This paper discusses the new aspects of the formulation for structural model updates presented by Larsson and Sas at the 10th International Modal Analysis Conference held in San Diego, California in February 1991. First, a closed-form representation of the reduced impedance matrix is presented. It clarifies the issues associated with the range of the excitation frequencies used in model updates. Second, the applicable range of excitation frequencies is considered through an example where low frequency local modes are present. Third, the computational aspects of this algorithm are studied. It is shown that displacement sensitivity vectors corresponding to the virtual loads are critical in the computation of the reduced impedance matrix sensitivity. Finally, applicability of this algorithm for approximate frequency responses in structural optimization is studied. Numerical results are included to illustrate the essence of this formulation.