Reverberation processing

This is generally the basis of an ACS system configuration.  It is common knowledge that the amount of reverberation desired is depending on the type performance.

You have to speak slowly in a reverberant environment like a cathedral to be intelligible, but a choir will sound great there.

The ACS reverberation processing adds reverberation in the amount necessary.
The length of reverberation is measured as the so called RT60 time. This is the time it takes in seconds for the sound level to decrease 60 dB after the sound source has stopped.
Below you see a list of generally accepted reverberation time requirements.

 Type of Performance
 RT 60 @ 500 Hz (sec.)
 Pop music /cinema 
 0.7 – 1.1
 1.0 – 1.4
 Comic and Baroque Opera
 1.2 – 1.4
 Grand Opera 
 1.4 – 1.6
 Symphonic (Baroque) 
 1.2 – 1.5
 Symphonic (Classical)
 1.5 – 1.8
 Symphonic (Romantic) 
 1.8 – 2.5
 Organ, (Choir)
 > 2

Early Reflections processing

Another important part of acoustics are the early reflections.
Early reflections is sound arriving within 100 ms after the direct sound. (The first reflections from ceilings and walls)
Early reflections are among others important for clarity presence and spaciousness.
The correct amount of early reflections will make the audience feel closer to the performers, it will give a clear definition of sound and give a perception  about the size of the hall.

The example of the cathedral in the last paragraph makes it clear that reverberation doesn’t improve clarity and presence, this is the reason why ACS has separate modules (control) for early reflections and reverberation (re-reflected later reflections)

In smaller shoe box shaped halls there are often sufficient naturally generated early reflections. In larger halls and for example the modern theatres with the typical diverging walls, you usually find a lack of early energy.
Of course the density of early reflections necessary is depended on the use of a venue.
In churches that like to have a more reverberant hall for hymn singing and choirs with an organ, early reflections may be of minor importance . In larger churches or cathedrals you usually also find a limited amount of early energy.
In concert halls on on the other hand early reflections are of major importance to hear the pure “definition” of sound.
ACS delivers the early reflection processing, which will  use a number of loudspeakers, placed on suited locations, adding reflections in a very flexible and controlled way.
In most  theatres and concert halls a combination of processing for reverberation and early reflections is used to provide the exact balanced mix between early reflections and reverberation.

Foldback Extension

In some venues the performance area is not a part of the acoustical environment where the audience is located. You often find situations like this in modern theatres where musicians will be seated on stage, in/under the stage tower. They are to some extend shielded from the acoustics of the hall because of  the proscenium opening.
For a good performance however it is important that the musicians know how their music is perceived by the audience. To solve this the Folback extension to the reverberation processing was developed.
Foldback loudspeakers will radiate an image of the reverberation in the hall back to the performers on stage, making it an acoustical unity. Next to that, the nice reverberation will also let musician on stage play “lighter” and put more detail in their performance.

Orchestra Shell Extension

It is important for musicians to hear each other and themselves. Therefore there should be sufficient early reflections in the performance area.
In theaters / concert halls often large mechanical orchestra shells are used for this purpose.
As an extension to the early reflection processing, ACS developed an electronic orchestra shell as an alternative for the mechanical shell.
A number of loudspeakers (typically 12-14) radiating early energy are placed in an U-shape around the performers.
The ACS orchestra shell will make sure one doesn’t have to move the often large and heavy parts of a mechanical shell and compared to a mechanical shell it will save both initial the expenses as well as the labor costs necessary to set up a mechanical shell. In case an ACS shell cannot be mounted permanently the relatively light weight elements (loudspeakers) can be placed quickly and when stored it only takes a very limited amount of storage space.