I am modeling the reflections from a piece of plastic in air. When I use the following material properties the simulation is unstable and diverges.

```
% source / reflector (PLA)
c_plastic = 2730; % [m/s] According to Olympus NDT
rho_plastic = 1190; % [kg / m^3] According to ONDA Tables
c_air = 345; % [m/s] According to ONDA Tables
rho_air = 1.293; % [kg / m^3] According to ONDA Tables
```

However, if I increase the density of the air by a factor of ten, then the simulation is stable:

```
% source / reflector (PLA)
c_plastic = 2730; % [m/s] According to Olympus NDT
rho_plastic = 1190; % [kg / m^3] According to ONDA Tables
c_air = 345; % [m/s] According to ONDA Tables
rho_air = 10*1.293; % [kg / m^3] According to ONDA Tables
```

This isn't a bad workaround since the speed of sound stays the same and the reflection coefficient largely stays the same (huge).

Alternatively I can reduce the density of the plastic and get a stable solution:

```
c_plastic = 2730; % [m/s] According to Olympus NDT
%rho_plastic = 1190; % [kg / m^3] According to ONDA Tables
rho_plastic = 1/10*1190; % [kg / m^3] According to ONDA Tables
c_air = 345; % [m/s] According to ONDA Tables
rho_air = 1.293; % [kg / m^3] According to ONDA Tables
```

My suspicion is that the temporal sampling rate is somehow not properly determined for materials with these extreme mismatches. I would suspect that this can be overridden to resolve the issue. Can anyone comment on this hypothesis or tell me how to do an override of the temporal sampling rate?

Thanks.