SE509855C2 - Resonator - Google Patents

Abstract

The present invention relates to a resonator for attenuation of preferably low-frequency noises. More specifically, the invention relates to a resonator, wherein the stream is split into one primary stream and one secondary stream, the secondary stream being produced by short-circuiting the flow tube of said stream. In this manner the part of the flow pipe delimited by the short-circuit in which said primary stream flows forms a resonance volume in a resonator of Helmholtz type. Thus, the main part of the stream may be made to flow through the resonance volume and this volume may be used for other types of attenuation. <IMAGE>

Description

509 sas 2 10 15 20 25 30 35 thereby by interference attenuate this frequency of the sound in the main tube.

Most known attenuators, however, combine several different attenuation methods to provide attenuators which attenuate sound within as wide a frequency band as possible.

An example of a muffler based on the introduction of flow resistance is given in the publication SE 441 205. The muffler described in this publication uses changes of direction and various transition cross-sections for the flow fl to thereby attenuate the sound. In this muffler, guards are also used as resonant volumes lying next to the head. A disadvantage of this type of damper is, however, that there is a large pressure drop across the damper, which leads to an unnecessarily high energy consumption.

There are many examples of mufflers that use interference. The publications GB 2 222 852 and DK 142 467 give examples of interference attenuators which divide the flow of flux into several parts which have traveled different distances during the reunification and thereby provide attenuation of the sound waves which are in opposite phase due to the path difference. In the latter writing, however, only the acoustic fl fate is divided, while the flow fl fate remains in principle undivided. This is done by providing the muffler with membranes which can transmit sound waves but are essentially impermeable to the flowing medium. A disadvantage of these dampers, however, is that the damping capacity is relatively poor.

Attenuators with expansion volumes are given examples of, among others, DE 716 329, SU 1423 755, FR 2 261414 and DE 3 711 029. These attenuators are based partly on interference by the formation of standing waves in the expansion volumes, and partly on the sound waves being reflected at the sudden cross-sectional changes in the flow tubes. In addition, several of these described dampers are combined with other damping methods such as absorption or velocity barriers. A disadvantage of these dampers, however, is that the expansion volumes take up a lot of space, which makes these dampers relatively large and clumsy.

A resonator provides very good attenuation around the resonant frequency. Examples of resonant attenuators, resonators, are given in the documents DE 760 362 and US 4 892 168. The resonator attenuator in DE 760 362 consists of a large volume lying around a flow tube which 10 15 20 25 30 35 3 509 ess is connected to this flow pipes via one or more of your openings.

A disadvantage of this resonator, however, is that it, like the expansion dampers above, has a large volume, which results in poor space utilization. The flow past the openings into the resonant volume also means that, especially when the flow rate of the flowing medium is high, undesired edge effects and disturbance phenomena easily occur, which lead to the resonator attenuating sound less effectively.

This latter disadvantage has been partially overcome in the resonator described in US 4,892,168. In the muffler described here, there are separate inlet and discharge openings for the resonant volume, through which a smaller flow will take place through this volume. As a result, the sound attenuation of the main flow flowing past the resonant volume will be less disturbed by disturbing effects around the openings. However, the disadvantage that the resonant volume takes up a lot of space at the same time as it cannot be used in any effective way for other attenuation also remains for this resonator.

OBJECT OF THE INVENTION The object of the present invention is to provide a resonator-type silencer which effectively attenuates, in particular, low-frequency sounds.

It is a further object of the present invention to design the resonant volume in such a way that it can be used simultaneously as an expansion chamber and / or absorption chamber.

A further object of the invention is to design the input to the resonant volume in such a way that disturbance phenomena, especially at high flow rates of the gas medium, are avoided.

The purpose of the invention is also to be flexible so that it can be easily adjusted for attenuation of various predetermined frequencies, and that it results in a low pressure drop across the muffler and thus reduces energy consumption.

Summary of the Invention Resonator with a resonant volume for passive sound attenuation of low frequency sound in a fl fate, the fl fate in at least one place being short-circuited, the fl fate being divided into a primary fl fate and a secondary fl fate, where the secondary flow is conducted through the short circuit 15 20 25 30 35 855 the closure delimited the primary flow constitutes the resonant volume, characterized in that the primary fate occurs through a flow tube and that the short circuit consists of a short-circuit tube, which is short in relation to the wavelength of the sound to be attenuated and whose diameter is smaller than the diameter of the flow tube.

