The main objective of this research is to perform an active control of the sound radiated by a trombone. For this instrument, most of the energy is radiated in the 50-3000 Hz frequency band, which will be the framework for this study. The goal is to reduce the total radiated power and modify the directivity of the radiated pressure. Several actuators placed near the bell will perform the active control. The optimisation of power reduction is first studied analytically by two methods using a model with monopoles. The study is further developed numerically beginning with a monopole model to finally get a more accurate model with a simplified trombone and control loudspeakers. This permits to take into account the diffraction of each part of the system. These models enable to compare and choose an optimal number of control loudspeakers as well as their optimal positions to get the targeted power radiation efficiency and directivity according to the frequency.