Technical field

The invention relates to silencers such as micro-perforated panels for attenuating noise, for example in devices that generate an air flow.

Background

Many devices produce noise as a by-product of their intended function, including devices that generate an airflow through ducts and housings, for example consumer devices such as domestic appliances.

Various noise mitigation techniques are currently employed in such devices. A common technique is to incorporate one or more sound absorbing bodies made from porous and/or dissipative materials, for example polyester fibres or foam. Increasing the sound transmission loss (STL) that is achieved within a device using sound absorbing bodies generally involves increasing the total volume of the noise absorption material. The ability to do so is constrained by the structure of the device, and it is often impractical to change the device structure significantly to accommodate a sound absorbing body. Adding a sound absorbing body may also provide an unwanted thermal insulating effect, thus complicating thermal management of the device.

Silencers in the form of micro-perforated panels (MPPs) are an alternative means of noise absorption. MPPs make use of an array of small perforations, or ‘microperforations’ in a solid wall to act as dissipative elements that attenuate noise. The perforations may be of the order of 0.5mm to 1 mm in diameter, for example, and typically collectively represent between 1% and 5% of the surface area of the panel, this proportion of cover being referred to as the ‘perforation ratio’.

Figures 1 and 2 show an example of a particular known type of MPP, specifically a cylindrical micro-perforated panel (CMPP) 10. The known CMPP 10 has an inner tube and right 18 ends. The inner tube 12 is therefore open, and so can serve as a flow duct, for example.

A central portion of the inner tube 12 is encircled by an outer tube 20 of larger diameter, with respective axial midpoints of the inner 12 and outer 20 tubes being aligned. Accordingly, end portions of the inner tube 22, 24 of equal length protrude from each end of the outer tube.

Radially-extending end faces 26, 28 connect each end of the outer tube 20 to the inner tube 12, to support the outer tube 20 relative to the inner tube 12 and thereby hold the inner 12 and outer 20 tubes in concentric relation. A tubular annulus is therefore defined between the inner and outer tubes 12, 20, the annulus being closed by the end faces 26, 28 to create a substantially sealed annular cavity.

The portion of the inner tube 12 contained within the annulus is shown in Figure 2, which reveals that this portion is perforated with an array of micro-perforations 30 that are arranged in a series of circumferentially-extending rows that are evenly spaced axially along the inner tube 12. Each perforation 30 is a circular through-hole of approximately 0.7mm diameter in this example.

Meanwhile, the end portions of the inner tube 22, 24 that are outside the annulus do not include such perforations and therefore have uniform, uninterrupted surfaces. Likewise, the outer tube 20 and the radially-extending end faces 26, 28 also have uniform, uninterrupted surfaces.

The perforations 30 of the inner tube 12 act as dissipative elements, as noted above. The perforations 30 also cooperate with the sealed volume (not shown) defined by the annulus to create a noise-damping resonator in a manner analogous to a Helmholtz resonator, in which the annulus represents a resonance cavity and the perforations 30 collectively define a neck of the resonator. The CMPP 10 therefore provides both reactive and dissipative noise attenuation.

The perforations 30 are sufficiently small to resist significant fluid exchange between the inner tube 12 and the annulus and thereby avoid flow separation in the inner tube 12, whilst being sufficiently large to link the inner tube 12 with the annulus to an extent that allows the perforations 30 and the annulus to form a resonator.

The CMPP 10 acts to attenuate noise entering the inner tube 12 through its open ends 16, 18. For example, such noise may be carried by a flow of air through the inner tube 12. In other arrangements, the inner tube 12 may be closed at one end and so acts to attenuate noise entering through the open end without an air flow.

The noise damping achieved by the CMPP 10 is concentrated in a certain frequency band that is determined by the physical characteristics of the CMPP 10, including the size, shape, number and distribution of the perforations and the dimensions of the annulus, particularly the radial depth of the annulus. These characteristics can therefore be adjusted to tune the noise-attenuating response of the CMPP 10 to target frequencies of interest, noting that a device typically generates noise predominantly at particular frequencies. In this way, reactive silencing can be applied that attenuates noise effectively in a target frequency band.

However, analogously to increasing the mass of traditional sound absorbing bodies, enhancing the noise attenuation provided by a CMPP typically entails increasing its size, which may be impractical.

In addition, the CMPP 10 of Figures 1 and 2 is typically only capable of attenuating noise effectively within a relatively narrow bandwidth, especially at medium and high frequencies, which may limit the applications in which the CMPP 10 can be useful. In this respect, many consumer appliances are configured to operate at relatively high power, which tends to produce noise at correspondingly high frequencies.

It is against this background that the present invention has been devised.

