To construct a synthetic guitar bridge model, we can first measure empirically the admittance of a real guitar bridge, or we can work from measurements published in the literature, as shown in Fig. 9.7. Each peak in the admittance curve corresponds to a resonance in the guitar body that is well coupled to the strings via the bridge. Whether or not the corresponding vibrational mode radiates efficiently depends on the geometry of the vibrational mode, and how well it couples to the surrounding air. Thus, a complete bridge model requires not only a synthetic bridge admittance which determines the reflectance ``seen'' on the string, but also a separate filter which models the transmittance from the bridge to the outside air; the transmittance filter normally contains the same poles as the reflectance filter, but the zeros are different. Moreover, keep in mind that each string sees a slightly different reflectance and transmittance because it is located at a slightly different point relative to the guitar top plate; this changes the coupling coefficients to the various resonating modes to some extent. (We normally ignore this for simplicity and use the same bridge filters for all the strings.)
Finally, also keep in mind that each string excites the bridge in three dimensions. The two most important are the horizontal and vertical planes of vibration, corresponding to the two planes of transverse vibration on the string. The vertical plane is normal to the guitar top plate, while the horizontal plane is parallel to the top plate. Longitudinal waves also excite the bridge, and they can be important as well, especially in the piano. Since longitudinal waves are much faster in strings than transverse waves, the corresponding overtones in the sound are normally inharmonically related to the main (nearly harmonic) overtones set up by the transverse string vibrations.
The frequency, complex amplitude, and width of each peak in the measured admittance of a guitar bridge can be used to determine the parameters of a second-order digital resonator in a parallel bank of such resonators being used to model the bridge impedance. This is a variation on modal synthesis [5,301]. However, for the bridge model to be passive when attached to a string, its transfer function must be positive real, as discused previously. Since strings are very nearly lossless, passivity of the bridge model is actually quite critical in practice. If the bridge model is even slightly active at some frequency, it can either make the whole guitar model unstable, or it might objectionably lengthen the decay time of certain string overtones.
We will describe two methods of constructing passive ``bridge filters'' from measured modal parameters. The first is guaranteed to be positive real but has some drawbacks. The second method gives better results, but it has to be checked for passivity and possibly modified to give a positive real admittance. Both methods illustrate more generally applicable signal processing methods.