An acoustic equalizer or EQ, refers to the alteration in frequency response of an amplifier so that the relative levels of certain frequencies are more or less pronounced than others. EQ can be applied to a single recorded channel, to a group of channels, or to an entire program to correct specific problems; in the recording room, to overcome deficiencies in a frequency response of a mic or the sound of an instrument or to alter sound purely for musical or creative reasons
There are two main types of equalizers, Parametric and Graphic:
The parametric equalizer lets you adjust most or all of its frequencies in a variable fashion. Due to its flexibility and performance, this EQ has become the standard design for most input strips, digital equalizers and work stations.
The graphic equalizer provides boost and cut level control over a series ofcenter frequencies that are equally spaced. This type of EQ is often used in applications that help with fine-tuning a system to compensate for the acoustics in a specific room.
Simply put, using equalization is about taking out the bad and leaving the good. For example; lets say a mic was placed in a bad spot during a recording session. If you could you would probably redo the track, but if you have run out of time, or say the existing tape is excellent except for this minor problem, you can EQ to take away a deficiency in the signal. However, adding too much boost tends to cause listening fatigue.
When an equalizer is used properly, EQ can enhance or restore the sonic balance of a signal. Keep in mind that it is difficult and time consuming to fix it at a later time. An equalizer should not be used as a cure-all for improper recording techniques. With or without EQ, it is always important to get the best possible sound on tape during a recording session.
Building a Recording Studio 