IE42550B1 - Acoustic attenuator - Google Patents

Abstract

1536164 Silencing gaseous currents C L GRANDCHAMPS 9 April 1976 [11 April 1975] 14600/76 Heading F1B [Also in Division F4] A silencer, e.g. for insertion into an air conduit of a ventilation or exhaust system, comprises at least three stages A, B, C of deflector vanes within a conduit element 1, the vanes of each stage being disposed obliquely, e.g. at 45 degrees, to the axis Z of conduit element 1 to bring about a 90 degree change in the direction of air flowing between the stages, the downstream edges of each vane in the first or intermediate stage lying in the same plane as, and (except for the outermost vanes 2A, 2A") midway between the upstream edges of two adjacent vanes in the following stage. The length I of each vane may be of the order of 1À5 times the distance D between neighbouring vanes within a stage. The walls of conduit element 1 and the air deflecting faces of the vanes may be lined with lead foil and all surfaces may be covered with acoustic absorbent material such as glass wool which may be impregnated with resin to resist abrasion.

Description

PATENT APPLICATION BY (71) CHRISTIAN LOUIS GRANDCHAMPS, A FRENCH CITIZEN OF 4 RUE DE BRETAGNE, CLAMART, HAUTS-DE-SEINE, FRANCE.

Pnct 12±p - 2 The present invention relates to an acoustic attenuator, particularly for insertion into an air conduit in a ventilation or exhaust system.

In order to reduce the transmission of sound through 5 air conduits, hotably through ventilation conduits, various different types of acoustic attenuator have already been proposed.

One first known type of acoustic attenuator comprises parallel thick walls made from an acoustically absorbent material which subdivide the section of the air conduit into several parallel channels of reduced section. In order lo attenuate sound waves the direction of propagation of which substantially coincides with that of the channels, there is a tendency to give this type of known attenuator a substantial length, which leads to a cumbersome and expensive apparatus.

Furthermore, the acoustic absorbing material which constitutes the thick walls should be protected against abrasion by air flow by means of protective elements such as grilles or perforated sheets of metal applied on the surfaces of the said walls. These protective elements do not only raise the cost price of the unit, but additionally give rise in the ventilation air flow promoted by high speeds to parasitic reflections of sound waves which reduce the acoustic attenuation effect. - 3 Anol.her known type of acoustic attenuator comprises essentially a conduit of substantially constant section but of which the lateral walls, lined with an acoustically absorbing material, form one after the other several sharp elbows, for example at 90°, which force the air to flow along a sinuous path favourable to absorption of the sound waves by the said lateral walls. However these sharp elbows have the inconvenience of giving rise, at least for certain speeds of air flow, fo eddy phenomena which considerably l<> disrupt tiie flow of the air and which it is only possible fo remedy by providing diversions in the air conduit, which would give rise to a substantial loss of power.

Acoustic attenuators are likewise known, particularly for insertion into an air conduit, which each comprise a conduit element with a straight line axis in which are disposed, one after the other in the axial direction of the said conduit element, several stages of deflector vanes, spaced regularly and parallel. In a first variety of this known type of acoustic attenuator, the vanes of two success20 ive stages are in contact and inclined symmetrically relative fo the axial direction of (.he conduit so that there forms, between the vanes, several guiding channels for sound waves, adjacent one anol.her and not communicating, the lateral respective walls of these channels forming sharp elbows which produce the discouragement previously noted. In the second variety, the vanes of two successive stages are inclined at different angles to the axis of the conduit, but as the exit edges of the vanes from each stage are displaced axially relative to the entry edges of the vanes of the following stage, the guiding' channels for the sound waves formed by the said vanes are interrupted at the level of each interval between two successive stages, which reduces efficiency of the acoustic attenuator of this known type. 43530 - 4 The acoustic attenuator according to the present invention, which in particular may be inserted into an air conduit, does not have tlie disadvantages of the acoustic attenuators which have been developed up till now for analogous applic5 ations.

