Method for determining modal parameters of a system under test by using only output signals, comprising the steps of - taking a first set of values of at least three output signals obtained under a first loading condition of said system, characterized by comprising the steps of: - determining for said first loading condition a parametric matrix model for said system, said parametric matrix model representing a space orthogonal to the space spanned by said output signals, said space being at least two dimensions smaller than the number of output signals, so that the influence of input spectra is reduced, - determining for said first loading condition, parameters of said parametric matrix model, - expressing a polynomial eigenvalue problem based on said parametric matrix model and the corresponding parameters for said first loading condition, - deriving additional information about said system under test by considering a second set of values of said output signals obtained under a second loading condition of said system and determining for said second loading condition a parametric matrix model and its parameters, or by considering prior knowledge about characteristics of said system under test, - adapting said polynomial eigenvalue problem using said additional information, until a complete set of equations is obtained, - obtaining said modal parameters by solving said adapted polynomial eigenvalue problem. - taking a first set of values of at least three output signals obtained under a first loading condition of said system, characterized by comprising the steps of: - determining for said first loading condition a parametric matrix model for said system, said parametric matrix model representing a space orthogonal to the space spanned by said output signals, said space being at least two dimensions smaller than the number of output signals, so that the influence of input spectra is reduced, - determining for said first loading condition, parameters of said parametric matrix model, - expressing a polynomial eigenvalue problem based on said parametric matrix model and the corresponding parameters for said first loading condition, - deriving additional information about said system under test by considering a second set of values of said output signals obtained under a second loading condition of said system and determining for said second loading condition a parametric matrix model and its parameters, or by considering prior knowledge about characteristics of said system under test, - adapting said polynomial eigenvalue problem using said additional information, until a complete set of equations is obtained, - obtaining said modal parameters by solving said adapted polynomial eigenvalue problem. - determining for said first loading condition a parametric matrix model for said system, said parametric matrix model representing a space orthogonal to the space spanned by said output signals, said space being at least two dimensions smaller than the number of output signals, so that the influence of input spectra is reduced, - determining for said first loading condition, parameters of said parametric matrix model, - expressing a polynomial eigenvalue problem based on said parametric matrix model and the corresponding parameters for said first loading condition, - deriving additional information about said system under test by considering a second set of values of said output signals obtained under a second loading condition of said system and determining for said second loading condition a parametric matrix model and its parameters, or by considering prior knowledge about characteristics of said system under test, - adapting said polynomial eigenvalue problem using said additional information, until a complete set of equations is obtained, - obtaining said modal parameters by solving said adapted polynomial eigenvalue problem.