Wearable air purifier

Abstract

A head wearable air purifier (10, Fig 1) comprising an ear cup assembly (12, 13, Fig 1) arranged to be worn over a wearer’s ear. The ear cup assembly comprises a fan assembly 15 having a radial fan 16

Description

B ACKGROUND Exposure to air pollution such as pollutant gases or airborne particulates presents a risk of harm to human health. An approach to reducing a person’s exposure to air pollution is to use a wearable air purifier to discharge filtered air towards a wearer’s face. SUMMARY In first aspect, there is provided a head wearable air purifier comprising an ear cup assembly arranged to be worn over a wearer’s ear, the ear cup assembly comprising a fan assembly having a radial fan operable to generate airflow for discharge to the wearer (e.g. to the wearer’s mouth and / or nose for inhalation by the wearer). By providing a fan assembly (for the provision of purified air) in an ear cup assembly, the fan assembly can be conveniently positioned over the ear of wearer. Such a position is secure (a wearer’s ear will typically aid in retaining the ear cup) and provides minimal obstruction, and thus inconvenience, to the user (for example, compared to a neck wearable device). The provision of a radial fan to generate an airflow can minimise the height of the fan assembly (height being taken in a direction extending laterally outwardly away from the wearer’s ear when worn). This can reduce, even further, any obstruction by the fan assembly (or ear cup assembly) and provide a more comfortable experience for the wearer. The term “radial fan” (also referred to as a centrifugal fan) defines a fan that is configured to discharge air in a direction that is substantially radial with respect to a rotational axis of the fan. Such radial fans are also typically configured to receive an airflow in an axial direction (parallel to the rotational axis of the radial fan). Thus, radial fans typically receive an airflow in an axial direction (i.e. flowing along the rotational axis of the fan), turn the airflow approximately 90 degrees, and discharge the airflow in a radial direction (i.e. perpendicular to the rotational axis of the fan). The phrase “over a wearer’s ear” may mean that the ear cup assembly extends across and substantially covers the wearer’s ear (i.e. in the same manner as the ear cups of circumaural or on-ear headphones) as opposed to, for example, being vertically above the wearer’s ear in use. Optional features of the first aspect will now be set out. These are applicable singly or in any combination with any aspect. The radial fan may comprise opposite proximal and distal sides. The proximal side may be closer than the distal side to a wearer’s ear when the head wearable air purifier is worn. The proximal side may be a side of the radial fan facing the wearer’s head when the head wearable air purifier is worn. The distal side may be a side of the radial fan facing away from the wearer’s head when the head wearable air purifier is worn. The distal side and / or the proximal side of the radial fan may be substantially planar. The radial fan may comprise a fan housing. The distal side of the radial fan may be provided by a distal wall of the fan housing. The proximal side of the radial fan may be provided by a proximal wall of the fan housing. One or both of the proximal and distal walls may be substantially planar. The proximal wall and distal wall may be substantially parallel. The proximal wall and distal wall may be spaced from one another to define an internal cavity of the fan housing. The radial fan may comprise an impeller. The impeller may be configured to rotate about a rotational axis. The fan (and thus the impeller) may be oriented such that the rotational axis extends in a direction that is substantially normal to the side of a wearer’s head when the head wearable air purifier is worn. The rotational axis may be substantially normal to the proximal / distal sides of the radial fan (and the proximal / distal walls of the fan housing). The impeller may comprise a hub and a plurality of blades extending radially from the hub. The fan may comprise a motor for driving rotation of the impeller. The hub may comprise (e.g. house) the motor. The impeller may be mounted within the internal cavity of the radial fan. The impeller (e.g. the blades of the impeller) may be configured to discharge air from the radial fan in a radial direction (i.e. substantially perpendicular to the rotational axis). The impeller may be configured to receive air in an axial direction (i.e. parallel to the rotational axis). The impeller may thus be configured to turn air from the axial direction to the radial direction. The radial fan may have a height (taken parallel to the rotational axis) that is substantially smaller than a diameter of the impeller (i.e. taken from a tip one blade to the tip of a diametrically opposite blade, sometimes referred to as blade span or sweep). The fan may have a height of between 4 and 8 mm, e.g. between 5 mm and 7 mm. Unless otherwise specified, height dimensions referred to herein are to be taken in a direction that extends laterally towards / away from the wearer’s head when the air purifier is worn). The height dimension may be parallel to the rotational axis of the fan. The impeller may have a diameter of between 20 mm and 50 mm, e.g. between 30 mm and 40 mm. The diameter may be a tip-to-tip diameter (i.e. taken as the distance between the tip of one blade of the impeller and the tip of an opposite blade of the impeller, sometimes