From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of exhaust system noise, 'boom' periods, and silencer performance on various car models.
| Identifier | ExFiles\Box 104\2\ scan0037 | |
| Date | 1st July 1932 | |
| -3- holes to one another and to the main pipe. By adjusting the size of the chambers, their position and the number of drilled holes, the various boom periods can be eliminated. The result is rather peculiar in that there is no boom in the car, though the exhaust is very nearly free, but if the car is driven full throttle with the windows open between walls the exhaust can be heard issuing from the tail pipe. Also if the car is accelerated away from an observer the same noise is audible. It is therefore necessary to baffle the silencer to bring this characteristic into line with R.R. existing practice and we are up against the previous problem to a certain extent, that to obtain silence in this respect without excessive back pressure we need more capacity on the P.II, though the P.II silencer on the 25 HP. gives good results. It will be rembered that while in the U.S.A. we commented on the free sound of the exhaust of the Cadillac to an outside observer, though the cars internally were free from any boom. The compromise we have so far arrived at for the 25 HP. is illustrated on sheet 3. With this we have been able to get rid of the front expansion box. We have had to fit baffles in the front, however, for the reason indicated above. Curve Rm.{William Robotham - Chief Engineer}71 shows a gain of about 4 B.H.P. at 75 M.P.H. with the resonant silencer over the existing standard. Curves Rm.{William Robotham - Chief Engineer}72 and 73 indicate where the losses occur. It will be observed that the chief restriction is in the exit to the swan neck and in the swan neck. We have tried a similar type of silencer on the P.II with complete success as regards boom periods, but the problem of noise at the fish tail is much more difficult on this car than on the 25 HP. We are, however, working on it. J.III was interesting because it has a particularly long exhaust system, actually about 150". This, functioning as an open organ pipe, has a fundamental frequency of 28.2/sec. at exhaust gas temperature and the first over-tone is at 56.4/sec. which corresponds to 3 explosions/rev. at 23 M.P.H. Therefore this is the speed at which the first boom might be expected within the running range and actually it was present with the ordinary P.II short type silencer to a very marked extent. These very low frequencies do not seem very easy to deal with which is perhaps why the Cadillac people keep their pipe from the manifold to the expansion box, short. | ||