Diffuser assembly

Abstract

A diffuser assembly is for receiving an airflow from a motor-driven impeller. The diffuser is shaped to define a primary channel 50 and an auxiliary channel 101. The primary channel comprises an air

Description

BACKGROUND Motor-driven impellers are used in household appliances to generate airflow, for example in hair dryers or vacuum cleaners. However, motor-driven impellers generate noise which can prove irritating to a user. It is desirable to reduce noise generated by motor-driven impellers in household appliances. SUMMARY A first aspect provides a diffuser assembly. The diffuser assembly is for receiving airflow from a motor-driven impeller. The diffuser assembly comprises: a primary channel comprising an air inlet for receiving the airflow from the motor-driven impeller, and an air outlet, the primary channel configured to guide airflow received at the air inlet to the air outlet; and an auxiliary channel extending from an open proximal end at a position along the primary channel and terminating at a closed distal end, the auxiliary channel following a curved path having a varying radius of curvature, and the auxiliary channel configured to function as an acoustic silencer at a characteristic acoustic frequency of the motor-driven impeller. Diffuser assemblies can improve the efficiency and effectiveness of a motor-driven impeller by receiving airflow, generated by the motor-driven impeller, at a primary channel, convert air velocity of the airflow into a rise in static pressure and improve uniformity of the airflow. In the first aspect, noise generated by a motor-driven impeller can be reduced, or attenuated, by the presence of the auxiliary channel connected to the primary channel of the diffuser assembly. In particular, noise at a characteristic acoustic frequency of the motor-driven impeller can be reduced. A characteristic acoustic frequency can be an acoustic frequency which has a largest amplitude, or a largest loudness, for example. It can be an acoustic frequency which has the largest apparent loudness to a user, for example. Generally, the acoustic silencing function of the auxiliary channel can be achieved by producing acoustic waves in the auxiliary channel which are out-of-phase with acoustic waves in the primary channel. In some examples, this is anticipated to arise due to a quarter-wave silencer configuration, but more generally may arise from a mixed contribution of different reactive, or resonant, acoustic silencing principles to which material choice, geometry, and constructional design, such as the manner in which components are connected to one another, can each contribute. For instance, a portion of the auxiliary channel may operate in a Helmholtz resonator manner, for example. In following a curved path with a varying radius of curvature, the auxiliary channel can occupy a smaller spatial footprint than, for example, an auxiliary channel following a curved path having a constant radius of curvature, or an auxiliary channel following a straight path. The spatial footprint can be, for example, a measure of extent of the auxiliary channel in a particular direction; for example, in a lateral direction relative to a longitudinal direction of the primary channel. This can reduce an overall size of the diffuser, for example, which can in turn reduce the overall size of a household appliance comprising the diffuser, for example. In some examples, a length of the auxiliary channel from the proximal end to the distal end is substantially equal to an odd integer number of quarter wavelengths of the characteristic acoustic frequency of the motor-driven impeller such that the auxiliary channel functions as a quarter-wave silencer. A length being substantially equal to an odd integer number of quarter wavelengths means a length of auxiliary channel which generates a notable reduction to noise at a characteristic frequency due to a quarter-wave reactive acoustic silencing effect. It will be appreciated that, in general, the attenuation effect of the auxiliary channel has an acoustic frequency bandwidth and so the length may not correspond exactly with an odd integer number of quarter wavelengths. In some examples, the length is substantially the same as the odd integer number of quarter wavelengths to within manufacturing tolerances. In some examples, the curved path is an involute of a circular locus, the radius of curvature of the path increasing with distance along the path from the circular locus. The curved path, and hence the auxiliary channel, is therefore in the form of a spiral which radiates out from a central point. The radius of curvature can change smoothly with distance along the auxiliary channel, which can improve performance of the auxiliary channel as an acoustic silencer by, for example, meaning boundary conditions for an acoustic wave travelling down, or propagating through, the auxiliary channel only vary slowly. This can reduce dispersion of the acoustic wave, for example, and thereby form a stronger acoustic wave in anti-phase with acoustic waves in the primary channel. In some examples, the radius of curvature increases linearly with distance. In some examples, the distal end of the auxiliary channel is located at a circumference of the circular locus. In other words, the auxiliary channel extends from an outward position, spiralling inwards to terminate at the circular locus, and its radius of curvature decreases towards the distal end. In some examples, the proximal end of the auxiliary channel is located at a circumference of the circular locus. In other words, the auxiliary channel extends from an inwards position at the circular locus, spiralling outwards to terminate at an outward position further from the circular locus, and its radius of curvature increases towards the distal end. In some examples, the auxiliary channel has parallel sidewalls. In such examples, a cross-sectional area of the auxiliary channel, in a cross-section perpendicular to a direction of the auxiliary channel along the path, is bounded by parallel opposite sidewalls. The cross-sectional area bounded by the parallel sidewalls can be shaped as a square, or rectangle, for example. Parallel opposite sidewalls can be understood to be following locally parallel curves, or offset curves. The auxiliary channel having parallel opposite sidewalls can improve performance of the auxiliary channel as an acoustic silencer by, for example, meaning boundary conditions for an acoustic wave travelling down the auxiliary channel can be substantially symmetric in a plane, for example, which can reduce dispersion of the acoustic wave, for example, and thereby form a stronger acoustic wave in anti-phase with acoustic waves in the primary channel. Moreover, the parallel opposite sidewalls can allow the auxiliary channel to tesselate with other auxiliary channels, for example, and can permit more straightforward manufacture, for example. In some examples, a