Equalisation basics: boost, cut and Q

An equaliser changes the balance between frequencies. It is the most used tool in mixing, and a few principles make it more effective.

Types of filters

Parametric equalisers let you set frequency, gain and Q. Graphic equalisers fix the frequencies and the width, so only the gain remains.

Q and bandwidth

A narrow Q affects few frequencies, and a wide Q many. Narrow boosts sound unnatural, because they emphasise single harmonics. Wide, gentle changes sound natural. Narrow cuts are often used to remove problems such as resonances, see Filter slope, bandwidth and Q.

Cut rather than boost

• Cuts remove what is in excess and usually sound more natural.
• Boosts raise noise and can overload later stages.
• Cutting something else is often better than boosting the thing you want.

Boosting at 3 kHz makes the voice more present; cutting at 300 Hz does the same by removing mud.

Frequency regions

Roughly, and with large variation between sounds:

Presence and absence

Boosting the range around 2 to 5 kHz makes a source sound more forward; cutting it moves the source back. This ties in with how we judge direction and distance, see How we hear direction.

Do not trust a straight response below 200 Hz

In rooms, the low-frequency response is dominated by standing waves, see Room modes explained. Equalising a monitor for a flat response at one position can produce large errors elsewhere in the room, and boosting a dip caused by interference does not fill it in. Low frequencies are best handled by position and treatment first, and equalisation second.

Other practical points

1. Listen at matched levels, see Equal-loudness contours. A boost always sounds louder, so it can seem better.
2. Compare with the bypass button.
3. Filters change phase as well as level; steep filters change phase more.
4. Fix the problem at the source, such as microphone placement, where possible.

Draft: written by an AI assistant, awaiting technical review.