Why the generated voltage is negative? (in a piezoelectric pressure sensor, frequency domain)

Hi,
I am using frequency domain study to analyse a piezo sensor. But it is so strange that in some frequencies the generated voltage is negative, while in other frequencies it is positive.
I do not understand why in frequency domain the sign will be different? What I applied is a 100 Pa pressure on the membrane.
Besides, the displace of the membrane is similar, in some frequencies negative, in some frequencies positive.
I think in frequency domain we enter the amplitude of a pressure, but why different frequencies give results with different signs?

Thanks!
Guofeng