Types of EQ.

When it comes to outboard EQ, there are 4 main types - The first three (Fixed, Sweepable Mid and Parametric) are logical progressions from one to the next where as a Graphical EQ unit serves a different function entirely. EQ units will nearly always be built into mixing desks / multi-trackers, regardless of the size and are considered to be the "paint-brushes" of the mixing world.

1. Fixed EQ

Fixed EQ's are about as basic as you get - you'll often find these on cheaper / smaller mixers and they're designed to tidy up the sound rather than make delicate adjustments (think of fixed EQ's like a big old paint-brush). Fixed EQ's will be set to certain frequencies (on my Behringer MX3282a, the 4 stereo channels have Fixed EQ's set at 80Hz, 500Hz, 3KHz and 12KHz) - you can not change the frequencies at which Fixed EQ's operate. Fixed EQ's will only have one knob which sets the desired amount of Gain at the frequency labeled on it.

This image shows the typical layout of a fixed EQ on a mixing desk - not this arrangment has 3 bands, Hi, Mid and Low, to cut or boost these frequencies, you simply turn the knob left or right respectivley.

2. Sweepable Mid's (aka semi-parametric)

An EQ with sweepable mids is the next logical step up from a fixed EQ and are found on medium priced desks. They still have their Low and High frequencies Fixed (usually at 80Hz and 12KHz) - but have one or two EQ's which are sweepable - meaning that you can select the frequency at which the EQ's operate (on my MX3282a you can sweep from 100Hz to 8KHz) - these are obviously a bit more precise than Fixed EQ as you can choose the frequency you wish to focus around. Sweepable EQ's will have 2 knobs - one to set the amount of gain you wish to apply and another to select the frequency you wish to effect.

This image shows the typical layout of a swept mid EQ on a mixing desk - note the two fixed Hi and Low EQ's and the 2 knobs in the middle - the upper one controls the gain and the lower (yellow) one selects the frequency.

3. Parametric EQ

Parametric EQ's are the daddy when it comes to EQ! These are found on the more expensive consoles and allow for precise EQ'ing of signals. Again, they will usually have 2 fixed EQ's at 80Hz and 12Khz, but will then have 1, 2 ,3 or even 4 (on really big consoles!) Parametric EQ's. Parametric EQ's are fairly similar to Sweepable EQ's in the respect that they have an Amount Knob and a Frequency Knob, but they will also have a 'Q' knob. The 'Q' refers to the "width" of the EQ that you are going to apply; a small 'Q' value (referred to as an 'open Q' will affect a large range of frequency around your selected frequency where as a higher 'Q' value (referred to as a 'tight Q' will affect a much smaller area of the frequency range. For example, if you set the frequency knob to 2 KHz and then boost using a tight 'Q', you will only affect the frequencies from 1.95 KHz to 2.15 KHz (this is just an example!) where as if you used an open Q you would probably affect frequencies from 1.3 KHz to 2.7 KHz)

This image shows the difference between a 'Tight Q' (on the left) and an 'Open Q' (on the right) and how they affect the surrouning frequencies.

4. Graphic EQ

Graphic EQ's are completely different to the other 3 types of EQ listed. They comprise of lots of sliders which represent the frequency spectrum from 20Hz all the way up to 20Khz and sometimes beyond. (some units have 32 such sliders, some have more, some have fewer). Graphic EQ's are not so much used on channels (like the other EQ's) but are more often used one the Main Outputs of a Mixing Console in a live situation to deal with any specific problems that are arising. They can also be used to help compensate for known 'tuning' problems within the mixing environment (eg: if you know there is a resonant peak at 120Hz in your mixing environment then you can take 2dB out of that frequency range without effecting the others.) Graphic EQ's can either be used to cut / boost specific frequencies, or the user can create a 'curve' to make more subtle changes to the character of the sound.

The Alesis M-EQ 230 Graphic EQ has 30 bands of EQ ranging from 20Hz to 20KHz.