Noise reducing devices such as sound barrier walls are common ways to mitigate traffic noise problems in residential areas next to arterial roads or railways. The basic performance of noise reducing devices is sound insulation and absorption. However, the existing measurement methods are not adequate for characterizing sound insulation and absorption performances of noise reducing devices. The sound insulation measurement method is different from the actual installation condition of the noise reducing devices that transmit the sound to the free field, and the sound absorption coefficient measurement method does not include the transmitted sound energy. This study raised the problem of the current acoustic performance measurement method of noise-reducing devices and arises the need for a new measurement method to complement it.