Brief Description of the Drawings Figs. 1 is a view of a resonator according to the present invention in cross-section coaxially with the axis of the chamber volume.

Fig. 2 is a view of a resonator according to the present invention in section perpendicular to the axis of the chamber volume.

Fig. 3 is a measurement diagram showing the attenuation of a resonator according to the present invention.

Description of Preferred Embodiments A preferred embodiment will now be discussed in more detail with reference to the accompanying drawings. The resonator consists, as shown in Fig. 1, of a closed cylindrical chamber volume 3, comprising a mantle surface 3a and two circular and slightly outwardly bulging end plates Bb, 3c. Into the chamber volume a flow tube 1 leads in the vicinity of one end plate Sc, and substantially perpendicular to the mantle surface. On the inside of the chamber volume, the flow tube 1 has a 90 degree tube bend 10. After the tube bend 10 is a straight flow neck 4, which leads down in the direction of the second end plate 3b.

The flow neck 4 opens into the chamber volume. Adjacent to this mouth is the mouth of a second flow neck 5, which flow neck is arranged parallel to the first flow neck 4. The second flow neck 5 is connected to a 180 degree pipe bend 11. This pipe bend is in turn connected to a pipe 2 which leads out of the chamber volume 3. Between the two pipe bends 10, 1 1 a short-circuit pipe 6 leads. The short-circuit pipe 6 leads a part of the flow a shorter way through the resonator. In the parts of the chamber volume facing the end plates there is absorption material 7, 8 for attenuating sound with high frequencies. This material can advantageously enclose the pipe bends 10, 11 and the short-circuit pipe 6. In this way, by the pipe bends 10, 1 1 and the short-circuit pipe 6 being perforated with small holes, absorption can take place by 10 15 20 25 30 35 5 509 855 sounds with higher frequencies also in these pipe sections. However, in order not to cause a further short-circuit fate through these perforations, a transverse wall 13 between the pipe bends 10 and 11, and a partition wall 14 perpendicular to the cylinder axis below the perforated parts of the pipes are suitably inserted. This provides separate volumes 15, 16 which prevent leakage currents due to the perforation. In this way, the flow is controlled in a controlled manner and the acoustic properties are easier to control.

The short-circuit tube should preferably be short relative to the wavelength of the sound to be attenuated. Briefly, however, this refers to the acoustic length, which does not coincide with the actual length, since the perforated parts of the pipe can be neglected when measuring the acoustic length. The acoustic length of the short-circuit tube 6 thus substantially coincides with the non-perforated part 6a thereof. The short-circuit pipe should also preferably have a considerably smaller diameter than the other advantages, as it is desirable that a larger part of the går passes the short-circuit pipe than through the same. In the exemplary embodiment, the short-circuit pipe has a diameter of approx. 20 - 40 mm, while the other pipes have a diameter of approx. 100 - 150 mm. In other words, the other pipes have a diameter that is about 4 times as large as the diameter of the short-circuit pipe. However, these values of the dimensions of the different tubes can be changed to optimize the resonator for different frequencies and fl fates. In the exemplary embodiment, the flow necks are approximately 250 mm long, while the chamber volume is approximately twice as long and has a diameter of approximately 400 mm. However, these values are also variable for optimizing the resonator.

When using the resonator, a flow flows into the resonator through the inflow tube 1. At the first tube bend 10, a part of the fl fate, a secondary fl fate, passes through the short-circuit tube 6 directly to the second tube bend 11. The remaining part of the fl fate, primary fl fate, however, it is led further into the chamber volume via the flow neck 4. At the mouth of the flow neck 4, the primary flow flows into the chamber volume 12, the chamber volume 12 acting as an expansion chamber and thereby attenuating some of the sound waves in the flow. The absorption material found on at least one of the walls of the chamber further helps to attenuate the sound, and in this case mainly sounds with a higher frequency. The second flow neck 5 then leads out of the chamber volume. At the subsequent pipe bend 11, the primary lead coincides with the secondary fate incoming via the short-circuit pipe. Since the path of the secondary fl fate through the short-circuit pipe is straighter and shorter than the path through which the primary fl fate flows, the secondary flow is acoustically the main fl fate while the primary fl fate is the main fl fate in terms of flow. Due to the fact that the secondary ak fate is the acoustic head fl fate, the primary fl fate can be regarded as a resonant volume of Helmholz character. The Helmholz resonator is based on a resonant volume located next to the acoustic head-fl, which can be regarded as a spring-mass system and which at certain resonant frequencies is made to oscillate and thereby attenuates the sound waves in the head-..

