4.7 dB(A)。Based on the principle of reciprocity, an experimental method was developed for the air-borne propagation path，and by doing so, the transfer functions were established from the sound sources to vehicle interior noise, and then the contributions from the different sources were got. According to the result of the noise sources identification, car body was redesigned by using techniques of the sound absorption and sound insulation. Case research showed that acoustical reduction factor of the vehicle floor increased by 1.83～9.74 dB(A), and that the vehicle interior noise fell 4.7 dB(A). 