Modal noise becomes a serious problem in multimode optical fiber systems. It is said that there are two main causes for modal noise, one being the optical source and the other fiber vibration. In this paper, the latter cause is discussed.Firstly the speckle fluctuation due to fiber vibration is expressed by a N-th order orthogonal matrix, and the effect of mode dependent loss at a connection point is expressed by coupling coefficients which are approximated by a geometrical progression, and the calculated results using the geometrical progression are in good agreement with Petermann's results. Using above outlined procedure, a carrier-to-noise ratio (CNR) for modal noise based on our model is formulated and the results calculated by our CNR formula are compared with Hill's results.Next, to investigate validities of this new model and of the modal noise estimation procedure, some experiments are carried out and calculated results are in good agreement with experimental ones. Moreover, the calculated modal noise spectra due to this model are in good agreement with the experimental ones.