This paper investigates numerically the Galerkin Least Square method for time-harmonic acoustic simulation within the virtual church of Royaumont abbey. The church's history is here reviewed. Since this church was destroyed after the french revolution, the methodology to build an accurate computer aided design model of the church of Royaumont abbey is clearly detailed. The Helmholtz equation is considered and the finite element analysis is carried out for solving this time-harmonic acoustic problem. Numerical experiments on the computer aided design model demonstrate the efficiency of the stabilized finite element method compared to the classical finite element method. © 2016 IEEE.