referred to as sweep). Each blade of the impeller may have a height of between 2 mm and 5 mm, e.g. between 3 mm and 4 mm, e.g. about 3.6 mm. The hub of the impeller may have a height of between 3 mm and 6 mm, e.g. between 4 mm and 5 mm. The ratio of the diameter of the impeller to the height of the fan may be between 8:1 and 4:1, e.g. between 7:1 and 5:1, e.g. about 6:1. The radial fan may comprise a distal airflow inlet on the distal side of the radial fan. In this way, air flow into the radial fan may be further from a wearer’s ear in operation (and when the head wearable air purifier is worn). This may reduce transmission of noise (for example, created by such airflow) to the wearer’s ear. The distal inlet may be arranged to face away from the wearer’s ear when the head wearable air purifier is worn. Again, this may reduce noise that is perceptible to the wearer. By providing an opening to the fan (i.e. the distal inlet) facing away from the wearer’s ear, noise generated by the fan and the airflow into the fan may be directed away from the wearer. The radial fan may comprise a proximal airflow inlet provided on the proximal side of the radial fan. The proximal airflow inlet may be provided in addition to the radial airflow inlet, or the proximal airflow inlet may be provided as an alternative to the radial airflow inlet. That is, the radial fan may comprise a distal inlet only, a proximal inlet only, or the combination of a distal inlet and a proximal inlet. The provision of both a proximal inlet and a distal inlet can distribute airflow between the proximal and distal sides of the radial fan. This may reduce the noise generated at each inlet. For example, noise can be generated by airflow at the periphery of the inlet (i.e. at an edge of the fan housing that defines the inlet) and can be dependent on the flow rate of air through the inlet. By providing both proximal and distal inlets, the flow rate through each inlet may be reduced, which can therefore reduce the noise generated by the airflow through the inlet. The radial fan may be soft mounted within the fan assembly housing. The radial fan may be mounted to the fan assembly housing by a resilient mounting element (e.g. compression spring, elastic (e.g. rubber) element, etc). The proximal and / or distal side of the fan may be mounted to the fan assembly housing (e.g. by a resilient mounting element). Such soft mounting can be desirable to allow the radial fan to move relative to the fan assembly housing of the fan assembly. This can protect the fan against shock loading (e.g. as a result of the fan assembly being dropped). Such relative movement may include relative movement in a direction towards / away from a wearer’s ear when the head wearable air purifier is worn (e.g. along the rotational axis of the fan). As may be appreciated, to accommodate such movement, a proximal space may be provided between the proximal side of the radial fan and the fan assembly housing, and a distal space may be provided between the distal side of the radial fan and the fan assembly housing. The proximal space and distal space may increase the overall height of the fan assembly. However, by providing both proximal and distal inlets, the proximal and distal spaces may be utilised for airflow to the radial fan. In this way, the proximal space and distal space may provide dual functionality (i.e. space for soft mounting and space for airflow to the fan). This can aid in minimising the overall height of the fan assembly compared to arrangements in which a soft mounted fan with a single inlet is provided. This is because in arrangements with a single inlet, greater airflow must be supplied through the corresponding proximal / distal space at which the inlet is located and that may require the space to be larger than necessary for soft mounting alone. The distal inlet may have a circular cross-sectional shape. The distal inlet may have an annular cross-sectional shape. The proximal inlet may have a circular cross-sectional shape. The proximal inlet may have an annular cross-sectional shape. The proximal and / or distal inlet may have other shapes, including non-circular shapes (e.g. rectangular, hexagonal, etc). The distal inlet may have a greater cross-sectional area than the proximal inlet. Thus, a greater flow rate of air may enter the fan through the distal inlet compared to the proximal inlet. This may mean more noise is generated at the distal inlet relative to the proximal inlet, away from wearer’s ear. This may reduce the noise that is perceptible by a wearer. The distal inlet may have a cross-sectional area of between 500 mm2 and 1200 mm2, or e.g. between 600 mm2 and 1000 mm2, or e.g. between 800 mm2 and 900 mm2. The proximal inlet may have a cross-sectional area of between 100 mm2 and 700 mm2, or e.g. between 300 mm2 and 500 mm2, or e.g. between 350 mm2 and 400 mm2. A ratio of the cross-sectional area of the distal inlet to the proximal inlet may be between 1.5:1 and 3:1, e.g. between 2:1 and 2.5:1. The ear cup assembly may comprise an airflow plenum. The radial fan may be received within the airflow plenum (e.g. substantially fully or partly). Receipt of the radial fan within the airflow plenum may be such that the distal and / or proximal airflow inlet is arranged to draw airflow from the airflow plenum. Supplying air to the radial fan via an airflow plenum may increase efficiency of the radial fan by reducing pressure drop of such airflow. Likewise, the provision of air via an airflow plenum (rather than e.g. dedicated passages) may