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 [Smith, 1994]. Such models are constrained by a fixed spatial step determined by the sampling rate. This usually prevents the length of the waveguide to vary progressively in time and the spatial resolution is very poor for standard sampling rate. The use of digital filters approximating fractional delays is one way of overcoming these limitations. Lagrange Interpolator Filters (LIFs) are approximations for fractional delay line filters according to a maximally flat error criterion [Laakso et al., 1996]. As waveguide filters imply feedback loops which may cause numerical instabilities, fractional delay lines must be passive filters. This paper focuses on a new passive implementation of LIFs based on a power series expansion of the ideal transfer function and on its time-varying properties.