Transmission loss characteristics of a reactive type muffler have been studied by Davis D. D. et al under the assumption that the tail pipe of the muffler is a non-reflective one. On the other hand, assuming that the pipe is completely reflective, Fukuda has developed an attenuation theory in his previous paper. In the case of a muffler with air flow, however, these equations are not always correct in estimating frequency characteristics of noise reduction. This is because of the change in these characteristics due to the air flow or a secondary noise inside the muffler. The authors have studied the influence of air flow on the relationship between the figure of the reactive type muffler and the noise radiated from it. They have also investigated how to identify the sources of the air flow noise inside the cavity. Furthermore, they have estimated air flow noise by means of calculation and experiment. As a result, many interesting conclusions have been obtained together with important data for the muffler design.