Summary of the invention

An aspect of the invention provides a noise attenuator, comprising: an inner element that comprises an axially-extending inner wall that encloses a hollow interior, at least a portion of the inner wall being perforated; an outer wall extending around the inner wall to enclose the perforated portion of the inner wall and to define a cavity surrounding the perforated portion of the inner wall; and a set of partition walls extending within the cavity from the inner wall to the outer wall to partition the cavity into sub-cavities. The set of partition walls comprises at least one circumferential partition wall that extends circumferentially around the inner wall.

Partitioning the cavity into sub-cavities changes the noise attenuation performance of the attenuator relative to the known CMPP described above, which has an unpartitioned cavity. For example, the or each circumferential partition wall may create sub-cavities that are in axial series, which may emulate the performance of a series of attenuators connected end-to-end.

More generally, the geometry of the sub-cavities differs from that of the overall cavity defined by the space between the inner element and the outer wall, which inherently produces different noise attenuation performance for a given overall package. This potentially enables the attenuator to be substituted for the known CMPP to provide improved noise attenuation performance in a particular application.

The partitioning of the cavity of the attenuator also offers improved noise attenuation performance without having to alter the characteristics of the inner element. So, if the inner element is a tube defining a flow duct, for example, beneficially enhanced noise attenuation can be achieved without disrupting flow through the inner element.

Partitioning the cavity may allow different frequencies to be attenuated, for example, and in particular higher frequencies than can be targeted using the known CMPP of equivalent dimensions. Alternatively, or in addition, the absolute noise reduction that is achieved may be improved in embodiments of the invention relative to the known CMPP.

The set of partition walls may comprise multiple circumferential partition walls. At least two circumferential partition walls of the set may extend in respective parallel planes. The circumferential partition walls may be regularly spaced axially along the inner wall.

At least one circumferential partition wall of the set may extend substantially orthogonally to a central axis of the inner element. At least one circumferential partition wall of the set may be inclined relative to a radial plane intersecting a central axis of the inner element.

At least one circumferential partition wall of the set may be annular.

The sub-cavities may be mutually isolated.

The set of partition walls may comprise at least one axial partition wall that extends axially along the inner wall. The at least one axial partition wall may extend parallel to a central axis of the inner element, and optionally in a plane that contains the central axis. The noise attenuator may comprise multiple axial partition walls that are angularly spaced around the inner wall. The axial partition walls may be regularly spaced around a central axis of the inner element. Axial and circumferential partition walls may intersect, for example to create a two-dimensional array of sub-cavities that extends axially along and circumferentially around the inner element.

The or each partition wall may extend orthogonally to the inner wall.

Each sub-cavity may extend continuously from the inner wall to the outer wall. The subcavities may all be similar in size and shape, or they may have a range of sizes and/or shapes.

The noise attenuator may comprise a plane of symmetry that is orthogonal to a central axis of the inner element. The plane of symmetry may intersect an axial midpoint of the inner element.

Optionally, the outer wall connects to the inner wall through axially-spaced end walls that extend transversely between the inner wall and the outer wall, in which case the outer wall and the end walls may collectively define an impermeable outer shell that encloses the cavity. Alternatively, the outer wall may connect directly to the inner wall to define an impermeable outer shell that encloses the cavity.

The inner wall may be tubular. The outer wall, and optionally also the cavity, may have circular symmetry around a central axis of the inner element.

The inner element may be arranged concentrically with the outer wall.

The inner element may be open at each axial end, for example to act as a flow duct. Alternatively, the inner element may comprise an end wall that closes an axial end of the inner element.

The cavity and perforations of the perforated portion of the inner wall may together define a noise-damping resonator.

The perforated portion of the inner wall may comprise no perforations having a width of more than 1 mm and/or no perforations having a width of less than 0.5mm.

In some embodiments, the perforated portion of the inner wall has a perforation ratio of between 1 % and 5%.

A transverse width of the outer wall may vary axially. For example, the width of the outer wall may vary non-linearly. At least one portion of the outer wall may be tubular, and the outer wall may comprise a non-tubular portion disposed between axially-spaced tubular portions. The outer wall may comprise tubular portions of varying diameters, for example to confer a square-wave shape to at least a portion of the outer wall. Adjacent tubular portions of the outer wall may be connected by radially-extending planar portions of the outer wall. The outer wall may comprise a curved portion that curves in a longitudinal plane, the curved portion extending between tubular or frustoconical portions of the outer wall. For example, the curved portion may define a recess, groove or protrusion that extends circumferentially around the outer wall. At least one portion of the outer wall may be frustoconical.

