So, you have the sound system hooked
up correctly and everything is turned on
and ready to go. You start to turn up the
mains and whoa! Feedback! What do you
do? What is the best way to get rid of that
nasty stuff?
The following assumes that you are already
using the proper mics and monitors, placed
correctly, in addition to good mains speakers
and adequate amplification. Having
realistic expectations of your gear doesn’t
hurt, either.
The drill starts out something like this: turn
all of the volumes down at the outputs of
the board. Next, center all of the graphic
EQs – it’s best to have a 31-band EQ.
You should also have a 31-band graphic
EQ on each output of the mixer – Left,
Right, Monitor 1 and Monitor 2. With the
mixer outputs down, turn all of the EQ,
crossover and amp gains to their proper
settings. Then slowly turn up the volume
one output at a time. Keep your hand on
the volume and turn it up very slowly until
one frequency starts to feedback, then try
to find that frequency on the graphic EQ
that corresponds to that output. This can
be an art in itself, with frequency detectors
being a great help and with some true diehards
being able to do this by ear. Some
exceptionally proficient soundguys have
even been known to call out the exact frequency,
as well as how many dBs it should
be turned down.
Once the offending frequency has been
identified, start with a 3dB reduction then
raise the gain a little. If you don’t need to
turn it back down, move onto the next
frequency that’s giving you trouble. Keep
repeating this process until you run out
of frequencies or several pop up at once
after you have eliminated most of the
trouble makers.
Next, we need to do the same thing again
for the mixer output. Here’s a little tip: if
you have already done this on the left
channel of the main output, you can usually
use the same settings for the right
(notice I said
usually). Now, recheck the
system and monitors to see if it will perform
to the level needed. If not, EQ out a
bit more of the problem frequencies. Keep
in mind that this can be a noisy and irritating
process and is best done without an
audience around.
Deal with a reputable music
store, and acutally listen to
their suggestions.
Now, let’s take a look at the actual
mechanics of sound that may be contributing
to the feedback. The speed of sound
changes with differences in temperature
and humidity. But how might that be
affecting the mix, you ask? Let’s say that
when you set up the club is empty and
the AC is on at a comfortable 65 degrees.
As the crowd begins to fill the venue, the
temperature begins to increase. The first
set goes great, but halfway through the
second set, feedback problems start. In
the interim, the temperature has climbed
to 78 degrees, allowing the feedback to
“scroll” up the frequency spectrum, doing
its best to ruin your mix. Now is the time
to be brave and to dial on the fly, which
isn’t for the faint of heart. Alternately, you
can turn down the volume on everything,
or you can even try to get through it by
turning down the monitor mixes. The band
will hate you, but since you’re the soundguy,
they probably do already.
Now is a good time to reiterate the fact
that the majority of feedback problems
occur due to loud main and monitor levels;
because of bad mic and monitor placement;
and from rooms with bad acoustic
properties, such as low ceilings, brick
walls, bad geometry and the generous use
of glass. Spend some time in a good room
moving your monitors to different positions
to get an idea of how placement can
affect sound and feedback.
Another option is trying new cables and
microphones. Deal with a reputable music
store and let them know your needs and
expectations. Actually listen to their suggestions.
Another piece of advice that
can’t be stressed enough is to buy good
gear – don’t skimp when it comes to EQs,
amps, speakers, and mics. Also, there is
a new generation of problem solvers, like
feedback eliminators and intelligent system
controllers. As with any new or existing
piece of gear, the most important aspect
is learning how to use it properly, so don’t
be afraid to spend time curled up with a
manual now and again.
To that end, there are many good books
available about this and other sound related
topics, with one of the best I have found
being the Yamaha
Sound Reinforcement
Handbook, by Gary Davis and Ralph Jones.
You can find it at your local bookstore or
from amazon.com. Shure also has some
good articles available in .pdf format available
at shure.com.
No go out and kill that nasty feedback!
Andy Anderson
Concert Sound
515-291-0464
www.concertsound.org
andent@lvcta.com