Equalization means altering the frequency response of a sound. It means adding more bass or lowering the treble on the signal. The simplest form of equalizer is the bass and treble controls on your home stereo.
It is very rare that any signal will pass through the recording chain without undergoing some form of equalization, either to enhance it, correct it, or make it blend better
with the other sounds. Equalizers are giving us possibilities to manipulate any frequencies within the entire audio bandwidth (16Hz to 20kHz). The timbre or harmonic balance of musical instruments can be altered with equalizers. They are probably the most important signal processing device in the whole recording chain. Equalizers can be used to compensate for many shortcomings in the tone of an instrument and can also drastically change sounds, making them fashionable by current-day standards or just plain weird.
Outboard equalizers are used primarily because they offer a particular sound quality and an additional selection of frequencies that cannot be addressed by the plug-in or the console’s EQ section. Some popular models are made by Pultec, API, Neve and Urei.
Since we use Equalizer to various instruments differs according to their frequency ranges here are a few generalizations:
Low Bass (16Hz to 60Hz) is felt more than heard. Extreme boosts within this range
can make the music sound powerful but muddy.
Bass (60Hz to 240Hz) contains the fundamentals. Boosting here adds fatness, while attenuating
will give a thin sound. Too much emphasis can create a boomy sound.
Low Mids (250Hz to 2kHz) consist of the low harmonics within most instruments.
Excessive frequency boosts within this range can quickly induce ear fatigue.
High Mids (2kHz to 4kHz) Too much boost in this band can impair vocal qualities
and if the frequency around 3k is boosted, listener ear fatigue will soon occur.
Presence (4kHz to 6kHz) Boosting here will apparently bring the music closer and
seem to raise the overall level. Attenuation at 5k will add transparency to a sound.
Highs (6kHz to 16kHz) Here the clarity and brilliance can be controlled, but sibilant
vocals will result if too much boost is used in this range.