A new technique has been investigated for improving the signals that can be obtained in air-coupled NDE. This relies on the broad bandwidth available from polymer-filmed capacitive transducers. The technique relies on a swept-frequency "chirp" signal, which is transmitted from a transducer in air, passes through the sample under test, and is then detected at the far side using a separate receiver in through-transmission. The new technique differs from existing time-domain correlation techniques, such as pulse compression, in that a single multiplication process is performed in the time domain to give a difference frequency signal. This can then be easily isolated in the frequency domain. It will be demonstrated that this new Swept Frequency Multiplication (SFM) approach gives excellent resolution between overlapping signals, and indeed has several potential advantages over pulse compression. Examples will be shown of air-coupled images, obtained using SFM processing, which demonstrate the wide application of the technique to air-coupled NDE.
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