simplify the structure of the fan assembly and may thus reduce the weight of the fan assembly. The airflow plenum may be defined within a plenum housing. The plenum housing may form part of the fan assembly housing. The radial fan may be mounted to the plenum housing (e.g. by a resilient mounting element). The plenum housing may comprise a plenum inlet for airflow into the plenum housing. The plenum inlet may extend around the fan (e.g. circumferentially around the fan). The airflow plenum may have a height of between 8 mm and 16 mm, e.g. between 10 and 14 mm, e.g. about 12 mm. The height of the airflow plenum may be taken between e.g. opposing distal and proximal walls defining the airflow plenum (e.g. such walls being parallel to the distal / proximal sides of the fan).The ratio of the height of the airflow plenum to the height of the radial fan may be between 1.5:1 and 3:1, or e.g. between 2:1 and 2.5:1. The height of the ear cup assembly may be between 30 mm and 90 mm, or e.g. between 40 mm and 80 mm, or e.g. between 50 mm and 70 mm, or e.g. between 55 mm and 65 mm. Such height may be determined as the maximum height of the ear cup assembly (i.e. the lateral distance between the distal-most part and proximal-most part of the ear cup assembly). The width of the ear cup assembly may be between 60 mm and 120 mm, or e.g. between 70 mm and 110 mm, or e.g. between 80 mm and 100 mm or e.g. between 85 mm and 95 mm. the width dimension may be taken perpendicular to the height dimension. The ratio of the width to height of the ear cup assembly may be between 1:1 and 2:1, or e.g. between 1.2:1 and 1.8:1, or e.g. about 1.5:1. The ear cup assembly may comprise an external inlet formed in an external wall of the ear cup assembly (i.e. for flow of external / ambient air into the ear cup assembly). The external inlet may be arranged for airflow into the ear cup assembly in a direction that is angled with respect to (i.e. non-parallel with) a rotational axis of the radial fan. Thus, the external inlet may be arranged such that an angle between the airflow direction into the external inlet and the rotational axis may be greater than 45 degrees, for example 45 to 135 degrees, or e.g. 80 to 110 degrees. The external inlet may be arranged for airflow into the ear cup assembly in a direction that is substantially perpendicular to the rotational axis. The external inlet may be arranged for airflow into the ear cup assembly in a substantially radially inward direction with respect to the rotational axis. The external inlet may extend in a substantially circumferential direction around the radial fan (i.e. the circumferential direction being a direction extending around the rotational axis). The external inlet may extend substantially fully around the radial fan in the circumferential direction (e.g. may extend for more than 70% of the circumference of a reference circle extending around the radial fan, or e.g. may extend for more than 80% or 90% of the circumference of the reference circle). Arranging the external inlet so as to circumferentially surround the radial fan may aid in maximising the volume of air that can be drawn into the fan assembly by the fan, while minimising the height of the fan assembly. The circumferential external inlet may have a greater cross-sectional area than, for example, would be the case with an axial inlet (i.e. aligned along the rotational axis). The greater cross-sectional area may help to minimise pressure drop in airflow supplied to the radial fan. Minimising pressure drop may be especially desirable when the air is filtered (as discussed further below) upstream of the radial fan (because such filtering may already create some pressure drop in the airflow). The external inlet may comprise an array of apertures. The array of apertures may be arranged in a regular pattern (e.g. may be a grid-like array of apertures). The external inlet may comprise one or more slots (e.g. extending circumferentially about the radial fan). In some embodiments, the external inlet may consist of a single aperture. The head wearable air purifier may comprise an external outlet. The external outlet may be connected to the fan outlet by a duct. The external outlet may form part of the ear cup assembly. The external outlet may form part of a nozzle e.g. releasably mountable to the ear cup assembly. Such a nozzle may be configured for delivery of air to a lower nasal and mouth region of the wearer. The external outlet may be angled away from the headgear, for example, downwards to the wearer’s mouth area when the head wearable air purifier is worn. The nozzle may be configured to be contactless to the wearer’s face (for example, spaced apart from the wearer’s mouth and / or nasal area when the head wearable air purifier is worn with the nozzle attached). The nozzle may be configured to be face-contacting (for example, the nozzle may be configured to contact the wearer’s mouth and / or nasal area when the head wearable air purifier is worn with the nozzle attached). The fan assembly may comprise a filter (e.g. for removing particulates from air). The filter may be as described below with respect to the third aspect. For the avoidance of any doubt, the filter may include any one or more of the features of the filter described as part of the wearable air purifier of the third aspect. The filter may be upstream of the radial fan, such that air supplied to the radial fan is filtered. This may prevent (or at least reduce the possibility) or particulates entering the radial fan, which could otherwise