The calculation method for natural frequency of seal in traps which we suggested in former reports is compared with the seal broken frequency in sinusoidal pressure test on Water Closet. Subsequently the suggested formula caluculated in the transition area between natural frequencies of the seal at the full and the half level has demonstrated that it can sufficiently evaluate the seal broken frequency in sinusoidal pressure test on a siphon type WC and a siphon jet type WC.
