Graphic EQ for Live Sound: System Tuning and Feedback Control

Graphic EQ for live sound serves multiple critical functions including system tuning, room correction, and feedback control. The visual representation of frequency adjustments gives graphic equalizers their name—the slider positions create a graphic display of the EQ curve. Understanding proper application prevents both under-utilization and over-processing.

Graphic EQ Fundamentals

A graphic equalizer divides the audio spectrum into fixed frequency bands, each controlled by an individual slider. The 31-band graphic EQ uses ISO standard frequencies spaced one-third octave apart, providing reasonable resolution for most live sound applications. Common bands include 31.5 Hz, 40 Hz, 50 Hz, 63 Hz, 80 Hz, 100 Hz, 125 Hz, continuing through 16 kHz.

Each slider typically offers plus or minus 12 to 15 dB of gain adjustment. The slider’s physical position indicates the amount of boost or cut at that frequency. Center position represents unity gain—no change to the signal.

The bandwidth or Q of each filter affects how much adjacent frequency content the adjustment influences. Graphic EQ filters use relatively wide bandwidth, meaning a cut at 1 kHz also affects frequencies above and below to some degree. This differs from narrow parametric filters that target specific frequencies precisely.

System Tuning Applications

System EQ on main outputs addresses the combined response of speakers, amplifiers, and room acoustics. No speaker produces perfectly flat response, and room acoustics add peaks and nulls at various frequencies. Graphic EQ can smooth these anomalies for more consistent audience experience.

Pink noise testing provides objective data for system tuning. Pink noise contains equal energy per octave across the frequency spectrum. Playing pink noise through the system while measuring with a calibrated microphone and analyzer reveals where frequency response deviates from flat.

Analyzer software displays real-time frequency response, showing peaks and valleys across the spectrum. Rational Acoustics Smaart, Room EQ Wizard, and other tools provide this capability. Comparing measured response to a flat target indicates where EQ adjustments might help.

System tuning cuts rather than boosts. If 500 Hz appears elevated relative to adjacent frequencies, cutting 500 Hz brings it into line. Boosting deficient frequencies requires more amplifier power and speaker output capability while potentially masking problems that cutting would reveal.

Monitor EQ Strategies

Each monitor mix may benefit from dedicated graphic EQ. Different performers request different monitor content, and different positions on stage couple acoustically to monitors differently. Individual EQs allow optimization per mix.

Ringing out monitors identifies feedback-prone frequencies for treatment. Slowly increasing monitor level while listening for ringing frequencies, then cutting those frequencies on the graphic EQ, extends usable gain before feedback. Typically three to eight cuts per monitor mix provides adequate control.

Aggressive cuts degrade monitor tone quality. If extensive cutting seems necessary, the problem likely lies elsewhere—monitor placement, microphone choice, or stage volume levels. More than 10 dB of cumulative cut suggests fundamental issues beyond what EQ can solve.

Monitor EQ affects what performers hear, not what the audience hears. Settings appropriate for feedback control in monitors may differ significantly from main system tuning. Each serves a distinct purpose.

Placement in Signal Chain

Graphic EQ typically inserts between mixer outputs and amplifier inputs or powered speaker inputs. This position allows the EQ to affect the complete mix, addressing system and room characteristics rather than individual source qualities.

Insert points on mixer subgroups allow graphic EQ on specific channel groups. Drum subgroup EQ shapes overall drum tone. Vocal subgroup EQ provides unified vocal processing. This approach offers more targeted control than main output EQ alone.

Digital mixers often include graphic EQ within the console, eliminating the need for external hardware. Assigning graphic EQ to outputs or buses provides flexible configuration. The internal implementation typically matches or exceeds external hardware quality.

Common Graphic EQ Mistakes

Creating extreme curves with wild boosts and cuts indicates misuse. If the EQ curve looks like a city skyline, something fundamental is wrong with the system or approach. Graphic EQ should make modest corrections, not dramatic transformations.

Boosting low frequencies to add bass taxes amplifier power and speaker capability. If bass seems lacking, investigate speaker placement, subwoofer configuration, or room acoustics before reaching for the EQ boost.

Compensating for poor speaker placement with excessive EQ rarely succeeds. Problems caused by reflections, phase cancellation, or coverage gaps respond poorly to equalization. Addressing the physical setup provides better results.

Copying settings between venues ignores the unique acoustic characteristics of each space. Room response varies dramatically between locations. Fresh analysis and adjustment at each venue produces appropriate results.

Hardware Options

The dbx 231s dual 31-band graphic EQ remains a live sound standard for its reliability and straightforward operation. Two channels in a single rack unit handles stereo main or two independent monitor mixes.

Klark Teknik DN360 and DN370 models represent professional-grade graphic equalization with tighter tolerances and lower noise than budget alternatives. Larger productions often specify these units.

Behringer FBQ series graphic EQs offer budget-friendly options with built-in feedback detection LEDs indicating which frequencies approach feedback. While not matching premium unit quality, these serve adequately for modest applications.

Digital alternatives include hardware DSP units from Lake, BSS, and Symetrix that provide graphic EQ functionality along with other processing. These units integrate multiple functions into compact, configurable packages.