  In the first-order phase transitions (PTs) colliding bubble is an important
gravitational wave (GW) source. Following bubble collision, domain walls can be
formed when degenerate vacua occur as a result of the breaking of a discrete
symmetry relevant to new physics at electroweak or higher scales. Using lattice
simulations, we study the dynamical evolution of domain walls and find that the
networks of the domain wall are formed around the completion of PTs and the
lifetime of the wall networks largely depends on whether or not the degeneracy
of true vacua is broken. Our numerical results indicate that domain wall
networks continue to produce GWs in the aftermath of PTs, leading to
dramatically changing the spectral shape and enhancing the magnitude by about
one order. The resulting GW power spectra are peaked at $kR_* \simeq \pi$,
above the peak wavenumber it has a decaying power law close to $k^{-1.2}$
followed by a slowly decreasing plateau with the UV cutoff at $kR_* \sim
\mathcal{O}(10^2)$
