An equalizer is a filter, or tone control, designed to shape sound-
either to remove or de-emphasize unwanted frequencies, or to boost
specific frequencies. The most common equalizer (EQ) in everyday
use is a tone control on a radio or TV, which selectively removes
high-frequency content. Bass and treble controls on stereo gear
adjust specific bands of frequencies. Guitar amps commonly have
bass, treble, and midrange controls, usually capable of boosting or
cutting the frequencies in preset bands. More complex is the graphic
equalizer, which may have 3, 5, 7, or even as many as 31 bands,
each representing a frequency center and offering boost and cut at
each of those centers, as this diagram shows.
It's not unusual for a graphic
equalizer's bands to actually
overlap. This makes them
somewhat imprecise for cer-
tain applications where preci-
sion equalization (such as
feedback control) is desired.
What tone controls and graphic equalizers have in common is that they affect frequencies that are prese-
lected by the manufacturer. In addition, the bandwidth-or spectrum-of frequencies affected by each con-
trol is preset. For example, a graphic equalizer may have a band that says 110 Hz (Hz is short for Hertz,
or cycles per second). That is its center frequency, and the slide control indeed boosts or cuts sound at
that frequency. However, it may boost and cut, to varying degrees, frequencies above and below that
center frequency. Therefore, when you adjust the 110 Hz slider, you may also be boosting and cutting
frequencies in a range from, say 80 Hz up to 140 Hz. The center frequency will be boosted or cut by the
greatest amount, and frequencies farther from the center will be affected to lesser degrees, depending
on the design and number of bands in an equalizer.
Another important aspect of graphic equalizers is that as a band is boosted or cut, its bandwidth often
changes-usually, the larger the boost or cut, the greater the bandwidth.
EQ / PREAMP / DIRECT BOX
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