be to the detriment of the operation of the fan. The filter may be directly downstream of the external outlet. The filter may define a void (e.g. central void) in which the radial fan is located (e.g. nested). The airflow plenum (and e.g. the plenum and plenum housing) may be located within the (e.g. central) void. Thus, the filter may comprise inner surfaces separated by the void in which the fan is located. The location of the fan in the (e.g. central) void may provide a relatively compact arrangement so as to minimise the overall size of the fan assembly (in particular, the height of the fan assembly). The fan assembly may comprise an airflow duct extending from an outlet of the radial fan. The filter may comprise a gap through which the airflow duct extends. The gap may extend at least partway along a thickness of the filter (e.g. thickness being taken in a direction along the rotational axis of the fan, or in other embodiments in a radial direction, perpendicular to the rotational axis). The radial fan may be arranged concentrically with the filter. Thus, the filter may extend at least partway around the fan. The filter may extend circumferentially around the fan. The filter may extend around the void in which the fan is located. The filter may be annular, so as to surround the void and / or the fan. The filter may extend around the fan from a first end portion to a second end portion. The gap may be defined between the first and second end portions. The end portions may represent ends (i.e. extremities) of the filter. Alternatively, the end portions may be portions of the filter defining sides of (and may be spaced either side of) a recess or hole formed in the filter. The filter may extend from the first end portion of the filter to the second end portion of the filter along a C-shaped path. The C-shaped path may form a major arc of a circle. Thus, the filter may extend for more than 50% of the circumference of a reference circle extending around the void and / or fan. The fan assembly may be a removable portion of the ear cup assembly. That is, the fan assembly may be releasably mountable (i.e. to the remainder of the ear cup assembly). Accordingly, the ear cup assembly may be worn by a wearer without the fan assembly mounted thereto. This may allow a user to effectively reduce the weight and / or volume of the ear cup assembly when the fan assembly is not required. The releasable mounting of the fan assembly may be provided by e.g. magnetic connection, bayonet fitting, snapengagement, etc. The ear cup assembly may comprise a speaker. Accordingly, the head wearable air purifier may be in the form of a pair of headphones (e.g. circumaural or on-hear headphones). The speaker may be arranged so as to be between the radial fan and the wearer’s ear when ear cup assembly is worn. Thus, the speaker may be arranged in proximity to the proximal side of the radial fan (as opposed to the distal side of the radial fan). In this way, the speaker may be distal from noise generated by airflow into the fan (i.e. when the radial fan includes a distal inlet only, or when both proximal and distal inlets are provided but the distal inlet is larger than the proximal inlet). The ear cup assembly may comprise a power source (e.g. a battery, such as a rechargeable battery). The power source may be operatively connected to the radial fan to supply power to the radial fan. The power source may be operatively connected to the speaker to supply power to the speaker. The ear cup assembly may comprise a speaker housing within which the speaker is housed. The fan assembly may be releasably mountable to the speaker housing (e.g. rotatably mountable, such as for rotation about an axis parallel to the rotational axis). As above, such releasable mounting may be provided by e.g. magnetic connection, bayonet fitting, snapengagement, etc. Thus, a wearer may be able to remove the fan assembly from the speaker housing to reduce the weight and / or volume of the ear cup assembly when the fan assembly is not required. The head wearable air purifier may further comprise headgear for wearing on a wearer’s head. The headgear may be in the form of a headband (i.e. to extend over the top of a wearer’s head). The fan assembly may be releasably mountable to the headgear. The fan assembly may rotatably mountable to the headgear (e.g. for rotation about an axis parallel to the rotational axis). Providing a fan assembly that is rotatably mountable may enable a wearer to adjust a rotational position of the fan assembly relative to the headgear, for example to selectively rotate the fan assembly. The ear cup assembly may be mounted to the headgear so as to locatable over a wearer’ ear when the headgear is worn. The mounting of the ear cup assembly to the headgear may be such that the ear cup assembly is rotatable relative to the ear cup assembly (e.g. may be rotatable about an axis that extends towards / away from a wearer’s head when the headgear is worn). The ear cup assembly may be a first ear cup assembly and the head wearable air purifier may comprise a second ear cup assembly mounted at an opposite side of the headgear to the first ear cup assembly. Each ear cup assembly may be as described above. In this way, each ear cup assembly may be worn over a respective left or right ear of a wearer. Accordingly, the head wearable air purifier may be in the form of a pair of headphones (when provided with speaker(s)) or e.g. ear warmers. As set forth above, the head wearable air purifier may comprise a nozzle. The nozzle may