Equalizers
Equalizers are used to fine tune a system, not to fix
design flaws. If you have to use a lot of equalization, there is a
problem with the system that should be solved first by relocating speakers,
changing crossover frequencies, amplifier gains, etc. Equalizers are
valuable instruments to flatten a system's frequency response (making the
levels the same at all frequencies). In competition, a measurement is
taken on how flat the response of a system is. More points are given to
a competitor with a flatter response. In a real system, a flat frequency
response is a starting point, but does not ultimately mean perfect sound since
human ears are not sensitive at the same level to all frequencies.
Many people believe that an equalizer is to boost power by
raising signal levels. 95% of the time an equalizer should be used to
cut levels rather than boost them. In a well designed system the
settings on an equalizer should not be too far from the zero dB line.
Frequency
Frequency is how many times per second a signal
(AC) switches from positive to negative and back, measured in Hertz (see
page on "Electrical Concepts" for a more extensive explanation). The
frequency range in which we are interested for audio is from 20 Hertz to
20,000 (20k) Hz. The lower the frequency, the slower the signal
oscillates.
The frequency spectrum is read using a logarithmic
scale, and is divided in octaves (doubling of the frequency). Octaves
are for example, 20, 40, 80, 160, 315, 630 Hz and so on. Equalizers
are divided in octaves, 1/2 octaves or 1/3 octaves. A 1 octave
equalizer can only control 7-9 bands (frequencies), while a 1/2 octave
equalizer can control 15 bands. A 1/3 octave equalizer would give you the
most control over the system, by being able to adjust 30 -31
bands.
Q
 Q is a measurement of how much
the equalizer band affects a range of frequencies. A high Q means that
the EQ can control a lower "envelope" of frequencies, while a low Q is a
larger envelope. Looking at the image on the right, Q is the thickness
of the affected frequencies. A smaller Q means a wider range of
frequencies boosted or cut, while a larger Q is a narrower shape.
Typical Q values are 1, 2 and 3.
Parametric vs. Graphic Equalizers
A graphic equalizer has usually fixed frequency and Q
value. The layout of a graphic equalizer is the typical sliding
controls arranged by frequency. The advantage of a graphic equalizer
is that in the way it is laid out, it is easy to see what frequency is being
boosted or cut and any person without much experience can adjust it.
Since a graphic EQ has fixed frequencies and Q, it has limitations on what
it can control.
A parametric equalizer consists of knobs that are turned
to desired levels, have adjustable frequencies and (usually) Q. The
advantage of parametric equalizers is a much greater control, since
frequencies and Q values can be adjusted. On the other hand, a
parametric equalizer is much harder to adjust than a graphic EQ, requiring
an experienced person and measuring equipment.
Mono and Stereo Equalizers
The main difference between mono and stereo EQs is that
a mono EQ has only one input and one output, and a stereo has two inputs and
two outputs. They both have their advantages and disadvantages:
A stereo EQ controls your whole system (both left and right channels) and it
is easy to adjust: Just turn the knob or slider and both left and right
channels are taken care of. If you want to adjust left and right
channels independently, you can't!
A mono EQ controls only one channel, so you need two of
them for the whole system. Since you have now two EQs it takes a lot
more time to setup the system. Many people use mono EQs for the
greater control they give over the system. Since left and right
speakers are not exactly at the same distance to our ears, two mono EQs can
help compensate for time delays and problems caused by speaker
placement. Buying two mono equalizers is more expensive than buying
one stereo EQ.
Low Level and High Level Output
A high level output equalizer takes either high level
(speaker) or low level (RCA) inputs and has a built in amplifier. The
output goes directly to the speakers and can not be hooked up to another
amplifier. These equalizers are cheap and cause more damage than good
to the sound system. They do "boost" signals, but all this does is add
distortion to the overall sound.
A low level input EQ takes RCA signals from the radio
and has RCA outputs that get hooked up to amplifiers. Since these equalizers
work at low signal levels, they introduce very little distortion, if any to
the system. They do cost more and require more wiring than a high
level EQ.
A third kind of equalizer gets hooked up directly to the
head unit via a special cable and is controlled by the head unit.
These EQs use low level signals and are usually of good quality. The
drawback is that if you want to upgrade the head unit or change brands, the
EQ will not be compatible with other brands or even with different models
from the same manufacturer.
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