528E
14
Equalization
Equalization is one of the most powerful tools available to the audio engineer. It is, quite possibly,
the fi rst signal modifi cation device that most people experience (aside from the volume control).
This experience takes the form of using the tone controls found on most consumer audio equip-
ment. Even in this primitive form, simple tone controls can shape and alter a sound, giving us
pleasure or pain, evoking emotion, or simply enhancing our listening pleasure.
The parametric EQ in the 528E provides both creative and corrective frequency shaping - it can be
used to create a more pleasing sound, and to correct frequency response problems. The equalizer
has a symmetrical ±15 dB boost/cut response.
The term “parametric” simply refers to the fact that the primary operating parameters of the equal-
izer may be altered by the user. The user adjustable parameters are:
• center frequency (or fc, expressed in Hz),
• bandwidth (sometimes called “Q,” or selectivity, expressed in octaves), and
• the amount of cut or boost (expressed indB).
These terms are defi ned as follows:
1. Center Frequency is defi ned as the frequency (in Hz) of the middle of the bell shaped response
curve formed by a fi lter.
2. Bandwidth is the width of the bell shaped curve, measured between its -3 dB points. The
measure of bandwidth in audio equalizers is usually given in octaves or parts of an octave.
3. Cut or Boost is given in dB, at the center frequency.
Equalization Tutorial
Equalization is nothing more than selectively (or not) amplifying a signal based on frequency. Since
audio signals consist of combinations of fundamental signals and their harmonics, changing the to-
nality or the spectral balance of a signal involves nothing more than altering the relationship of the
fundamental to its harmonics, and of the harmonics to themselves. Each harmonic is re spon si ble for
one aspect of the audible character of a signal; knowing these relationships allows you to quickly
zero-in on the correct frequency range of the signal and apply boost or cut to enhance or correct
what you are hearing.
The audio spectrum has several critical portions that are responsible for our perceptions of sounds
that we hear:
2
Range Frequencies Musical Location
Very Low Bass 16-64 Hz 1st and 2nd octaves.
Bass 64-256 Hz 3rd and 4th octaves.
Midrange 256-2048 Hz 5th, 6th, and 7th octaves.
“Lisping” Quality 3000 Hz Between the 7th and 8th octaves.
Presence Range 4750-5000 Hz Between the 8th and 9th octaves.
Brilliance 6500-16 kHz Part of the 9th through the 10th octave.
Power and Fullness
In the very low bass region lies the threshold of feeling, where the lowest sounds, like wind, room
effects, and distant thunder, are felt, rather than heard. In the upper half of the fi rst octave of this
range, research has shown that the fundamentals of piano, organ and even the harp reach well into
this range. Harvey Fletcher (of Fletcher-Munson fame) charted the sensitivity of the ear for various
parts of the spectrum at levels that are lower than those of reality. Fletcher’s compensation curves
(the well known Fletcher-Munson curves) show that for equal loudness in this range at lower
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