ACS or Acoustic Control Systems is a system designed to give the ability to alter the acoustic characteristics of a performance venue. Typically used to enhance the reverberation time in acoustically dry halls or to enable multi-purpose halls to be used for different types of performance.

RK Sound Engineering Ltd are agents for ACS in the UK and Ireland.

Acoustic Control Systems: An introduction to ACS

In any hall there will be sound reflections from walls, ceilings etc. and the way in which the sound reflects and re-reflects in relation to the position of the sound source and the position of a listener is what is perceived as the acoustics of the hall.
ACS changes the acoustics by adding sound reflections to the natural ones. To make sure this is done in a way that the listener will perceive it as natural acoustics, it is very important that this happens at the correct place with the correct signal mix and timing.

A number of microphones (typically 8-30) are used to pick up the source sound which is then processed, amplified and rendered using a number of strategically placed loudspeakers which are usually installed to be visually unobstrusive and to blens with the surroundings.

In a typical system several acoustic presets are programmed which together with the original acoustics of the venue enable it to be used for a variety of differing performance types all at the push of a button. ACS can for example change the very dry acoustics of the typical modern theatre which is best suited to speech into the very lively acoustics of a good concert hall. Or that of a dry sounding modern church into the reverberant sound usually associated with the more traditional church.

Because different venues require different solutions, various hardware and software modules were developed. One, or a combination of these modules are installed in the main processing system and together they will form the variable acoustics system.ACS systems are known for their natural sound whcih largely accounts for the popularity of the ACS system.

Installation of an ACS system is usually carried out in co-operation with the customer's acoustic consultant and RK Sound Engineering Lts who provides the local project management and installation services. ACS engineers provide support during the whole process and check and tune the system after the installation is completed. ACS also provides post installation support beacuse they believe that the best publicity comes from a satisfied user.

The function of the various system modules is described in the paragraphs below.

Reverberation processing

This is the general basis of an ACS system. It is common knowledge that the amount of reverberation desired is dependent upon the type of performance being staged. e.g. 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 module adds reverberation to the correct amount. 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 60dB after the sound source has stopped.
Below you see a list of generally accepted reverberation time requirements.

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 (amongst others) are 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 feeling about the size of the hall.

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

In small shoe box shaped halls there are often sufficient naturally generated early reflections. In larger halls and for example some modern theatres with their typically 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 are of minor importance. In larger churches or cathedrals you usually also find a limited amount of early energy. In concert halls on the contrary early reflections are of major importance to hear the pure "definition" of sound.

ACS provides the early reflection unit to mitigate for the lack of early reflections. Driving a number of loudspeakers, placed in suitable locations, the unit can add reflections in a very flexible and accurate way. In most projects where the ACS system is installed in theatres / concert halls a combination of processing for reverberation and early reflections has been used.

Foldback Extension

In some venues the performance area is not part of the acoustical environment where the audience is seated. You often find in modern theatres that the musicians are seated on stage or in/under the stage tower. They are shielded from the acoustics of the hall by 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 an extension to the reverberation processing was developed which is the Foldback extension.
Foldback loudspeakers will radiate an image of the reverberation in the hall back to the performers on stage, making it an acoustical unity. The effect of this is that the reverberation allows the musician to play with a llighter touch.

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 theatres / 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, 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 negate the need to move the often large and heavy parts of a mechanical shell and compared to a mechanical shell will save initial expenses and subsequent labour costs.

A system rack with 24 power amplifier channels in the lowest 2 sub-racks.