Lowell EQ1 - Manual (Page 6)

Instruction Sheet
IS-EQ1
Issued: 1-20-14
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: 636-257-6606 Click: www.lowellmfg.com
6
Tuning a Sound System for “Flat” Response using the EQ1
A commercial sound system is typically installed for one of two purposes: Either a sound exists at one location
and needs to be spread throughout a facility where the original sound cannot be heard (as in the case of a voice
from a paging microphone, a prerecorded message, or background music being distributed throughout an office
complex), or the sound that exists is not loud enough to be heard by all of the listeners (as in the case of a
sound reinforcement system with a performer using a live microphone on a stage with listeners throughout the
auditorium). In both cases, it is important that the original sound is reproduced or reinforced in such a manor
that the sound heard from the speakers has the same frequency content as the original voice or music program.
Modern electronic sound equipment that is operating properly amplifies sound system signals without “coloring”
or changing the frequency response of the original signal. There are, however, two items that can drastically
affect the output of the sound system. The first is the system loudspeakers. There is no perfect loudspeaker
transducer that can turn an electrical signal into a sound wave without affecting the frequency response of the
signal somewhat. Some speakers are better than others, but none reproduce sound perfectly. The second item
is the acoustical space that the speakers are covering. Anyone that has spent time in a reverberant gymnasium
or in a building with glass walls, knows how dramatically a sound can be distorted by the acoustical
environment. Equalization cannot erase all of these acoustical challenges, but there are certain improvements
that can be made by using a graphic equalizer.
When the sound from an electronic sound system is produced by an imperfect loudspeaker transducer into an
imperfect acoustical space, the amplitude of the sound at certain frequencies can be either reduced or
increased due to mechanical issues in the loudspeaker itself or as a result of acoustical issues in the room. In
the early days of sound system use, engineers developed the graphic equalizer to deal with the destructive
effects of non-perfect loudspeakers and poor acoustics. The graphic equalizer divides the audio frequency
spectrum (the sound that humans can hear) into isolated 1/3 octave bands. That allows the user to adjust
selected 1/3 octave bands (slices) of the audio signal that have been artificially boosted or attenuated by the
loudspeakers or the acoustics.
Audio Engineers have developed an audio test signal called “Pink Noise” which is a noise that contains equal
energy per octave. Most humans (because of the way our ears work)
would report that “Pink Noise” sounds
“FLAT”. In other words, it sounds like it has as much energy in the low frequencies as it does in the high
frequencies. A complete discussion of audio theory is beyond the scope of this paper, but suffice it to say that
“Pink Noise” has become the standard test signal used to tune a system equalizer. A device called a “Real
Time Analyzer” (RTA for short) was developed that is calibrated to Pink Noise and can detect the output that is
produced by the sound system and break the signal up into the same standardized ISO 1/3 octave band centers
as the sliders control on the graphic equalizer.
The steps to tuning a sound system for “Flat Response” (In other words for equal energy per octave like Pink
Noise) are fairly simple. Pink Noise is fed to the input of the sound system with all tone controls (bass, treble, or
mids) on the sound system mixer set to flat (no boost or gain). The test microphone for the RTA is placed on
axis with one of the loudspeakers at a typical listener’s position. If the display on the RTA shows a flat
horizontal line, the sound system has produced a pure Pink Noise signal and no tuning of the equalizer is
required. In reality,
that never happens or there wouldn’t be a need for an equalizer. In normal practice the
response curve on the RTA will show a mostly flat line but there will be a series of peaks and dips to the line
that have been caused by the imperfect response of the loudspeaker or by issues with the acoustics. A peak in
the curve can be reduced by slightly lowering the slider on the equalizer that corresponds to the same 1/3
octave frequency band as that of the peak showing on the RTA. A dip in the curve can be corrected by slightly
raising the slider that corresponds to the same 1/3 octave frequency band as that of the dip on the RTA.
Repeating this process will eventually “Equalize” the frequency response of the sound system so that a flat pink
noise input will result in a flat pink noise output from the sound system. Now that the sound system has been
tuned using the equalizer, the output of the sound system should have the same frequency response content as
the live or recorded voice or music material that is fed into the sound system. This discussion has been a bit of
a simplification. Many good text books have been written on the subject of sound system equalization and a
study of those would be a worthwhile endeavor.