We investigate the close relationship between the potential and absorption cross section for test fields in an AdS_3 bubble(a 5D black hole) and an exact AdS_3. There are two solutions in type IIB string theory: an AdS_3 bubble corresponds to the dilatonic solution, while an exact AdS_3 is the non-dilatonic solution. In order to obtain the cross section for an AdS_3 bubble, we introduce the {out}-state scattering picture with the AdS_3-AFS matching procedure. For an exact AdS_3, one considers

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... e {in}-state scattering picture with the AdS_3-AdS_3 matching. Here the non-normalizable modes are crucially taken into account for the matching procedure. It turns out that the cross sections for the test fields in an AdS_3 bubble take the same forms as those in an exact AdS_3. This suggests that in the dilute gas and the low energy limits, the S-matrix for an AdS_3 bubble can be derived from an exact AdS_3 space.