High level basics about EQ (Equalization)

Equalization – what is it? 

In basic terms it is a way to compensate for deficiencies in an audio signal. Many make the mistake of using to overcompensate instead of flattening the signal so that there are no bias (I will elaborate more later). 

You have seen these devices even in your windows media player. To the far right they control low, middle voice and other tones in this range, left symbols and other high pitched sounds.  In Technical terms it is defines as the process of changing the frequency envelope of a sound in audio processing. 

To quote Wikipedia – “The term "equalizer" is often incorrectly applied to audio filters, such as those included on DJ mixing equipment and hi-fi audio components. However, these "equalizers" are typically general all-purpose audio filters, which can be arranged to produce the effect of low pass, high pass, band pass and band stop filters. Such filters are true equalizers only when arranged to reverse the effects of internal circuitry on sound output. 

In Multitrack recording and sound reinforcement systems, individual channels have equalization for aesthetic reasons, while the combined mix of sound is processed through equalization for practical reasons. Any acoustic space will cause some sound frequencies to be louder than others. This is due to standing waves produced by the size of the room and the materials in it. Equalization is used to compensate for the discrepancies of a room's acoustics.” 

A graphic equalizer gets its name because of the graphic representation of the intensity of each frquency level. 

Typically in a well designed system once it is installed a spectrum analyzer is used in concert with a graphic equalizer to adjust the equipment and compensate for the acoustics of a room and make sure the signal is flat. Allow me to explain in some non technical detail. When a system is configured to have a flat frequency response it means there are no biases and no signal augmentations. As a result in an ideally designed system you could use a proper microphone and then record a song or vocalist and then when you play back the recorded sound it will sound exactly the same as the live performer or instruments.

Trivia: 

At a live event, where microphones and speakers operate simultaneously equalizers are used to ensure that there are no frequency bands where there is a round trip gain of greater than 1 to avoid feedback. Also all audio records, or vinyls, have had equalization applied to before the consumers' record was made because of the limitations of equipment.