Handbook for Sound Engineers

194 Chapter 7

Vice versa, it closes the input gate and opens the output
gate when the microphone signal is falling, Fig. 7-60.^62

An acoustical feedback is thus avoided, but this
system fails to enhance the lateral energy with contin-
uous music, which makes it unsuitable for music perfor-
mances. It is no longer used.

7.4.2.3.3 LARES

The LARES system by the Lexicon Company uses
modules of the standardized Room Processor 480L
which, when fed by special software, simulates the
desired decay curves, Fig. 7-61. A large number of
loudspeakers are required in the wall and ceiling areas.
The input signals are picked up by just a few micro-
phones in an area near the source.63,64 On account of the
time-variant signal processing (a large quantity of inde-
pendent time-variant reverberation devices), the adjust-
ment of reverberation times is not exactly repeatable.
Common computer-controlled measuring software
(based, e.g., on MLS) is thus unable to measure decay
curves. Apart from the ASC system, LARES installa-
tions are very widespread in Europe and the United
States. Well known are the systems installed in the
Staatsoper Berlin, the Staatsschauspiel Dresden, and the
Seebühne (floating stage) in Mörbisch/Austria.

7.4.2.3.4 System for Improved Acoustic Performance
(SIAP)

The basic principle of SIAP consists in picking up the
sound produced by the source by means of a relatively
small number of microphones, processing it appropri-
ately (by means of processors which convolute, that is
overlay electronically the room-acoustical parameters of
a room with target parameters) and then feeding it back
into the hall by an adequate number of loudspeakers,
Fig. 7-62. The aim is to produce desired natural acous-

tical properties by electronic means. For obtaining

spatial diffusivity a large number of different output

channels are required. Moreover, the maximally attain-

able acoustic amplification is dependent on the number

of uncorrelated paths. Compared with a simple feedback

channel, a system with 4 inputs and 25 outputs is able to

produce a 20 dB higher amplification before feedback

sets in. This holds true, of course, only under the

assumption that each and every input and output path is

sufficiently decoupled from the other input/output

paths. Each listener seat receives sound from several

loudspeakers, each of which irradiates a signal some-

what differently processed than any of the others (!).^65

7.4.2.3.5 Active Field Control, AFC

The AFC system by Yamaha^66 makes active use of

acoustic feedback for enhancing the sound energy

density and thereby also the reverberation time. When

using the acoustic feedback it is, however, important to

avoid timbre changes and to insure the stability of the

system. To this effect one uses a specific switching

circuit, the so-called Time Varying Control (TVC)

which consists of two components:

• Electronic Microphone Rotator (EMR).


• Fluctuating FIR (fluc-FIR).

The EMR unit scans the boundary microphones in

cycles while the FIR filters impede feedback.

For enhancing the reverberation, the microphones are

arranged in the diffuse sound field and still in the

close-range source area (gray dots in Fig. 7-63 on the

right). The loudspeakers are located in the wall and

ceiling areas of the room. For enhancing the early reflec-

tions there are four to eight microphones arranged in the

ceiling area near the sources. The signals picked up by

these are passed through FIR filters and reproduced as

Figure 7-60. Basic block diagram of the Acoustic Control System (ACS) illustrated for a loudspeaker–microphone pair.

ACS

processor

Real Hall

Sound

source

Signal

Convolution

Reflections

Reflection

simulator

Hall with the desired

acoustical properties

Parameters