Electromagnetic interference measurements (EMI) for EMC qualification of products have to be made in normalized test sites such as an open area test site or a semi-anechoic chamber. Site validation is defined by European and international standards for the frequency range between 30 MHz and 1 GHz. Using Computational ElectroMagnetics (CEM), it is now possible to predict chamber performance. The work presented in this paper is on the optimisation of the dimensions and the surface of ferrite tile absorbers, due to the important impact of these parameters on the cost of an EMC chamber.