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Technical Discussions
Alex
Israel, Joined Sep 2017, 4

Alex

Israel,
Joined Sep 2017
4
14:31 Jan-09-2019
Water US TT material frequency attenuation

Hi,
I used a water through transmission method to measure and calculate frequency dependent ultrasound attenuation in several materials, by applying FFT to each waveform and comparing the amplitude of the signals at each frequency (Water TT vs Sample+Water TT).
In case of highly attenuating materials I had to increase the gain in Sample+Water TT case such that the amplitude of Sample+Water TT signal at Gain Y was equal to the amplitude of Water TT signal at Gain X (X<Y). In this case the difference Y-X in dB is the ratio of amplitudes between the two signals.
The problem is, that now I don't know how to compare the signals in frequency domain. Due to increased gain the signal in Sample+Water TT case has higher amplitude than in Water TT case.
Merely decrease the amplitude of first signal by Y-X dB and compare the result to second signal seems me wrong since the amplitude of received signal in frequency domain is not proportional to increase in Gain in time domain...
Any ideas?

    
 
 
Edward Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1252

Edward Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1252
15:04 Jan-09-2019
Re: Water US TT material frequency attenuation
In Reply to Alex at 14:31 Jan-09-2019 (Opening).

Alex, in a paper I prepared for NDT.net (https://www.ndt.net/article/ndtnet/2016/17_Ginzel.pdf) I used a similar configuration and several of the materials were highly attenuative. The paper references an Excel template that Ben Turnbull prepared for me to do what you are also doing in TT. The template was uploaded to the forum at
(http://www.ndt.net/forum/thread.php?admin=&forenID=0&msgID=64178&rootID=64178#64178)
The template is open to see all the equations used. If you would like to take a look at the format you will see that there is a correction factor that you can apply to adjust the FFT for gain differences between the reference (water) and sample. One subsequent revision was made to the template since it was uploaded so if it looks useful just give me an email and I can send you the latest edition.

    
 
 

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