be configured to deliver airflow from each fan assembly to the wearer (e.g. to a lower nasal and mouth region of the wearer). The nozzle may be attachable at left and right ends thereof to a respective one of the ear cup assemblies. The nozzle may be attachable at left and right ends thereof to a respective one of the fan assemblies. In some embodiments the nozzle may be fixed to the fan assemblies, and removal of the nozzle from the headgear may be provided by release of the fan assemblies from the headgear / speaker housings. The nozzle may define air inlets at the left and right ends that communicate with the air outlets of the fan assemblies (i.e. to receive purified airflow from the fan assemblies). The nozzle may be configured to extend from the left and right ends width-wise across the wearer’s face, and may comprise an air outlet at a central region approximately mid-way between the left and right ends to be positioned approximately directly in front of and facing the wearer’s lower nasal and mouth region, in use. In this way, the nozzle may function to duct filtered airflow from each of the fan assemblies (which may be supported at the sides of the wearer’s head), to the central outlet of the nozzle, so as to discharge the airflow from each fan assembly towards the lower nasal and mouth region of the wearer’s face. The head wearable air purifier of the first aspect may be as otherwise described below with respect to the third aspect. That is, the head wearable air purifier of the first aspect may include any one or more feature of the wearable air purifier of the third aspect. For example, the fan assembly of the first aspect may be the same as that of the third aspect. In a second aspect there is provided a detachable fan assembly for a head wearable air purifier, the detachable fan assembly comprising: a mounting portion for detachable mounting to headgear of a head wearable air purifier; and a radial fan operable to generate airflow for discharge to a wearer of the headgear when the detachable fan assembly is mounted to the headgear. The fan assembly of the second aspect may be as otherwise described with respect to the first aspect. Thus, for example, the fan assembly of the second aspect may include one or more of the optional features of the first aspect described above. The detachable fan assembly may, for example, form part of an ear cup assembly such that the detachable fan assembly may be worn over the wearer's ears when the head wearable air purifier is worn. The ear cup assembly may comprise a speaker housing supported by the headgear. The detachable fan assembly may be releasably mountable to the speaker housing. In a third aspect there is provided a wearable air purifier, comprising: headgear for wearing on a wearer’s head; and a fan assembly supported by the headgear, the fan assembly comprising: a fan operable to generate an airflow; an airflow duct extending (e.g. from an outlet of) the fan such that air discharged by the fan in use, flows along the airflow duct; and a filter to filter the airflow, the filter comprising inner surfaces separated by a void (e.g. central void) in which the fan is located, and the inner surfaces separated by a gap through which the airflow duct extends, the gap extending at least partway along a thickness of the filter. For comfort, it is generally desirable for any device worn by a wearer to present minimal obstruction to the movement of a user. This is especially the case for devices worn on a wearer’s head. The wearable air purifier of the third aspect provides such minimal obstruction by way of a compact fan assembly (in particular, a fan assembly of minimal height, taken in the direction of the thickness of the filter). This is partly achieved by providing a filter having inner surfaces separated by (and thus defining) a void into which the fan of the fan assembly is located. In this way, the fan may effectively be at least partly nested within a central void of the filter so as to reduce an overall height of the fan assembly. To further minimise the height of the fan assembly, the filter is also configured to accommodate an air flow duct of the fan assembly. In particular, as set forth above, the gap (separating the inner surfaces) extends at least partway along (i.e. into) the thickness of the filter, meaning that the duct and filter can effectively at least partly overlap in the thickness direction. As may be appreciated, such an overlapping arrangement reduces the overall height of the fan assembly (i.e. compared to an arrangement, for example, in which the duct was to extend fully above or below the filter). Further, the provision of a filter with inner surfaces separated by a void in which the fan is located effectively means the filter partly surrounds the fan. Such an arrangement can increase the amount of airflow available to the fan (which can reduce pressure drop in the air supplied to the fan), when compared e.g. to arrangements in which a single point of access is provided for airflow to the fan. Optional features of the third aspect will now be set out. These are applicable singly or in any combination with any aspect. Each of the fan assembly, fan and filter may comprise a proximal side and an opposite distal side. Each proximal side (i.e. of the fan assembly / fan / filter) may be closer than the respective distal side (i.e. of the fan assembly / fan / filter) to a wearer’s head when the wearable air purifier is worn. The thickness direction of the filter may extend normal to (and between) the proximal and