http://www.k-wave.org/forum/topic/output-signal-in-the-chirp-signal Support for the k-Wave MATLAB toolbox en-US Tue, 12 May 2026 23:15:42 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/output-signal-in-the-chirp-signal#post-5342 Fri, 04 Dec 2015 16:27:08 +0000 Anthony 5342@http://www.k-wave.org/forum/ <p>Hello rajiva, </p> <p>could you also post the code of the function you call 'my_chirp'? It does not run without it...</p> <p>Best regards,<br /> Anthony </p> http://www.k-wave.org/forum/topic/output-signal-in-the-chirp-signal#post-5341 Thu, 03 Dec 2015 14:16:23 +0000 rajiva 5341@http://www.k-wave.org/forum/ <p>I have looking into the effect of the size of the source for the output signal for giving input chirp signal.<br /> I am getting quite different output signal why? here is the code.<br /> clear all;clc;close all<br /> % set the size of the perfectly matched layer (PML)<br /> PML_X_SIZE = 20; % [grid points]<br /> PML_Y_SIZE = 20; % [grid points]<br /> % set total number of grid points not including the PML<br /> Nx = 128 - 2*PML_X_SIZE; % [grid points]<br /> Ny = 128 - 2*PML_Y_SIZE; % [grid points]<br /> % set desired grid size in the x-direction not including the PML<br /> x = 25e-3; % [m]</p> <p>% calculate the spacing between the grid points<br /> dx = x/Nx; % [m]<br /> dy = dx; % [m]</p> <p>% create the k-space grid<br /> kgrid = makeGrid(Nx, dx, Ny, dy);<br /> % define the properties of the propagation medium<br /> medium.sound_speed = 1500; % [m/s]<br /> medium.alpha_coeff = 0.75; % [dB/(MHz^y cm)]<br /> medium.alpha_power = 1.5;<br /> % create the time array<br /> [kgrid.t_array, dt] = makeTime(kgrid, medium.sound_speed,[], 100e-6);</p> <p>% define a single source point<br /> source.p_mask = zeros(Nx, Ny);<br /> source.p_mask(end - Nx/4, Ny/2) =1;</p> <p>% define a time varying sinusoidal source<br /> source_freq1 = 0.5e6; % [Hz]<br /> source_freq2=4e6;<br /> source_mag = 2; % [Pa]<br /> %input_signal =sin(2*pi*source_freq1*kgrid.t_array);%<br /> input_signal=my_chirp(kgrid.t_array,source_freq1,source_freq2,100e-6);<br /> source.p = source_mag*input_signal;</p> <p>%%<br /> % filter the source to remove high frequencies not supported by the grid<br /> source.p = filterTimeSeries(kgrid, medium, source.p);</p> <p>% define a single sensor point<br /> sensor.mask = zeros(Nx, Ny);<br /> sensor.mask(Nx/4, Ny/2) = 1;</p> <p>% define the acoustic parameters to record<br /> sensor.record = {'p', 'p_final'};</p> <p>% % run the simulation<br /> sensor_data = kspaceFirstOrder2D(kgrid, medium, source, sensor);<br /> %%<br /> % plot the final wave-field<br /> figure;<br /> imagesc(kgrid.y_vec*1e3, kgrid.x_vec*1e3, sensor_data.p_final + source.p_mask + sensor.mask, [-1 1]);<br /> colormap(getColorMap);<br /> ylabel('x-position [mm]');<br /> xlabel('y-position [mm]');<br /> axis image;</p> <p>% plot the simulated sensor data<br /> figure(1);<br /> [t_sc, scale, prefix] = scaleSI(max(kgrid.t_array(:)));</p> <p>subplot(2, 1, 1), plot(kgrid.t_array*scale, source.p, 'k-');grid on<br /> xlabel(['Time [' prefix 's]']);<br /> ylabel('Signal Amplitude');<br /> axis tight;<br /> title('Input Pressure Signal');</p> <p>subplot(2, 1, 2), plot(kgrid.t_array*scale, sensor_data.p, 'r-');grid on;<br /> xlabel(['Time [' prefix 's]']);<br /> ylabel('Signal Amplitude');<br /> axis tight;<br /> title('Sensor Pressure Signal');<br /> %%<br /> figure(2)<br /> [f_input, as_input] = spect([input_signal, zeros(1, 2*length(input_signal))], 1/kgrid.dt);<br /> subplot(211),plot(f_input/1e6, as_input./max(as_input(:)), 'k-');<br /> title('spectrum of chirp input signal')<br /> xlabel('frequency MHz')<br /> ylabel('normalized value')<br /> axis('tight')</p> <p>[f_input, as_input] = spect([source.p, zeros(1, 2*length(source.p))], 1/kgrid.dt);<br /> subplot(223),plot(f_input/1e6, as_input./max(as_input(:)), 'k-');<br /> title('spectrum of source signal')<br /> xlabel('frequency MHz')<br /> ylabel('normalized value')<br /> axis('tight')</p> <p>[f_output, as_output] = spect([sensor_data.p, zeros(1, 2*length(sensor_data.p))], 1/kgrid.dt);<br /> subplot(224),plot(f_output/1e6, as_output./max(as_output(:)), 'r-');<br /> title('spectrum of sensor output signal')<br /> xlabel('frequency MHz')<br /> ylabel('normalized value')<br /> axis('tight')</p> <p>display(dx)</p> <p>But for smaller size (i am taking 1 grid size) i am getting output signal increases with the frequency. but for lager size( for grid size 3 to 5) i am getting output signal amplitude decreases with the frequency (that what we expect). </p>