The acoustic attenuator according to the present invention likewise comprises a conduit element with a straight line or continuously curved axis, in which are disposed one after another in the axial direction of the said conduit element, at least three stages of deflector vanes, the vanes in each stage being parallel and regularly spaced. The attenuator is characterised in that the vanes of successive stages are disposed so that they are symmetrically inclined to the said axial direction, preferably at an angle of about +45° or about -45°, and in such a fashion that the exit edges of the vanes of any stage but the outlet stage are substantially in the same plane as the entry edges of tho vanes of the following stage, some of the exit edges being further located substantially midway between the corresponding entry edges of the vanes of the following stage.

Because of the successive stages of vanes which it comprises, the acoustic attenuator according to the present invention has a raised efficiency without being cumbersome and notably with a slightly elevated value of the ratio of the length pf the conduit element to the greatest transverse dimension of this conduit element. These advantageous properties of the acoustic attenuator according to the present invention result notably in the following two effects: on the one hand the air flow entering into the attenuator is divided by the deflector vanes, into several entering streams which each is then divided into two separate streams by the entry edge of the vane of the second stage, which is interposed between the exit edges of the vanes of the first stage, channelling the entering stream under consideration. in the 2 5 5 0 - 5 last stage, the two separate streams flow together again, and whether the attenuator according to the present invention comprises only three stages of vanes or, more generally, an odd number of stages, each stream of air emerging between two vanes of the last stage is substantially constituted by the air arising from a same entry stream. However, during crossing the attenuator comprising for example an odd number 2p + 1 of stages, each air stream has undergone a number p of successive divisions and reunitings which have promoted the dispersion and accordingly the attenuation of sound waves. Each stream of air which has been deflected by a deflector vane of (.he first stage of (.lie attenuator and which has then emerged substantially parallel to (.he said vane i.s then deviated by the corresponding vane of file immediately following stage by an angle substantially equal to 90° if the vanes are inclined at 45° to the axial direction of the element of the conduit. This phenomenon, which is reproduced in each of the vane stages of the attenuator, likewise favours the absorption of sound waves, without the disadvantages resulting, which are noted above, due to the eventual appearance of eddy phenomena at the level of the deflector vanes, as will be explained in more detail below.

Because of the fact that the exit edges of the vanes of a first, stage are located substantially in the middle of Lhe intervals between the entry edges of the vanes of the immediately following stage, each stream of entering air is divided by the corresponding vane of the second stage into two separate streams of practically the same size which has as its effect a regulation of the flowing of the air streams and thus avoids the appearance of eddy phenomena.

It is naturally possible to increase the efficiency of the acoustic attenuator according to the present invention by raising the number of stages of vanes to a value greater than 42S50 3. The arrangement according to the present invention however provides the substantial advantage of permitting this increase in the number of stages of vanes without giving rise to substantial increase of the cumbersomeness of the attenuator,notably the length of its conduit element, by suitably multiplying the number of vanes of each stage, as will be explained in more detail below.

In one preferred embodiment of the acoustic attenuator according to the present invention, the vanes of each stage have a length 1 greater than the shortest distance D between two neighbouring vanes, for example: 1=1.5 D. This arrangement avoids any direct transmission of sound waves across the conduit element.

Further to increase the efficiency of the acoustic attenuator according to the present invention, linings several centimetres thick of an acoustically absorbing material such as glass wool, optionally overlaying linings of lead, for example in the form of sheets of lead of thickness of the order. of 1 mm, are provided on at least certain of the surfaces of the internal walls of the element of the conduit and of the surfaces of the deflector vanes.

In addition, in a preferred embodiment the external surfaces of the linings of acoustically absorbent material are pretreated to resist abrasion, for example by impreg25 nation of the glass wool with a resin. This arrangement ensures efficient protection of the acoustically absorbing linings against abrasion by the particles which are often entrained in ventilation air, this result being obtained in a particularly simple and economic fashion avoiding the use of protective elements such as grilles or perforated metal sheets which have the disadvantages mentioned above.

Merely by way of example, there is described helow and Ί « (i J <) - 7 schematically illustrated In the attached drawings one embodiment of the acoustic attenuator according to the present invention.

Figure 1 shows schematically the embodiment in section and in a vertical plane; Figure 2 is a plan view of this embodiment; Figures 3 and 4 are views of details on a larger scale corresponding respectively to the sections according to line III-III of Figure 2 and according to line IV-IV of Figure 1.

