  In this paper we calculate the gravitational-wave power-spectra corresponding
to different types of transitions between the inflationary regime and the
radiation era. We study four cases, where inflation is followed by a stiff
matter phase, or by a dust dominated phase, or by a combination of the two,
before the universe enters the radiation era. Use is made of differential
equations for the Bogoliubov coefficients, allowing us to model all the
transitions between the different eras as continuous. New features appear, for
frequencies above 10^{7} rad/s in an otherwise flat part of the spectrum.
