Equalisation

Based on the root word, equal, an equaliser is an audio device whose function is to equal out the tonal characteristics of a sound. That was the idea back in the days when they were first conceived as a tool used to get flat response in telephone lines and to make up for the deficiencies in audio equipment and acoustic spaces.

Nowadays it could more aptly be named an "unequaliser" since they are more often used creatively to alter the relative balance of frequencies to produce desired tonal characteristics in sounds.

An equaliser has the ability to boost and/or cut the energy (amplitude) in specified frequency ranges by employing one or more filter circuits. There are many different types of EQ's in use today in many widely varying applications, but they fundamentally all do the same thing.

There are three types of eq we will be covering here: basic filters, parametric eq, and graphic eq.

Filters A filter is an electronic device designed to reduce a signal's energy at a specific frequency in a given bandwidth. A true filter acts as a subtractive device, and does not add anything to the signal. In some filters however, an amplifier is incorporated into the circuit, which allows the frequency to be boosted or cut. This is called an active filter. Filters of different frequencies are often combined to create equalisers.

Filters are found on most consoles in the form of High Pass and Low Pass filters. High Pass filters "pass" the highs, or allow the highs to pass through the signal chain, while filtering out the low frequencies. Low pass filters do the opposite. Sometimes the frequency is sweepable, meaning that the point at which the filter starts acting, is variable.

Parametric Eq Parametric eq is found on each input channel of most mixing consoles. This is a type of equaliser having several "parameters" for control of various filters that can be applied to audio signals. Parametric equalisers are most widely used in situations where very fine control over the audio signal is needed. the Allen and Heath GL2400 EQ, shown at left, is quasi-parametric.

This channel uses another kind of equalisation called fixed band eq, which also happens to be shelving eq. 12k and above can be cut or boost up to 15db, and 80hz and below can be cut or boost by up to 15db as well. Then there are the two sweepable mid eq pots, sweeping from 35Hz to 1 k, and from 500 Hz all the way up to 15k. the Allen and Heath GL2400 EQ, shown at left, is quasi-parametric.

In order for an equaliser to be parametric it must at least have control over gain, Q, and frequency. In most cases each of these controls are on rotary pots, but there are a few "graphic style" parametric equalisers on the market. Some equalisers have selectable frequencies that can be adjusted, but no Q control. These are known as quasi-parametric or sweepable equalisers. The Allen & Heath consoles we have at VUT are this kind, with no Q control.

In order for an equaliser to be parametric it must at least have control over gain, Q, and frequency. "Q" refers to the width of the frequency band being adjusted. Fully parametric eqs have variable Q.

What does Q really mean? The resonance of an electronic circuit. "Q" actually refers to quality factor. Q is a measure of the sharpness of a resonant peak. The term Q is often used interchangeably with "bandwidth". This is not entirely correct. It is more accurate to say that Q determines bandwidth (a subtle but distinct difference).

Q is most often used in reference to synthesizer filters (sometimes referred to as resonance) and equalisers, but it also applies to capacitors (a measure of efficiency, the ratio of capacitive reactance to resistance at a high frequency) and speakers (a measure of directivity).

Parametric Equalisers: Some Examples These are standalone Eq units, performing the same function to the eq component in a console.

Channel eq from the Solid State Logic (SSL) Console.

But the worst is still to come; an electronic filter is not phase- linear. Phase shifts below and above the centre frequency of every EQ filter. The higher the Q (31 band EQ) the more phase shift will occur. Every band that boost or cuts adds its phase distortion to the end result. Just as you think it can't get worse; it can! Steep filters have another flaw; the ringing effect. A high Q bandfilter (an EQ stage) uses input energy to slowly get into resonance, and when the input signal vanishes, the bandfilter will use the stored energy to resonate a little further.

To illustrate this just take an empty beer bottle; blow over the top to get the whooo resonant sound of the bottle. You will notice that the tone will slowly rise in volume and when you stop blowing it will slowly die out. It is impossible to start and stop this resonant tone abruptly. Well, the electronic equivalent - the 31 band EQ- shows the same problem 31 times...

When doing your sound check, try to always put the signal you are eq'ing back into context. Changing things in isolation may result in a badly eq'd general mix. Use restraint with live eq- and if you see patterns developing across your channels, consider changing the graphic across FOH instead. If you are doing monitors from front of house, everything you eq on a channel will be also be changing in whatever monitor send it is in as well. When tuning with a graphic, it can be a good learning tool to switch the graphic in and out of bypass to see how much gain you are losing and to make sure you are making the sound better and not worse!