http://www.k-wave.org/forum/topic/filtertimeseries-output-varied-if-the-timesteps-change Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 00:20:09 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/filtertimeseries-output-varied-if-the-timesteps-change#post-7086 Fri, 18 Oct 2019 19:51:15 +0000 Bradley Treeby 7086@http://www.k-wave.org/forum/ <p>Hi Dirk,</p> <p>I've created a minimum working example:</p> <pre><code>clearvars; close all; % spatial grid properties dx = 0.5e-3; Nx = 500; c0 = 1500; % temporal grid sc = 1; dt = 0.3 * dx / c0 / sc; Nt = 200 * sc; % create grid kgrid = kWaveGrid(Nx, dx); kgrid.setTime(Nt, dt); medium.sound_speed = c0; % create source f_max = medium.sound_speed / (2 * kgrid.dx); source_freq = f_max / 2; source_p = sin(2 * pi * source_freq * kgrid.t_array); % apply filter source_filt = applyFilter(source_p, 1/kgrid.dt, f_max, &#39;LowPass&#39;, &#39;Plot&#39;, true); % plot figure; plot(source_p); hold on; plot(source_filt);</code></pre> <p>If you set <code>sc = 1</code>, you get the expected behaviour. If you set <code>sc = 30</code>, the signal amplitude is reduced similar to what you report. (Note, <code>filterTimeSeries</code> uses <code>applyFilter</code>).</p> <p>The problem is that the filter transition width is set as a proportion of the sampling frequency. If you also reduce this relative to the increase in sampling frequency, everything is as it should be:</p> <p><code>source_filt = applyFilter(source_p, 1/kgrid.dt, f_max, &#39;LowPass&#39;, &#39;Plot&#39;, true, &#39;TransitionWidth&#39;, 0.1 / sc);</code></p> <p>There is a more general point about whether this is intuitive behaviour of course!</p> <p>Brad. </p> http://www.k-wave.org/forum/topic/filtertimeseries-output-varied-if-the-timesteps-change#post-7048 Fri, 13 Sep 2019 13:32:00 +0000 dbuender 7048@http://www.k-wave.org/forum/ <p>Hi, </p> <p>I am not sure if i use somthing wrong, but in the example "Diffraction Through A Slit Example", when the timestep dt drops I encounter a drop in the amplitude of the source. This occurs after the line 129 (<code>source.p = filterTimeSeries(kgrid, medium, source.p);</code>)</p> <p>As far as i understood filterTimeSeries schould only effekt Signals witch are consist of frequencies higher or near the maximum supported frequency.</p> <p>Kind regards<br /> Dirk </p>