cross-sectional area of the auxiliary channel is uniform along a majority of a length of the auxiliary channel from the proximal end to the distal end. In such examples, the cross-sectional area of the auxiliary channel is in a cross-section perpendicular to a direction of the auxiliary channel along the path. In some examples, the cross-sectional area is uniform because sidewalls of the auxiliary channel bounding the area remain of substantially uniform dimensions along the length. The auxiliary channel having a substantially uniform cross-sectional area can improve performance of the auxiliary channel as an acoustic silencer by, for example, meaning boundary conditions for an acoustic wave travelling down the auxiliary channel remain substantially constant along the length of the auxiliary channel, for example, which can reduce dispersion of the acoustic wave, for example, and thereby form a stronger wave in anti-phase with acoustic waves in the primary channel. In some examples, the auxiliary channel comprises a first portion at the proximal end extending from the primary channel in a first direction, and a second portion extending from the first portion in a second direction orthogonal to the first direction. This can allow for a folded configuration, for example, in that the auxiliary channel exists in portions which extend orthogonal to one another. In some examples, the second portion follows the curved path to the distal end. In some examples, the first portion is a minority of an overall length of the auxiliary channel, and the second portion is a majority of the overall length of the auxiliary channel. In some such examples, the second portion extends parallel to the neighbouring primary channel. This can allow the diffuser assembly to have relatively compact spatial footprint in that a lateral extent of the diffuser assembly, measured perpendicular to a portion of the primary channel, is reduced. In some examples, the first portion is connected to the second portion by a comer portion, and the comer portion comprises an oblique surface connecting a first neighbouring surface of the first portion and a second neighbouring surface of the second portion, the oblique surface at an angle to the first neighbouring surface and the second neighbouring surface. The oblique surface can reduce a number of surfaces which face head-on, or are perpendicular to the propagation direction of, an incoming acoustic wave. This can improve performance of the auxiliary channel as an acoustic silencer by, for example, improving reflection of an acoustic wave down the full length of the auxiliary channel which can produce an acoustic wave in anti-phase with acoustic waves in the primary channel. In some examples, the proximal end of the auxiliary channel is covered by an acoustically permeable membrane. In such examples, the acoustically permeable membrane can form a boundary between the auxiliary channel and the primary channel. This can prevent, or otherwise reduce, the separation of airflow in the primary channel at or around the proximal end of the auxiliary channel. Separation of airflow can increase noise due to turbulence, and can interfere with destructive interference of acoustic waves in the primary channel with acoustic waves in the auxiliary channel. In some examples, a majority of a length of the auxiliary channel from the proximal end to the distal end is parallel to a neighbouring portion of the primary channel. This can reduce a spatial footprint of the diffuser assembly as the primary channel and auxiliary channel can be arranged in a same overall direction, or same overall plane, for example. In some examples, the auxiliary channel is a first auxiliary channel of a plurality of auxiliary channels, each auxiliary channel of the plurality of auxiliary channels: extending from an open proximal end at a position along the primary channel and terminating at a respective closed distal end, following a respective curved path having a varying radius of curvature; and configured to function as an acoustic silencer at a characteristic acoustic frequency of the motor-driven impeller. Providing a plurality of auxiliary channels can improve an overall noise reduction, for example by producing a larger number of acoustic waves out-of-phase with acoustic waves in the primary channel. In some examples, each auxiliary channel extends from a respective proximal end, each respective proximal end being at a respective position along the primary channel. In other examples, two or more auxiliary channels may extend from a shared proximal end to respective distal ends. In some examples, the proximal ends of the plurality of auxiliary channels are spaced regularly along a perimeter of the primary channel. This can improve performance of the plurality of auxiliary channels as an acoustic silencer by, for example, generating acoustic waves which are out-of-phase with acoustic waves in the primary channel in a substantially uniform manner around the primary channel. In some examples, the diffuser assembly has rotational symmetry in the arrangement of auxiliary channels about the primary channel. A diffuser assembly having rotational symmetry can have acoustic silencing performance less sensitive to orientation of the diffuser assembly relative to the motor-driven impeller, for example. In some examples, the first auxiliary channel and a neighbouring auxiliary channel are separated by a dividing wall, a first side of the dividing wall being a sidewall of the first auxiliary channel and a second side of the dividing wall being a sidewall of a neighbouring auxiliary channel. In other words, the first auxiliary channel and the neighbouring auxiliary channel share and are separated by a common dividing wall. This can allow the auxiliary channels to be more compactly spaced, allowing for a greater number of auxiliary channels to be provided into a given area, for example, compared with each auxiliary channel having separate respective sidewalls. In some examples, each auxiliary channel is separated from a respective neighbouring auxiliary channel by a respective dividing wall, a first side of the respective dividing wall being a sidewall of the auxiliary channel and a second side of the respective dividing wall being a sidewall of the respective neighbouring auxiliary channel. In some examples, the plurality of auxiliary channels are tessellated together to fill an area, each auxiliary channel sharing and being separated from a common sidewall with neighbouring auxiliary channels. This can allow for a greater number of auxiliary channels to be provided into a given area, for example, compared with when auxiliary channels are separated by gaps, for example. In some examples, each auxiliary channel of the plurality of auxiliary channels is congruent. In other words, each auxiliary channel has substantially similar dimensions to one another. In this way, the plurality of auxiliary channels can have similar acoustic