Using only part of saved solution Comsol v4.4

Hi all,

I am studying the acoustic response of a spherical shell. I am doing an acoustic-solid interaction study (frequency domain). I would like to study the acoustic field generated by an oscillating shell in its quadrupole mode.

I would like to go in two steps. In the first, I use a background plane wave in order to excite the shell to one of its modes. In the second, I give the displacement field calculated in the first step as a initial value, disable the background field and solve again in the frequency domain. I hope that the approach is not totally wrong :)

In the acoustic module I am using, my dependent variables are the displacement field and the pressure field. Looking into the forum, however, I only found posts about keeping the whole solution of a previous step as an initial condition for the second step, regarding the dependent variables. I would like to keep only the displacement field because I would like the pressure field for the second step to be 0. How is it possible to do that?

Thank you in advance for your help,
Alessio Caciagli