distal sides of the filter. Thus, the gap may be formed along a direction that extends towards / away from a wearer’s head when the wearable air purifier is worn. The filter may comprise one or more filter media and at least one filter frame that houses the one or more filter media. The filter media or each filter medium may be porous. In some embodiments, the filter may comprise a single filter medium which may extend for e.g. substantially an entire length of a filter frame. Alternatively, the filter may comprise a plurality of filter media (housed by a filter frame or housed by a plurality of filter frames). The one or more filter media may comprise any of a particulate filter medium and a chemical filter medium. The one or more filter media may be configured to remove VOCs and / or NOx from the air. The one or more filter media may comprise carbon. The one or more filter media may comprise (e.g. non-woven) electrostatic media (e.g. with a coating, such as a polypropylene or polytetrafluoroethylene (PTFE) coating). Alternatively, the one or more filter media may comprise wool. The one or more filter media may be pleated (e.g. the filter may comprise a pleated particulate filter medium or a plurality of pleated particulate filter media). Folds (e.g. fold lines) of the one or more pleated particulate filter media, may extend in the thickness direction of the filter. The one or more filter media may comprise a distal side and an opposite proximal side (i.e. the proximal side being closer than the distal side to the wearer when the air purifier is worn). One or both of the proximal and distal sides may be sealed to the one or more filter frame (e.g. by way of one or more resilient sealing members). In some embodiments, one or more portions of the filter (e.g. one or more portions of the filter frame) may define the airflow duct. In other words, the duct may, in some examples, be integrally formed with the filter (e.g. the filter frame). The central void may have a depth in a direction of the thickness of the filter. The fan may be (e.g. substantially wholly) contained within the depth of the void. That is, the fan may have a height (i.e. taken parallel to the direction of the thickness of the filter), that is equal to or less than the depth of the void, and the full height of the fan may be within the depth of the void. Thus, the fan may not protrude, in the thickness direction, beyond the filter (e.g. beyond the proximal or distal sides of the filter). Accordingly, the fan may be fully nested within the filter such that, in effect, the fan does not contribute to the overall height of the fan assembly. The thickness of the filter may be between 10 mm and 20 mm, e.g. between 13 mm and 17 mm. The fan may be (e.g. substantially wholly) contained within the void. Thus, in addition to the height of the fan, as discussed above, the lateral extent of the fan (e.g. a width, length and / or diameter of the fan, taken perpendicular to the height of the fan) may be substantially fully contained within the void. The void may have a cross-sectional shape that is substantially circular (i.e. taken perpendicular to the direction of the thickness of the filter). The void may be disc shaped. The void may have other shapes (e.g. hexagonal, octagonal, etc.). The gap (formed in the filter) may have a depth in a direction of the thickness of the filter. A portion (of a length) of the airflow duct extending through the gap may be substantially wholly contained within the depth of the gap. In other words, the portion of the duct extending through the gap may have a height (in the thickness direction) that is fully contained within the depth of the gap. In this way, the duct (or at least the portion of the duct extending through the gap) may not contribute to the overall height of the fan assembly. The gap may be formed by a recess extending partway along the thickness direction of the filter. That is, the recess may extend into one of the distal and proximal sides of the filter. The gap may alternatively be in the form of a hole extending through the filter in a direction perpendicular to the thickness of the filter (e.g. in a radial direction). In such embodiments, the duct may therefore extend through the filter. In embodiments in which the gap is formed by a recess or by a hole, the inner surface(s) of the filter may form portions of a single continuous inner surface. In other words, while the inner surface(s) may be separated by the gap, other portions of the continuous inner surface may extend across the gap (i.e. when the gap is such that it does not extend fully through the filter in the thickness direction). In alternative embodiments, the gap may extend fully through the filter in the thickness direction of the filter. In other words, the depth of the gap may be the same as the thickness of the filter (e.g. taken between the distal and proximal sides). Such an arrangement may maximise the depth of the gap (i.e. in the thickness direction) and thus the height of the duct able to be received fully in the gap. The gap may be contiguous with the void. The gap may be fluidly connected to the void, or the gap may be fluidly disconnected from the void. The filter may extend around the void in which the fan is located. The filter may thus extend at least partway around the fan. The filter may extend circumferentially around the fan. The filter may extend around the void in which the fan is located. The filter may be annular, so as to (e.g. substantially fully) surround the void and / or the fan). The