performance in that each can provide similar response to acoustic waves in the primary channel. The auxiliary channels, being congruent, may be in rotated alignment relative to one another. In some examples, each auxiliary channel of the plurality of auxiliary channels extends from a proximal open end at a respective position along the primary channel to a respective closed distal end, following an involute curved path to a circular locus, the distal end of each auxiliary channel at the circumference of the circular locus, each auxiliary channel separated from neighbouring auxiliary channels by dividing walls, wherein a first side of each dividing wall is a sidewall for a first of the respective neighbouring auxiliary channels and a second side of each dividing wall is a sidewall for a second of the respective neighbouring auxiliary channels, and each auxiliary channel of the plurality of auxiliary channels is dimensioned substantially similarly such that the plurality of auxiliary channels are congruent. In some examples, at least a second auxiliary channel of the plurality of auxiliary channels is configured to function as an acoustic silencer at a further acoustic frequency different to the characteristic acoustic frequency. This can allow a greater range of frequencies to be reduced, for example. The further acoustic frequency, in some examples, is a second characteristic acoustic frequency of the motor-driven impeller, such as a higher-order harmonic frequency. In other examples, the further acoustic frequency can be an emergent acoustic frequency which may result from operation of the diffuser assembly, for example. In some examples, the characteristic frequency at which the auxiliary channel is configured to operate as an acoustic silencer is a first harmonic tone of the motor-driven impeller. In other words, the auxiliary channel is configured to operate as an acoustic silencer at a fundamental frequency of the motor-driven impeller. In some examples, the characteristic frequency at which the auxiliary channel is configured to operate as an acoustic silencer is between 1000 and 2000 Hz. In some such examples, the characteristic frequency at which the auxiliary channel is configured to operate as an acoustic silencer is between 1250 and 1750 Hz. In some examples, the diffuser assembly comprises a supplementary channel extending from a proximal end at a position along the primary channel and terminating at a closed distal end, and following a differently shaped path to the auxiliary channel and configured to function as an acoustic silencer. The supplementary channel can augment the acoustic silencing provided by the auxiliary channel. The different shape of the supplementary channel can provide a different auditory response, such as a more or less pronounced attenuation across a broader or narrower spectral range. In some examples, the supplementary channel can provide a majority of the attenuation effect compared with the auxiliary channel, whereas in other examples, the auxiliary channel provides a majority of the attenuation effect compared with the supplementary channel. In some examples, the supplementary channel extends from an opposite side of the primary channel to the auxiliary channel. The supplementary channel, in having a different shape to the auxiliary channel and arranged on an opposite side of the primary channel, may be arranged to fit into a different portion of the diffuser assembly in order to increase usage of a volume occupied by the diffuser assembly. In other words, a spatial footprint of the diffuser assembly may be increased. In some examples, the supplementary channel has a substantially linear portion. In some examples, a majority of the supplementary channel extends parallel to an adjacent portion of the primary channel. This can reduce a spatial footprint of the diffuser assembly. In some examples, the supplementary channel follows a path having multiple linear sections connected by comer sections. The supplementary channel, similarly to the auxiliary channel, can thereby comprise a folded configuration in which a total length of the supplementary channel is contained within a smaller spatial footprint, as measured by an extent in a direction away from the proximal end of the supplementary channel, for example. In some examples, the supplementary channel is configured to function as an acoustic silencer at a different frequency to the auxiliary channel. Similar to the auxiliary channels configured to function as an acoustic silencer at different frequencies to the characteristic frequency, this can improve the acoustic silencing response of the diffuser assembly. This can allow noise at other characteristic frequencies to be attenuated, for example. In some examples, the diffuser assembly comprises a plurality of supplementary channels. A second aspect provides a system comprising a motor-driven impeller for generating an airflow, and the diffuser assembly of the first aspect, the diffuser assembly arranged to receive airflow from the motor-driven impeller. The motor-driven impeller comprises a motor and an impeller, the impeller attached to a rotor shaft of the motor. In some examples, the primary channel of the diffuser assembly comprises a proximal portion comprising the air inlet, and a distal portion comprising the air outlet. The proximal portion and distal portion are arranged sequentially to form an airflow path from air inlet to air outlet, the proximal portion arranged nearest the motor-driven impeller and extending away from the motor-driven impeller in a radial direction, perpendicular to an axial direction of a rotor shaft of the motor-driven impeller; and the distal portion extends from the proximal portion in the axial direction, perpendicular to the proximal portion. The at least one auxiliary channel extends from the proximal portion, a minority of the auxiliary channel extending in the axial direction and a majority of the auxiliary channel extending in the radial direction. A third aspect provides an appliance comprising the diffuser assembly of the first aspect or the system of the second aspect. In some examples, the appliance is a vacuum cleaner, and in other examples the appliance is a hair dryer. More generally, the appliance may be a household appliance. The appliance can experience lower noise levels during operation by a user. Advantages described for an aspect are equally applicable to other aspects, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic cross-sectional view of a system comprising a motor-driven impeller and a diffuser assembly according to the present disclosure; Figures 2 and 3 are schematic plan views of the diffuser assembly of Figure 1; Figure 4 is a schematic cross-sectional view of a first further diffuser assembly according to the present disclosure; Figure 5 is a schematic cross-sectional view of a portion of a second further diffuser assembly according to the present disclosure; Figures 6a and 