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Equalizers:
The first thing that you should know
is that an equalizer is not designed to make a system louder. An equalizer
will not make an amplifier put out any more power than without it if your
head unit has a sufficient drive signal. An EQ is designed to smooth out
the frequency response of your system. The most popular EQ is the preamp
level graphic equalizer. It takes the signal from the RCA jacks on your head unit. This
type of EQ is sometimes called a passive equalizer but this is not accurate.
A preamp EQ is full of active filter circuits. All of the equalization
is done by those active filters. There are different types of preamp EQs.
Graphic Equalizers
The graphic equalizer gets its name because its sliders will
form a graphic representation of the boost or cut in the audio output.
Each of the sliders boost or cut a small section of the audio spectrum at a predetermined
frequency. Usually the lower frequency (bass) sliders will be on the left.
The high frequency (treble) sliders are on the right.
The diagram below shows how the output boost/cut curve relates to the slider position.
Note:
The sliders on the equalizer above are set up (roughly) in the typical 'smiley face' positions. Generally, when an equalizer is set up like that, either the speaker system is severely lacking on both ends of the audio spectrum or the person who set the EQ didn't know what they were doing. The position of the sliders above were simply randomly set in those positions and therefore shouldn't lead you to believe that that's the way to set the EQ in any particular system. Read the 'Spectrum Analyzer' and 'Tuning with RTA' pages of the site for more detailed information.
Parametric Equalizers:
Another type of EQ
is a parametric. A parametric EQ will allow for the adjustment of boost
and cut of different bands like a graphic EQ but it allows the shifting
of the frequency up or down the audio band. It may also allow for the adjustment
of "Q" (the quality of the curve of the band passed by the filter). One
drawback of the parametric EQ is that they usually have fewer bands than
a graphic EQ but they can be extremely useful in fine tuning a system with only
a few flaws in its frequency response.
The diagram below shows the difference in the output response with different Qs. You can see that a high Q curve is very narrow and a low Q curve is wider. If you need to fix a very narrow dip or peak in your system's response, the high Q filter will work best. If you have a wider band that needs to be tamed, the low Q setting would be a better choice.
Calculating 'Q':
The 'Q' of a filter or curve can be calculated by the formula...
Q = (center frequency)/((high frequency 3dB down point) - (low frequency 3dB down point))
For the following curve, you can see that -3dBhigh is 1750hz and the -3dBlow is 750hz. If we punch those numbers in the formula we get...
-3dBhigh = 1750hz
-3dBlow = 750hz
Q = (center frequency)/((-3dBhigh) -
(-3dBlow))
Q = 1000/(1750 - 750)
Q = 1
For the following curve, you can see that -3dBhigh is 1100hz and the -3dBlow is 900hz. If we punch those numbers in the formula we get...
-3dBhigh = 1100hz
-3dBlow = 900hz
Q = (center frequency)/((-3dBhigh) -
(-3dBlow))
Q = 1000/(1100 - 900)
Q = 5
You can see that the higher Q filter has a much narrower 'skirt'.
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