Optionally, at least a portion of the outer wall has an undulating or square-wave profile in a longitudinal plane. An undulating profile may be generally sinusoidal, for example, or an irregular wave-shape. By configuring the outer wall with a non-uniform transverse width that varies along the axial length of the inner element, the shape of the overall cavity formed between the inner element and the outer wall can be altered relative to the tubular cavity of the known CMPP described above. In turn, altering the shape of the cavity changes the noiseattenuation performance of the attenuator for a given overall package, potentially enabling the attenuator to be substituted for the known CMPP to provide improved noise attenuation performance in a particular application. Accordingly, varying the width of the outer wall may provide a further enhancement in noise attenuation beyond that provided by partitioning the cavity.

It is noted that the transverse width of the outer wall refers to its width in a radial direction with respect to a central longitudinal axis of the inner element. The variation in the width of the outer wall may be modest such that the outer wall is close to being tubular, but not perfectly tubular as in the known CMPP. For example, the outer wall may include a single recess but otherwise be tubular. Conversely, in embodiments with a varying outer wall width, the width of the outer wall may vary by at least 5%.

Alternatively, the width of the outer wall may be uniform axially, and so the outer wall may comprise a tube for example.

The invention also extends to a device comprising the noise attenuator of the above aspect.

It will be appreciated that preferred and/or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.

Brief description of the drawings

Reference has already been made to Figures 1 and 2, which show a known CMPP. One or more embodiments of the invention will now be described, by way of example only, with reference to the remaining accompanying drawings, in which like features are assigned like numerals, and in which: Figure 3 shows a device including a silencer;

Figure 4 shows an exploded view of a rear section of the device of Figure 3, revealing the silencer;

Figure 5 shows the silencer of Figure 4 in isolation in perspective view;

Figure 6 shows the silencer of Figure 5 in axial cross section;

Figure 7 shows a side view of an alternative silencer that could be used in the device of Figure 3;

Figure 8 is a graph comparing the performance of the silencers of Figures 1 , 5 and 7;

Figure 9 shows a perspective view of another alternative silencer that could be used in the device of Figure 3;

Figures 10 to 12 show variants of the silencer of Figure 9; and

Figure 13 is a graph comparing the performance of the silencers of Figures 1 , 9, 10 and 11.

Detailed description

In general terms, embodiments of the invention provide noise attenuators, or ‘silencers’, comprising improved CMPPs relative to the example shown in Figures 1 and 2. Generally, embodiments of the invention provide CMPPs that have a similar basic structure to the known CMPP 10 described above, in that the silencers comprise a double walled arrangement that creates a substantially sealed cavity encircling a perforated inner tube. Accordingly, in a similar way to the known CMPP 10, in CMPPs according to the invention perforations act as dissipative elements and also cooperate with a surrounding cavity to form a noise-damping resonator, so that both dissipative and reactive attenuation is applied to noise entering the inner tube.

However, relative to the known CMPP 10, in embodiments of the invention the nature of the cavity is altered to improve noise attenuation performance. Improving performance may entail any one or more of: extending the bandwidth over which noise is effectively attenuated; shifting that bandwidth to higher frequencies; and increasing the average and/or maximum STL that is achieved.

Effective noise attenuation may entail achieving an STL of at least 5dB, for example, and preferably at least 10dB, although the magnitude of the attenuation required for the silencer to be considered effective will vary for different applications. The bandwidth over which a silencer effectively attenuates noise is therefore defined by a range of frequencies for which an STL of at least 5dB is achieved.

The improvement in performance can be achieved without increasing the overall dimensions of the CMPP. Equally, embodiments of the invention provide silencers that can achieve the same noise attenuation as the known CMPP 10, but with a smaller package. Accordingly, the silencers provide clear benefits in contexts where packaging space is limited.

In one approach for improving performance, altering the nature of the cavity entails replacing the outer tube of the known CMPP 10 with an outer wall having a non-uniform diameter along its length, such that the cavity formed around the inner tube is not entirely tubular, but is instead a different, typically more complex shape. Any shape other than a simple tube as in the known CMPP 10 may be used. For example, the outer wall may have a curved, notched, recessed, wave-shaped or zig-zag profile. Altering the shape of the cavity in this way has been found to increase the STL that can be achieved without increasing the overall dimensions of the CMPP. For example, the shape of the outer wall may be configured to provide a different impedance to acoustic waves relative to the tubular outer wall of the known CMPP 10. The specific shape that is selected can be tuned according to the requirements of the application, for example to target particular frequencies. Reshaping the cavity therefore offers a way to achieve a required level of noise attenuation in a particular application, without increasing packaging requirements for the CMPP.