Referring to Figures 1 and 2 a conduit element 1 is shown having, for example, metal walls and a rectangular internal section. The well known elements such as assembly clips, clasping flanges etc. nre not shown, these permitting the conduit element 1 to be inserted in use in an air conduit, for example between two conduit elements of a ventilation installation, in order to attenuate the transmission of noise through the said air conduit which could arise for example from an area ventilated by the installation. On the other hand it has been proposed that the conduit element 1 should be inserted in an air conduit having a vertical axis Z, in such a way that the said conduit element 1 is crossed by the air flow in the vertical ascending direction. This arrangement is however optional, the conduit element 1 of the attenuator according to the present invention being able to be located with any inclination whatever of its axis Z to the vertical.

On the inside of conduit element 1 there are disposed according to the present invention one after another in tho direction of axis Z three stages of deflector vanes A, B and C; each stage such as A comprises a certain number of parallel vanes 2A, 2A’, 2A..., regularly spaced and - 8 42550 inclined at 45° to the direction of axis Z. The respective vanes of two successive stages, for example A and B, B and C, are inclined symmetrically as shown relative to the direction of the axis Z, while, in the embodiment being considered, the directions of the respective vanes of the two successive stages are perpendicular one to another.

On the other hand, the exit edges such as 3A, 3A* of the vanes 2A, 2A’ of a first stage such as A are interposed without contact between the entry edges such as 3B, 3B* of che vanes such as 2B, 2B* of the following stage denoted B. In this preferred embodiment, each entry edge such as 3B of a vane 2B of a stage such as B is located substantially in the centre of the interval between the exit edges 3A and 3A’ of the vanes 2A and 2Α» correspond15 ing to the immediately preceding stage denoted A. Because of this arrangement,- it can be seen that i f one designates the shortest distance between two neighbouring vanes of the same stage D, such as between vanes 2A and 2Α», the exit edge 3A of the vane 2A of a stage such as A is located at a distance dj=D/2 from the corresponding vane 2B of the immediately following stage B while the entry edge 3B of a vane 2B of a stage such as B is likewise located at a distance d2=D/2 from the corresponding vane 2A’, of the immediately preceding stage denoted A. Finally, in this embodiment, each of the vanes such as 2A of each of the stages such as A has a length 1 = 1.5 D. This dimensioning largely suffices fco prevent the direct propagation of sound waves parallel to the direction of the axis Z. One can see in effect that it is necessary for this that the 3θ length 1 of each of the vanes should be at least equal to their shortest distance D.

Et is evident from Figure 1 that in the embodiment in question, each of the air streams such as Fj+F’j , which have - 9 42530 a .substantially vertical, ascending direction at. their entry in the lower edge, of the conduit element A are deflected between two neighbouring vanes at the first stage A by the vanes towards the right (Figure 1) substantially at an angle equal to 45°: if one considers for example the stream of air Fj+F*!, which is channelled by the two vanes 2A, 2A·, when It arrives at the frontier between the stages of vanes A and B, which is represented by the dash dot line f^ (Figure l) the said stream of air is divided by the entry edge 3B of the vane 2B of the following stage B into two fractions and F’^ respectively which are substantially equal, since the distances d* and d^ are equal. Γη the second stage B, the different fractions of Lhe air streams thus separated are again deflected, by about 90° towards the left (on Figure l) and the fract ions such as F’^ and i'jj of two neighbouring air streams which are channelled hy two neighbouring vanes 2B and 2B* from stage B flow together in this stage into a single stream which is again separated into its two constituent fractions, substantially equal, F*^ and F., by the entry edge of vane 2C’ of the third stage C. This latter produces again a deviation of the different fractions of the air· stream of about 9θ° towards the right, but in addition, its vanes produce the reunion of the fractions in which the air streams entered, Fj+F'j, F2+ F*2’ which were separated at the level of the intermediate stage of the vanes B.

