Numerical dispersion

Push the source Frequency up (or the Velocity down) to lower the points-per-wavelength (PPW), and watch numerical dispersion ripple behind the wavefront and stretch the recorded trace into a long tail. The PPW badge turns amber below ~8 and red below ~5.

Then switch between 2nd and 4th order at the same settings: the higher-order stencil suppresses most of the dispersion, which is why production solvers use wide stencils.

The dashed line on the trace marks the exact arrival time (offset ÷ velocity); a well-sampled pulse sits on it with little trailing energy.