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).
Detailed explanation of a sound attenuating unit for an internal combustion engine's exhaust system.
| Identifier | ExFiles\Box 147\1\ scan0077 | |
| Date | 10th April 1933 guessed | |
| 417,935 4 will respond to a sound wave whose frequency is substantially the mean of maximum and minimum frequencies of the sound waves which it is proposed that the multiple-compound resonance unit shall attenuate. Of course, under some circumstances, extraneous factors, such as limitations of space, may render it desirable to employ constituent resonance units of proportions and dimensions other than those which will reduce the number of constituent resonance units to a minimum. Irrespective of what considerations dictate the proportions and dimensions of the constituent resonance units, the number of the constituent resonance units is determined by ascertaining, experimentally or otherwise, the number necessary to reduce the intensity of the sound waves, whose frequencies are within the range with which it is proposed that the multiple-compound resonance unit shall deal, to such an extent that they will be unobjectionable. It is in place here to mention that the efficiency of resonance units which are connected to a sound wave passage through which a fluid travels with considerable velocity is affected by the number, nature and size of the passages by which the constituent resonance chambers are acoustically connected to the passage. In general, it may be stated that in such an installation a resonance unit is most efficient if the constituent resonance chambers are connected to the passage by relatively closely spaced perforations of relatively small size extending through a relatively thin wall. We consider it preferable that the wall be of no greater thickness than is necessary to give it the necessary mechanical strength, and that the perforations be approximately 1/16'' in diameter and spaced apart ½'' from center to center. It should, however, be noted that experiments have indicated that relatively closely spaced perforations of relatively small size are preferable, primarily, not because of their size, but because when perforations of relatively small size are employed the fluid flowing through the sound wave passage disturbs the vibrating body of fluid in the perforations and the communicating resonance chambers to a lesser extent than when perforations of relatively large size are employed and, consequently, that perforations of relatively large size may be satisfactorily employed if they are so formed and/or arranged that the fluid flowing through the sound wave passage cannot substantially disturb the vibrating body of fluid in the resonance unit. This may be accomplished in any one of several ways, e.g. by providing flanges around the perforations, preferably on the outer side of the walls of the sound wave passage, by providing on the upstream sides of the perforations, by increasing the thickness of the walls of the sound wave passage through which the perforations extend, or in the special manner shown in the drawing, viz., by providing a double-walled sound wave passage in which the perforations which extend through the inner wall are of larger diameter and spaced farther apart (e.g., as shown in the drawing 3/16'' perforations spaced apart 1½'' from center to center) than the perforations which extend through the outer wall. It may here be mentioned that, while as shown in the drawing, the perforations in the walls of the sound wave passage are of uniform size and are uniformly spaced, it may be desirable in order to impart to the several resonance units the desired response characteristics, to provide perforations of varying size and/or spacing at different points in the length of the sound wave passage. From what has been said above, it will be understood that each of the several resonance units which constitute the section B of the sound wave attenuating unit illustrated in Figures 1, 2 and 3 will be so proportioned and dimensioned that it will respond to and attenuate sound waves of one or more of the lower frequencies which occur in the exhaust of the engine 11, and that the resonance units which constitute the section A will be so proportioned and dimensioned and of such number that the multiple compound resonance unit which constitutes, in conjunction with the complex multiple-compound resonance unit which the sound wave attenuating unit as a whole constitutes, will attenuate all the objectionable sound waves which occur in the exhaust system of the engine which are not affected by the resonance units which constitute the section B. The passage through which the exhaust gases of an internal combustion engine are conducted to the atmosphere constitutes, in effect, a tube which is closed at the source of sound. Since such a tube responds to and reinforces any sound whose frequency is equal to the frequency of the fundamental or any harmonic of the tube, the passage by which the exhaust gases of the internal combustion engine are conducted to the atmosphere will reinforce any exhaust noises resulting from sound waves whose frequencies are equal to the 417,935 5 the fundamental or any harmonic of the passage and may therefore render objectionable some noises which would be unobjectionable if the passage were longer or shorter. Experiments have shown that the reinforcing effect can be counterbalanced and the intensity of noises which result from sound waves whose frequencies are equal to the frequency of the fundamental or a harmonic of a tube can be reduced by providing an expansion chamber at a nodal point (i.e. a point of maximum pressure change) of the fundamental or harmonic, and, furthermore, that such an expansion chamber will function with maximum efficiency if it constitutes an element of a resonance unit which is so proportioned and dimensioned that it will respond to and attenuate sound waves whose frequency equals that of the fundamental or harmonic at whose nodal point it is located. Although it is a matter of common knowledge, it may be well here to point out that the frequency of the fundamental of a tube of the type here under consideration (that is a tube closed at the end where the sound has its source and open at the other end) is determined by the formula: f = v / 4L in which f = the frequency v = the velocity of sound, and L = the length of the tube that the frequencies of the harmonics of such a tube is determined by the formula: f = v(2n+1) / 4L in which f, v and L have the same significance as in the preceding formula, and in which n is the integer 1, 2, 3, etc. for the first, second, third, etc. harmonics; that the fundamental has a nodal point at the source of sound; that each of the harmonics has a nodal point at the source of sound and, in addition, one or more nodal points between the source of sound and the end of the passage toward which the sound waves travel; and that the distance between the successive nodal points of any harmonic is equal to one-half of its wave length which may be determined from its frequency by the following formula: λ = v / f in which v and f have the same significance as in the preceding formulas and λ is the wave length. For the purpose of locating nodal points, the exhaust passage of an internal combustion engine may be considered as a tube closed at the engine end, where the sound has its source, and open at the other end. While it is not entirely clear in all types of exhaust systems just what point in the exhaust passage should be considered as the closed end of the tube to determine the effective length thereof, it seems that generally the effective length of the tube is approximately equal to the length of the passage from its atmospheric end to the discharge end of the exhaust manifold plus a distance equal to the volume of the exhaust manifold divided by the cross-sectional area of the exhaust pipe. In cases in which the indicated method of determining the effective length of the exhaust system considered as a closed tube is not applicable, the effective length may be readily determined by experiment. Practically, it is not essential that the expansion chamber be located precisely at a node of the fundamental or a harmonic of the sound wave whose intensity it is intended to reduce. As a matter of fact, I have found that it is more essential in order to obtain satisfactory results, to avoid locating the chamber too near an anti-node than to locate it precisely with respect to a node of the fundamental or a harmonic of the sound wave whose intensity it is intended to reduce. The sound wave attenuating unit illustrated in Figures 1, 2 and 3 was designed for use in the exhaust system of an internal combustion engine whose characteristics were such that the intensity of the fundamental of the exhaust passage was so inconsiderable that it was unnecessary to reduce it to render the resulting noise unobjectionable. It was, however, found that the intensity of the third and fifth harmonics of the passage was so great that they resulted in objectionable noises and therefore that it was desirable to reduce it. To this end there were provided two resonance units 27—44—43 and 33—45—46 which were so proportioned and dimensioned that one of the principal frequencies of the former equaled that of the third harmonic and one of the principal frequencies of the latter equaled that of the fifth harmonic and so located in the sound wave attenuating unit that they communicated with the interior of the tube 29—52 at points separated by a distance equal to that between one of the nodal points of the third harmonic and the adjacent nodal point of the fifth harmonic and the exhaust system was so designed that when | ||