An equalizer will allow you to increase or decrease specific frequencies.  For example, you can increase only the high frequencies of a cymbal in order to make it “sizzle” more. 

A Parametric EQ can has different sections that each contain knobs to select a frequency, increase or decrease it, and even select the width of the change (the “Q” or “bandwidth”).

A Graphic EQ consists of sliders, each set to increase or decrease a specific frequency.

A High Pass Filter will allow higher frequencies to pass through and stop lower frequencies.  A Low Pass Filter will allow low frequencies to pass through and stop higher frequencies. 

High and Low frequencies can be changed at specific freqiuencies (peak) or across all frequencies higher or lower than the eq setting (shelf).

Text Box:  The sound shown in the upper frequency response curve has been changed by an EQ in three ways.  There is a drop in the frequencies below 100hz with a wide bandwidth, a drop in the frequencies above 16khz with a more medium bandwidth, and a peak (push) in the frequencies around 3khz with a slightly narrow bandwidth.

Each adjustable element of an EQ is called a BAND.  For example, a 3-band EQ will usually contain a slider or knob for the Bass, Midrange, and Treble.  Each band will have it’s own frequency, bandwidth, and gain controls.  Note that some of these settings may be pre-set and un-adjustable.  Graphic EQs use sliders that do not allow you to change the bandwidth.  Some EQs have a Treble knob that will only allow you to adjust gain and not select the specific frequency being adjusted or the bandwidth of the adjustment.

An EQ can have either Treble and Bass controls that can act as a SHELF or a PEAK.  A SHELF will affect not only the frequency indicated but also all of the frequencies either above or below.  For example, if you have a Treble EQ set for Shelf and turn up the gain at 10k Hz, all the frequencies higher than 10k Hz will also be brought up.  If you have a Bass EQ set for Shelf and turn down 100 Hz, all the frequencies below 100 Hz will also be brought down. 

An EQ band that is set to PEAK will create a bump up or down only at that frequency.  The width of the bump is determined by the Bandwidth.

NOTE that an EQ PEAK is not the same as a VOLUME PEAK, which is spike in sound level.