Modal space - in our own little world

When using frequency domain curvefitting techniques, many software packages allow the incorporation of extra terms in the polynomial in order to account for out of band effects. This is very useful in order to obtain accurate modal parameters. However, the user can specify many additional extra terms in order to improve the fit of the data. While this may “look” better, it is questionable where or not the parameters are actually better. So let’s discuss the basic underlying equation and concept behind using residuals for modal parameter estimation. The basic frequency response equation can be written as