http://www.k-wave.org/forum/topic/absorption Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 01:42:51 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/absorption#post-5860 Tue, 21 Mar 2017 09:51:15 +0000 Bradley Treeby 5860@http://www.k-wave.org/forum/ <p>Yes, that's correct. </p> http://www.k-wave.org/forum/topic/absorption#post-5858 Mon, 20 Mar 2017 22:57:58 +0000 sait 5858@http://www.k-wave.org/forum/ <p>There is 2 definitions:<br /> -----------------<br /> medium.alpha_power - power law absorption exponent<br /> medium.alpha_coeff - power law absorption coefficient [dB/(MHz^y cm)]<br /> -----------------<br /> and medium.alpha_power is 'y' value in '[dB/(MHz^y cm)] '. </p> <p>Isn't it ?</p> <p>sait </p> http://www.k-wave.org/forum/topic/absorption#post-5857 Mon, 20 Mar 2017 22:27:58 +0000 Bradley Treeby 5857@http://www.k-wave.org/forum/ <p>Hi sait,</p> <p>Try typing the following at the command line:</p> <pre><code>db2neper(1, 2) db2neper(1, 1.5)</code></pre> <p>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.</p> <p>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 <code>medium.alpha_power</code>. See the examples in the toolbox to see how this is used.</p> <p>Brad. </p> http://www.k-wave.org/forum/topic/absorption#post-5856 Mon, 20 Mar 2017 22:00:54 +0000 sait 5856@http://www.k-wave.org/forum/ <p>Thank you for your reply. </p> <p>Actually I change the value 2 to 1.5. But why it is happining I don't understand.</p> <p>I searched in pstdElastic2D and there is neper2db, we can easly change y parameter. But I can't find in the kspaceFirstOrder2D. </p> <p>Which part should I change in kspaceFirstOrder2D ? </p> <p>*I want to make the power law exponent (1.5). </p> http://www.k-wave.org/forum/topic/absorption#post-5855 Mon, 20 Mar 2017 21:12:36 +0000 Bradley Treeby 5855@http://www.k-wave.org/forum/ <p>Hi sait,</p> <p>It is possible to change the power law exponent in the fluid code (<code>kspaceFirstOrderND</code>) but not in the elastic code (<code>pstdElasticND</code>). Manually changing the y value within <code>pstdElastic2D</code> 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 <code>pstdElastic2D</code>.</p> <p>Hope that helps,</p> <p>Brad. </p> http://www.k-wave.org/forum/topic/absorption#post-5852 Sat, 18 Mar 2017 14:04:31 +0000 sait 5852@http://www.k-wave.org/forum/ <p>Hi,</p> <p>Can I change the absorption coefficient dB/(MHz^2 cm) to dB/(MHz^1.5 cm) ?</p> <p>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 . </p> <p>How can I solve this problem ?</p> <p>Thank you for your attention. </p>