6b are schematic plan views of the diffuser assembly of Figure 4; Figure 7 is a schematic cross-sectional view of a further supplementary channel according to the present disclosure; and Figures 8 and 9 are schematic illustrations of appliances comprising a diffuser assembly according to the present disclosure. DETAILED DESCRIPTION A system 20 comprising a motor-driven impeller 14 and a diffuser assembly 10 according to the present disclosure is shown schematically in Figure 1. The motor-driven impeller 14 is mounted on a shaft 16 of a motor 18. The motor 18 comprises a stator assembly (not shown) including one or more coils, windings, which can generate a magnetic field when energised, and a permanent magnet (not shown) is mounted to the shaft 16. The permanent magnet interacts with the magnetic field and results in a rotational motion of the shaft 16 about a longitudinal rotational axis R. In general, the motor 18 is configured to generate a rotational motion of the shaft 16 to thereby rotate the motor-driven impeller 14. It will be appreciated that the precise configuration of the motor 18 is immaterial to the present disclosure, and that rotation of the motor-driven impeller 14 can be brought about in a variety of manners. The motor-driven impeller 14 has blades which generate an airflow when the motor-driven impeller 14 is rotated by the motor 18. The longitudinal rotational axis R can be considered to define an axial direction, as referred to herein, and directions perpendicular to the longitudinal rotational axis R can be considered radial directions, as referred to herein. The diffuser assembly 10 has a primary channel 50 and a plurality of auxiliary channels 100. The primary channel 50 has an air inlet 52 and an air outlet 54. The air inlet 52 is arranged relative to the motor-driven impeller 14 such that it can receive airflow generated by the motor-driven impeller 14. The air inlet 52 is an annulus arranged circumferentially around the motor-driven impeller 14 in a plane perpendicular to the longitudinal rotational axis R. The primary channel 50 extends from the air inlet 52 to the air outlet 54 and has a shape for guiding airflow received at the air inlet 52 to the air outlet 54. In this example, the primary channel 50 extends from the air inlet 52 in a radial direction, away from the longitudinal rotational axis R, before extending in an axial direction to the air outlet 54. The air outlet 54 is an annulus forming a circumference around the longitudinal rotational axis R, as can be seen in Figure 3, which illustrates a plan view in plane B-B. A series of vanes 12 are disposed within a region of the primary channel 50 upstream of and near to the air outlet 54, the vanes 12 being regularly spaced through the primary channel 50. The air inlet 52, air outlet 54, and primary channel 50, as well as any intermediate structures such as the vanes 12, may be dimensioned and shaped differently in other examples, and are not limited to the examples illustrated here. For instance, the vanes 12 may be irregularly spaced through the primary channel, or be distributed according to some arbitrary pattern of spacing. The plurality of auxiliary channels 100 extend from the primary channel 50. A single auxiliary channel 101 of the plurality of auxiliary channels 100 will hereafter be described. In this example, each auxiliary channel of the plurality of auxiliary channels 101 is substantially identical, but in other examples, described later, some or each of the plurality of auxiliary channels may be different to one another. The auxiliary channel 101 extends from a proximal end 103 and terminates at a distal end 105, as can be seen in Figures 1 and 2. The proximal end 103 is located at a position along an exterior wall of the primary channel 50 and is open, such that the auxiliary channel 101 is in fluidic communication with the primary channel 50. Thus air within the primary channel 50 can flow into and out from the auxiliary channel 101. The distal end 105 is closed, in that the auxiliary channel 101 terminates at an end wall 105-3. Thus there is one way into and out from the auxiliary channel 101, which is at the proximal end 103. The auxiliary channel 101 is arranged with the proximal end 103 located further from the longitudinal axis R than the distal end 105 is from the longitudinal axis R, but in other examples the proximal end 103 may be closer to the longitudinal axis R than the distal end 105 is to the longitudinal axis R. As can be seen in Figure 1, the auxiliary channel 101 extends from a side of the primary channel 50 which is located away from, in the axial direction, the motor-driven impeller 14 and the air outlet 54 of the primaiy channel 50. A first portion 106a of the auxiliaiy channel 101 extends from the proximal end 103 for a distance in the axial direction, and a second portion 106b of the auxiliary channel 101 extends radially inwards, towards the longitudinal axis R for a distance along a curved path P which will be described shortly. The first portion 106a has a smaller length than the second portion 106b, such that a majority of the auxiliary channel 101 is parallel to and adjacent with the primary channel 50. A comer portion 106c joins the first portion 106a, extending in the axial direction, to the second portion 106b, extending in the radial direction. The auxiliary channel 101 is bounded by sidewalls 107, 108, 109, 110. The sidewalls 107-110 define a cross-section of the auxiliary channel 101, the cross-section being perpendicular to a length of the auxiliary channel which runs from the proximal end 103 to the distal end 105. In this example, the auxiliary channel has a quadrilateral cross-section, in particular a rectangular cross-section, and is bounded by four sidewalls: a first 107 and second 109 lateral sidewalls, and top 110 and bottom 108 sidewalls. The rectangular cross-section is taller than it is wide, in that the lateral sidewalls 107, 109 are longer than the top and bottom sidewalls 108, 110, in the plane of the rectangular cross-section. In other examples, the auxiliary channel 101 can have different cross-sections. For example, the auxiliary channel 101 may be tubular and have a circular cross-section, for example, in which case the auxiliary channel 101 is bounded by a single circular sidewall, or a square cross-section, in which the lateral, top, and bottom sidewalls 107-110 are of equal dimensions. The second portion 106b of the auxiliary channel 101, in extending from the comer portion 106c to the distal end 105, follows a curved path P which is a plan view of plane A-A. More specifically, the curved path P is a path followed by a centre of the second portion 106b of the auxiliary channel 101, the centre being a midway position, or a geometric centre, between the lateral sidewalls 107,109. The curved path P has a varying radius of curvature, in that the radius of curvature is different at different positions along the path P. The lateral