We now present a theorem characterizing the condition of losslessness in terms of the eigenvalues and eigenvectors of the scattering matrix.

(60) |

Conversely, assume for each eigenvalue of , and that there exists a matrix of linearly independent eigenvectors of . Then the matrix diagonalizes to give , where . Multiplying, we obtain . Thus, the condition of lossless scattering (26) is satisfied for which is Hermitian and positive definite.

Theorem 3 can be extended to lossless junctions of lossy waveguides, as the reader can easily verify by applying the above proof to (27).

It is worth noting that most research in *feedback delay networks* for artificial
reverberation has been concerned only with orthogonal feedback matrices
[42,33] rather than on the more general class of matrices
satisfying the losslessness condition (26)
[78,106,79]. This is an excessive
restriction since many of the matrices naturally arising from models
of lossless physical junctions are not orthogonal. As can be seen
above, physical scattering matrices are orthogonal only if
we restrict attention to propagation in equal-admittance media
[
], or to propagation of normalized waves.

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