A Guide to EQs

What EQ can do and what it can’t…

A-guide-to-Equalisers
EQ can be used to create or correct sounds, but is not the only way to do either. When well used at the recording stage EQ can produce sounds characterised as full, round, warm and natural; these are then open to any manipulation you choose to put them through.

When not so well used, the results may be either ‘thin’ (too narrow in spectrum) or may lack crucial characteristics or defining areas, e.g. bass guitars with no bottom end, kick drums with no click or guitars with too much fizz and not enough note. Only limited help is available after that, no matter how brilliant your gear is.

Thick sounds can be thinned more easily than thin ones can be fattened: EQ doesn’t generate or add frequencies. In practice, good designs can help reduce over prominent peaks, or sculpt and fit signals around other sounds with which they then work better in combination. The hard truth, however, is that it is up to you to create the sounds you want and to record them when you hear them. Jam today please, don’t wait till later.

In the end music is not separate from the sound it makes; EQ decisions can really help music that is working and make it more vivid and powerful. However, if the music isn’t working it still won’t ‘sound’ good no matter what you do to it.


Types of EQ

  • Active – uses amplifiers to boost or cut selected frequencies
  • Passive – does not use amplifiers and can only cut selected frequencies: this is considered by some to be ‘kinder’ to the sound. Most passive EQs have an active gain stage to compensate for signal loss.
  • Dynamic – some more recent units only boost the selected frequency when it is present. This approach makes units less noisy, and is a godsend when restoring top end to old or degraded tapes.
  • Baxandall EQ – simple shelf type circuit designed by Peter Baxandall in the 1950s and adopted by Trident and other manufacturers. Currently appearing in TLA valve consoles.
  • Sallen Key Filters – high or low pass filters that achieve 12 dB per octave roll off.

They also differ visually in their layouts:

  • Graphic – have a number of vertical sliding volume controls (up to thirty-one on some types) which only cut or boost one fixed frequency. ‘Graphic’ refers to the way the vertical controls ‘graphically’ display the shape of the equalisation curve. In mini cabs this is a straight line along the top. In practice, graphic EQs are not widely used in musical applications, and are generally employed to adjust PA systems or studio monitors to achieve accurate response in particular spaces. However a decent graphic can be a godsend in the rack for trimming bass drum, bass guitar or other instruments after more normal EQ
  • Parametric – with variable parameters. The term ‘parametric’ should strictly apply only to EQ with three variable parameters, i.e. including a Q width control, but in everyday speech it has come to mean ‘with knobs’. Three main layouts, Filter, Shelf and Sweep (see below for fuller descriptions).

Other terms:

  • Semi-Parametric – a unit with two variable controls, typically Frequency Sweep and Cut/Boost. The width of the Q is determined by the amount of boost or cut, becoming narrower with higher dB values. This approach is usually quite adequate in normal applications, and most mid-priced desks use this type of EQ on mid frequencies.
  • Q – measurement of how much the frequencies to either side of the selected, centre frequency are also affected. Q shape can be broad or narrow and is determined either by a separate control, or by the amount of cut/boost applied.

What are the EQ controls for?

The options offered by Parametric type units relate to three different approaches to signal processing, namely Filter, Shelf and Sweep EQ.

Filter

Fat Boy Slim made a fortune from extreme usage of this effect.

  • Can only cut, not boost, frequencies.
  • Has a variable control for frequency and maybe a switch for roll-off rate.
  • Selects one variable point on the audio spectrum. How savagely or gently the unwanted sound is removed beyond this point is expressed in dB per octave, the most common rate being 6dB per octave.

Forms:

  • High Pass Filter (HPF) – allows only frequenceis higher than the point selected to pass through it. Its standard use is to take the rumble or hum out of any sound source, or if wiggled in real time can make a nice ahw-ahw effect.
  • Low Pass Filter (LPF) – allows only frequencies lower than the point to pas through. Its standard use is to reduce noise in quiet quiet passages or to take the fizz off any source with excess high frequencies. If wiggled in real time it can make a nice wah-wah effect.
  • Band Pass Filter (BPF) – only lets an area around a central frequency through. This usually comes in special units.
Sweep or Peak
  • Has one or two variable controls for boost/cut and frequency, and may have extra controls for slope.
  • Operates at either the top or the bottom end of the audio spectrum, and boosts or cutsone centre frequency which can be either pre-set, selectable, or variable.
  • Affects the sound at that frequency and all frequencies beyond it to the edge of audibility. How sharply the effect kicks in can be varied.
  • When used to cut frequencies, a Shelf can clarify the overall sound greatly, especially on the low end since boosting at the extreme edges of the range of instruments and voices can introduce large amounts of unfocused sound, and should be avoided. Neither bats nor whales buy records

 

Sweep or Peak
  • The most flexible type, with the most adjustable parameters. It will have two or three variable controls for boost/cut, frequency and Q, with maybe one switch for Q shape.
  • Select one central frequency and boost or cut this frequency and a variable area either side of it.
  • The broadness of this affected area is referred to as its ‘Q’ and is either continuously variable or selectable, usually between a thin pointy shape or a broader shape like a bell.
  • Narrow (low) Q values sound unnatural and unsettling when boosted, and are best used for removing particular unwanted frequencies, like drum overtones or whistling amp noises.
  • Most sweep EQs actually narrow the frequencies affected with greater amounts of gain or cut, and this sometimes means that the Q control is dispensed with, as on a Mackie 8 Buss desk. Using extreme cut values at narrow Q values is referred to as ‘notch’ filtering.

Sweep EQs are designed for fishing about in the middle frequencies for characteristic tones that bring out clarity. The middle range is always the most crowded, and is also the range to which the human ear is most sensitive. Careful combinations of cutting and boosting can vastly increase the intelligibility of complex soundscapes. However, over emphasising this area can produce physical pain in the listener at high sound pressure levels.


View our full range of EQs here.

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