side walls 107,109 of the auxiliary channel are substantially parallel with the curved path P, in that they are curves which locally track the curve path P, and so also have varying radius of curvatures along their lengths. The radius of curvature is larger towards the proximal end 103, and becomes smaller towards the distal end 105. In particular, the curved path P followed by the auxiliary channel 101 in the example of Figures 1-3 is an involute of a central circular locus 120. In other words, the auxiliary channel 101 follows a spiral path around and towards the circular locus 120. The radius of curvature varies smoothly at a substantially linear rate, and varies monotonically. In other examples, the radius of curvature varies in other ways, for instance non-monotonically, such as increasing and then decreasing, or decreasing and then increasing. Due to the smoothly varying curved path, the sidewalls 107, 109 can be considered, at least along a majority of the length of the second portion 106b of the auxiliary channel 101, to be substantially parallel with each other. Substantially parallel can mean that they are within a few degrees of parallel. At the proximal end, for a minority of the length of the auxiliary channel, the sidewalls 107,109 flare slightly from parallel. The distal end 105 is located at the circumference of the central circular locus 120, and the proximal end 103 is located away from the circular locus 120. In other examples, the arrangement could be reversed, in that the auxiliary channel 101 could extend from a proximal end at a location along the primary channel 50 adjacent to the circular locus 120 to a distal end away from the circular locus 120, the path P followed by the auxiliary channel 101 being otherwise unchanged. The end wall 105-3 at the distal end 105 is conformal to an adjacent portion of the circumference of the circular locus. The end wall 105-3 interfaces with the sidewalls in a substantially perpendicular manner, as well as being substantially perpendicular to the direction of the path P at the distal end. The auxiliary channel 101 has an overall length which is a distance, along a geometric centre of the bounding side walls 107-110, from the proximal end 103 to the distal end 105. This includes lengths of the first portion 106a, the comer portion 106c, and the second portion 106b. The auxiliary channel 101 is a first auxiliary channel of the plurality of auxiliary channels 100, and is neighboured on a first side by a second auxiliary channel 101-2 and neighboured on an opposite, second side by a third auxiliary channel 101-3. The second auxiliary channel 101-2 hence extends from a respective proximal end 103-2 to a respective distal end 105-2, and the third auxiliary channel 101-3 similarly extends from respective proximal end 103-3 to a respective distal end 105-3. The respective distal ends 105-2,105-3 are located around the circumference of the circular locus 120, and the respective proximal ends 103-2, 103-3 are located at respective positions along the primary channel 50 and away from the circular locus 120. As described earlier, each auxiliary channel is substantially identical, or congruent, and so the first 101, second 101-2, and third 101-3 auxiliary channels each extend along a respective involute curve, or a respective spiral path, from the circular locus. Together, the plurality of auxiliary channels 100 spiral inwards towards the circular locus 120 along respective spiral paths. The proximal ends of the auxiliary channels are spaced regularly such that, relative to the axial direction R, the proximal ends are evenly distributed around the longitudinal axis R, and hence have an even distribution across the primary channel 50. The auxiliary channel 101 shares the first lateral sidewall 107 with the second auxiliary channel 101-2, the second auxiliary channel 101-2 on the opposite side of the first lateral sidewall 107 to the first auxiliary channel 101, and shares the second lateral sidewall 109 with the third auxiliary channel 101-3, the third auxiliary channel 101-3 on the opposite side of the second lateral sidewall 109 to the first auxiliary channel 101. In this way, the three auxiliary channels 101, 101-2, 101-3 are tessellated, and separated by common sidewalls 107, 109, which can be considered dividing walls between the auxiliary channels 101, 101-2, 101-3. Extending this to the entire plurality of auxiliary channels 100, the plurality of auxiliary channels 100 can be tessellated to substantially fill a surface area. As can be seen in Figure 2, an area surrounding the circular locus 120 is filled with a spiral arrangement of auxiliary channels 100 which are divided by common dividing walls 107, 109. In use, the motor-driven impeller 14 is rotated by the motor 18 to generate an airflow. In doing so, acoustic noise is generated, and in particular noise at a first shaft speed harmonic tone, also known as a “motor tone”, is generated. The frequency, or frequency envelope, of this tone is characteristic of the motor-driven impeller 14. It will be appreciated that the characteristic frequency typically has some degree of bandwidth to it. The tone can result from, for example, vibrations due to mass imbalance in the rotor / impeller, which may be known as structure-borne acoustic noise, or result from the airflow, in particular complex flow phenomena which can induce noise such as turbulence and separation. The characteristic frequency can be in a range of 1000-5000 Hz, such as around 1500 Hz. The characteristic frequency can be dependent upon, but is in general not the same as, a revolution speed of the motor-driven impeller 14. Higher-order harmonic tones can be generated as well, and can also, or alternatively, be the target of noise attenuation described herein. In addition, the motor-driven impeller 14 generates broadband noise throughout the audible frequency range of 20 - 20,000 Hz. This broadband noise can result from random fluctuations of pressure in the airflow as is travels through the motor 14 and diffuser assembly 10, for example. The airflow generated by the motor-driven impeller 14 is received by the air inlet 52 of the primary channel 50. The function of the primary channel 50 is to convert air velocity of the airflow into a rise in static pressure and improve uniformity of the airflow. In this example, the primary channel 50 features vanes 12 which facilitate this by slowing down incident airflow moving through the primary channel 50. The auxiliary channels 100 each have a length Lx from the proximal end 103, along the curved path, to the distal end 105, which matches an odd-integer number of quarter of a wavelength of the characteristic frequency of the motor-driven impeller 14. In general, length L of the auxiliary channels follows: n L = —A, n = 1,3,5 ... 