Dependence of the amplitude of gravitational waves from preheating on
  the inflationary energy scale
  Stochastic gravitational wave backgrounds (SGWBs) receive increasing
attention and provide a new possibility to directly probe the early Universe.
In the preheating process at the end of inflation, parametric resonance can
generate large energy density perturbations and efficiently produce
gravitational waves (GWs) which carry unique information about inflation. Since
the peak frequency of such GWs is approximately proportional to the
inflationary energy scale, $\Lambda_{\mathrm{inf}}$, GWs from preheating are
expected to be observed by interferometer GW detectors in low-scale
inflationary models. We investigate the dependence of the amplitude of such GWs
on $\Lambda_{\mathrm{inf}}$, and find that the present energy spectrum of these
GWs does not depend on $\Lambda_{\mathrm{inf}}$ only in the case of
$\Lambda_{\mathrm{inf}}$ is above a critical value $\Lambda_{c}$, a parameter
depending on the resonance strength. We numerically obtain $\Lambda_{c}$ in
terms of the model parameters in linear approximation and then conduct lattice
simulations to verify this result. For
$\Lambda_{\mathrm{inf}}\lesssim\Lambda_{c}$, the amplitude of GWs quickly
decreases with $\Lambda_{\mathrm{inf}}$ and becomes challenging to observe. In
turn, observing such GWs in interferometer detectors also helps to determine
$\Lambda_{\mathrm{inf}}$ and the resonance strength during the preheating.
