The interpretation of ultrasonic signals in the inspection for flaws in bonded regions of multilayered specimens is difficult because of signal energy loss due to material attenuation, undesired reverberations within certain layers, and overlapping responses from different interfaces because of finite transducer bandwidth. The flaws are usually air gaps, lack of adhesion, and porosity within the bonding agent.

While ultrasonic NDE signal interpretation can always be improved with appropriate instrumentation — broader bandwidth transducers for increased resolution, for example — signal processing allows for further enhancement using a digital computer. Material attenuation can be compensated for by using digital filters that preferentially allow the higher frequency components in the ultrasonic signal, similar to “preemphasis” filters used in communications. Deconvolution of signal response broadens the effective bandwidth of the transducer and can be used to minimize dominant reverberations within a layer. The use of the cepstrum — a relatively new signal processing method — allows for the separation of overlapped responses which are visually difficult to separate.

The use of these signal processing methods is demonstrated for inspecting bronze-rubber structures with a small layer of epoxy in between.