  The solar magnetic activity consists of two periodic components: the main
cycle with a period of 11 yr and a shorter cycle with a period of ~2 yrs. The
origin of this second periodicity is still not well understood. We use almost
15 yrs of long high quality resolved data provided by the Global Oscillation
Network Group (GONG) to investigate the solar cycle changes in p-mode frequency
with spherical degree l=0-120 and in the range 1600muHz < nu < 3500 muHz. For
both periodic components of solar magnetic activity our findings locate the
origin of the frequency shift in the subsurface layers with a sudden
enhancement in the amplitude of the shift in the last few hundred kilometers.
We also show that the size of the shift increases towards equatorial latitudes
and from minimum to maximum of solar activity. On the other hand, the
signatures of the 2 yr cycle differ from the one of the 11 yr cycle in the
magnitude of the shift, as the 2 yr cycle causes a weaker shift in mode
frequencies and a slower enhancement in the last few hundreds kilometers. Based
on these findings we speculate that a possible physical mechanism behind the
quasi biennial periodicity (QBP) could be the beating between different dynamo
modes (dipole and quadrupole mode)
