Parametric Equalizers are most often used when very fine adjustments of an audio signal are needed.
There are a lot of equalizers, both analog and digital. Like the name says, Parametric Equalizers have several “parameters” to control several filters that can be applied to an audio signal.
In order to be named parametric, an equalizer has to have at least the following filters: Gain, Q and Frequency.
The audio signal which is delivered by a plugged instrument or a player of any kind can be amplified. This amplification, done by an electronic circuit, is called “gain”. Often, gain is expressed as decibel value, for example 5 dB of gain. Another word that could be used for gain is Amplitude, the strength of an audio signal.
Refers to the resonance of an audio signal. The technical explanation is that Q is the energy stored divided by the energy dissipated per cycle in a network. Q also means quality factor, determining and being the unit of measure of the bandwidth (frequency span) of a signal. Bandwidth is the difference between the upper and lower frequencies of an audio signal. For example, the bandwidth of a signal that ranges between 3400 Hz and 300 Hz is 3100 Hz. Another explanation of what Q is could be this one: If you pluck a guitar string, it will vibrate for a long time. It dissipates just a little amount of its energy, meaning it is like a high Q mechanical filter. When you put your finger on the string it stops vibrating very quickly. Your finger has lowered the resonance of the vibrating string, removing a lot of energy. Analogue, this would be a low Q mechanical filter. Another classic high Q filter would be the Wah-Wah-Pedal of a guitar, for example.
The number of times something occurs per unit of time. For example, if an audio signal has a frequency of 1kHz, its period or swing rate is 1000 times per second. In the musicians realm, the frequency of the vibrations of an audio signal is directly related to the thing we hear as pitch. This relationship is not linear.