EP0072274B1 - Reactive silencer or sound attenuator for a pulsating gas flow - Google Patents

Abstract

1. A reactive sound attenuator or silencer, i.e. a reflection or sound wave echo attenuator or silencer, for a pulsed flow-rate of gas submitted to mean and high frequencies, namely of about 300 to 8000 Hz, characterized by a system of several identical sets (1, 2, 3, 4, 5, 6, 7, 8) disposed in parallel, connected with at least an inlet collector (34) and/or an exhaust collector (35), each of the sets being composed of a succession of expansion rooms (1, 2, 3) of different dimensions connected by an axial duct (4, 5, 6), each of the rooms having such dimensions that it is accorded for a field of frequency to be attenuated, whereas the joining ducts (4, 5, 6) have a weak cross-section with respect to the one of the rooms, the system of several sets of rooms (1, 2, 3) connected with ducts (4, 5, 6) being carried out in order that the lenghts of the rooms and ducts are determined so that it can be provided an alveolar imbrication of the elements which are staggered the one with respect to the other in order that the rooms reach the level with the ducts of the adjacent sets and inversely, so as to circumscribe said system in the minimal volume.

Description

Technical area.

The present invention relates to a silencer or attenuator of its reactive sound, that is to say with reflection or echo of sound wave, for pulsed flow of gas subjected to medium and high frequencies, namely approximately 300 to 8000 Hz .

Problem.

Many machines use or process gases by giving them impulses which are almost always in the sound domain and cause nuisance. This is why, it has been brought for a long time to join to these machines silencers or sound attenuators which must be made as effective as possible in the attenuation of sound by altering as little as possible the efficiency of the machine.

This problem is frequently encountered in heat engines and also in compressors.

The invention proposes to solve it although, as we will see, this has been a concern for many technicians for decades. These technicians had to solve the elimination of medium and high frequencies ranging between 300 and 8000 Hz.

State of the prior art and drawbacks.

Existing silencers can be divided into two main categories:

• - Les silencieux actifs ou réactifs à chicane et à chambre ne comportant pas de matières absorbantes.

The first category concerns passive silencers containing insulating and absorbent materials.

• - Active or reactive mufflers with baffles and chambers that do not contain absorbent materials.

Thus, in silencers based on the principle of absorption damping of noise by absorption (passive silencer), the gases are brought in by means of tubes which have holes at the periphery through which the sound wave can exit while being distributed from uniformly and is absorbed to some extent by absorbent which surrounds these tubes. These absorption silencers are used in particular as main silencers to dampen a wide range of noise. However, they have the drawback of being bulky because it is necessary to give the tubes which have holes a relatively large section to limit the speed of passage of the gases in order to avoid entrainment of absorbent materials which causes pollution of the gas and deterioration of the muffler.

In addition to these absorption or passive sound absorbers, there are reflection or baffle absorbers that operate on an active principle where the sound waves are reflected by attenuating the posterior waves. We know that to be able to use the principle of these so-called reflection silencers, the acoustic waves must propagate in plane waves. In these silencers, conduits are provided which bring the gases into chambers or reflection spaces from which the gases can escape in order to flow more regularly with a strongly attenuated sound energy. The principle used for the sound absorbers by reflection rests on the fact that the sound is reduced thanks to modifications of section and changes of direction. By virtue of the varied reactance encountered in each of the elements, there is an attenuation of the sound energy. For large gas flows, the muffler section is such that only low frequencies can be treated.

The principle of these silencers is illustrated in Figures 1 to 6, of the accompanying drawings.

In FIG. 1 of these drawings, a chamber of section S, of length I, is shown, which is connected to two conduits of section s. It is recognized that such a chamber can absorb sound waves whose wavelength is in relation to the length 1 and corresponds to a frequency f ₁ . The attenuation efficiency is a function of the expansion ratio, that is to say the ratio of the S / s sections. It is immediately understood that it is advantageous to have a relatively small section S to limit the size of the chamber but that, on the other hand, the flow rate of the device is limited.