filter may extend around a rotational axis of the fan. The rotational axis of the fan may be parallel to the thickness direction of the filter, or in other embodiments may be perpendicular to the thickness direction of the filter). The filter may extend around the fan from a first end portion to a second end portion. The gap may be defined between the first and second end portions. The end portions may represent ends (i.e. extremities) of the filter. Alternatively, the end portions may be portions of the filter defining sides of (and may be spaced either side of) a recess or hole formed in the filter. The filter may extend from the first end portion of the filter to the second end portion of the filter along a C-shaped path. The C-shaped path may form a major arc of a circle. Thus, the filter may extend for more than 50% of the circumference of a reference circle extending around the void and / or fan. At least one of the first and second end portions may seal against an outer surface of the airflow duct. The first and second end portions may form part of the filter frame (e.g. may be opposite end surfaces of the filter frame). Thus, the filter frame may seal against the outer surface of the airflow duct. In such embodiments, the gap may have a width (taken perpendicularly to the thickness direction) that is substantially the same as a width of the duct. A maximum width (e.g. a maximum distance between the first and second end portions of the filter) of the gap may be between 15 mm and 50 mm, or e.g. between 20 mm and 40 mm, or e.g. between 25 mm and 35 mm. The gap may subtend an angle of between 30° and 60°, or e.g. between 40° and 50°, with respect to a central axis of the void (i.e. taken in the direction of thickness of the filter). The airflow duct may extend laterally with respect to a rotational axis of the fan. The air flow duct may extend tangentially with respect to the fan. The airflow duct may be sloped relative to the radial direction (i.e. a direction perpendicular to the rotational axis). For example, the airflow duct may slope towards the proximal side of the fan assembly. The filter may be upstream of an inlet of the fan. Thus, air entering the inlet of the fan may be filtered air. This may eliminate (or at least reduce) particulates in the air entering the fan, which may be beneficial to operation of the fan. A central axis of the filter (or of the void) may be coaxial with a rotational axis of the fan. Alternatively, the central axis of the filter (or the void) may be offset radially from rotational axis of the fan. The central axis of the filter may extend through the geometric centre of the filter. The central axis of the void may extend through the geometric centre of the void. The fan assembly may comprise an external inlet formed in an external wall of the fan assembly (i.e. for flow of external / ambient air into the fan assembly). The external inlet may be arranged for airflow into the fan assembly in a direction that is angled with respect to (i.e. non-parallel with) a rotational axis of the fan. Thus, the external inlet may be arranged such that an angle between the airflow direction into the external inlet and the rotational axis may be greater than 45 degrees, for example 45 to 135 degrees, or e.g. 80 to 110 degrees. The external inlet may be arranged for airflow into the fan assembly in a direction that is substantially perpendicular to the rotational axis of the fan. The external inlet may be arranged for airflow into the fan assembly in a substantially radially inward direction with respect to the rotational axis of the fan. The external inlet may extend in a substantially circumferential direction around the filter and / or fan. The external inlet may comprise an array of apertures. The array of apertures may be arranged in a regular pattern (e.g. may be a grid-like array of apertures). The external inlet may comprise one or more slots (e.g. extending circumferentially about the filter and / or fan). In some embodiments, the external inlet may consist of a single aperture. The fan may be a radial fan. The fan of the third aspect may be the same as that described in the first aspect. Thus, for example, the fan may include any one or more of the features of the radial fan of the first aspect (including optional features of the first aspect). As set forth above, the fan may comprise opposite proximal and distal sides (and the proximal side may be closer than the distal side to a wearer’s head when the headgear is worn). One or both of the distal sides of the fan may be located within the void of the filter. The fan may comprise a distal airflow inlet on the distal side of the fan. The fan may alternatively or additionally comprise a proximal airflow inlet provided on the proximal side of the radial fan. When the fan comprises both a proximal and distal inlet, the distal inlet may have a greater cross-sectional area than the proximal inlet. The radial fan may be soft mounted within the fan assembly housing. The radial fan may be mounted to the fan assembly housing by a resilient mounting element (e.g. compression spring, elastic (e.g. rubber) element, etc). The ear cup assembly may comprise an airflow plenum. The fan may be received within the airflow plenum (e.g. substantially fully or partly). Receipt of the fan within the airflow plenum may be such that the distal and / or proximal airflow inlet is arranged to draw airflow from the airflow plenum. The airflow plenum may be defined within a plenum housing. The plenum housing may form part of the fan