Suppose a guitar bridge couples an ideal vibrating string to a single resonance, as depicted schematically in Fig.9.5. This is often an accurate model of an acoustic bridge impedance in a narrow frequency range, especially at low frequencies where the resonances are well separated. Then, as developed in Chapter 7, the driving-point impedance seen by the string at the bridge is
That is, the driving-point impedance is the series combination of a mass , spring , and dashpot (§7.2). More general bridge impedances can be modeled as a sum of such terms. Since the bridge is passive, is positive real (§C.11.2).