No vibrating string in musical acoustics is truly rigidly terminated, because such a string would produce no sound through the body of the instrument.^{7.15}Yielding terminations result in coupling of the horizontal and vertical planes of vibration. In typical acoustic stringed instruments, nearly all of this coupling takes place at the bridge of the instrument.

Figure 6.20 illustrates the more realistic case of two planes of vibration which are linearly coupled at one end of the string (the ``bridge''). Denoting the traveling force waves entering the bridge from the vertical and horizontal vibration components by and , respectively, the outgoing waves in each plane are given by
In physically symmetric situations, we expect . That is, the transfer function from horizontal to vertical waves is normally the same as the transfer function from vertical to horizontal waves.
If we consider a single frequency , then the coupling matrix with is a constant (generally complex) matrix (where denotes the sampling interval as usual). An eigenanalysis of this matrix gives information about the modes of the coupled system and the damping and tuning of these modes [547].
As a simple example, suppose the coupling matrix at some frequency has the form
where and are any complex numbers. This means both string terminations are identical, and the coupling is symmetric (the simplest case in practice). The eigenvectors are easily calculated to be^{7.16}
and the eigenvalues are and , respectively.
The eigenvector corresponds to ``in phase'' vibration of the two string endpoints, i.e., , while corresponds to ``opposite phase'' vibration, for which . If it happens to be the case that
then the inphase vibration component will decay faster than the oppositephase vibration. This situation applies to coupled piano strings [547], as discussed further below.
More generally, the two eigenvectors of the coupling frequencyresponse matrix
correspond to two decoupled polarization planes. That is, at each frequency there are two eigenpolarizations in which incident vibration reflects in the same plane. In general, the eigenplanes change with frequency. A related analysis is given in [547].
By definition of the eigenvectors of , we have
where denotes the th eigenvalue of the couplingmatrix at frequency , where . Since the eigenvector holds the Fourier transform of the incoming waves for mode of the coupledstring system, we see that the eigenvalues have the following interpretation:
The th eigenvalue of the coupling matrix equals the frequency response seen by the th eigenpolarization.In particular, the modulus of the eigenvalue gives the reflectance magnitude (affecting mode damping), and the angle of the eigenvalue is the phase shift of the reflection, for that mode (affecting tuning of the mode). Use of coupling matrix eigenanalysis to determine mode damping and tuning is explored further in §C.13.