4 where A is the wavelength corresponding to a frequency at which noise attenuation is desired. In this example, the auxiliary channels 100 are arranged at the first integer number of quarter wavelengths, n = 1. For instance, a characteristic frequency of 1500 Hz has a wavelength of approximately 230 millimetres - the auxiliary channels have a length according to a quarter of this wavelength of approximately 57.5 millimetres. Accordingly, an acoustic wave travelling down the auxiliary channel 101 is reflected by the end wall at the distal end 105, whereupon it returns to the primary channel 50 having travelled half a wavelength. The acoustic wave is therefore out-of-phase with acoustic waves within the primary channel 50. The reflected acoustic wave destructively interferes with acoustic waves in the primary channel 50 to thereby bring about an acoustic attenuation effect at the characteristic frequency. The auxiliary channels 100 thereby function as quarter-wave silencers at the characteristic frequency. The gradual curvature of the auxiliary channels 100 mean that the sidewalls 107-110, or boundaries, of the auxiliary channels 100 are effectively parallel along the length which can reduce dispersion of the acoustic wave, which can improve the attenuation effect of the auxiliary channels. Furthermore, the change in curvature allows a greater length to be contained within a smaller area, which can reduce an overall size of the diffuser assembly 10 for a given length of auxiliary channel 101. Similarly, maintaining a constant cross-sectional area can reduce dispersion of the acoustic wave. In some examples, the sidewalls 107-110 and / or cross-sectional area vary sufficiently slowly and smoothly due to the curved path that dispersion of acoustic waves can be reduced. The aforementioned even distribution of auxiliary channels 100, distributed regularly around a circumference about the axis R and the primary channel 50, and being rotationally symmetric about a central circular locus 120, means that they can provide a more uniform sound attenuation effect. It will be appreciated that the plurality of auxiliary channels 100 illustrated in Figures 1 and 2 represents an efficient and effective arrangement of auxiliary channels 100, but more generally a single auxiliary channel 101 having a varying radius of curvature can be effective alone, and other arrangements of such an auxiliary channel 101 are possible. Some alternatives are described later. Figure 4 illustrates a diffuser assembly 10b comprising supplementary channels 201, 301 which can augment auxiliary channels 100, as well as illustrating other aspects of design which can be used in both supplementary 201,301 and auxiliary channels 100. The diffuser assembly 10b of Figure 4 comprises all the features of the example of Figure 1, with the addition of the supplementary channels 201,301 described hereafter. A first supplementary channel 201 extends from a proximal end 203 at a location along the primary channel 50 to a distal end 205. The proximal end 203 of the first supplementary channel 201 is on an opposite side of the primary channel 50 to the proximal end of the auxiliary channel 101, and generally the supplementary channels 201,301 are on an opposite side of the primary channel 50 to the auxiliary channels 101. The first supplementary channel 201 extends in a substantially linear fashion from the primary channel 50 in an axial direction. The supplementary channel has a length L2 which matches the length of the auxiliary channels, and so also functions as a quarter-wave silencer at the characteristic frequency of the motor-driven impeller 14. A second supplementary channel 301 extends from a proximal end 303 at a location along the primary channel to a distal end 305. The second supplementary channel 301 has a “folded” path which comprises two parallel linear portions joined by a comer portion 307. The folded configuration of the secondary supplementary channel 301 means that it has a reduced extent in the axial direction R compared with a purely linear channel of the same overall length, which can reduce an overall size of the diffuser assembly 10b. The comer portion 307 includes oblique surfaces 309 which are angled relative to neighbouring sidewalls of the supplementary channel 301. Considering the presence of an oblique surface in either a supplementary channel 301 or an auxiliary channel 101, the oblique surfaces 309 can mean that an acoustic wave is not incident normally upon a surface of the channel at the comer portion. This can further improve the attenuation effects of the channel by reducing premature back reflection of the acoustic wave, occurring before the end wall at the distal end 105. In some examples, such as one illustrated by Figure 7, rather than having an angled oblique surface joining neighbouring sidewalls, the comer portion 307 may comprise a curved surface 309b smoothly transitioning between the neighbouring sidewalls so as to preserve cross-sectional area of the supplementary channel 301 as much as possible, which can minimise change in impedance and improve performance for low acoustic frequencies, for example. Such approaches can be implemented in the auxiliary channel as well, such as introducing an oblique surface 111 into a comer portion 112 of the auxiliary channel 101. The second supplementary channel 301 comprises an overall length L3 which, in this example, is different to Lu and hence the second supplementary channel 301 operates as a quarter-wave silencer at a different frequency to the auxiliary channels 100, or to the first supplementary channel 201. The different frequency could be a second characteristic frequency of the motor-driven impeller 14, for example, or could be a frequency which results from interaction of the airflow with the auxiliary channels 100, for example. More generally, the different frequency could be any acoustic frequency for which attenuation is desired. As can be seen from Figure 4, the first and second supplementary channels 201, 301 extend parallel to neighbouring portions of the primary channel 50 comprising the vanes 12 and air outlet 54. The first and second supplementary channels 201, 301 are between the neighbouring portion of the primary channel 50 and the motor-driven impeller 14, and can thereby make use of space which might otherwise have been empty and facilitating a compact arrangement of the supplementary channels 201, 301, as can be seen in Figure 6a, which is a plan view of plane C-C. In some examples, for instance in those having the same overall form of primary channel and motor-driven impeller as illustrate in Figures 1 and 4, the supplementary channels 301b, 201b can have cross-sections which are portions of a circular arc around the longitudinal axis R and bounded by two radial wall sections, as seen in the plan view of Figure 6b. More generally, the supplementary channels may, similarly to the auxiliary channels, take upon a variety of cross-sectional designs. The supplementary channels 201, 301 described here are provided for non-limiting explanatory purposes; any number of supplementary channels 