Change in Eigenfrequency due to added mass

Hello everybody,

I have a very simple doubt, which is as follows.

I have calculated the Eigenfrequency of a clamped plate. Now, I would like to add another body, which has a definite surface and weight, on top of the plate to see how the Eigenfrequency is changing. However, I am not able to know how to add this weight in comsol 4.0a.

In the attached figure, you will be able to see, I have added this weight as a face load on boundary 2. However, the Eigenfrequency is not changing.

All form of help would be happily appreciated.

Thanks in advance,

Kiran