Live Sound Mixing: FOH, Stage Monitors & Feedback Control

1. The FOH vs. Monitor Mix Separation Architecture

Concert audio operates on two completely separate mixing philosophies driven from a common analog stage split or Dante digital snake:

• Front-of-House (FOH): Focuses on the paying audience in the auditorium. Tracks are processed with post-fader aux sends, stereo panning, master bus compression, and spatial hall reverbs.
• Stage Monitors (Foldback / IEMs): Focuses entirely on the performers on stage. Mixes use pre-fader auxiliary sends so that adjustments made to the house mix do not alter what musicians hear in their ears or floor wedges.

2. The Physics of Acoustic Feedback & Ringing Out Wedges

Acoustic feedback occurs when sound from a stage monitor speaker re-enters the vocal microphone at a gain level greater than unity (Loop Gain ≥ 1), creating a runaway oscillating squeal or low rumble.

Live sound engineers eliminate feedback before the show using the Ringing Out procedure:

1. Place the vocal microphone in its exact performance stand position facing the floor wedge.
2. Slowly raise the monitor aux master level until the room begins to resonate with a faint ringing pitch.
3. Identify the oscillating frequency using a real-time analyzer (RTA) or ear training.
4. Insert a narrow parametric notch filter (Q = 10 to 14) or 1/3-octave graphic EQ band on the monitor bus and cut 3 dB to 6 dB at that exact resonant frequency.
5. Push the monitor level higher and repeat for the next 2 to 3 resonant frequencies, gaining 6 dB to 9 dB of stable headroom before feedback occurs.

3. Microphone Polar Patterns: Cardioid vs. Supercardioid Placement

Matching monitor wedge physical placement to microphone capsule rejection angles is essential:

4. Delay Tower Time Alignment Physics

In large concert venues and outdoor festivals, secondary loudspeaker towers (Delay Towers) cover audience members seated 150+ feet away.

Because sound from the main stage PA travels through air at 1.13 feet per millisecond (343 m/s), sound from the main stage arrives roughly 133 milliseconds after the delay tower speaker fires. If uncorrected, the audience hears a disorienting, smearing double-echo.

System engineers insert a digital delay into the tower processor matching the physical distance:

5. Cardioid Subwoofer Arrays: Cleaning Up the Stage

Standard subwoofers are omnidirectional, radiating low bass equally forward into the audience and backward onto the stage, creating muddy bass resonance that bleeds into drum and vocal microphones.

Concert sound designers build Cardioid Subwoofer Arrays: by placing one backward-facing subwoofer between two forward-facing subwoofers, inverting its electrical polarity, and adding a precise millisecond delay matching the cabinet depth, the sound waves cancel completely behind the array on stage while summing constructively toward the audience.

Calculated: the delay tower margin, the polar nulls and a cardioid sub model

We worked out three of this page's claims on 2026-10-01. This is arithmetic and a simple model. No PA, microphone, stage or subwoofer was involved. The speed of sound comes from the dry air formula. The polar patterns are the first order family, pressure = a + (1 - a) cos(theta), with a of 0.5 for a cardioid and 0.37 for a supercardioid. The sub array is two point sources, the second inverted and delayed by their spacing divided by the speed of sound, which is a simpler layout than the page's three cabinets.

The delay tower arithmetic holds: 150 ft is 133 ms at 20 C. The margin does not leave much room, though. Across 15 to 30 C the delay for the same distance moves by 3.4 ms, so a tower set at 30 C and left alone as the evening cools to 15 C loses 3.4 ms of the 5 ms and keeps only 1.6 ms, while one set at 15 C and left to warm gains the same amount. Re-measure the delay when the temperature changes, and whether a larger margin suits the system is a choice to make in the room.

The null angles are right. A cardioid is silent at 180 degrees and a supercardioid at 126 and 234 degrees. That gives the page's flanking wedge advice a number: a supercardioid is only 12 dB down directly behind, and 22 dB down 45 degrees to either side of the rear, so two wedges there reject more than one wedge behind. Whether the Beta 58A and e945 are supercardioid with a null at 126 degrees was not checked against their data sheets, and real microphones depart from the ideal curve.

The cardioid sub model cancels completely behind the pair, and toward the audience it adds, as the page says, but by only 1.28 to 1.78 times one sub across 63 to 100 Hz, which is less than the doubling that two subs in phase would give. A delay error of one sample (21 microseconds) leaves the rear rejection near 43 dB at 80 Hz, 0.1 ms leaves about 30 dB and 0.5 ms leaves about 15 dB, so cabinet spacing and the room, and not the delay resolution, decide how well it works. The page's three cabinet layout, the room and real cabinets were not modelled.

Reference. Calculated 2026-10-01 by arithmetic and a two source model. No PA, microphone or subwoofer was used, and the microphone models and the three cabinet layout were not checked.

Working Vocabulary

Key live sound reinforcement and concert engineering terms introduced in this guide:

• Gain Before Feedback (GBF). The maximum amplification level achievable in a sound system before acoustic feedback oscillation begins.
• Ringing Out. The process of intentionally exciting stage monitors to identify and notch out resonant room frequencies using parametric or graphic EQ.
• Pre-Fader Aux Send. An auxiliary channel routing where signal is tapped before the channel fader, ensuring monitor levels remain independent of house fader moves.
• Delay Tower. Supplemental loudspeaker arrays positioned further back in a venue time-delayed to align with sound arriving from the main stage PA.
• Cardioid Subwoofer. A directional subwoofer configuration that uses physical displacement, delay, and polarity inversion to cancel rear bass radiation.
• Haas Effect (Precedence Effect). The psychoacoustic phenomenon where the human ear localizes sound to whichever loudspeaker source arrives 1 to 5 milliseconds first.

Sources & Further Reading

• Audio Buffer Size & Latency Calculator
• Audio Cable Loss & Capacitance Calculator
• Drum Recording: Mic Techniques & Phase Alignment
• Boom Mic Operating & Location Sound Recording