The air streams which emerge from the upper end of the conduit element 1 in a substantially vertical ascending direction have thus undergone in all, four successive deflections, the two intermediate deflections at the level of the frontiers fj and f^ between the different stages of veins A and C being substantially of 90°. The acoustic attenuation attained across the conduit element 1 does not only result from the multiple deflections, but also from the division -10of the air streams entering at the level of the intermediate stage B. ft is convenient to remark that the deflections (of about 90°) which theairstreams undergo at their respective entries into stages B and C do not give rise to the risk of eddy formation as in the case where deflections of the same size are produced in the previously known attenuators, which have been mentioned above, by a wall of an elbow in a conduit; in effect, the formation of eddies, which can considerably reduce the desired effect of acoustic attenuation, can only take place if the wall which the air stream comes to impinge on substantially perpendicularly is prolonged by a second wall substantially perpendicular to the first, as is the case in the walls of the '’elbows previously used.

Naturally the effect of acoustic attenuation obtained with the apparatus according to the present invention may be increased again by increasing the number of its vane stages. Et is notable that, this result can be obtained without substantially increasing the size of the acoustic attenuator according to the present invention and notably the length or the height L of its conduit element 1. In effect, the preferential condition according to which the vanes of each stage ought to have a length 1 greater than the shortest distanceD of two neighbouring veins may be satisfied by setting: 1) 1 = D (1 + a), a being a positive number. ft is easy to see that for an angle of inclination of the vanes exactly equal to 45θ, if one designates by B the largest transverse interior dimension of the conduit element - ιι J and by N flic numberof vanes of (lie odd numbered stages, the following relationship is satisfied in these latter stages: 2) E=(N-1 )D\ZT+ l/v£.

From equations 1 and 2 it is easily deduced that: D 3) E = __- (2N + a _i) \J2 On the other hand, if the number of stages of vanes of the acoustic attenuator according to the present invention i.s designated by p_, then: P 4) L~-p. - = -=- D(1 + a) s/z' supporting the sl.ages to be joined as in Figure 1 From equations j) and 4) above, tin· size ratio of the acoustic attenuator may lie deduced: P ) L/E-( 1 +a). --2N + a - 1 Equation 5) shows (hat il is possible Lo provide acoustic attenuators according to the present, invention which have similar size ratios that is to say lengths of about L, for a same transverse dimension E, but which comprise different numbers of stages p: it is sufficient. for this to provide each stage of these different, attenuators with different numbers of vanes N, such that the ratio p/(2N + a -l) has a value about the same for these different attenuators. In other words, the efficiency of the acoustic attenuator according to the present invention may 4355° be multiplied by raising its number of stages p without at the same time raising its space requirements, with a condition all the time of at the same time increasing the number N of vanes of each stage in appropriate fashion.

The slight encumbrance in length or in height, of Lhe acoustic attenuator according to the present invention is particularly advantageous in the case where the substantial lengths which became necessary in at least certain of Lhe acoustic attenuators previously built entrained for this latter substantial losses in power, and even the appearance of noise due to air flow, which was counter to the desired acoustic attenuation effect.

Equation 5) above shows additionally that the amount of space which the acoustic attenuator according to the present invention takes up is however much less when the ; number a is very small j it has already been indicated that one should use for preference 1—1.5 D that is to say according to equation l)l a=|. In this case, the size ratio is given hy the equation 3p 6) L/E= —-4N - 1 i.e. in the case illustrated in Figure 1 that is to say for p=3 and N=3: L/E=9/11=O. 8Ϊ8.

In this preferred embodiment according to the invention which is illustrated in Figures 1 to 4 each of the walls such as 1^ (Figure 3) of. conduit element 1 is covered by a composite lining comprising a sheet of lead 5, for example of 1 mm thickness, which is affixed by any appropriate means to the said wall and which is itself covered by a layer 6 of 2 or 3 cm thickness of an acoustically absorbing material such as glass wool, for example adhered on the sheet of lead 5. 2 5 51) - 13 Preferably the external surface 7 of the layer of glass wool 0 i.s prelreat.ed to resist abrasion by solid particles entrained by the air flow which goes across the acoustic attenuator; this surface pretreatment of the layer of glass S wool consists for example of impregnation thereof with a resin.

