  By strictly adhering to the microscopic theory of composite fermions for the
Landau-level filling fractions nu_e = p/(2 p + 1), we reproduce, with
remarkable accuracy, the surface-acoustic-wave (SAW)-based experimental results
by Willett and co-workers concerning two-dimensional electron systems with nu_e
close to 1/2. Our results imply that the electron band mass m_b, as distinct
from the composite fermion mass m_*, must undergo a substantial increase under
the conditions corresponding to nu_e approximately equal to 1/2. In view of the
relatively low aerial electronic densities n_e to which the underlying SAW
experiments correspond, our finding conforms with the experimental results by
Shashkin et al. [Phys. Rev. B 66, 073303 (2002)], concerning two-dimensional
electrons in silicon, that signal sharp increase in m_b for n_e decreasing
below approximately 2 x 10^{11} cm^{-2}. We further establish that a finite
mean-free path l_0 is essential for the observed linearity of the longitudinal
conductivity sigma_{xx}(q) as deduced from the SAW velocity shifts.