It is assumed that the chamber has a relatively large length 1 to attenuate a relatively small frequency f ₁ .

To widen the bandwidth, one can mount in series (Figures 3 and 4) two chambers, the second chamber having a length 1 ₂ shorter than so that the frequencies are attenuated according to the spectrum which is represented in Figure 4 where, as in FIG. 2, the attenuation in dB is shown on the ordinate and the attenuated frequency on the abscissa. The lower (300 Hz) and upper (8000 Hz) limits between which the device must be effective have also been shown.

Following the same reasoning, we can adopt the basic principle which is represented in Figures 5 and 6, by adding a third chamber which has a length 1 ₃ shorter than the length 1 ₂ and which dampens frequencies f ₃ greater than f ₂ . The spectrum of damped frequencies is represented in figure 6 which one can comment in the same way as what was done for figure 2.

This theoretical arrangement leads to adopt a bulky construction of the sound damper to obtain the same damping properties as with an absorption damper, which results in a greater expenditure of materials and therefore makes the dampers more expensive by reflection known so far.

The problem therefore comes down to obtaining the highest possible S / s ratio, the number of chambers of different dimensions in series as large as possible and, however, limiting the pressure losses and the bulk, all this by reducing the price of returns from the device.

This is the aim which the present invention proposes to achieve.

It should be noted that an improvement consists in providing tapered chamber bottoms, as shown in dotted lines in FIG. 1, in order to reduce the pressure losses without however affecting efficiency.

Statement of the invention.

The silencer or sound attenuator of the invention is characterized by a system of several identical assemblies arranged in parallel, connected to at least one intake manifold and / or one exhaust manifold, each of the assemblies being constituted by a succession of expansion chambers of different dimensions connected by an axial duct, each of the chambers having dimensions such as it is granted for a frequency domain to be attenuated while the connecting conduits have a small section compared to the section of the chambers.

Said chambers can be prismatic and / or cylindrical and their section depends on the upper frequency limit capable of being attenuated while their length depends on the frequency domain capable of being attenuated, it being understood that these latter frequencies are below the upper limit of frequencies to be processed. If they are cylindrical, they are preferably of revolution with frustoconical bottoms adapting to axial tubular conduits.

A very important characteristic of the invention resides in the fact that a system of several sets of chambers connected by conduits is produced, a system in which the lengths of the chambers and of the conduits are determined so that an alveolar nesting can be ensured elements which are offset from each other so that the chambers are placed at the level of the conduits of the adjacent assemblies and vice versa, so that the system occupies the minimum volume.

To practically manufacture these systems, there are two main solutions.

The first consists in making the silencer by sections perpendicular to the axis of the conduits and that said sections are assembled by known means in order to constitute a sound-absorbing block. In this case, the sound-absorbing block can be either prismatic, generally parallelepipedic, or cylindrical.

The second consists in making the silencer by identical plane elements, in the form of plates each having in hollow, on their two faces, complementary parts of the assemblies connected by conduits, said complementary parts being arranged to adapt exactly to the corresponding parts of the adjacent planar elements for reconstructing the system of sets of chambers and conduits by means of assembly and relative positioning. It is more convenient for the planar elements to have, in hollow, on each of their faces, half of each assembly constituted by a section of the chambers and conduits along a diametral plane thereof, said elements comprising means of assembly by tie rods ensuring the juxtaposition and the tightening to thus constitute a blockplarallélépipédique sound absorber.

Preferably; the intake and exhaust manifolds consist of boxes respectively covering the inputs and outputs of the sound-absorbing block. To make said boxes in a simple manner, an elongated box is provided. the central part of which the sound damping block is placed, leaving upstream and downstream two chambers serving as collectors having a relatively large length for processing and damping the low frequencies.

This elongated box generally serves as a base for the machine producing the sound pulses of gas.

Solution to the problem, advantages and industrial result.

