http://www.k-wave.org/forum/topic/relationship-between-attenuation-and-wave-velocity Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 09:03:24 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/relationship-between-attenuation-and-wave-velocity#post-7570 Sat, 06 Jun 2020 18:31:12 +0000 Bradley Treeby 7570@http://www.k-wave.org/forum/ <p>Hi emanuelepeschiera,</p> <p>For power law values very close to 1, there is a strong dependence of the sound speed on frequency.</p> <p>Try opening the <a href="http://www.k-wave.org/documentation/example_na_modelling_absorption.php">http://www.k-wave.org/documentation/example_na_modelling_absorption.php</a> and replacing the absorption properties (around line 91) with your values:</p> <pre><code>% define the absorption properties of the propagation medium switch loop case 1 medium.alpha_coeff = 0.57; medium.alpha_power = 1.01; case 2 medium.alpha_coeff = 0.57; medium.alpha_power = 0.99; case 3 medium.alpha_coeff = 0.57; medium.alpha_power = 1.01; medium.alpha_mode = &#39;no_dispersion&#39;; end</code></pre> <p>This more or less confirms exactly what you're seeing. For the case of <code>no_dispersion</code>, the sound speed comes out at the background value (i.e., unchanged). If you move the power law exponent a bit further away from 1, the effect is much less pronounced.</p> <p>The no dispersion flag just turns off one of the terms in the absorption operator. If you read the <a href="http://www.k-wave.org/papers/2010-Treeby-JASA.pdf">original paper</a>, to first order, one of the terms accounts for absorption and the other to dispersion.</p> <p>Brad </p> http://www.k-wave.org/forum/topic/relationship-between-attenuation-and-wave-velocity#post-7536 Mon, 01 Jun 2020 11:47:23 +0000 emanuelepeschiera 7536@http://www.k-wave.org/forum/ <p>Hi,<br /> for a thesis project we are simulating ultrasound waves propagation in bubbly media, focusing on multiple scattering phenomena.<br /> The background medium is set to muscle.</p> <p>We're currently trying to introduce attenuation in muscle using<br /> alpha_coeff = 0.57,<br /> alpha_power = 0.99 (or alpha_power = 1.01).</p> <p>What we notice is an increase/decrease of the wave velocity (depending on alpha_power being &gt;/&lt; 1) respect to the case without attenuation.<br /> We know that attenuation is made by absorption + dispersion, and dispersion could change the velocity of the different harmonic components (Kramers-Kronig relationships). This would result in a 'distorsion' of the transmitted pulse, and this effect would be more evident as frequency increases.<br /> Nevertheless, in our code the velocity of the harmonics seems to be constant, resulting in a progressive shift, as space increases, independent from the frequency (we use central frequencies ranging from 1 to 5 MHz).</p> <p>Our question is: is the velocity variation due to dispersion? Or is it related to other reasons such as numerical ones?</p> <p>Another thing we tried is to set alpha_mode to 'no_dispersion'. This didn't change the results, making it seem like the delay is not due to dispersion.<br /> The doubt here is that we don't know if 'no_dispersion' works: for instance, the documentation suggests to set alpha_mode = 'no_dispersion' if you want to model an alpha_power exactly 1. Trying this, the code returns the same error of the case alpha_power = 1 and dispersion present.</p> <p>Thank you for the answers! </p>