Hi,
Can I change the absorption coefficient dB/(MHz^2 cm) to dB/(MHz^1.5 cm) ?
Actually I change db2neper code's y variable 1.5 in pstdElastic2D code, but when I run the simulation after 2-3 minutes screen getting darker and darker .
How can I solve this problem ?
Thank you for your attention.
Hi sait,
It is possible to change the power law exponent in the fluid code (kspaceFirstOrderND) but not in the elastic code (pstdElasticND). Manually changing the y value within pstdElastic2D will make the absorption coefficient much larger, but it won't actually change the frequency dependence of the absorption. The image getting darker is classic sign of instability, likely caused by the very high absorption coefficient. In general, you can avoid instability by reducing the size of the time step. However, in your case, it's likely that your absorption coefficient is too large as a result of the change to pstdElastic2D.
Hope that helps,
Brad.
Thank you for your reply.
Actually I change the value 2 to 1.5. But why it is happining I don't understand.
I searched in pstdElastic2D and there is neper2db, we can easly change y parameter. But I can't find in the kspaceFirstOrder2D.
Which part should I change in kspaceFirstOrder2D ?
*I want to make the power law exponent (1.5).
Hi sait,
Try typing the following at the command line:
db2neper(1, 2)
db2neper(1, 1.5)You will see that the second gives you a much higher value for the absorption coefficient, which in your case, leads to instability. Note, this doesn't change the frequency dependence of the absorption in the code, just the value of the absorption coefficient.
You shouldn't need to change any of the simulation functions, just the scripts that call them. To change the power law exponent in the fluid codes, set the value of medium.alpha_power. See the examples in the toolbox to see how this is used.
Brad.
There is 2 definitions:
-----------------
medium.alpha_power - power law absorption exponent
medium.alpha_coeff - power law absorption coefficient [dB/(MHz^y cm)]
-----------------
and medium.alpha_power is 'y' value in '[dB/(MHz^y cm)] '.
Isn't it ?
sait
Yes, that's correct.
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