On the other hand, each of the vanes such as 2B’ (Figure 4) is constituted by a relatively thin metal sheet 8, which is fixed by any appropriate means to two major lateral walls la and lb (Figure 2) of the conduit element 1, this fixture being realised for example using corner plates each having a branch riveted to one of the lateral walls noted of conduit dementi (particularly in an aperture of the acoustic absorbing lining o), its other branch being foi· example riveted to one of the ends of the sheet 8. file face of the sheet 8 on which the air flow impinges directly, i.e. its face turned downwardly in the case of flic arrangement illustrated in Figure 1, is likewise covered by a sheet of lead 9, for example of 1 mm thick20 ness, fixed by any appropriate means; finally the face of the sheet 8 which is turned upwardly and the free face of the sheet of lead 9 are covered respectively by linings If) and 10, of acoustically absorbing material, notably 3 U glass wool, for example adhered thereto, the exterior surface 11^ and 11^ of these two layers of glass wool 1Ο0 and lOjj being preferably rendered resistant to abrasion by resin impregnation.

The present invention is not limited to the embodiment previously described. In particular the structure of the composite lining provided on at least- certain of the surfaces of the internal wails of the conduit element and the surfaces of the vanes is an optional material; these linings can be provided only on the internal walls of the conduit 42530 - 14 element or again only on the vanes; they may comprise solely sheets of lead or solely an acoustically absorbing material which can be if desired different from glass wool. The best results are however obtained with the composite linings described above. The lining of lead especially, deadens low acoustic frequencies while the lining of glass wool particularly deadens the high acoustic frequencies.

The deadening effect thus obtained has proved its maximum efficiency in a range of measurement extending from 30 Ha to 16,000 Hz.

Without departing from the scope of the present invention as defined in the appendent claims it is likewise possible to give the vanes of different stages (A to C irt Figure 1) inclinations relative to the direction of the axis Z which differ by a relatively small angle from the optimum value, indicated above, which is i 45°.

It is to be understood that the invention includes an air ventilation system comprising one or more acoustic attenuators according to the invention.

Claims (11)

1. An acoustic attenuator particularly for insertion in an air conduit, which comprises a conduit element having a straight line or continuously curved axis, in which are disposed one after another in t.he axial direction of the conduit, element, at. least, three stages of deflector vanes, l.lie vanes in each stage being paral lel and regularly spaced, wherein the vanes of successive stages are disposed so as to lie symmetrically inclined to the axial, direction of the conduit element and so that the exit edges of the vanes of any stage but the outlet stage are substantially in the same plane as tlie entry edges of the vanes of the following stage, some of the exit edges being further located substantially midway between the corresponding entry edges of the vanes of the following stage.

2. An acoustic attenuator according to claim 1 wherein the vanes of successive stages are symmetrically inclined at an angle of substantially 45° to the axial direction of the conduit element.

3. An acoustic attenuator according to claim 1 or 2 wherein the vanes have a length greater than the shortest, distance between two neighbouring vanes in a same stage.

4. · An acoustic attenuator according to claim 3 wherein the vanes arc of length substantially 1.5 times t.he distance between neighbouring vanes in a same stage,

5. An acoustic attenuator according to any one of claims 1 to 4, wherein lead linings are provided on at least some of the surfaces of the internal walls of the conduit element and t.he surfaces of the vanes. - ιό

6. An acoustic attenuator according to claim 5, wherein the 1inings consist of lead sheets of thickness substantially 1 mm.

7. An acoustic attenuator according to any one of claims 5 1 to 6, wherein thick layers of acoustic absorbent material are provided on at least some of the surfaces of the internal walls of the conduit element and of the surfaces of the vanes.

8. An acoustic attenuator according to claim 7, wherein 10 the acoustic absorbent material is glass wool

9. - An acoustic attenuator according to claim 7 or 8 wherein the external surfaces of the linings of an acoustic absorbent material are pretreated to resist abrasion by impregnation with a resin. 15

10. An acoustic attenuator substantially as hereinbefore described with reference to the accompanying drawings.

11. An air ventilation system comprising one or more acoustic attenuators according to any preceding claim.