Ultrasound activated thermography is based on intense acoustic wave interaction with defects. Internal friction in defect areas results in a local temperature rise. The lockinapproach is applied by using a low-frequency amplitude modulation of acoustic excitation to enhance the sensitivity of detection of weak heat sources (ultrasound lockin-thermography). A new approach to detect regions of damage includes the frequency dependence of heat generation: defects like cracks show characteristic frequencies where mechanical losses have a maximum. In order to activate all defects, a wideband frequency-sweep is used. Intact regions of the material under inspection do not significantly change their properties with frequency, so that a defect selective methodology is obtained.