The signals emitted from the ring-down cavities under differnt conditions were investigated using pulsed light from an optical parametric oscillator. The ring-down signals coupled with complicated modulations were observed in the experiment. Some of the mode frequencies can be explained by the formula deduced from the eigenmode theory of resonant cavity. It is proposed that the complicated mode beating was due to the slight splitting of the transverse mode owing to the astigmation and aberration of the ring-down cavity optical system.