An irregularly shaped cavity can also be tuned to provide noise attenuation across a wider range of frequencies and/or to target higher frequencies, which again extends the applications in which the CMPP may be used for a given packaging space.

A second approach for improving the performance of the known CMPP 10 is to partition, or segment, the cavity. For example, ribs or fins may extend radially from the inner tube within the cavity to divide the space within the cavity into mutually-isolated volumes. A segmented cavity may alternatively be regarded as a series of CMPPs, in which each segment of the cavity defines a respective CMPP. Again, this approach improves the performance of the CMPP without altering its overall dimensions, and externally a CMPP with a segmented cavity may appear identical to the known CMPP of Figure 1.

Partitioning the cavity can provide similar performance improvements to changing the shape of the cavity, including increasing the STL that can be achieved, widening the bandwidth in which noise is effectively attenuated and enabling higher frequencies to be targeted for noise attenuation. This approach therefore also enables the CMPP to be used in high-power devices that produce noise at frequencies above the level at which the known CMPP 10 can effectively attenuate.

The two approaches can be combined for further optimisation of performance, by configuring a CMPP with a cavity that is both segmented and irregularly shaped.

Both approaches improve the performance of the silencer without altering the inner tube, which is particularly beneficial where the inner tube acts as a flow duct as there will be no impact on the flow.

The above approaches may also produce a CMPP that is more robust than the known CMPP 10 of Figure 1 , particularly where ribs or fins are used to segment the cavity as they have the secondary benefit of reinforcing the outer wall. Accordingly, CMPPs of embodiments of the invention are less vulnerable to damage from impact or vibration and so may be suitable for use in applications for which the known CMPP 10 may be too fragile.

Specific examples of implementations of the above techniques are described in more detail later. First, to illustrate the context for the invention, Figure 3 shows a device 110 in which silencers according to the invention may be implemented.

In this example, the device 110 is an environmental care device, specifically a dust separation device. It is emphasised, however, that silencers according to the invention may be used in a wide range of devices and contexts in which noise attenuation is desired, and so the device of Figure 3 is used purely as an example.

The device 110 forms a part of a vacuum cleaner (not shown), and includes a main motor (not shown) of the vacuum cleaner that is arranged to generate suction to draw in a flow of air, along with filtration apparatus (not shown) for filtering and cleaning the air flow. The device 110 may be docked with a main body of the vacuum cleaner in one operating mode, and may be detached from the main body and connected to attachments such as a ‘wand’ in another operating mode.

The device includes a housing assembly having a main casing 112 that accommodates the motor and a portion of the filtration apparatus, a filter assembly 114 at the rear of the device, which is shown at the top in Figure 3, and a handle 116 for manipulation by the user.

The filter assembly 114 provides a post-motor filter stage for the device, and includes vents 118 collectively defining an outlet of the device through which filtered air is discharged, in use. As shown in the exploded view of Figure 4, the filter assembly 114 is detachable from the main casing 112. Figure 4 also reveals a silencer 120 that is installed at an interface between the filter assembly 114 and the main casing 112 of the device. The silencer 120 is therefore positioned to attenuate noise in the air flow immediately upstream of the outlet 118 of the device 110.

The silencer 120 is shown in isolation in Figures 5 and 6, which are now referred to collectively. The silencer 120 has an inner element defined by an inner tube 122 of circular crosssection, which extends along a central axis 124 between open left 126 and right 128 ends. The inner tube 122 therefore defines a wall of the inner element that partially encloses a hollow cylindrical interior of the inner element, and represents an inner wall of the silencer.

The silencer 120 is arranged to receive an airflow in this example, specifically a filtered airflow that is about to be discharged through the outlet 118 of the device 110. Accordingly, the inner tube 122 acts as a flow passage. In the orientation shown in Figures 5 and 6, the open left end 126 of the inner tube 122 defines an inlet of the flow passage, and the open right end 128 of the inner tube 122 defines an outlet of the flow passage, although this particular silencer is entirely reversible.

It is noted that in other embodiments the inner element may include an end wall that closes the left end 126 or the right end 128 of the inner tube, if the inner element is not to act as a flow duct as in the present example.

Figure 6 reveals that the inner tube 122 is perforated with an array of perforations, or ‘micro-perforations’ 130 that are arranged in a series of circumferential rows that are evenly spaced axially along the inner tube 122. The perforations 130 are regularly spaced both axially and circumferentially, with spaces between adjacent perforations 130 being shorter in the axial direction than in the circumferential direction in this example.

Each perforation 130 is defined by a hole of circular cross-section that extends through the full wall thickness of the inner tube 122, and which has a diameter of 0.7mm in this example. The spacing, size and number of perforations 130 is selected to provide a perforation ratio between 1% and 5% in this example.

