An experimental modal analysis method for engineering systems; the method comprises the following steps; • Performing a sine test within a certain frequency range so that the response amplitude of any measurement point on the system remains constant and repeating this test for different constant response amplitude levels, • Measuring the harmonic excitation force spectra corresponding to different constant response amplitude levels during sine tests, • Measuring the frequency response functions corresponding to different constant response amplitude levels during sine tests, • Creating the harmonic force surface for each measurement point by combining harmonic force spectra corresponding to different constant response amplitude levels, • Determining the system's modal constant, natural frequency and modal damping ratio as a function of the response amplitude by applying modal analysis to the frequency response functions measured at each constant response amplitude level, • Determining the frequency response functions of the system at a force level by cutting the constructed harmonic force surfaces with a constant force plane corresponding to that force level, • Synthesizing frequency response functions of the system corresponding to a constant force level using parameters of modal constant, natural frequency and modal damping ratio determined as a function of the response amplitude, • Verifying the modal parameters determined by comparing the frequency response functions synthesized at a selected constant force level using the modal constant, natural frequency and modal damping ratio determined as a function of the response amplitude with the frequency response functions obtained by cutting the harmonic force surface with the constant force plane corresponding to the same force level. • Performing a sine test within a certain frequency range so that the response amplitude of any measurement point on the system remains constant and repeating this test for different constant response amplitude levels, • Measuring the harmonic excitation force spectra corresponding to different constant response amplitude levels during sine tests, • Measuring the frequency response functions corresponding to different constant response amplitude levels during sine tests, • Creating the harmonic force surface for each measurement point by combining harmonic force spectra corresponding to different constant response amplitude levels, • Determining the system's modal constant, natural frequency and modal damping ratio as a function of the response amplitude by applying modal analysis to the frequency response functions measured at each constant response amplitude level, • Determining the frequency response functions of the system at a force level by cutting the constructed harmonic force surfaces with a constant force plane corresponding to that force level, • Synthesizing frequency response functions of the system corresponding to a constant force level using parameters of modal constant, natural frequency and modal damping ratio determined as a function of the response amplitude, • Verifying the modal parameters determined by comparing the frequency response functions synthesized at a selected constant force level using the modal constant, natural frequency and modal damping ratio determined as a function of the response amplitude with the frequency response functions obtained by cutting the harmonic force surface with the constant force plane corresponding to the same force level.