A computer implemented method, apparatus, and computer usable program code for testing a material. A signal is sent into the material using the transmitter. The signal has a frequency range that falls within a selected frequency range to form a transmitted signal. An actual response to the transmitted signal is received at a sensor. A simulated response of the material to the transmitted signal is generated using a functional model capable of modeling responses of the material to different frequency ranges falling within the selected frequency range. The simulated response is compared to the actual response to determine if a change has occurred in the material.