The inner tube 122 is therefore broadly similar to that of the known CMPP 10 of Figure 1 , although the inner tube 122 of Figures 5 and 6 does not include end regions without perforations, and is instead perforated along its entire length. Such end regions could optionally be added, however, depending on the connections that need to be made in practice.

Planar, annular end walls 132, 134 extend radially outwardly from each end of the inner tube 126, 128, the end walls 132, 134 being identical to each other. Each end wall 132, 134 is spaced slightly inboard of the respective end of the inner tube 126, 128, so that a short portion of the inner tube 122 protrudes beyond each end wall. It is noted that these short protruding portions of the inner tube 122 do not include perforations 130.

A continuous outer wall 136 extends between and connects the radially outer ends of the end walls 132, 134. The outer wall 136 and the end walls 132, 134 collectively enclose a cavity 138 that extends around the inner tube 122, the cavity 138 being sealed aside from the perforations 130 in the inner tube 122. In this respect, the outer wall 136 and the end walls 132, 134 do not include perforations and as such have uniform, uninterrupted surfaces.

The outer wall 136 and the end walls 132, 134 therefore collectively define an impermeable outer shell that extends around the central axis 124 to enclose the inner tube 122 and the cavity 138. Accordingly, the outer wall 136 defines an axially-extending portion of the outer shell, whereas the end walls 132, 134 define transversely-extending end portions of the outer shell. In other embodiments, the outer shell may not have distinct end walls and may instead be defined by an outer wall that is shaped to connect to the inner tube 122 directly.

In this example, the outer wall 136 has circular symmetry around the central axis 124. It follows that the outer wall 136 is arranged concentrically with the inner tube 122. The outer wall 136 is also symmetrical about a plane that is orthogonal to the central axis and intersects an axial centre of the inner tube 122. Accordingly, respective axial midpoints of the inner tube 122 and the outer wall 136 are aligned.

In contrast with the outer tube 20 of the known CMPP of Figure 1 , the outer wall 136 of the silencer of Figures 5 and 6 has a diameter that varies along the central axis 124. In this respect, the outer wall 136 comprises a pair of tubular end portions 140, 142 of constant diameter that are connected by a recessed central portion 144. The central portion 144 has a generally semi-circular profile in axial cross-section, and curves inwardly to reach a minimum diameter at a point coinciding with the axial midpoint of the outer wall 136. The central portion 144 therefore creates a circumferential central groove in an exterior surface of the silencer 120.

The end portions 140, 142 and the recessed portion 144 have similar axial lengths, and form a continuous wall 136 having a generally wave-shaped profile. It follows that the cavity 138 enclosed by the outer wall 136 is not tubular, but irregular in shape. Due to the circular symmetry of the outer wall 136, the cavity 138 is annular in this example. The inner boundary of the cavity 138 is defined by the inner tube 122 and thus the cavity 138 has a uniform inner diameter along the central axis 124.

In this example, the inner tube 122, the outer wall 136 and the end walls 132, 134 are all rigid and have similar wall thicknesses, although in other embodiments this may not be the case. The inner tube 122, the outer wall 136 and the end walls 132, 134 are formed from the same material in this example, although dissimilar materials may be used. For example, the silencer 120 may be formed from plastics materials or a range of other materials using an additive manufacturing process such as ‘3D printing’. Alternatively, the silencer 120 may be assembled from multiple components, for example moulded or extruded components. The silencer 120 may also be formed from sheet metal, for example sheet aluminium, in which case the perforations 130 may be formed by an etching process.

The silencer 120 acts to attenuate noise carried by air flowing through the inner tube 122, such noise having been generated by the motor of the device 110 for example. As noted above, in other arrangements a silencer may be provided with an inner tube having a closed end, in which case the silencer acts to attenuate noise entering through the open end without an air flow.

As for the known CMPP 10 of Figures 1 and 2, in the silencer of Figures 5 and 6 the perforations 130 and the cavity 138 cooperate to form a noise-damping resonator. In this respect, the cavity 138 represents a resonance cavity and the perforations 130 collectively define a neck of the resonator. The perforations 130 are sufficiently small to resist significant fluid exchange between the inner tube 122 and the cavity 138 and thereby avoid flow separation in the inner tube 122, whilst being sufficiently large to link the inner tube 122 with the cavity 138 to an extent that allows the perforations 130 and the cavity 138 to form a noise-damping resonator.

Meanwhile, the perforations 130 of the inner tube 122 also act as dissipative elements. Accordingly, as for the known CMPP 10, the silencer 120 provides both reactive and dissipative noise attenuation.