201, 301 could be provided, having different lengths, different geometries, interfacing with the primary channel 50 at different locations, and so on. The supplementary channels 201, 301 could be disposed within the arrangement of auxiliary channels 100, for example, and do not need to be provided on an opposite side of the primary channel 50. Figure 5 illustrates a further feature which can be implemented into one or more of the auxiliary 100 or supplementary channels 201, 301 of the diffuser assembly 20. An acoustically permeable membrane 180 covers the proximal end 103 of the auxiliary channel 101. Air movement from the primary channel 50 down the auxiliary channel 101 is restricted, or otherwise prevented, by the acoustically permeable membrane 180, but nevertheless acoustic waves can be transmitted through the acoustically permeable membrane 180. This can reduce the occurrence of flow separation of airflow, which can be a source of acoustic noise itself, within the primary channel 50, for example, particularly in a region 56 close to the proximal end 103 of the auxiliary channel 101. Figure 8 illustrates a vacuum cleaner 1000 comprising a diffuser assembly according to the present disclosure, for example of Figures 1-7. In the vacuum cleaner 1000 of Figure 8, the diffuser assembly 10 is part of a compressor assembly housed inside a main body of the vacuum cleaner, the compressor assembly for drawing air into and through the main body. The vacuum cleaner 1000, in comprising the diffuser assembly, can have a reduced acoustic noise at particular frequencies which can be less irritating to a user of the vacuum cleaner 1000. In other examples, other appliances are envisioned, such as a hairdryer 2000 as depicted in Figure 9, for example, which can benefit in a similar manner to the vacuum cleaner 1000. In the hairdryer 2000 of Figure 9, a compressor assembly comprising the diffuser assembly 10,10b is encased within a handle of the hairdryer. The diffuser assembly 10 can be manufactured in a variety of methods and from a variety of materials which will be apparent to the skilled person. For example, the diffuser assembly 10 can be manufactured by injection-moulding. In such examples, the lateral sidewalls 107, 109 of the auxiliary channels 100 may be slightly drafted, for example, to aid in removal from a mould, whilst otherwise being substantially parallel to one another. Similarly, interfaces between sidewalls 107-110 may be filleted, which can also aid in manufacturing the diffuser assembly. It will be appreciated that selecting a stiffer material, or a structure with a stiffer construction, can aid in reflection of acoustic waves and can thereby improve the acoustic silence performance of the diffuser assembly 10. In other examples, a softer material, such as rubber, could be used, in particular, for example, where the softer material functions as a mounting isolation feature and can damp acoustic modes of the diffuser assembly 10, for example. The diffuser assembly 10 could comprise a mixed construction, wherein some portions comprise a first material and some portions comprise a second or further materials, in accordance with desired qualities of the diffuser assembly. The diffuser assembly 10 may be designed with a fixing arrangement, for example integrally formed within the diffuser assembly, to allow it to be attached to the impeller and motor elements. The above examples are to be understood as illustrative examples of the present disclosure. Further examples are envisaged. In the described example, the auxiliary channel comprises a first portion 106a joined to a second portion 106b by a comer portion 106c, the second portion 106b following a curved path. The auxiliary channel, in other examples, may comprise just a single portion following a curved path of varying curvature, for example. In the described examples, the auxiliary channels extend primarily within a plane parallel to A-A, as can be seen in the cross-section of Figures 1 and 2. In other examples, the auxiliary channels 100 could extend in three dimensions - for example, the auxiliary channels 100, following curved paths of varying curvature, could be helical. For example, in the described example a plurality of auxiliary channels 100 extend from a single primary channel 50. In other examples, the diffuser assembly 10 may comprise more than one primary channel 50 receiving airflow from the motor-driven impeller 14, each primary channel 50 comprising one or more auxiliary channels 101,100. In some examples, the diffuser assembly may comprise just a single auxiliary channel 101. In the described examples, the auxiliary channels 100 share common lateral sidewalls 107, 109. In other examples, each auxiliary channel 100 has its own set of lateral sidewalls. In the described examples, the auxiliary channels 100 are congruent and have a same length Lr. In other examples, some of the auxiliary channels 100 have different lengths, for example a length Lx which matches a different acoustic frequency, such as a different characteristic acoustic frequency. In the described examples, each auxiliary channel 100 extends from a respective proximal end 103. In other examples, multiple auxiliary channels may extend from a single common proximal end 103, each extending to a respective distal end 105. For example, the multiple auxiliary channels would extend in different directions from the common proximal end 103. In the described examples, the primary channel 50 has a rotationally-symmetric form and the auxiliary channels 100 are disposed in a rotationally symmetric manner around the primary channel. In other examples, the primary channel 50 may have other forms, including extending in a substantially linear fashion from the motor-driven impeller 14, for example. The auxiliary channels 100, similarly, can be disposed in a variety of manners around the primary channel 50. In the described examples, the motor tone which the auxiliary channels 100 are configured to attenuate is a fundamental harmonic tone. In other examples, higher-order harmonic can be targeted by appropriately sized auxiliary channels 100 in the diffuser assembly. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination 5 of any other embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A diffuser assembly for receiving an airflow from a motor-driven impeller, the diffuser assembly comprising:a primary channel comprising an air inlet for receiving the airflow from the motor-driven impeller, and an air outlet, the primary channel configured to guide airflow received at the air inlet to the air outlet; andan auxiliary channel extending from an open proximal end at a position along the primary channel and terminating at a closed distal end,the auxiliary channel following a curved path having a varying radius of curvature, andthe auxiliary channel configured to function as an acoustic silencer at a characteristic acoustic frequency of the motor-driven impeller.

