00:44 May-12-1997 Rolf Diederichs Director, Editor, Publisher, Internet, PHP MySQL NDT.net, Germany, Joined Nov 1998 ^{602}

Model generated by an inverse Fast Fourier Transformation

Concerning the article "Approximative Modeling for the Practical Ultrasonic Inspections" by H.Wüstenberg, A.Erhard. The A-Scans are in the model generated by an inverse Fast Fourier Transformation. What are the advantages and drawbacks of this approach?

Re: Model generated by an inverse Fast Fourier Transformation : Concerning the article "Approximative Modeling for the Practical Ultrasonic Inspections" by H.Wüstenberg, A.Erhard. : The A-Scans are in the model generated by an inverse Fast Fourier Transformation. : What are the advantages and drawbacks of this approach?

: Rolf Diederichs

Pleas find here an first attempt of answering the following question:

1. The model can be linked to well known pulse spectra of UT-probes, which commonly are available as data measured and also - depending from the manufacturer - calculated by a KLM-quadrupol model or similar equivalent circuits for the probe. The influence of the bandpass filter characteristic of the receiving amplifier within the UT-equipment can easely be studied.

2. The phase behaviour of reflection or mode conversion factors and of the point source directivities for the different wave modes and its influence can be considered without any difficulty

3. This enables us to cut the totality of the process into different parts and to multiply or add them in order to get the transfer function of the whole system.

4. The necessary number of frequency samples to be calculated can be adapted to the extension of the time domain to be considered for the A-Scan. E.g. many of the examples in the above mentioned contribution are using 64 samples in the frequeny domain and 128 in the time domain, which corresponds e.g. to 16 mm sound path length. That means that the use of the inverse FFT enables us to reach very short computational times.

The drawbacks have to listed by someone else favorising a time domain approach.