This paper deals with the silencing effect of exhaust mufflers of automobiles. The purpose of this experiment is especially to investigate the effect of the flow rate of exhaust gas on a changes in noise attenuation in mufflers from the practical point of view. Ten fundamental types of reactive mufflers are tested by an engine, which has a cylinder volume of about 900 cc. The results are compared with those of a test by a pulsating air flow at various flow rates. The spectrum of noises in the pulsating flow test is similar to that of exhaust noises by the engine test as shown in Fig. 1. But in general, the sound level in the pulsating flow test is lower in the frequency range, but higher in the high frequency range than the level in the engine test (Fig. 2). The noise attenuation L_e (Eq. 1) in the engine test varies with engine load, and is generally in value under the full load than in no load condition, especially in the high frequency range above about 300 Hz (Fig. 4). These tendencies are observed clearly in the case of the mufflers with the inlet and outlet pipes installed straight through. The noise attenuation L_p in the pulsating flow test also decreases with an increase in flow rate (Fig. 4). With the noise spectrum divide into two frequency ranges, the low and high frequencies bounded by 500 or 1, 000 Hz, the acoustic power of the exhaust noise W' is classified in thems of a function of the mean flow rate as shown in Eq. 2〜5. The acoustic powers, W_L and W_H, of exhaust noises from an exhaust pipe without a muffler consist of the components proportional to the 2nd, 4th or 6th power of the mean flow rate as shown in Fig. 5. And the acoustic power W_m of noises generated in a muffler can be explained in terms of the component proportional to the 6th power of the flow rate (Fig. 6). The noise attenuation in a muffler is expressed as shown in Eq. 8. The amount of attenuation L in a muffler is comparable between the engine test and the pulsating flow test under the condition expressed by Eq. 10 (Fig. 9). In this case the coefficient of correlation between the results of both tests is given as about 0. 78 for the whole mufflers (Fig. 10).