The embodiment of the invention allows a great ef, efficiency of the silencers since the S / s ratio is important and that the disadvantage of a small section s is avoided by arranging in parallel a large number of equivalent sets. The diameter of the elementary silencers is calculated to allow propagation in plane waves up to high frequencies, 8000 Hz for example. It is possible to extend this limit at high frequencies by using elementary silencers of smaller diameters.

The nesting of the different chambers and conduits has made it possible to considerably reduce the overall dimensions. It should be noted that all of the conduits and chambers have the same length whatever the flow to be passed since this flow can vary by multiplying the number of joined sets. In practice, mufflers are obtained which are two to ten times shorter, for the same efficiency, than the device simply applying the theoretical principle.

As the passive silencer principle is not used by the presence of absorbent products, higher passage speeds can be adopted in the assemblies without risk of entraining this absorbent material. This improves the longevity of the silencer and the purity of the gas leaving the device.

It should be noted that this device makes it possible to process high frequencies since it is possible to reduce the length of the rooms in a simple way. Finally, the cost price is considerably reduced, above all, as we have seen if the device is integrated into the base of the device.

The invention will be better understood with the aid of the description below which gives some nonlimiting examples of practical embodiment thereof and which are illustrated by the accompanying drawings.

Brief description of the figures.

• La figure 1 est une vue schématique d'une chambre d'un silencieux actif ou à réflexion.
• La figure 2 est un graphique montrant comment le son est atténué par le dispositif de la figure 1.
• La figure 3 représente schématiquement un silencieux actif à deux chambres en série traitant des fréquences différentes.
• La figure 4 est un graphique montrant l'évolution de l'atténuation du son du silencieux schématisé à la figure 3.
• La figure 5 est une troisième version du silencieux à trois chambres successives pour traiter trois zones de fréquences f ₁, f₂; f₃, les chambres ayant des longueurs allant en diminuant I, I₂, 1₃.
• La figure 6 est le graphique représentant l'atténuation du son relative au dispositif schématisé à la figure 5.
• La figure 7 est une vue en perspective d'un bloc amortisseur parallélépipédique avec une coupe médiane représentant la répartition imbriquée de façon alvéolaire des différents conduits et chambres.
• La figure 8 est une coupe transversale verticale d'un élément en forme de plaques longitudinales.
• La figure 9 est une coupe suivant un plan longitudinal, d'un élément transversal susceptible de réaliser le bloc amortisseur de la figure 7 suivant une autre version.
• La figure 10 est une vue en élévation d'un élément transversal, similaire à celui de la figure 9, destiné à constituer un bloc amortisseur cylindrique à section circulaire.
• La figure 11 est une vue éclatée d'un caisson allon- géservant de socle à la machine produisant les pulsations sonores de gaz, et contenant le bloc amortisseur de son de l'invention.
• La figure 12 est une vue de profil du caisson de la figure 11 qui est alors assemblé.

In these drawings:

• Figure 1 is a schematic view of a chamber of an active silencer or reflection.
• Figure 2 is a graph showing how the sound is attenuated by the device in Figure 1.
• FIG. 3 schematically represents an active silencer with two chambers in series processing different frequencies.
• FIG. 4 is a graph showing the evolution of the attenuation of the sound of the silencer shown diagrammatically in FIG. 3.
• FIG. 5 is a third version of the silencer with three successive chambers for treating three zones of frequencies f ₁ , f ₂ ; f ₃ , the chambers having lengths decreasing I, I ₂ , 1 ₃ .
• FIG. 6 is the graph representing the attenuation of the sound relating to the device shown diagrammatically in FIG. 5.
• Figure 7 is a perspective view of a block parallelepiped damper with a median section representing the alveolar nested distribution of the different conduits and chambers.
• Figure 8 is a vertical cross section of an element in the form of longitudinal plates.
• Figure 9 is a section along a longitudinal plane, of a transverse element capable of producing the damper block of Figure 7 in another version.
• Figure 10 is an elevational view of a transverse element, similar to that of Figure 9, intended to constitute a cylindrical damper block with circular section.
• FIG. 11 is an exploded view of an elongated box serving as a base for the machine producing the sound pulsations of gas, and containing the sound-absorbing block of the invention.
• Figure 12 is a side view of the box of Figure 11 which is then assembled.

