It is known that a simple spring-mass system may be reconstructed uniquely (apart from a single scaling factor) from the poles and zeros of the frequency response function corresponding to sinusoidal forcing at an end. The (squares of the) poles yield the eigenvalues of a tridiagonal matrix, A, while the zeros yield the eigenvalues of the matrix, A*, with the last row and column deleted. There are proven numerical methods for reconstructing A. The authors show that, if forcing is applied at an interior point, then the reconstruction is unique if that point is not a node of any eigenmode; if it is, there is a family of systems with the required properties. In either case the system may be constructed using modifications of proven techniques.