I am trying to simulate a bi-concave acoustic lens in 2D by introducing a higher sound speed for the lens. Medium sound speed is 1500m/s, density 1000kg/m^3 and lens sound speed is 2590m/s and density 884m/s. A point source (smoothened to avoid aliasing) is used as the initial pressure. I observed that there are multiple oscillations in the PA signal generated from the point source. ie instead of a single N-shaped pulse there is an oscillation with gradually decaying amplitude. To confirm the signal is not from multiple reflections from the heterogeneous part of the medium (impedance mismatch), I detected the signal before it reaches the lens. However, the signal remains the same. To further verify other parameters of the simulation, I simply eliminated the lens (heterogeneous part of the medium) and the signal falls back to the expected PA signal ( as in photoacoustic Waveforms in 1D, 2D and 3D Example ).
Please help me to understand what caused the oscillating nature of PA signal. And how to solve this problem?