This is a presentation of some works in active control. Most of them were conducted by members of the LMA. We will show how these previous works have led to the project which is called ''Active anechoic room'' and is now carried out in the laboratory. The final aim of this project is to develop an active control system in order to increase sound absorption in the LMA anechoic room at low frequencies. Indeed, the characteristics of an anechoic room is to reduce the echoes coming from the walls in a very large frequency range. In the middle and high frequency range, this is very well achieved by covering the walls with absorbing materials. At very low frequencies (below 100Hz for example), this is more difficult but the active control systems are quite efficient at these frequencies and can be used as an additional tool to improve the acoustic performances of the passive system that is the coating on the walls. Apart from its practical applications, the study addresses more general questions related to sound synthesis and representations of sound fields and sources.