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Equalization
We've talked a lot about equalization.
Equalization is defined as increasing or decreasing the gain of a
band of frequencies. As an example, many
library and recommended settings suggest that you can boost 3 kHz
to increase the attack of an electric guitar.
3 kHz is in the middle of the "upper mid-range" frequencies of
human hearing that is approximately 2 kHz - 4 kHz. Boosting
this octave tends to make many instruments "sharper" or have more
"projection" or "attack." It's a human hearing thing.
Quality Equalization
I define
Quality equalization as boosting the correct set of harmonic
frequencies on an instrument to achieve a desired tone quality
change in the instrument. The difference is you are taking
into account the frequencies actually being generated by the
instrument. This allows you to gain the desired effect (more
"attack") while maintaining even tone and natural sound of the
instrument as much as possible. For instance, its
quite possible to boost 3 kHz on a guitar to get more attack, but
as an unwanted side effect, get the instrument sounding "uneven"
as it plays different notes.
Figure
1, below, shows that the octave containing 3 kHz would be
approximately boosting the 5th harmonic of the pitches being
played by the guitar doing a typical rhythm part.
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Figure 1 - Fifth
Octave of the Guitar |
Quality Factor
The quality
factor ("Q") in equalization defines the sharpness of the band of
frequencies that are affected by the EQ. The term comes from
the days when "reactive elements" were commonly used to make the
equalizer have a center frequency that was boosted or cut more
than any other audio frequency. How "pure" this reactance
was (it's "quality factor") determined how sharply the circuit
"tunes" to just one frequency, thus a higher "Q" meant the
equalizer affected less frequencies. Figure 2 below shows
how the "Q" parameter in EQ adjusts the amount of frequencies
around the center frequency which are being boosted or cut by a
similar amount as the center frequency.
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Figure 2 - The Q
and bandwidth |
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Q=0.7=BW of 2 octaves |
Q=1.4=BW of 1 octave |
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Q=1.0=BW of 1.33 octaves |
Q=2.0=BW of 0.67 octaves |
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The Correct Bandwidth For
Quality EQ
When we think of EQ in
terms of the harmonics boosted, we think of the bandwidth that we
need. We set the bandwidth wit the "Q" control and must be
able to relate Q settings to specific bandwidths, as shown in
figure 2.
To accent one set of
harmonics (like the fifth harmonic in a guitar) with our
equalizer, we would need a bandwidth of one octave. As long
as the pitches played by the instrument were all in one octave, it
would accent all of the notes in the musical part with a similar
tone change. The rhythm guitar (and even a melody guitar) in most
popular productions only plays notes within a one octave span.
Thus a Q value of 1.4 would be a good "recommended" or default Q
value in an EQ library program designed to accent a guitar's
sharpness.
Tuning The Bandwidth
Sometimes there is a good reason to alter the bandwidth used in
the equalization. When accenting the guitar attack in a tune with 3
guitars, we must be careful not to use a similar center frequency
on each each guitar to prevent masking problems. In this
case, we would want to narrow the bandwidth as we use different
center frequencies for the different guitars' equalization. How
much we can narrow this bandwidth, while retaining an even boost
of tone, depends on the pitch range played by the guitar that we
are equalizing.
In a rock guitar
progression of CFG, the rhythm or "backbeat" guitar often just
sounds the chords according to the chord progression. Since
the range of pitches between C and G in the same octave spans less
than 2/3 of an octave, a cue of 2.0 (2/3 octave bandwidth).
Most often the "voicing" of the chords is such that the range of
pitches played is even less than that, meaning that a cue of 2.8
(half an octave) works well for a rhythm guitar.
On the other hand, some melody guitar lines can exceed a range of
one octave. This is especially true for the lead guitar
parts. In this case, increasing the bandwidth to 1 1/3
octaves (Q=1.0) is needed. When equalizing choir, background
vocals and string orchestration parts, the range of pitches is
even greater, and it may require a Q setting as low at 0.7 (2
octaves).
Tuning The Center Pitch
In most cases a default EQ
library setting of Q-1.4 works for increasing the attack of a
guitar even though different tunes, in different keys, operate
over a different range of pitches. When you, however, use
sharper bandwidths (higher Q settings), changing the center
frequency can get better results. In the case of the guitar,
the center frequency could be "tuned" between 2500 Hz and 4000 Hz
to give the best results for the particular tune that you are
mixing.
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