This study is dedicated to the development and experimental validation of an active acoustic anechoic method to reproduce, in an enclosed space, free field conditions, that is without reflections from the walls.The use of an anechoic room is a classical solution but it is not sufficient at low frequencies, in particular, below the cutoff frequency of the room which is directly related to the thickness of the absorbent material on the walls. In order to combine passive and active noise reduction techniques, the goal of this study is to control the field scattered by the walls in this frequency range using active techniques. A specific difficulty with this active control problem concerns the measurement of the diffracted field which is not a directly accessible physical quantity. In this work, a strategy for estimating the diffracted pressure is developed from measurements of total pressure on a surface surrounding the source to be characterized. Active control techniques are then used to drive an array of actuators to minimize the estimated diffracted field. The active anechoic method used here consists in identifying in an precalibration step the mathematical operator which relates the diffracted pressure to the total pressures. At the active control stage, the operator is then used as a filter to be applied on the total pressure measurements. For this first experimentin real size, the goal is to control the diffracted field by the reflecting wall of a semi-anechoic room with 9 secondary sources in the frequency range 40-200 Hz. The sources are situated on the reflecting wall. The diffracted pressure is first estimated from measurements from 16 microphones located in a plane parallel to the reflecting wall. Several control strategies are applied and evaluated to characterize the acoustic radiation of a low frequency source.