The job of the equalizer is to correct the tone, working on the sound frequencies. The term equalizer can refer to any form of tone control. The simplest form is a condenser connected to a control that filters the sound frequencies by cutting the signal.
An amplifier can have many tone controls that work on the low, middle, and high frequencies, but there are also more sophisticated methods to alter the frequency response.
Equalizers are divided into three types:
- analytical equalizers
- graphic equalizers
- parametric equalizers .
The analytical equalizer has a normal emphasis-cut control, plus an additional control that allows the middle frequency of the normal control to be raised or lowered in the audio spectrum. This is found on most mixers.
The graphic equalizer originally was designed for use in recording studios, but today can be found on many guitar amplifiers and as an effect (pedal or rack).
The parametric equalizer (name incorrectly given to the analytical equalizer) has one more control than the graphic EQ (the bandwidth, called "Q"), it also has a different approach on the manual level.
The graphic and parametric EQ's are the two types that we will examine more closely because guitarists use these more frequently than the analytical EQ. Now lets distinguish two operational modes, or intervention methods, that are either active or passive.
Active Equalizers
active equalizers amplify or attenuate the selected frequency. In the sound spectrum (term that graphically visualizes the frequencies), the frequencies have been calculated from 20hz (hertz) to 20K (20 Kilohertz, which is 20,000hz). There are higher frequencies, but they can be harmful to the human ear.
For the examples we will use a graph of this type:
The horizontal lines define the value of emphasis (positive) or cut (negative) of the frequencies, identified by values between +15 and -15. The vertical lines indicate the frequency from 20hz to 20K.
Lets return to the active equalizers that, as already stated, amplify or attenuate the selected frequency. If, for example, we want to mainly hear the 500hz frequency (therefore having more emphasis on that particular low tone), we need to increase that frequency on the equalizer to the corresponding value.
The active equalizer does nothing more than raise the volume of the 500hz solos, allowing us to hear them better with respect to the previous value.
The next question is this: When must we intervene with the equalizer?
Answer: any time that we feel the need to add or remove particular frequencies. But how do you know when to do this?
Without the use of expensive equipment (spectrometer, audioscope), you have to do it by ear and by taste. Lets look at some examples for active equalizers.
Suppose we have a sound loaded with bass, and without any middle-high. Graphically it would look like this:
The solution is to reduce the low frequencies, whose range varies from about 20hz to 500hz, and to increase the presence (middle-high) from 2Khz to 10Khz.
The job of the active equalizer will be to decrease the volume of the low frequencies and to increase the volume of the middle-high frequencies.
passive equalizers are the filters that limit the passage of the planned frequency. The opposite of active equalizers, passive EQ's don't amplify the frequency, but oppose it: set to zero it gives the maximum opposition (meaning the filter is closed), while at 10 it is neutral (meaning the filter is completely open), allowing the entire frequency to pass.
Most valve amplifiers control the tones in this manner. Suppose we want to regulate one of the three classical tone controls of the amplifier (bass, middle and treble). Rotate to the right the control for the middle tones, which doesn't increase the middle frequencies but instead filters it less.
If you maximize the value (10) the filter opens completely, allowing the entire frequency to pass. It is therefore incorrect to say: "increase the bass", or: "lower the treble."
The tone control of the guitar is also passive, unless an active circuit is used. If the tone control is closed, the treble doesn't pass, but if we open everything, we hear the true sound of the instrument.
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