Acoustic module: COMSOL beginners

1. Difference between acoustic pressure and sound pressure graph?
2. can we simulate/compute small region of the big geometry.
3. how to give sine wave input?
4. how mesh size effect the analysis?
5. what is the effect of using sound soft bounary?
6. why we are getting reflections in sound soft boundary?
7. what should be the mesh element size?
8. what should be the time step for time domain analysis?
9. what is element, is it point or small surface?
10. what is the function of plane wave radiation node?
11. what is incident pressure under 'plane wave radiation" node?