However, the noise attenuation performance of the silencer 120 is improved by the irregular shape of the cavity 138 compared with the tubular annulus of the known CMPP 10. The acoustic performance of the silencer is dependent on the specific geometry of the cavity, including the variation in the depth of the cavity, the sharpness of bends and corners and the overall volume. Accordingly, the shape can be varied to tune the noise attenuating response of the silencer to suit the requirements of each application.

In this respect, Figure 7 shows a variant 150 of the silencer of Figures 5 and 6, in which the cavity (not shown) has a different shape that is tuned for different noise attenuation characteristics. Specifically, the profile of the outer wall 152 in axial cross-section has the general form of a square wave, thereby forming a cavity that has a series of alternating narrow and wide tubular sections along the central axis 124.

The silencer of Figure 7 also includes unperforated end portions 156, 158 of the inner tube 122 protruding axially beyond the extremities of the outer wall 152 at each end of the silencer 150.

The noise attenuation that is achieved by the silencers of Figures 5 and 7 relative to the known CMPP of Figure 1 is represented in Figure 8.

As Figure 8 shows, the STL achieved by the known CMPP 10 is concentrated in a frequency band extending from 500Hz to approximately 1300Hz, with a peak STL of approximately 17dB at 900Hz.

The silencer of Figure 5 with the wave-shaped outer wall 136 offers significantly improved performance, with a significantly greater STL achieved at all frequencies compared with the known CMPP 10 and an STL of at least 10dB through a range extending from 500Hz to 1400Hz. The silencer of Figure 5 therefore offers wideband noise attenuation and is superior to the known CMPP 10 for any application.

The silencer of Figure 7, meanwhile, offers similar absolute STL values to the known CMPP 10, but at higher frequencies. The silencer of Figure 7 also achieves an isolated peak STL of 20dB in a narrow band around 1700Hz, which far exceeds the STL achieved by the known CMPP 10 or the silencer of Figure 5 at that frequency. The square wave shape of the silencer of Figure 7 may therefore be useful in applications having noise peaks at particular high frequencies that can be targeted.

Figure 9 shows a partial view of an alternative silencer 210 that could be used in the device of Figure 3.

As for the silencer of Figure 5, the silencer 210 shown in Figure 9 also has an inner element defined by an inner tube 222 of circular cross-section that extends along a central axis 224 between open left 226 and right 228 ends. The inner tube 222 therefore defines a wall of the inner element that partially encloses a hollow cylindrical interior of the inner element, and represents an inner wall of the silencer 210.

The silencer 210 is also arranged to receive an airflow in this example, specifically a filtered airflow that is about to be discharged through the outlet 118 of the device 110. Accordingly, the inner tube 222 acts as a flow passage. In the orientation shown in Figure 9, the open left end 226 of the inner tube 222 defines an inlet of the flow passage, and the open right end 228 of the inner tube 222 defines an outlet of the flow passage, although this particular silencer is entirely reversible.

The inner tube 222 is perforated with an array of micro-perforations 230 that are arranged in a series of circumferential rows that are evenly spaced axially along the inner tube 222. The perforations 230 are regularly spaced both axially and circumferentially, with spaces between adjacent perforations 230 being shorter in the axial direction than in the circumferential direction in this example. Each perforation 230 is defined by a hole of circular cross-section that extends through the full wall thickness of the inner tube 222, and which has a diameter of 0.7mm in this example. The spacing, size and number of perforations 230 is selected to provide a perforation ratio between 1% and 5% in this example.

Planar, annular end walls 232, 234 extend radially outwardly from each end of the inner tube 222, the end walls 232, 234 being identical to each other. Each end wall 232, 234 is spaced slightly inboard of the respective end of the inner tube 226, 228, so that a short portion of the inner tube 222 protrudes beyond each end wall. This extended portion of the inner tube 222 is not perforated.

The outer wall of this silencer is not shown in Figure 9, although it is broadly similar to that of the known CMPP of Figure 1. Accordingly, the outer wall has a constant diameter along the central axis 224 and so defines an outer tube. The outer tube extends continuously between and connects the radial outer ends of the end walls 232, 234, and is therefore arranged concentrically with the inner tube 222. Accordingly, respective axial midpoints of the inner tube 222 and the outer wall are aligned. A tubular annulus is therefore defined between the inner 222 and outer tubes, the annulus being closed by the end faces to create a substantially sealed annular cavity 238.

In the silencer of Figure 9, the annular cavity 238 is partitioned by a set of partition walls extending within the cavity 238, each partition wall acting as a partition that divides the cavity into smaller sub-cavities 244 that are separated from each other by the partition walls and, in this example, sealed from each other and therefore mutually- isolated.

