by Major Venudhar Singh, Research Associate (Sur.Aur.Saaz Team)

In this part, I will give a brief idea about equalization (or EQ in short). This device forms an important interface between the mixing console and the recording equipment/amplifier. Unfortunately more often than not it is used as a show piece by many pseudo-techies who wish to add modules upon modules on top of each other like the leaning tower. Then there are some finicy people who toy around with an EQ to get that 'BOOMING' effect or the 'TRING TRING' effect without a slightest compunction about the type of amplifier & speakers they are using. An act of balance between aesthetics and some technical knowledge can shape the sound beautifully.

Most of the time BASS and TREBLE controls in an amplifier take care of the tonal quality of the music and the personal preferences but an equalizer can further enhance the tonal quality by giving you a greater control over the full range of audible spectrum of the frequencies between 60 hertz and 16000 hertz. Beyond that only dogs can enjoy as they have the capability to sense higher frequencies as compared to human beings. So coming back to the serious business of understanding the function of an EQ I wish to start with the very basics.

An equalizer is a set of filters and the boosters circuits to alter the frequancy response in selected ranges of frequencies called bands. You see a simple built EQ in a music system with 4 , 5 or 6 bands or if you can afford you may buy the device separately which may have 12 bands separately for left and right channels. Whatever be the case , an EQ helps you to cut or boost the frequencies of your choice for individual aural pleasure or for the purpose of compensation of certain frequencies. For the professionals , EQ plays a vital role in sound transfer from one media to other media. Stage performers also use it to add brightness to various instruments and the vocals. The frequency repsonse that you require may have to be built up from several different filters.

For example, you are attempting to take backup from gramophone records to the tape. If you have a signal with lot of rumble (low frequency) and the hiss (high frequency) then you pass the signal through two filters to filter out rumble and the hiss so that a clean sound can be transfered. To filter out low frequencies a high-pass filter (or bass-cut filter) is used while for filtering the higher frequencies a low-pass filter (or treble-cut filter) is used. So filters and the boost circuits both have their uses in specific situations and their effective combined use is what is normally understood as EQ. AS a general rule you do not alter the part of signal that is correct. Thus you follow the policy of minimum intervention. It is better to get the recording correct at the fisrt place. This can be ensured by correct placement of mic and proper GAIN adjustments alongwith the correct settings of channel faders on your mixer console.

In our example of rumble and hiss, the correct method would be to use filters. There could be a case where the recording was satisfactory in the BASS and TREBLE but there was a slight , say 3 dB , lack of mid-range signal.Then you would use a boost circuit on the mid-range. Theoritically we use a filter that operates at a particular frequency with a 'GO and 'NO GO' characteristic. This is known as 'BRICK-WALL' filtering. In practice we do not use such filters in audio because they actually sound awful and unnatural.

Practical filters have a more gentle roll-off so they are not as audible. They are universally described by the sharpness of their action. To describe this we have to undersatnd the term 'OCTAVE'. Musicians know about octave, like a five octave or a seven octave keyboard (more octaves more keys and hence more price),eg piano and professional keyboards have seven octaves.

An octave is the pitch interval between two sounds if one sound is double the frequency of other. On a piano or a keyboard you play the same note such as 'A' on low , mid and high octaves but the frequency gets doubled in a geometric progression. This gives us a convenient measure of a range of frequencies up and down from a specified one. An octave does not have units . In this respect it is similar to dB (remember my last article?); you could say that an octave was a 6dB increase or decrease in frequency.

We can say that a certain filter rolls-off the frequency response by so many dB per octave. The more dB per octave the steeper is the roll-off , or slope , of the filter. A slope of 20dB/octave is very steep and audible , a slope of 16dB/octave is a normal steep filter slope , a slope of 3dB/octave is a gentle roll-off. If you could recaptulate, what is minus 3dB? forgot? It is the drop in signal level that produces half the original power , the half-power point.

You will come across this -3dB point when you read the specs of an amplifier. But here it's a convenient reference point for identifying the filter frequency. The filters are said to be 440Hz high-pass filters with slopes of 12 and 3dB/octave. Note that the actual reduction in level starts at a higher frequency than 440Hz but that the response is 3dB down at 440Hz.

In practice the more useful filters have switched frequencies with a set slope. A typical good specification would be as follows:-
* high-pass: -3dB at 35Hz , 65Hz, 140Hz, 400Hz, slope of 16dB/octave.
* low-pass: -3dB at 5kHz, 8kHz, 12kHz, 16kHz, slope of 16dB/octave.

Remember that the frequency scale is not linear when you see a frequency repponse graph in the technical specs of the equipment . It is on a logarithmic scale like the relationship between ordinary. ratios and the dB.

Types of EQ
Graphic Equalizers : This is the most commonly used device with a number of vertical slide controls each altering a specific frequency. The position of controls gives a graphic representation of the unit's effect on the frequency response. Each slider boosts up a particular frquency when moved up from its zero position. It attenuates or cuts the frequency when moved down the zero position. The amount of change is marked on the scale in terms of dB (up +12dB and down -12dB). The response of each section overlaps slightly into the next frequency band so the overall response is smoothed out. The aim is to get the sound with minimum movement of the slides from their mid position.

A cheap graphic will have controls spaced more than an octave apart. The better ones have a control for each half or even one third of an octave. Thus the expensive equalizers will have more sliders going upto 31 in number. But you must patch the device carefully as per the acoustic requirement only.

Other types are Shelving EQ, Peak and Dip EQ and Parametric EQ which I feel will be best understood at a later stage once you have masterd the art of using a Graphic EQ. We will cover these EQ in the next series.