  We study a particular nonlinear dispersion relation $\omega_p(k_p)$ -- a
series expansion in the physical wavenumber $k_p$ -- for modeling first-order
corrections in the equation of motion of a test scalar field in a de Sitter
spacetime from trans-Planckian physics in cosmology. Using both a numerical
approach and a semianalytical one, we show that the WKB approximation
previously adopted in the literature should be used with caution, since it
holds only when the comoving wavenumber $k\gg aH$. We determine the amplitude
and behavior of the corrections on the power spectrum for this test field.
Furthermore, we consider also a more realistic model of inflation, the
power-law model, using only a numerical approach to determine the corrections
on the power spectrum.