The partition walls include three circumferential partition walls 240 that are evenly spaced in longitudinal series along the central axis 224, between the two end walls 232, 234. The circumferential partition walls 240 extend in respective axially-spaced radial planes that are parallel to the end walls 232, 234. The dimensions of the circumferential partition walls 240 are identical to those of the end walls 232, 234 in this example, and so the circumferential partition walls 240 connect to the outer tube and thus partition the annular cavity 238 between the inner tube 222 and the outer tube. The partition walls also include axial partition walls 242 that extend axially along the inner tube 222 between the two end walls 232, 234, parallel to the central axis 224 and therefore orthogonally to the circumferential partition walls 240. The axial partition walls 242 are evenly spaced circumferentially around the inner tube 222, in this example at 90° intervals. It is noted that the axial partition walls may be unevenly spaced around the inner tube in other embodiments. The axial partition walls 242 have the same radial depth as the circumferential partition walls 240, and therefore extend to the outer tube to partition the annular cavity 238.

The axial and circumferential partition walls 240, 242 intersect each other perpendicularly, so that the partition walls 240, 242 and the end walls 232, 234 collectively enclose an array of sub-cavities 244 of substantially equal volume. The sub-cavities 244 are sealed aside from the perforations 230 in the inner tube 222. In this respect, the outer tube, the partition walls 240, 242, and the end walls 232, 234 do not include perforations and as such have uniform, uninterrupted surfaces. As for the example of Figure 5, the outer tube and the end walls 232, 234 together form an impermeable outer shell that encloses the inner tube 122 and the sub-cavities 244.

In this example, the array of sub-cavities 244 comprises four circumferential rows and four axial rows, giving sixteen sub-cavities 244 in total. In this way, the cavity 238 between the inner 222 and outer tubes is partitioned or segmented into smaller mutually-isolated cavities 244. It is noted that the number and geometry of the subcavities 244 may be altered to suit the requirements of each application. So, for example, the sub-cavities may not be of uniform size and/or shape. Also, in some embodiments some or all of the partition walls may be configured to create gaps that connect the sub-cavities to each other.

Similarly to the silencer of Figure 5, the components of the silencer of Figure 9 are all formed from the same material. Alternatively, the silencer 220 may be assembled from multiple components, for example moulded or extruded components. The silencer 220 may also be formed from additive manufacturing processes as noted above. The silencer of Figure 9 attenuates noise carried by air flowing through the inner tube 222 in a similar manner to the known CMPP 10 of Figure 1 , namely by providing both reactive and dissipative noise attenuation through cooperation between the perforations of the inner tube 222 and the cavity 238 enclosed by the outer tube, as is explained above. In the example given in Figure 9, the noise attenuation performance of the silencer 220 is enhanced by partitioning or segmenting the cavity 238, to provide multiple relatively small resonance cavities 244 instead of the single cavity of the known CMPP 10.

As noted above with reference to the irregularly shaped cavity of Figure 5, the acoustic performance of the silencer is dependent on the specific geometry of the cavity. Altering the shape of the cavity relative to the known CMPP 10 as in the Figure 5 example changes the geometry of the cavity, which provides a corresponding change in the acoustic performance of the silencer. In a similar way, segmenting the cavity as in the example shown in Figure 9 also effectively alters the shape and geometry of the cavity, whilst also effectively creating a greater number of cavities. This, in turn, alters the acoustic performance of the silencer. Accordingly, the number, position and orientation of the partition walls, and therefore the number, size and arrangement of the smaller cavities into which the space between the inner and outer tubes is divided, can be varied to tune the noise attenuating response of the silencer to suit the requirements of each application.

To illustrate this principle further, Figures 10, 11, and 12 all show partial views of variants of the silencer of Figure 9, in which the silencers have partition walls in different configurations that tune the silencer for different noise attenuation characteristics. Similarly to Figure 9, the outer wall is excluded from Figures 10, 11, and 12 for clarity.

Specifically, the silencer 320 of Figure 10 corresponds to the silencer of Figure 9 but has the axial partition walls removed. Accordingly, the silencer 320 of Figure 10 has circumferential partition walls 340 similar to those of Figure 9, which extend circumferentially around the inner tube 322 and are evenly spaced axially along the inner tube 322 between the two end walls 332, 334. These partition walls 340 are annular and planar and are parallel with each other, and perpendicular to the central axis 324 of the inner tube 322.

The silencer 420 of Figure 11 corresponds to the silencer of Figure 9 but has the circumferential partition walls removed. Accordingly, the silencer 420 of Figure 11 has axial partition walls 442 similar to those of Figure 9, which therefore extend axially along the inner tube 422 between the two end walls 432, (right end wall not shown), and are evenly spaced circumferentially around the inner tube 422. These partition walls 442 are planar and their lengths are parallel with the central axis 424 of the inner tube 422.

