Abstract: Abstract: Transmission loss (TL) is a key reference for structure design and noise reduction performance analysis of a muffler. Taken an expansion chamber muffler as an illustration, and based on finite elements method, the interior sound field of the muffler was calculated and analyzed, the transmission loss was virtually measured by three-point method and traditional four-point method under one load condition. Three-point method was to be calculated by measuring the complex sound pressure of two points upstream and one point downstream of the muffler pipe. It can only get the accurate results in the event that the muffler termination was ideal anechoic. Traditional four-point method under one load condition was to be calculated by measuring the complex sound pressure of two points upstream and two points downstream of the muffler pipe at the same time. Compared with three-point method, the transmitted reflected wave downstream from the actual tail pipe effects was taken into account in traditional four-point method, but its coupling effects on the wave upstream were ignored. And the measurement accuracy was obviously influenced by the sound absorption performance of the muffler termination. The lower the sound absorption coefficient of the termination, the worse is the measurement accuracy. So, exact results can not be achieved at low frequencies by adding sound absorption material at the muffler termination. On these bases, in order to eliminate the coupling effects on the wave upstream from the transmitted reflected wave downstream, four-point method under two load conditions was proposed. It calculates transmission loss by combining the complex sound pressure of two points upstream and two points downstream of the muffler pipe under two difference termination conditions, such as an open termination and a total reflection termination. Then the transmission loss of a typical expansion chamber was measured with this method. And the experimental result was well concordant with the theoretical and the finite elements analysis results. At the same time, according to the simulation results of the interior sound field, the arrangement principles of microphones on the muffler pipe were put forward. It was concluded that: the distances from the measurement points to the expanding cross-sections should be longer than the diameter of shrinkable pipe in order to guarantee the well plane wave at the measurement points; the distances between two microphones upstream and downstream should be shorter than half-wavelength of max analysis frequencies in order to avoid the false results (the emergence of jump points) at some special frequencies. These can provide references for the actual measurement of transmission loss.