Ed Ginzel R & D, - Materials Research Institute, Canada, Joined Nov 1998, 1254
Ed Ginzel R & D, - Materials Research Institute, Canada, Joined Nov 1998 1254
19:40 Jan-03-2015 Re: Photoelastic Visualisation of Phased Array Sound Fields: Part 19 – Formation of grating lobes In Reply to Ali at 08:24 Jan-03-2015 (Opening).
An explanation of grating lobes is presented in the Jan 2015 edition of NDT.net.
Several people deserve special thanks. First I would like to thank Rolf Diederichs for publishing the material. By means of the combined formats of written word and video imaging, Rolf has managed to make the NDT.net noticeably superior to the traditional hard-copy journals available to readers.
The idea to produce an explanation of grating lobes is thanks to Ali Golshani. Ali asked me if it would be possible to see grating lobes using photoelastic visualisation. For many years, PAUT instructors have simply stated the sort of explanation provided in the official definitions, i.e. "parasitic replications of the main beam caused by spatial under sampling (pitch between elements compared to wavelength)". This has not been very helpful for students to understand their origin. At the time Ali asked me I was not sure it could be done...I hope the explanation has met his expectations.
As with so many of my projects, I again resorted to Civa to help with my explanation and personal understanding of acoustic phenomena. Using Civa, I was able to configure a delay law that would produce a pronounced grating lobe with the limited selection of phased-array probes I have. I would like to thank the good people of the CEA and EXTENDE for their ongoing help with my use of Civa.
Finally, the video of a simple 74° beam in transverse mode is in itself not a great illustration of the lobes that form. In order to illustrate how constructive interference causes the lobes I used a technique of image addition first developed by Dr. Till Schmitte. I would like to thank Till for pointing out this very useful tool of image processing.
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