Axions from wall decay
  We discuss the decay of axion walls bounded by strings and present numerical simulations of the decay process. In these simulations, the decay happens immediately, in a time scale of order the light travel time, and the average energy of the radiated axions is $<\omega_a > \simeq 7 m_a$ for $v_a/m_a\simeq 500$. $<\omega_a>$ is found to increase approximately linearly with $\ln(v_a/m_a)$. Extrapolation of this behaviour yields $<\omega_a> \simeq 60 m_a$ in axion models of interest. 
S. Chang, C. Hagmann and P. Sikivie