System Tuning
Definition
System tuning, or system optimisation, is the adjustment of a sound reinforcement system so that its tonal balance and timing are consistent across the audience area before a show is mixed on it. The core technique is the dual-channel FFT transfer function measurement. The same signal is sent to the system and, as a reference, directly to the analyzer, while a measurement microphone in the room picks up the output of the system. The analyzer compares the two signals, and the difference between them is the response of the system, room included.
The guide
The measurement gives three traces. Magnitude shows the tonal balance, the frequency regions where the system is too bright or too dull. Phase shows the timing relationship across frequency and indicates whether the different parts of the system arrive together. Coherence expresses, frequency by frequency, how consistent the measurement is, and it shows where noise, reverberation or reflections make the data unreliable. From these traces the operator equalizes the main tonal imbalances toward a target curve, then aligns arrival times so that the sources add instead of cancelling.
Delay is the main alignment tool. The subwoofers are aligned with the main loudspeakers so that the two sum in the crossover region, and fill and distributed loudspeakers are delayed to the mains so that the audience hears a single coherent arrival. Tuning takes place before the show, in a quiet venue, so that during the show a fader move has the expected effect on a neutral, predictable system.
The rule
The system is tuned before the show so that mixing starts on a predictable system.
Glossary terms
Sources
- Rational Acoustics, Measurement 101: Types of Measurement : la fonction de transfert (FFT bicanale) compare un signal de référence et le signal mesuré · support.rationalacoustics.com
- Rational Acoustics, What is Coherence? Cohérence : corrélation mesure/référence, repère le bruit, la réverbération et les réflexions · support.rationalacoustics.com
- B. McCarthy, Sound Systems: Design and Optimization, 3e éd., Routledge, méthode d'optimisation : égalisation, délais, alignement des sources · blackwells.co.uk
- M. van Veen, Subwoofer Alignment: The foolproof relative/absolute method : alignement en phase subs/principales dans la zone de raccordement · merlijnvanveen.nl