We present a method to reduce noise in helioseismic power spectra using a n
on-orthogonal wavelet transform based on quadratic spline functions. The qu
ality of our method is tested by applying it to artificially generated time
-series approximating solar p-modes. The mode frequencies and line widths o
btained from least-squares fits to the smoothed spectra are compared with t
he corresponding parameters deduced from maximum likelihood fits to the ori
ginal spectra. The results from both approaches are very similar and sugges
t that there is no major bias in either of these rather independent approac
hes. As a practical example we denoise parts of the power spectrum obtained
from the two first years of operation of the GOLF instrument onboard SOHO.