Most everyone has heard of an equalizer.  In fact, almost every media player offers one of these to give the consumer better control over their sound.  While some know what this is for, most don’t really have a clue!  They’re usually graphic equalizers which consist of anywhere from 5 to 10 bands.  Each band is a fader which can be pulled up or down.  These are filters, which, when pulled up, create a peak, and when pulled down, create a notch.  The peak/notch filter is the main function of an equalizer.  There are hardware units with a very large band count, like 32, and are used for controlling feedback in a PA system while tuning a room.  The reason the number of bands is important is because the resonant frequencies of the room can be very specific.  By breaking the spectrum into smaller increments, the sound man has more choices of what frequency to attenuate.

In a studio environment, a parametric EQ is the weapon of choice.  Parametric simply means that the frequency of the band can be selected by the engineer.  Some classic mixers include an equalizer section within each track where the low frequency, as well as the high, is chosen for you; meaning, graphic.  But, they include a mid sweep knob for selecting which frequency will be used for the middle.  This mid sweep makes the middle band a parametric filter.  Many hardware and software equalizers include several of these parametric bands and offer even more control by including other types of filters as well.  Typically, there may be a high pass and a low pass filter.

High pass filters are controls that allow the high frequencies to remain, but, which, turn down some of the lower frequencies.  Usually a separate knob is offered for parametric control here, as well.  Low pass are the opposite, and allow the bass frequencies to stay, but roll of some of the highs.  One other function of parametric filters that you will often see, is bell width, or slope.  This control gives the choice of how quickly or slowly a change in the frequencies is affected when approaching from the neighboring frequencies.  This means you can create a very sharp point at a frequency of your choosing without affecting the surrounding frequencies.  Or, you may opt to smooth out the transition by creating a long and gradual slope which reaches its extreme point at the chosen frequency.

For the inexperienced, I highly recommend using an illustrated equalizer, such as Blue Cat Audio’s, to get familiar with how these filters look and feel.  The curves will be drawn out so that, as you change the bell shape, the curve of affected frequencies will be shown on the display.  These tools are useful for creating other interesting filters as well.  By sloping frequencies downward only to step up to a very high point, and gradually slope back down to normal, a step, or tilt EQ can be created.  This can be a useful tool during mastering for correcting problems.  Band rejection can also be accomplished with some of these types of equalizers.  Rejection is the complete removal of offending frequencies; usually used for noise control.  Finally, these tools, as well as some higher end hardware EQ units, include dedicated bands used just for shelving.  Shelf filters allow everything above or below the selected frequency to be pulled up or down evenly across the board.

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