  It has been reported that traveling waves propagate periodically and stably
in sub-excitable systems driven by noise [Phys. Rev. Lett. \textbf{88}, 138301
(2002)]. As a further investigation, here we observe different types of
traveling waves under different noises and periodic forces, using a simplified
Oregonator model. Depending on different noises and periodic forces, we have
observed different types of wave propagation (or their disappearance).
Moreover, the reversal phenomena are observed in this system based on the
numerical experiments in the one-dimensional space. As an explanation, we
regard it as the effect of periodic forces. Thus, we give qualitative
explanations to how reversal phenomena stably appear, which seem to arise from
the mixing function of the periodic force and the noise. And the output period
and three velocities (the normal, the positive and the negative) of the
travelling waves are defined and their relationship with the periodic forces,
along with the types of waves, are also studied in sub-excitable system under a
fixed noise intensity.
