© 2003 TDM Audio, Inc.
Graphic Equalizer Owner’s Manual
Page 13
With PA equalization, avoid boosting altogether if possible. It is sometimes advisable to boost
the extreme lows (50 and 63 Hz) or the extreme highs (12.5 kHz and 16 kHz) to deal with a PA
system that lacks extension on the ends of the spectrum, but if you do this, use only a slight
boost. Excessive boosting of low frequencies can eat up amplifier headroom quickly without
providing much benefit to the overall sound. It can also damage some low-frequency speakers
from overexcursion. Excessive boosting of high frequencies can damage high-frequency
speakers, ears, or both. Any boosting can cause problems with feedback. The rule with boosting
is to use extreme discretion and make sure you know what you are doing.
Use as little equalization as you can get away with to achieve good sound. Equalization can
easily make things worse if not used carefully. The “if it’s not broken, don’t fix it” rule applies
here. If it sounds good flat, then leave it flat (although in practice, most systems can be enhanced
using at least some equalization).
When using the unit for instrument equalization, a different set of rules applies. Boosting is often
used with instruments to achieve a particular effect. Also, it is typical to see radical equalization
curves used with electronic instruments. In these cases, the equalizer can actually be considered
part of the instrument: It helps create the sound. Be careful with acoustic instruments and
especially microphones and acoustic guitar pickups because a lot of boosting can still cause
feedback problems.
Using the Notch Filters
When a particular frequency is problematic (for example, a frequency where feedback is a
problem), a
notch filter
can be used to eliminate it from the signal. Notch filters are different
from regular graphic equalizer bands in four ways.
1. Their frequency bands are 1/6 of an octave wide instead of 1/3 of an octave. This means
that they cut frequencies much more selectively.
2. They provide up to 40 dB of cut instead of 12.
3. Their center frequencies are sweepable. This lets you zero in on the exact frequency
causing the problem.
4. They are for cutting only. Boosting is not an option.
Notch filters should be used whenever you need to zero in on an exact frequency and cut it
drastically without affecting nearby frequencies. They are called notch filters because when you
plot their frequency vs. gain characteristic you see a deep, thin, notch at the selected frequency.
The TDM 30GE series graphic equalizers provide 3 sweepable notch filters per channel. Each
notch filter provides control over a different range of selectable frequencies. The high and low
points of the ranges are printed at the top and bottom of the notch filter frequency sliders. During
normal operation, the cut sliders for these filters are all the way up signifying no cut. To use a
notch filter, set its frequency slider to the frequency that you want to cut, then lower its cut slider
to select the amount of cut. After selecting the amount of cut, you can go back and fine-tune the
frequency slider “by ear” to get the exact frequency that you want.
