  We looked for signatures of Quasi-Biennial Periodicity (QBP) over different
phases of solar cycle by means of acoustic modes of oscillation. Low-degree
p-mode frequencies are shown to be sensitive to changes in magnetic activity
due to the global dynamo. Recently have been reported evidences in favor of
two-year variations in p-mode frequencies. Long high-quality helioseismic data
are provided by BiSON (Birmingham Solar Oscillation Network), GONG (Global
Oscillation Network Group), GOLF (Global Oscillation at Low Frequency) and
VIRGO (Variability of Solar IRradiance and Gravity Oscillation) instruments. We
determined the solar cycle changes in p-mode frequencies for spherical degree
l=0, 1, 2 with their azimuthal components in the frequency range 2.5 mHz < nu <
3.5 mHz. We found signatures of QBP at all levels of solar activity in the
modes more sensitive to higher latitudes. The signal strength increases with
latitude and the equatorial component seems also to be modulated by the 11-year
envelope. The persistent nature of the seismic QBP is not observed in the
surface activity indices, where mid-term variations are found only time to time
and mainly over periods of high activity. This feature together with the
latitudinal dependence provides more evidences in favor of a mechanism almost
independent and different from the one that brings up to the surface the active
regions. Therefore, these findings can be used to provide more constraints on
dynamo models that consider a further cyclic component on top of the 11-year
cycle.
