Frequency domain analysis in general PDE form

Hi all,

As you may know, the PDE(general) module has the form of (a*d^2(u)/dt^2+d*du/dt+nabla.nabla(u)=f ). This format provides the possibility for implementing our own equation in time domain studies. I want to use the general form PDE for a frequency domain. So the equation will not have the derivations of time anymore. For example, for the case of a pressure acoustics, the general equation is ((1/c^2) * d^2(p)/dt^2-nabla.nabla(p)=0 ) . Considering the pressure as harmonic p=p*exp(-i*omega*t), we can write the frequency domain equation for the above as:

-(1/c^2)*(omega^2)*p-nabla.nabla(p)=0)

The only way for implementing it in General PDE is puting the (+(1/c^2)*(omega^2)*p) as f.

Here the question is that:

What is the proper variable to put as "omega" in the equations that Comsol can understand it as the "frequency" and give the right solution? I know "freq" but I am not sure if it works and if I can get right results from Frequency Domain studies.
In general: How to implement the frequency domain study with the general PDE form?