Active Field Control (AFC) Reverberation Enhancement System Using Acoustical Feedback Control

Active Field Control (AFC) Reverberation Enhancement System Using Acoustical Feedback Control

What is AFC? Active Field Control Electro-acoustical sound field enhancement system *Enhancement of RT and SPL reverberance, loudness Early reflections spaciousness * Improvement of undesirable acoustic conditions Under-balcony uniformity Stage acoustics Easiness of playing Under development at Yamaha for more than 15 years Installed in approximately 50 venues in Japan New device AFC-1 has been released in the US

Definition of three categories developed in the field of Sound Field Control A-SF P-SF S-SF Categories Assistance of Sound Field Production of Sound Field Synthesis of Sound Field Definition Control of room acoustic condition based on the given room condition Production of spatial sound effects, mainly for a theater, movie, etc. Synthesis of requested condition in a given, or highly absorbent, room Yamaha AFC System

AFC Key Features 1. Conservation of original acoustics in the room because it uses an acoustic feedback loop 2. Wide range controllability using a FIR filter 3. Effective at any position in the room 4. Stability against coloration using Time Varying Control 5. Two separate systems, Rev. and ER 6. Easy operation

Conservation of original acoustics

Two ways to increase Reverb Time Less absorptive Absorptive Room size Larger room size

(db) Wide range of controllability using FIR filter AFC has two ways of enhancing reverberation (db) 0 adding energy to each FIR tap extending RT with increasing SPL Decreasing absorption (Sec) 0 (Sec) expanding gaps between FIR taps extending RT without increasing SPL Increasing room volume

Time Varying Control Using feedback is essential for AFC Loop gain control for maintaining stability is the key for AFC Two types of Time Varying Control are included in AFC to ensure smooth loop gain They ensure enough margin against instability

TVC :EMR (Electronic Microphone Rotator) I/O combination is changed periodically

TVC : fluctuate-fir Each FIR tap moves periodically on time axis (Tdi,Fi) Tdi: variable time range(msec) Fi: modulation frequency(hz) time

Closed loop gain with TVC

Stability against the increase of Loop Gain

Basic Idea for Speaker and Microphone Placements 5-6m (16-20ft)

Effective at any position in the room Yamaha AFC Conventional way Reverberation is controlled using feedback Effective area is limited

Block diagram of AFC system

Core Device: AFC1

Block diagram using AFC1

AFC Tuning Process Architectural condition is fixed Equalization of frequency characteristic of each speaker Adjustment and measurement of loop gain Adjustment and measurement of reverberation Confirmation of acoustical condition by listening to the sound using instruments actually

Loop Gain Monitor (LGM) Microphone HA Loop Gain Monitor EQ SFP EQ SP AFC1(1) AFC1(2) PC Pink noise FFT or RTA LGM has been developed based on DME32 Digitally connected with AFC1 To change the loop on/off of each channel To select the oscillator signal and its input channel

Application of AFC

Easy Operation

Example of RT Characteristics

Experimental Project Yamaha Hall A total solution for the system including AFC, SR, surround effects, etc. is realized utilizing digital network m-lan.

YAMAHA HALL 524 seats, Original RT 500Hz =0.7s Finished by wooden slats which caused high absorption Too dead for acoustical instruments AFC has been installed in order to realize the sound field optimum for both cinema and classical music. Experimental project using AFC1 & m-lan,, which have been installed at the first time in the world.

System configuration AFC and SR system are digitally connected. The system located in different spaces are also connected with m-lan.

pattern change control panel for AFC AFC1 Optical mlan converter AFC / Amplifiers rack

T 60 measurement results

Conclusion The development of TVC and utilization of digital equipment improve sound quality. AFC is gaining acceptance by classical musicians, who typically have had some reservation about the use of electro-acoustics. As the next phase of development, a total solution for the system including AFC, SR surround effects should be proposed utilizing digital network.

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