A time series of GONG Dopplergrams for the period 10-14 May 1997 from Udaip
ur and Big Bear sites has been used to measure the velocity fluctuations in
a sunspot (NOAA active region 8038) and quiet photosphere simultaneously.
We observe that the power of pre-dominant p mode is reduced in the sunspot
as compared to quiet photosphere by 39-52% depending on the location of the
sunspot region on the solar disk. We also observe a relative peak frequenc
y deviation of p modes in the sunspot, of the order of 80-310 mu Hz, which
shows a linear dependence on the magnetic field gradient in the active regi
on. The maximum frequency deviation of 310 mu Hz on 12 May appears to be an
influence of a long-duration solar flare that occurred in this active regi
on. We interpret this relative peak frequency deviation as either due to po
wer re-distribution of p modes in the sunspot or a consequence of frequency
modulation of these modes along the magnetic flux tubes due to rapidly var
ying magnetic field structure.