For an ideal Helmholz resonator, the reduction number, ie n; i (f / fo -fo / fY l where the transmission factor r is: cA = 47gr0A0le the attenuation, R can be written: R = l0l0 l + 2 'and fuc .. A ”zff V16 where the designations in the equations stand for: c = the speed of sound A = the manifold to the area of the resonant volume Ao = the area of the main tube 10 = the manifold to the length lo of the resonant volume plus an end correction IC V = the volume of the chamber f = the frequency fo = the resonant frequency lc can be neglected when the flow rate is high.

It can thus be seen that for a Helmholtz resonator it applies that the reduction is greatest for frequencies around the resonant frequency, while the attenuation for frequencies further away from this frequency decreases rapidly.

This behavior also exhibits the resonator according to the present invention, which can be seen in Fig. 3. In Fig. 3 the attenuation is shown as a function of the frequency, and one can see a clear maximum value for the attenuation at a frequency corresponding to against the resonant frequency of the resonator.

The resonator in the exemplary embodiment is preferably made of something durable, rigid and durable but at the same time inexpensive material, such as sheet metal.

The above-described embodiment of the present invention has the advantages that it can effectively attenuate sound even when the flowing medium has a high flow rate. This is done by a large part of the fl fate being guided through the resonant volume, whereby disturbance effects at the openings to the resonant volume are avoided, and the resonant volume can be used simultaneously for other damping techniques, such as expansion chambers, to accommodate absorbent material or the like. the attenuation over a large frequency range. This is done by the flow tube being short-circuited, the intermediate part of the flow tube constituting the resonant volume of the resonator. Furthermore, the resonator according to the present invention can be easily adapted by changing the dimensions of the flow tubes, short-circuit tubes, chamber volume, etc. to provide effective attenuation at different sound frequencies. The resonator according to the present invention also causes a very low pressure drop, which leads to a lower energy consumption.

Several variants of the above-described embodiment of the present invention are possible. The resonant volume that forms part of the flow tube does not need to be used for other types of attenuation. If this is the case, these other types of attenuation do not have to include absorption and expansion either, but other types of attenuation techniques can be used to attenuate the sound in the current flow that occurs through the resonant volume. Nor, in the case of a resonant volume, the flow necks need to open adjacent to each other, but the orifices can be located anywhere in the resonant volume and at any distance from each other. Furthermore, as already mentioned, the dimensions of the components included in the resonator according to the present invention can of course be changed in order to optimize the resonator for different application areas and for attenuation of sounds with different frequencies. These and other related 509,855 variants of the present invention must be considered to be clearly within the scope of the invention as set forth in the appended claims.

Claims (9)

lO 15 20 25 30 35 9 i soaåsss PATENTKRAV A resonator with a resonant volume for passive sound attenuation of low-frequency sound in a flow, where the fl fate in at least one place is short-circuited, the fl fate being divided into a primary charge and a secondary fl fate, where the secondary flow is conducted through the short circuit and the short circuit fl delimited by the short circuit fl constitutes the resonant volume, characterized in that the primary sker fate occurs through a flow tube (4, 5) and that the short circuit consists of a short circuit tube (6), which is short in relation to the wavelength of the sound to be attenuated and whose diameter is smaller than the diameter of the flow tube. 2. A resonator according to claim 1, characterized in that the primary fate is greater than the secondary fate. 3. A resonator according to claim 1, characterized in that the flow tube (4, 5) within the short-circuited section of the same leads in to and out of a closed chamber volume (12), which volume functions as an expansion chamber. A resonator according to claim 3, characterized in that the chamber volume (12) on at least one of its walls is covered with absorption material (7) for attenuating higher frequency sounds. 5. A resonator according to claim 1, characterized in that it is made of sheet metal. A resonator according to claim 1, characterized in that the flow tube (4, 5) has a diameter at least 3 times larger than the short-circuit tube (6). 7. A resonator according to any one of the preceding claims, characterized in that it is primarily intended for attenuation in internal combustion engine systems. A resonator according to claim 1, characterized in that the short-circuit tube (6) is curved and perforated with holes to make the acoustic length shorter. 9. A resonator according to claim 8, characterized in that the short-circuit tube, at least around the part thereof perforated with holes, is surrounded by absorption material (7) for attenuating higher frequency sound.