http://www.k-wave.org/forum/topic/attenuation-in-heroginious-medium Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 00:11:03 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/attenuation-in-heroginious-medium#post-5662 Fri, 09 Sep 2016 20:36:53 +0000 Bradley Treeby 5662@http://www.k-wave.org/forum/ <p>Hi benji,</p> <p>If you're using <code>kspaceFirstOrder3D</code>, the simulation will not be unconditionally stable when you add absorption. You can use the function <code>checkStability</code> to check if the time step is small enough to ensure stability.</p> <p>There is a also small phase error that is introduced when using <code>kspaceFirstOrder3D</code> due to the way the absorption term is implemented (see the k-Wave manual and the <a href="http://www.k-wave.org/documentation/example_na_modelling_absorption.php">Modelling Power Law Absorption Example</a> for more details). This can be counteracted by reducing the size of the time step. </p> <p>As an alternative, you can use <code>kspaceSecondOrder</code>. This is restricted to homogeneous media, but is unconditionally stable and encodes the absorption operator exactly.</p> <p>Three things to check:</p> <p>(1) Do you get the same results if your halve the time step (i.e., have you performed a convergence check)?</p> <p>(2) Do you seem the same behaviour if you use <code>kspaceSecondOrder</code>?</p> <p>(3) If you open the Modelling Power Law Absorption Example, set <code>example_number = 3</code> and then run the simulations with and without <code>medium.alpha_mode = &#39;no_dispersion&#39;</code>, do you see any difference in the output?</p> <p>Brad. </p> http://www.k-wave.org/forum/topic/attenuation-in-heroginious-medium#post-5644 Fri, 19 Aug 2016 18:15:55 +0000 benji 5644@http://www.k-wave.org/forum/ <p>hi again,<br /> after experimenting a little more, here are some more details:<br /> 1. the simulation is stable for alpha_power&gt;1.05 or alpha_power&lt;0.9 (also for alpha_coeff greater than 3dB)<br /> 2. the recorded pressure signal measured is delayed for alpha_power&lt;1, and exibits a negative delay for alpha_power greater than 1 (delay is relative to the signal measured without attenuation, but the same medium otherwise).as alpha_power is bigger (than 1) - so does the delay (and opposite when alpha_power&lt;1).<br /> 3. the signal is still expanded in time domain (why?)<br /> 4. the signal becomes not casual as alpha_power becomes smaller (or bigger) than 1.<br /> 5. defining: medium.alpha_mode = 'no_dispersion', makes no differance to the simulation! i get exactly the same results with and without it. also with that line, i observe a loss obeying the relation: a*x[cm]*f^b</p> <p>thank u<br /> benji </p> http://www.k-wave.org/forum/topic/attenuation-in-heroginious-medium#post-5643 Thu, 18 Aug 2016 16:35:00 +0000 benji 5643@http://www.k-wave.org/forum/ <p>hi,<br /> I am trying to add attenuation to a hetroginious soft tissue model.<br /> as a first step i have allocated a homoginious attenuation coefficient to the hole model:<br /> medium.alpha_coeff=a*ones(Nx,Ny,Nz);<br /> medium.alpha_power=1.02;<br /> medium.alpha_mode = 'no_dispersion';</p> <p>i am observing the next (unexpected) behaviour:</p> <p>1. for attenuation levels greater than 3db/(cm*MHz) - the simulation is unstable and pressure goes to infinity.</p> <p>2. when attenuation levels are less or equal to 3db/(cm*MHz) :<br /> a. the measured signal starts earlier (as if the wave propagates faster, but the speed in<br /> defined for the medium is exactly the same). as bigger the attenuation is, the faster it<br /> "arrives" the<br /> sensor.<br /> b. it seems as if the signal is being expanded in time axis (meaning p(a*t)) - and<br /> analogiously squeezed in frequency domain.<br /> c. the signal becomes not causal, and is not exactly zero in times before the signal arrives<br /> the sensor.</p> <p>I would really appriciate your help understanding the above phenomena. i expected having some dispersion, with every frequency attenuatted according to a*x[cm]*f[MHz], but that behviour is different.</p> <p>could i count on phase accuracy of the simulation to measure phase differance between different points?</p> <p>thank u very much for your kind help</p> <p>benji </p>