In this talk we consider 3D interior and exterior Helmholtz problems, reformulated in terms of a Boundary Integral Equation (BIE). For their numerical solution, we rely on a collocation Boundary Element Method (BEM) formulated in the general framework of Isogeometric Analysis (IgA-BEM), adopting in particular conforming multi-patch discretizations [3] . As it is well known (using BEM as well as IgA-BEM), the matrices of the resulting linear system are fully populated and non-symmetric, a drawback that prevents the application of this strategy to large scale realistic problems. As a possible remedy to reduce the global complexity of the method, we propose a numerical scheme based on the hierarchical matrix (H-matrix) technique [4]. Using a suitable admissibility condition, it starts with hierarchically partitioning the matrix into full- and low-rank blocks. The former are stored and computed in conventional way, meanwhile the latter are approximated by the Adaptive Cross Approximation (ACA) methodology [1] which successfully compresses the dense matrices of the multi-patch IgA-BEM approach. Furthermore, the cost of the matrix-vector product is reduced [2] and this allows us to increase the overall computational efficiency of the Generalized Minimal Residual Method (GMRES) [5], adopted for the solution of the linear system. Several numerical examples are given to demonstrate the accuracy and efficiency of the proposed methodology.