Figure 5. Nonlinear Wave Modulation Spectroscopy (NWMS). Frequency spectra from wave modulation tests of undamaged (a) and damaged (b) engine bearing caps. The inset (top left) illustrates a full spectrum. The boxed within the inset illustrates the sideband portion of the spectrum shown in (a) and (b). In the experiment, multiple frequencies were input into the sample simultaneously in continuous-wave mode. This is the reason there are many many sidebands.