k-Wave User Forum » Topic: Different propagation time delays with medium_alpha

k-Wave User Forum » Topic: Different propagation time delays with medium_alpha_coeff variations http://www.k-wave.org/forum/topic/different-propagation-time-delays-with-medium_alpha_coeff-variations Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 02:31:10 +0000 http://bbpress.org/?v=1.0.2 <![CDATA[Search]]> q http://www.k-wave.org/forum/search.php Bradley Treeby on "Different propagation time delays with medium_alpha_coeff variations" http://www.k-wave.org/forum/topic/different-propagation-time-delays-with-medium_alpha_coeff-variations#post-6394 Sun, 08 Apr 2018 18:31:26 +0000 Bradley Treeby 6394@http://www.k-wave.org/forum/ Hi marioj, This is due to sound speed dispersion (the physical kind, not the numerical kind). This accompanies power law absorption to obey causality. Changing the amount of absorption changes the amount of dispersion (i.e., the speed at which the wave travels). In an absorbing medium, the speed will also depend on frequency (e.g., see <a href="http://www.k-wave.org/documentation/example_na_modelling_absorption.php">this example</a>). This dispersion is also visible in experiments (e.g., see <a href="http://bug.medphys.ucl.ac.uk/papers/2011-Treeby-UMB.pdf">this paper</a>). In k-Wave, it's possible to turn this off (at least to first-order) by setting: <code>medium.alpha_mode = 'no_dispersion'</code> Hope that helps, Brad. marioj on "Different propagation time delays with medium_alpha_coeff variations" http://www.k-wave.org/forum/topic/different-propagation-time-delays-with-medium_alpha_coeff-variations#post-6381 Wed, 04 Apr 2018 17:12:39 +0000 marioj 6381@http://www.k-wave.org/forum/ Hello, Using 3D simulation with a circular emitter and a square sensor I am computing the pressure at the same point for media with different level of attenuation "medium.alpha_coeff". The parameter "medium.alpha_power is the same for all the situations and is equal to 1.01. As an exemple here are 3 signals obtained for different values of "medium.alpha_coeff" (0, 1 and 2 dB/cm), where is possible to see the delay between them: <a href="https://meocloud.pt/link/4cf3c14c-52db-41f5-9df6-06217bc37e27/3%20signals.tif/" rel="nofollow">https://meocloud.pt/link/4cf3c14c-52db-41f5-9df6-06217bc37e27/3%20signals.tif/</a> Why this variation of time of flight for different values of "medium.alpha_coeff", while whole remaining parameters remain the same? I appreciate some help in this matter. Best regards. Here is the simulation code: %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % grid dimensions XX=3.2e-3; YY=3.2e-3; ZZ=30e-3; % grid point spacing scat=200e-6; dx=scat; dy=scat; dz=scat; % create the computational grid Nx=round(XX/dx); Ny=round(YY/dy); Nz=round(ZZ/dz); kgrid = makeGrid(Nx, dx, Ny, dy, Nz, dz); kgrid.t_array = 0:40e-9:20e-6; % circular transducer R=round(1.5e-3/scat); disc = makeDisc(Nx, Ny, Nx/2, Ny/2, R); source.p_mask = zeros(Nx, Ny, Nz); aa=zeros(Nx, Ny, Nz-1);% source.p_mask=cat(3,disc,aa); % % define properties of the input signal source_strength = 10e6; % [Pa] tone_burst_freq = 1e6; % [Hz] tone_burst_cycles = 2; % create the input signal using toneBurst source.p = toneBurst(25e6, tone_burst_freq, tone_burst_cycles); % define the properties of the propagation medium medium.sound_speed = 1500*ones(Nx, Ny, Nz); % [m/s] medium.density = 1000*ones(Nx, Ny, Nz); % [kg/m^3] % receiver sensor.mask = zeros(Nx, Ny, Nz); sensor.mask(1:16,1:16,100:100)=1 % % attenuation alpha= a.f^b medium.alpha_coeff =2; medium.alpha_power = 1.01; % input arguments input_args = {'DisplayMask', source.p_mask, 'DataCast', 'single','PMLInside', false,'PMLAlpha',2,'PMLSize', 20} % run the simulation sensor_data = kspaceFirstOrder3D(kgrid, medium, source, sensor, input_args{:}); sensor.record ={'p'} pp=sum(sensor_data);