k-Wave User Forum » Topic: Unusual Output Signals

k-Wave User Forum » Topic: Unusual Output Signals http://www.k-wave.org/forum/topic/unusual-output-signals Support for the k-Wave MATLAB toolbox en-US Tue, 12 May 2026 23:38:31 +0000 http://bbpress.org/?v=1.0.2 <![CDATA[Search]]> q http://www.k-wave.org/forum/search.php bencox on "Unusual Output Signals" http://www.k-wave.org/forum/topic/unusual-output-signals#post-331 Fri, 24 Feb 2012 10:54:52 +0000 bencox 331@http://www.k-wave.org/forum/ Hi Eldhose, Sorry for the delay. A few questions. (1) I'm not quite sure what you're trying to do, and which bits of the signal you don't believe. (Although I agree the oscillations at the front don't look right.) (2) Have you tried smoothing the point source using <code>smooth</code>? (3) Why are the point source and bowl detector both to one side of the domain? It looks as though it's twice as wide as it needs to be. Other things you could try to aid diagnosis: (a) double the thickness of the PML, (b) halve the frequency. Do they make any difference? Ben eldhochhan on "Unusual Output Signals" http://www.k-wave.org/forum/topic/unusual-output-signals#post-322 Wed, 22 Feb 2012 05:19:01 +0000 eldhochhan 322@http://www.k-wave.org/forum/ Hi Ben, Any suggestions for the above problem?? What actually i am trying to explore is that when a source at let say 19mm away from the detector propagates a signal in a particular frequency. I need to check the frequency of the signals being captured by the detector placed 19mm away like whether it is the same frequency of the signals that was being propagated? Or how much is the loss?? I have pasted the MATLAB code below and it would be really very helpful if u can help me. clear all; close all; clc; % ========================================================================= % SIMULATION % ========================================================================= % create the computational grid Nx = 320; % number of grid points in the x direction Ny = 320; % number of grid points in the y direction Nz = 320; % number of grid points in the z direction dx = 6.52e-5; % grid point spacing in the x direction [m] dy = dx; % grid point spacing in the y direction [m] dz = dx; % grid point spacing in the z direction [m] kgrid = makeGrid(Nx, dx, Ny, dy, Nz, dz); % define the properties of the propagation medium medium.sound_speed = 1500; % [m/s] %medium.sound_speed(1:Nx/2, :) = 1800; % [m/s] %medium.density = 1000*ones(Nx, Ny, Nz); % [kg/m^3] %medium.density(:, Ny/4:end) = 1200; % [kg/m^3] % create the time array [kgrid.t_array, dt] = makeTime(kgrid, medium.sound_speed); Nt = length(kgrid.t_array); % create a concave sensor sphere_sz = 194; sphere_radius = 96; sphere = makeSphere(sphere_sz, sphere_sz, sphere_sz, sphere_radius); % carve most of the sphere off then add it to a mask of the correct size sphere(:,98:end,:) = 0; sensor.mask = zeros(Nx, Ny, Nz); sphere_offset = 11; sensor.mask(Nx/20:Nx/20+sphere_sz-1, Ny/20:Ny/20+sphere_sz-1, sphere_offset:sphere_offset+sphere_sz-1) = sphere; % define a time varying sinusoidal source source_freq = 10e6; source_mag = 10; source.p = source_mag*sin(2*pi*source_freq*kgrid.t_array); source.p = filterTimeSeries(kgrid, medium, source.p); % place the first point source near the focus of the detector source1 = zeros(Nx, Ny, Nz); source1(112, 306, 108) = 1; % place the second point source off axis %source2 = zeros(Nx, Ny, Nz); %source2(Nx/2-10, Ny/2, sphere_offset+sphere_radius) = 1; % run the first simulation source.p_mask = source1; input_args = {'PMLSize', 10, 'DataCast', 'single', 'PlotSim', false}; sensor_data = kspaceFirstOrder3D(kgrid, medium, source, sensor, input_args{:}); % average the data recorded at each grid point to simulate the measured % signal from a single element focussed detector sensor_data = sum(sensor_data, 1); % run the second simulation %source.p_mask = source2; %sensor_data2 = kspaceFirstOrder3D(kgrid, medium, source, sensor, input_args{:}); % average the data recorded at each grid point to simulate the measured % signal from a single element focussed detector %sensor_data2 = sum(sensor_data2, 1); % run the simulation %sensor_data = kspaceFirstOrder3D(kgrid, medium, source, sensor); % define the frequency response of the sensor elements center_freq = 10e6; % [Hz] bandwidth = 10; % [%] sensor.frequency_response = [center_freq, bandwidth]; % re-run the simulation sensor_data_filtered = kspaceFirstOrder3D(kgrid, medium, source, sensor, input_args{:}); % calculate the frequency spectrum at the first sensor element [f, sensor_data_as] = spectrum(sensor_data(1, :), 1/dt); [f, sensor_data_filtered_as] = spectrum(sensor_data_filtered(1, :), 1/dt); % ========================================================================= % VISUALISATION % ========================================================================= % plot the detector and on-axis and off-axis point sources voxelPlot(sensor.mask + source1); view([14, 20]); % plot the time series corresponding to the on-axis and off-axis sources figure [t_sc, t_scale, t_prefix] = scaleSI(kgrid.t_array(end)); plot(kgrid.t_array.*t_scale, sensor_data, '-'); hold on %plot(kgrid.t_array.*t_scale, sensor_data2, 'r-'); xlabel(['Time [' t_prefix 's]']); ylabel('Average Pressure Measured By Focussed Detector [au]'); legend('Source on axis', 'Source off axis'); figure; [t_sc, scale, prefix] = scaleSI(max(kgrid.t_array(:))); plot(scale*kgrid.t_array, sensor_data(1, :), 'k-', scale*kgrid.t_array, sensor_data_filtered(1, :), 'r-'); ylabel('Pressure [au]'); xlabel(['Time [' prefix 's]']); legend('Original Time Series', 'Filtered Time Series'); % plot the amplitude spectrum figure; [f_sc, scale, prefix] = scaleSI(max(f(:))); plot(scale*f, sensor_data_as, 'k-', scale*f, sensor_data_filtered_as, 'r-'); ylabel('Amplitude Spectrum'); xlabel(['Frequency [' prefix 'Hz]']); legend('Original Time Series', 'Filtered Time Series'); set(gca, 'XLim', [0 30]); Thanks, Eldhose eldhochhan on "Unusual Output Signals" http://www.k-wave.org/forum/topic/unusual-output-signals#post-302 Thu, 09 Feb 2012 18:41:05 +0000 eldhochhan 302@http://www.k-wave.org/forum/ Hi Ben, Thanks for the reply. Actually what i have done is: i) changed the dx value ii) changed the number of grid points iii) also the positions of the source and sensor mask to simulate a real case. I have a specific distance (mm) from the source where the sensor has to be kept. So i just divided the distance by 'dx' to get the distance in pixels to place the detector. Yes, actually i using a frequency close to the maximum supported frequency. Let say my required frequency is 10MHz and so the maximum supported frequency is 11.5MHz. I didn't had any other choice because when i was trying to use a higher maximum supported frequency, it was taking very long hours for computation and at the end showed "OUT OF MEMORY". And for the small oscillations in the beginning, even when i zoomed in and out it was still present. I will try to use the 'RampPPW' option as u have said. bencox on "Unusual Output Signals" http://www.k-wave.org/forum/topic/unusual-output-signals#post-300 Thu, 09 Feb 2012 15:29:20 +0000 bencox 300@http://www.k-wave.org/forum/ Hi Eldhose, A few things occur to me. How did you change the example? Could it be that for certain sensor-detector arrangements this is what you would expect to see? Are you using a frequency close to that maximum supported by the grid (as output to the desktop)? When using a sinusoidal signal, it has to start somewhere and that will generate a transient containing higher frequencies. Could it be this causing the problem? Try ramping up the amplitude slowly over a number of time points using the <code>'RampPPW'</code> option in the function <a href="http://www.k-wave.org/documentation/filterTimeSeries.php">filterTimeSeries</a> and see what happens. Are the small oscillations at the very beginning just a plotting issue: if you zoom in are the oscillations still there or do they go away? Ben eldhochhan on "Unusual Output Signals" http://www.k-wave.org/forum/topic/unusual-output-signals#post-298 Thu, 09 Feb 2012 08:47:31 +0000 eldhochhan 298@http://www.k-wave.org/forum/ Hi Brad and Ben, I have tried to use the example "Focussed Detector in 3D" for simulating the captured signals by a single element transducer. I found that there are few unusual peaks and trough in the amplitudes of the signals before the "source on axis" signals are shown. I have pasted the output signals below and those highlighted in red circle seems to be very unusual. Could you please help me to explain the cause?? pls follow the link for the picture: <a href="http://tinypic.com/r/rhpum1/5" rel="nofollow">http://tinypic.com/r/rhpum1/5</a> Thanks, Eldhose