  Cosmic strings are a common prediction in many grand unified theories and a
promising source of stochastic gravitational waves (GWs) from the early
Universe. In this paper, we point out that the GW signal from cosmic strings
produced at a comparatively low energy scale, $v \lesssim 10^9 \textrm{GeV}$,
exhibits several novel features that are not present in the case of high-scale
cosmic strings. Our findings notably include (i) a sharp cutoff frequency
$f_{\rm cut}$ in the GW spectrum from the fundamental oscillation mode on
closed string loops and (ii) an oscillating pattern in the total GW spectrum
from all oscillation modes whose local minima are located at integer multiples
of $f_{\rm cut}$. These features reflect the fact that string loops produced in
the early Universe fail to shrink to zero size because of GW emission within
the age of the Universe, if their tension is low enough. In addition, they
offer an exciting opportunity to directly probe the discrete spectrum of
oscillation modes on closed string loops in GW observations. For strings
produced at a scale $v \sim 10^9\textrm{GeV}$, the novel features in the GW
spectrum are within the sensitivity reach of future experiments such as BBO and
DECIGO.