2. The diffuser assembly of claim 1, wherein a length of the auxiliary channel from the proximal end to the distal end is substantially equal to an odd integer number of quarter wavelengths of the characteristic acoustic frequency of the motor-driven impeller such that the auxiliary channel functions as a quarter-wave silencer.

3. The diffuser assembly of claim 1 or 2, wherein the curved path is an involute of a circular locus, the radius of curvature of the path increasing with distance along the path from the circular locus.

4. The diffuser assembly of claim 3, wherein the distal end of the auxiliary channel is located at a circumference of the circular locus.

5. The diffuser assembly of claim 3, wherein the proximal end of the auxiliary channelis located at a circumference of the circular locus.

6. The diffuser assembly of any previous claim, the auxiliary channel has parallel sidewalls.

7. The diffuser assembly of any previous claim, wherein a cross-sectional area of the auxiliary channel is uniform along a majority of a length of the auxiliary channel from the proximal end to the distal end.

8. The diffuser assembly of any previous claim, wherein the auxiliary channel comprises a first portion at the proximal end extending from the primary channel in a first direction, and a second portion extending from the first portion in a second direction orthogonal to the first direction.

9. The diffuser assembly of claim 8, wherein the first portion is connected to the secondportion by a comer portion, and the comer portion comprises an oblique surface connecting a first neighbouring surface of the first portion and a second neighbouring surface of the second portion, the oblique surface at an angle to the first neighbouring surface and the second neighbouring surface.

10. The diffuser assembly of any previous claim, wherein the proximal end of the auxiliary channel is covered by an acoustically permeable membrane.

11. The diffuser assembly of any previous claim, wherein a majority of a length of the auxiliary channel from the proximal end to the distal end is parallel to a neighbouring portion of the primary channel.

12. The diffuser assembly of any previous claim, wherein the auxiliary channel is a firstauxiliary channel of a plurality of auxiliary channels, each auxiliary channel of the plurality of auxiliary channels:extending from an open proximal end at a position along the primary channel and terminating at a respective closed distal end;following a respective curved path having a varying radius of curvature;and configured to function as an acoustic silencer at a characteristic acoustic frequency of the motor-driven impeller.

13. The diffuser assembly of claim 12, wherein each auxiliary channel extends from a respective proximal end, each respective proximal end being at a respective position along the primary channel.

14. The diffuser assembly of claim 12 or 13, wherein the proximal ends of the plurality of auxiliary channels are spaced regularly along a perimeter of the primary channel.

15. The diffuser assembly of any one of claims 12 to 14, wherein the diffuser assembly has rotational symmetry in the arrangement of auxiliary channels about the primary channel.

16. The diffuser assembly of any one of claims 12 to 15, wherein the first auxiliary channel and a neighbouring auxiliary channel are separated by a dividing wall, a first side of the dividing wall being a sidewall of the first auxiliary channel and a second side of the dividing wall being a side wall of a neighbouring auxiliary channel.

17. The diffuser assembly of claim 16, wherein each auxiliary channel is separated from a respective neighbouring auxiliary channel by a respective dividing wall, a first side of the respective dividing wall being a sidewall of the auxiliary channel and a second side of the respective dividing wall being a sidewall of the respective neighbouring auxiliary channel.

18. The diffuser assembly of any one of claims 12 to 17, wherein each auxiliary channel of the plurality of auxiliary channels is congruent.

19. The diffuser assembly of any one of claims 12 to 17 wherein at least a second auxiliary channel of the plurality of auxiliary channels is configured to function as an acoustic silencer at a further acoustic frequency different to the characteristic acoustic frequency.

20. The diffuser assembly of any previous claim, wherein the characteristic frequency at which the auxiliary channel is configured to operate as an acoustic silencer is a first harmonic tone of the motor-driven impeller.

21. The diffuser assembly of any previous claim, wherein the characteristic frequency at which the auxiliary channel is configured to operate as an acoustic silencer is between 1000 and 5000 Hz.

22. A system comprising a motor-driven impeller for generating an airflow and the diffuser assembly of claims 1-21, the diffuser assembly arranged to receive airflow from the motor-driven impeller.

23. The system of claim 22, wherein the primary channel of the diffuser assembly comprises a proximal portion comprising the air inlet, and a distal portion comprising the air outlet, the proximal portion and distal portion arranged sequentially to form an airflow path from air inlet to air outlet,the proximal portion arranged nearest the motor-driven impeller and extending away from the motor-driven impeller in a radial direction, perpendicular to an axial direction of a rotor shaft of the motor-driven impeller; andthe distal portion extending from the proximal portion in the axial direction, perpendicular to the proximal portion;and wherein the at least one auxiliary channel extends from the proximal portion, a minority of the auxiliary channel extending in the axial direction and a majority of the auxiliary channel extending in the radial direction.

24. An appliance comprising the system of claim 22 or 23, or the diffuser assembly of claims 1-21.

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