  We analytically calculate the spectrum of stochastic gravitational waves
(GWs) emitted by expanding string loops on domain walls in the scenario where
domain walls decay by nucleation of string loops. By introducing macroscopic
parameters characterizing the nucleation of the loops, the stochastic GW
spectrum is derived in a way that is independent of the details of particle
physics models. In contrast to GWs emitted from bubble collisions of the false
vacuum decay, the string loops do radiate GWs even when they are perfectly
circular before their collisions, resulting in that more and more contribution
to the spectrum comes from the smaller and smaller loops compared to the
typical size of the collided loops. Consequently, the spectrum is linearly
proportional to the frequency at the high-frequency region, which is peculiar
to this GW source. Furthermore, the results are compared with the recent
nano-Hertz pulsar timing array signal, as well as the projected sensitivity
curves of future gravitational wave observatories.