Description of some embodiments.

Referring to FIG. 7, we recognize the different assemblies composed of a large chamber 1, an average chamber 2 and a small chamber 3, these chambers being connected by conduits 4, 5, 6 and preceded by intake 7 and exhaust 8 ducts.

Chamber 1 has a length II; chamber 2 has a length 1 ₂ ; chamber 3 has a length 1 ₃ .

The sections of these rooms are identical.

The ends of these chambers are connected to conduits 4, 5, 6 by frustoconical parts such as 9 which improve the flow of gases and therefore the efficiency. These rooms are therefore identical.

Still in FIG. 7, other assemblies 1 to 8a have been shown which are staggered relative to the assembly 1 to 8 in order to achieve a rigorous nesting of the different assemblies. We note in particular that the conduit 4 has the same length as the chamber 1 a while the conduit 5a has the same length as the chamber 1 and the conduit 5 has the same length as the chamber 2a, that the conduit 6a has the same length as chamber 2 and that chambers 3a and 3 have the same length. The conduits 7 and 7a, in the same way as the conduits 8 and 8a have different lengths to make up for the offset between the two assemblies.

The other sets are arranged in the same way as the sets 1 to 8, on the one hand and 1 a to 8a, on the other hand.

In a first version, the damping block 10 is parallelepipedic and is composed of planar elements 11 (FIG. 8) which have in hollow on each of their faces a half of each set 1 to 8 and a half of each set 1 a to 8a , these sets of chambers 1, 2, 3, la, 2a, 3a and conduits 4, 5, 6, 7, 8, 5a, 6a, 7a and 8a being cut along a diametrical plane.

These sets cut in hollow on the face 12, as just described above, are also cut in hollow on the face 13 but staggered staggered, as shown in Figure 8, to gain the maximum of space.

The elements 11 also include positioning means (not shown) to each other and assembly which are generally threaded rods with nuts which pass through the notches 14, 15, 16, 17.

There have been shown elements 11 which have advantageously been foundry. One can however imagine similar elements produced by stamping which then gives the chambers 1 to 3, 1 a to 3a and the conduits 4 to 8 and 5a to 8a of polygonal sections, preferably hexagonal like the cells of a nest of bees.

We have seen that the damper block 10 was parallelepiped. One can also achieve a similar result by constituting the block 10 by elements 18 which are placed transversely by performing a slicing of the block 10 this is identified by the joint planes 19 to 30. In this regard, and by measurement of simplification, FIG. 7 brings together the two embodiments, one with elements 11 arranged longitudinally along the joint planes such as 31 and 32, and the other by transverse elements such as 18 with joint planes 19 to 30. In the latter case, longitudinal tie rods are used.

With this last embodiment by transverse elements 18, it is also possible to use elements 181 (FIG. 10) which then have a circular section by staggered distribution of the sets of chambers and conduits as it is clearly represented in this same FIG. 10.

Other embodiments can be imagined without departing from the scope of the invention. This would be, for example, an achievement by elements such as 18 with polygonal, oval section, etc.

Nevertheless, it is convenient to produce a parallelepipedal damping block 10 in particular in the embodiment which will now be described and illustrated in FIGS. 11 and 12.

In this embodiment, the damping block 10 is placed in the central part of an elongated box 33 leaving, upstream, an intake manifold 34 and, downstream, an exhaust manifold 35. The manifold 35 is of elsewhere a chamber having a relatively large length for processing the low frequencies which are not processed in the damping block 10.

