is the increase or decrease of signal strength for a portion of (a band of) audio frequencies. The audio we record (the sound made by instruments or voices) is complex. By this we mean that it is composed of energy at different audio frequencies.
If we take a bass control (a simple equalizer) and turn the knob clockwise, we will get an increase in strength of the signal (or the signal component) that has lower frequencies (usually any component below about 500 Hz). Thus equalization effects the tone because it changes the level relationship of the fundamental and harmonic frequencies.
A Shelf Equalizer
boosts (or reduces) energy at the set frequency and all audio frequencies above it (a high-frequency self control) or all audio frequencies below it (a low-frequency shelf control).
A Peak Equalizer
boosts (or reduces) at the set frequency and a band of frequencies close to the set frequency.
The "Q" control
sets the width of the band of frequencies that will be boosted or reduced; in other words it affects the amount of frequencies around the center frequency which will have a similar amount of boost or reduction. The width of the band is given in octaves. Q affects this width, but high Q numbers mean a narrower band of frequencies will be affected. To start, you will need to be able to identify the "Q" settings that go as narrow as 1/2 octave to as wide as 2 octaves.
Bandwidth in Octaves, n, can be expressed with the following formula:
X = (4*Q^2 + 1)^0.5
n = log base 2 of ((x+1)/(x-1))
Plugging in values of Q will lead to the following:
Q Setting Bandwidth
0.667 2 Octave
1.044 1 1/3 Octave
1.414 1 Octave
2.871 1/2 Octave
The amount of boost or cut is determined by the "dB" setting of the equalizer. Most equalizers can boost or reduce energy by 12 dB (up to 4 times/one-quarter the level). Some equalizers can boost or reduce up to 15 dB.