  We study the properties of the stochastic gravitational wave background
(SGWB) produced by domain walls (DWs) during inflation without forming a
network. We numerically simulate the DW production caused by a second-order
phase transition and calculate the SGWB spectrum using a
$1000\times1000\times1000$ lattice. We show that the SGWB can be observed
directly by future terrestrial and spatial gravitational wave detectors and
through the B-mode spectrum in CMB. This signal can also explain the common
noise process observed by pulsar timing array experiments. With numerical
simulations, we derive an empirical formula for the strength and qualitative
features of the SGWB spectrum. The details of the SGWB spectrum also contain
information about the later evolution of the universe.
