Fundamentals of Audio Production. Chapter 12

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Fundamentals of Audio Fundamentals of Audio ProductionProduction

Chapter Twelve:Chapter Twelve:

Live Sound ReinforcementLive Sound Reinforcement

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Live sound terminology

• Terms are often interchangeable

• Public address (PA) systems – most often describes systems for voice – i.e. sports venues, paging systems

• Sound reinforcement – describes systems for live music– i.e. concerts and live performances

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Controlling feedback

• Feedback is the result of the speakers’ output being picked up by microphones and re-amplified by the system

• Feedback is self-sustaining oscillation of the system

• Feedback is controlled by proper placement of the microphones is relation to the speakers

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Controlling feedback

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Controlling feedback

• Use of cardioid microphones helps eliminate feedback by rejecting sound from the front-of-house speakers and monitors

• The null or off-axis side of the microphone should be place toward the live speakers

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Tuning the system

• Each room has unique modes resulting from its dimensions and shape

• Modes may cause some frequencies to “ring” or resonate – other frequencies may be difficult to propagate or hear

• Precise “tuning” of the system with a 1/3 octave equalizer can compensate for room modes by boosting or cutting some frequencies

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Tuning the system

• Systems may be tuned by playing white noise, and then measuring the response with a real time analyzer

• RTAs capture the sound using a calibrated accurate microphone

• RTAs provide a visual display of the frequency response

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Tuning the system

PC-based RTA display, left and right channels

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Loudspeakers and monitors

• In many large systems, the signal from the console is split by external crossovers, then routed to appropriate high, mid, or low frequency enclosures in the speaker array

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Crossovers

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Monitor mixing

• In small to medium systems, the sound is mixed for the on-stage monitors on the front of house console

• “Pre-fader” signals from individual microphones may be mixed and routed to the monitors without being affected by the main faders

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Pre-fader versus post-fader

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Monitor mixing

• In large complex live systems, the monitor mix is created on a separate console, usually located just off-stage

• The signals from the microphones are split at the snake box, and routed to the two separate consoles

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Monitor mixing

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Monitor mixing

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Monitor mixing

• In ear monitors, or ear buds, have become popular alternatives to on-stage floor wedges

• Keeps stage sound levels lower

• Requires a limiter to assure that feedback or other loud transients do not damage hearing

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Monitor mixing

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Sound check

• The system is tuned to the room

• Gain and output levels are set

• Automated consoles recall settings from previous shows or set changes

• Caveat: Some performers will play much louder during performance than sound check

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Avoiding equipment damage

• Amplifiers should be turned on last after consoles and all other outboard equipment

• Amplifiers should be turned off first before powering down consoles and other gear

• This will avoid speaker-damaging “pops” from the power surges

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Level management

• More and more venues are placing restrictions on sound pressure levels

• Some because of noise ordinances in local municipalities

• Some because of the fear of liability for hearing damage to patrons

• Many live sound engineers keep a sound pressure level meter at the FOH console

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Ground loops

• Ground loops cause hum in the system due to differing potentials on different power circuits

• If possible, all equipment should share a common earth ground point

• If hum is present, connecting the console and amplifiers to the same circuit may eliminate it

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Connections

• Professional systems use balanced connections

• Wiring contains two conductors for hot (+) and cold (-) signal, plus a shield

• Unbalanced cabling only uses one conductor and a shield

• Unbalanced connections are more susceptible to hum and interference

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Types of shielded cabling

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XLR connectors

Female (left) and male (right) use 3 contacts, with pin “1” connected to

shield

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Connectors

1/8-inch tip-ring-sleeve

1/4-inch tip-ring-sleeve

1/4-inch tip-sleeve

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Connectors

RCA connector is popular in consumer electronics – but not widely used in professional systems

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Connectors

Speakon (left) is used for speaker connections, DIN (right) is used for connecting MIDI signals

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Impedance matching

• The method by which speakers are connected to the amplifier will affect the impedance or “load” on the amplifier

• Mismatching the load on the amplifier can rob the system of power, or cause damage to the amplifier

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Impedance matching