WILLY T talks us through a mixing desk’s CHANNEL EQ. Check out last week’s explanation of everything EQUALISATION -related if you missed it!

To use EQ effectively, we need to really listen to sound! The trick is to identify what we are trying to achieve. Really think about what is wrong, needs adjustment, or just what effect we are trying to create. After a good think, try to analyse which parts of the frequency spectrum to adjust. Our ears are for judging sounds, not our eyes. So don’t pay too much attention to the position of the knobs in the EQ section of the mixing desk. Sounds obvious I know; but many people get busy trying to remember an EQ ‘setting’ rather than pulling a sound.

It is a good practice at soundcheck to set all the Channel EQ controls to their centre – flat – positions and leave them there until you have a decent balance of the sounds. Get a rough mix going from the levels alone, THEN start EQing. Avoid spending a lot of time equalising a sound in isolation. Remember, equalisation affects the tone because it changes the relationship between the levels of the fundamental and harmonic frequencies. When we do listen to a channel by itself, apply corrective EQ to remove unwanted rumbles, ‘mains hum’, spill or whatever, but don’t get bogged down in making it sound ‘right’. The right sound for a particular instrument or source will depend on the other instruments and their balance within the total mix.

On mixing desks it is common to see different types of EQ all on the one channel. The names of the various types of EQ come from the amount of control we have over the signal, and how many sections or ‘bands’ we split the sound into. For example a 4-Band EQ splits the sound into four – Hi, Hi-mid, Low-mid and Low. (Your home stereo’s Bass and Treble controls make it, in effect, a 2-band Equaliser.)

Also called ‘Lo-cut’ filters they are a switchable, shelving filter designed to cut out bass frequencies and let everything higher than the nominated frequency be heard. HPFs are usually fixed at between 60 and 120 Hz. with a very steep ‘roll-off’ or rejection of frequencies below the cut-off frequency. Their only control is whether they are In or Out of circuit. This type of EQ is useful for getting rid of the low-frequency stage rumble picked-up by stage mics via the floor stands … like the singers’ foot-stomps.

Fixed Band or Shelf equalisers are usually found on the better mixing desks as the Hi and Low ends of the equaliser section. Shelving bands have ‘shelf’ or ‘hinge’ frequencies at above or below which the EQ can be cut or boosted by a user-definable amount. Shelving EQs play an important role as they effect a broad range of frequencies, useful when we want to cut some general bottom-end from the signal or to add some ‘air’ to the voices or hi-frequency instruments.

The most common type of mid-frequency EQ found on mixing desks is the Sweep EQ. As well as being able to cut or boost the signal, a sweep EQ allows us to change the centre frequency of the equaliser’s Peak & Dip Curve for more accurate control of specific frequency ranges. (For the tech-heads: the most equalising occurs around the centre of a bell curve with the effect decreasing further away from the centre frequency.)

A Parametric EQ, like the sweep EQ, allows us to cut/boost and alter the centre frequency of the bell curve. Parametrics also allow us to vary the amount of frequencies on either side of the Centre Frequency that are cut or boosted. The amount of frequencies affected on either side of the centre frequency is known as the Bandwidth or Q. Because the filter response is curved or bell-shaped, the actual frequency bandwidth is measured between the points on the graph where the signal level has fallen by 3dB.

Rather than being continuously variable the Q selector is often pre-set at Hi-Q or Low-Q (bandwidth). They are also sometimes referred to as wide or narrow Q or the bandwidth is sometimes given in octaves. Until the digital revolution they were mostly found on higher-end mixing desks, but are now more common on digi-gear. (For the tech-heads: a ‘Q’ of Setting of 0.7 = 2 Octaves Bandwidth, a ‘Q’ Setting of 1.0 Q = 1 1/3 Octaves Bandwidth, a ‘Q’ of Setting of 1.4 Q = 1 Octave Bandwidth, a ‘Q’ of Setting of 2.8 = 1/2 Octave Bandwidth.)

It is common on most "digi-desks" to have 4-band parametric EQ sections that can be applied in most, if not all areas of the mixing desk. The EQ is available across input channels, groups, outputs, auxiliary and FX sends, return and stereo outputs. The EQ sections can also be set up to operate with Hi and Lo frequency shelving EQ sections with parametric Lo-mid and Hi-mids.

EQ adjusts the level of a signal at different frequencies, so it will affect how the sound sits in the mix. Remember; if you’ve got an EQ Bypass switch on your mixing desk, use it! Switch the EQ in and out of circuit to compare your carefully sculpted artwork with the ‘flat’ setting, to make sure that you’re really are improving things! While you’re doing that you can always try to impress punters by telling them you’re "A-Bing" the EQ! As usual, experiment and find your sound.

WILLY T runs Giraffe Media. Providing Words, Pics and Live-Audio Recording Services to the entertainment industry. Email  for more information.

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