shelf: raises or lowers everything above (high shelf) or below (low shelf) a chosen frequency.
- Bell (peak): boosts or cuts a band around a center frequency.
- Notch: a very narrow cut that removes a single problem frequency, such as a resonance or hum.
Filters have a slope, measured in decibels per octave, which sets how quickly they cut past the cutoff. A steeper slope is more drastic. Common slopes are 6, 12, 24 and 48 dB per octave.
Q and bandwidth
On a bell or notch filter, Q controls how wide the affected band is. A high Q gives a narrow band, good for precise surgical work. A low Q gives a wide, gentle band, which sounds more natural for tone shaping. The bandwidth is the range of frequencies affected, and higher Q means smaller bandwidth.
Subtractive and additive EQ
Subtractive EQ removes problem frequencies. Additive EQ boosts frequencies you want to emphasize. A common guideline is to cut before you boost: if something sounds dull, consider whether another sound is covering it and reduce that, rather than adding brightness. Cuts tend to sound more natural than large boosts, and boosting adds overall level, which affects your gain staging. This is a guideline, and boosts are perfectly valid, especially with broad, gentle shapes.
Masking
Masking happens when two sounds share the same frequency range and one makes the other harder to hear. A bass and a kick drum both occupy the low end, and a guitar and a vocal compete in the mids. Rather than raising everything, use EQ to give each source its own space: cut a little where another instrument needs to be heard, or make small complementary moves in each.
Listen in context
A sound that seems perfect on its own may not work in the mix, and one that sounds thin alone may be exactly right alongside others. Make EQ decisions with the full mix playing, not with the track soloed. Check with the EQ bypassed regularly, and match the loudness of the processed and bypassed versions, because the louder one tends to sound better.
A simple workflow
- Apply a high-pass filter where there is no useful low end.
- Find any ringing or harsh frequency with a narrow cut, then widen the Q until it sounds natural.
- Shape tone with broad moves of a few decibels.
- Check against the rest of the mix.