Many studies have been undertaken on the modeling of physical systems by means of waveguide filters. These methods consist mainly in simulating the propagation of acoustic waves with digital delay lines [1]. These models are constrained to have a spatial step determined by the sampling rate which is a serious drawback when a high spatial resolution in the geometry of the model is needed or when the length of the waveguide needs to vary. Lagrange Interpolator Filters (LIFs) are approximations for fractional delay digital filters according to a maximaly flat error criterion [2]. This paper focuses on a new implementation based on a power series expansion of the ideal transfer function, on its numerical stabilty and on its time-varying properties.