  The trace magnetic power spectrum in the solar wind is known to be
characterized by a double power law at scales much larger than the proton
gyro-radius, with flatter spectral exponents close to -1 found at the lower
frequencies below an inertial range with indices closer to $[-1.5,-1.6]$. The
origin of the $1/f$ range is still under debate. In this study, we selected 109
magnetically incompressible solar wind intervals ($\delta |\boldsymbol
B|/|\boldsymbol B| \ll 1$) from Parker Solar Probe encounters 1 to 13 which
display such double power laws, with the aim of understanding the statistics
and radial evolution of the low frequency power spectral exponents from
Alfv\'en point up to 0.3 AU. New observations from closer to the sun show that
in the low frequency range solar wind turbulence can display spectra much
shallower than $1/f$, evolving asymptotically to $1/f$ as advection time
increases, indicating a dynamic origin for the $1/f$ range formation. We
discuss the implications of this result on the Matteini et al. (2018)
conjecture for the $1/f$ origin as well as example spectra displaying a triple
power law consistent with the model proposed by Chandran et al. (2018),
supporting the dynamic role of parametric decay in the young solar wind. Our
results provide new constraints on the origin of the $1/f$ spectrum and further
show the possibility of the coexistence of multiple formation mechanisms.