Figure 12 shows further ways in which the configuration of partition walls in a silencer may be varied. This includes a first example 520, shown to the left in Figure 12, with partition walls 540, 542 similar to the example of Figure 9, but with different dimensions. A second example, shown to the right in Figure 12, shows a silencer 620 that is oriented with the central axis 624 aligned vertically, and with partition walls 640, 642 that are inclined relative to the central axis 624. Specifically, the silencer 620 has circumferential partition walls 640 that extend circumferentially around the inner tube 622 in respective planes extending parallel to each other and at an acute angle to the central axis 624, so that the circumferential partition walls 640 are not horizontal in the orientation shown in Figure 12. The silencer 620 also includes axial partition walls 642 that extend axially along the length of the inner tube 622 and are evenly spaced circumferentially around the inner tube 622, the axial partition walls 642 being inclined at an acute angle to the vertical. The circumferential partition walls 640 and axial partition walls 642 are therefore not perpendicular to each other in this example.

The acoustic performance of the examples shown in Figure 12 will differ from the silencer of Figure 9, demonstrating the flexibility of the segmenting approach to tune the performance of the silencer as may be desired. The example of Figure 12 also demonstrates that a ‘circumferential partition wall’ does not necessarily extend perfectly circumferentially, but may be slightly inclined to a radial plane.

Correspondingly, an ‘axial partition wall’ may not be precisely parallel to the central axis of the silencer. The noise attenuation that is achieved by the silencers of Figures 9, 10, and 11 relative to the known CMPP of Figure 1 is represented in Figure 13.

As Figure 13 shows, the STL achieved by the known CMPP 10 is concentrated in a frequency band extending from 500Hz to approximately 1300Hz, with a peak STL of approximately 17dB at 900Hz.

The silencer of Figure 9 with both circumferential and axial partition walls 240, 242 offers significantly increased peak attenuation, at a higher frequency range, when compared to the known CMPP 10. Specifically, the STL peaks at approximately 37dB at 1300Hz, and is no lower than 3dB through a range extending from approximately 1100Hz to 2100Hz.

The silencer 320 with only circumferential partition walls 340 shown in Figure 10 also offers significantly improved performance compared to the known CMPP 10, at a substantially similar frequency range offered by the silencer of Figure 9. The silencer of Figure 10 peaks at approximately 27dB STL at around 1400Hz.

The silencers of Figures 9 and 10 may therefore both offer wideband noise attenuation and can target higher frequencies than the known CMPP 10 without altering the overall packaging size for the silencer.

The silencer 420 with axial partition walls 442 only that is shown in Figure 11 offers a different noise attenuation performance to the silencers of Figures 9 and 10, and the known CMPP of Figure 1. The silencer of Figure 11 achieves an isolated peak STL of 27dB in a narrow band around 2200Hz, which far exceeds the STL achieved by the known CMPP 10 or the silencers of Figures 9 and 10 at that frequency. The silencer 420 of Figure 11 also achieves an STL of at least 4dB between approximately 1050Hz and 2350Hz. In comparison, the STL for the silencers of Figures 1 , 9, and 10 drop below 4dB at approximately 1400Hz, 2000Hz, and 2000Hz respectively. The silencer of Figure 11 may therefore be useful in applications having noise peaks at even higher frequencies than the applications that may be targeted by the silencers of Figures 1 , 9, and 10. It can be seen that the addition of partition walls to a cavity of a double-walled silencer enhances sound attenuation performance, and/or enables higher frequencies to be targeted, when compared to a silencer with a single non-partitioned cavity. Silencers including partitioned cavities may therefore be useful in applications with noise peaks at high frequencies, for example. Accordingly, the number, orientation, and geometry of these partition walls can be varied to tune the noise attenuating response of the silencer to suit the requirements of each application.

The two approaches described above can be combined, in that a silencer may have a cavity that is both partitioned and non-tubular. This further enhances the ability to tune the acoustic performance of the silencer by incorporating both variation in the geometry of the cavity, and variation in the number and geometry of partition walls.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

For example, the invention is not restricted to improved CMPPs, and in some embodiments provides noise attenuators having inner and/or outer walls that are non- tubular. For example, the inner tube of the above described embodiments may be replaced with a box-shaped inner element having a square or oblong cross-section.

Although in the examples described above the perforated portions of the inner tubes have evenly distributed perforations, in other arrangements the perforations may be distributed unevenly, for example to be concentrated in certain areas. The perforations may also have a variety of shapes, and the perforations of an attenuator may not all be of the same shape and/or size.