The box 33 is closed at both ends by irons U 37 and 38 which serve as feet for the assembly and whose upper wings 39 and 40 contribute to the fixing of a cover 41 which comprises a blocking element 42 of the block 10 The latter is also blocked by an angle 43 fixed to the bottom of the gutter 36. This gutter, decoupled from the box 33 by a viscoelastic product, serves as a double wall to reduce the acoustic radiation.

The admission of gases into the chamber 34 is done through the hole 44 while the exhaust from the chamber 35 is done through the exhaust tube 45 which also penetrates largely in overhang to the inside of the exhaust collecting chamber 35. The gas path is moreover materialized by all the arrows in FIG. 12.

There is a valve connection or an access hole 46 which can be plugged by a plate (not shown) which is fixed by the studs 47. It is also if provided cross members 48, 49, 50 which support the mechanics.

The box 33 is therefore massive and its inertia is considerably increased by the weight of the apparatus which rests thereon. This contributes to the improvement of sound absorption.

Claims (10)

1. A reactive sound attenuator or silencer, i.e. a reflection or sound wave echo attenuator or silencer, for a pulsed flow-rate of gas submitted to mean and high frequencies, namely of about 300 to 8000 Hz, characterized by a system of several identical sets (1, 2, 3, 4, 5, 6, 7, 8) disposed in parallel, connected with at least an inlet collector (34) and/or an exhaust collector (35), each of the sets being composed of a succession of expansion rooms (1, 2, 3) of different dimensions connected by an axial duct (4,5,6), each of the rooms having such dimensions that it is accorded for a field of frequency to be attenuated, whereas the joining ducts (4, 5, 6) have a weak cross-section with respect to the one of the rooms, the system of several sets of rooms (1, 2, 3) connected with ducts (4, 5, 6) being carried out in order that the lenghts of the rooms and ducts are determined so that it can be provided an alveolar imbrication of the elements which are staggered the one with respect to the other in order that the rooms reach the level with the ducts of the adjacent sets and inversely, so as to circumscribe said system in the minimal volume.

2. A silencer according to claim 1, characterized in that the rooms (1, 2, 3) are prismatic and their cross-section depends on the upper limit of frequencies to be attenuated, whereas their length depends on the field of frequencies apt to be attenuated, on the understanding that these last frequencies are underneathh the upper limit of the frequencies to be treated.

3. A silencer according to claim 1, characterized in that the rooms (1, 2, 3) are cylindrical of revolution, with bottoms (9) in the shape of a truncated cone suiting to tubular axial ducts (4, 5, 6).

4. A silencer according to any one of the claims 1 to 3, characterized in that it is carried out by slices (18) perpendicular to the axis of the ducts (4, 5, 6) and said slices are assembled by a known means in order to constitute a sound damper block (10).

5. A silencer according to claim 4, characterized in that the sound damper block (10) is prismatic, generally like a parallelepiped.

6. A silencer according to claim 4, characterized in that the sound damper block (10) is cylindrical.

7. A silencer according to any one of the claims 1 to 3, characterized in that is is carried out by plane identical elements (11) in the form of plates each comprising in hollow on their two faces (12, 13) complementary parts of the sets connected by the ducts (4, 5, 6), said complementary parts being disposed for exactly suiting to the corresponding parts of the adjacent plane elements so as to reconstitute the system of the sets of rooms and ducts owing to relative assembling and positioning means.

8. A silencer according to claim 7, characterized in that the plane elements comprise in hollow on each of their faces (12, 13) a half of each set made up of a section of the rooms (1, 2, 3) and ducts (4, 5, 6) along to a diametrical plane of those, said element (11) comprising assembling means by tie-rods providing the juxtaposition and the tightening for thus constituting a block like a parallelepiped which damps down the sound.

9. A silencer according to claim 7, characterized in that it is provided an elongated box (33), in the central part of which the sound damper block (10) is disposed, by leaving upstream and downstream two rooms (34, 35) serving as collectors having a relatively important length for treating and damping down the low frequencies.

10. A silencer according to claim 9, characterized in that the elongated box (33) serves as stand for the machine producing the sound pulses of gas.

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