assembly housing. The radial fan may be mounted to the plenum housing (e.g. by a resilient mounting element). The plenum housing may comprise a plenum inlet for airflow into the plenum housing. The plenum inlet may extend around the fan (e.g. circumferentially around the fan). The filter may extend around the plenum (and e.g. the plenum housing). The filter may be provided between the external inlet and the plenum. The fan assembly may be releasably mountable to the headgear. The fan assembly may be rotatably mounted to the headgear (e.g. may be rotatable about an axis parallel to the rotational axis relative to the headgear). The wearable air purifier may comprise a speaker. The speaker may be in a speaker housing, which may be supported by the headgear. The fan assembly may be releasably mountable to the speaker housing. The fan assembly may be rotatably mountable to the speaker housing (e.g. may be rotatable about the rotational axis relative to the speaker housing). The fan assembly may form part of an ear cup assembly. Together, the speaker housing and fan assembly may form the ear cup assembly. The ear cup assembly may be as described above in the first aspect (e.g. may include one or more of the features of the ear cup assembly described in the first aspect). The ear cup assembly may be mounted to the headgear so as to locatable over a wearer’ ear when the headgear is worn. The ear cup assembly may be a first ear cup assembly and the head wearable air purifier may comprise a second ear cup assembly mounted at an opposite side of the headgear to the first ear cup assembly. Each ear cup assembly may be as described above (and may include a fan assembly as described above). In this way, each ear cup assembly may be worn over a respective left or right ear of a wearer. Accordingly, the head wearable air purifier may be in the form of a pair of headphones (when provided with speaker(s)) or e.g. ear warmers. The wearable air purifier may comprise a nozzle. The nozzle may be configured to deliver airflow from each fan assembly to the wearer (e.g. to a lower nasal and mouth region of the wearer). The nozzle may be attachable at left and right ends thereof to a respective one of the ear cup assemblies. The nozzle may be attachable at left and right ends thereof to a respective one of the fan assemblies. In some embodiments the nozzle may be fixed to the fan assemblies, and removal of the nozzle from the headgear may be provided by release of the fan assemblies from the headgear / speaker housings. The nozzle may define air inlets at the left and right ends that communicate with the air outlets of the fan assemblies (i.e. to receive purified airflow from the fan assemblies). The nozzle may be configured to extend from the left and right ends width-wise across the wearer’s face, and may comprise an air outlet at a central region approximately mid-way between the left and right ends to be positioned approximately directly in front of and facing the wearer’s lower nasal and mouth region, in use. In this way, the nozzle may function to duct filtered airflow from each of the fan assemblies (which may be supported at the sides of the wearer’s head), to the central outlet of the nozzle, so as to discharge the airflow from each fan assembly towards the lower nasal and mouth region of the wearer’s face. The wearable air purifier of the third aspect may be as otherwise described above with respect to the first aspect. That is, the wearable air purifier of the third aspect may include any one or more features of the head wearable air purifier of the first aspect. For example, the fan assembly of the third aspect may be the same as that of the first aspect. In a fourth aspect, there is provided a detachable fan assembly for a wearable air purifier, the detachable fan assembly comprising: a mounting portion for detachable mounting to headgear of a wearable air purifier; a fan operable to generate an airflow; an airflow duct extending from (e.g. an outlet of) the fan such that air discharged by the fan in use, flows along the airflow duct. The detachable fan assembly may comprise a filter to filter the airflow. The filter may comprise inner surfaces separated by a void in which the fan is located, and the inner surfaces separated by a gap through which the airflow duct extends. The gap may extend at least partway along a thickness of the filter. In some embodiments, the filter may be removably mountable in the fan assembly. The duct may be formed upon mounting of the filter in the fan assembly. The fan assembly of the fourth aspect may be as otherwise described with respect to the third aspect. Thus, for example, the fan assembly of the second aspect may include one or more of the optional features of the first aspect described above. In a fifth aspect, there is provided a wearable air purifier, comprising: headgear for wearing on a wearer’s head; and a fan assembly supported by the headgear, the fan assembly comprising: a fan operable to generate an airflow; an airflow duct extending from the fan such that air discharged by the fan is configured to flow along the airflow duct; and a filter to filter the airflow, wherein the filter is configured to partially wrap-around the fan such that the airflow duct extends through a gap in the filter. The wearable air purifier of the fifth aspect may be as otherwise described above with respect to the first or third aspect. That is, the wearable air purifier of the fifth aspect may include any one or more features of the wearable air purifier of the third aspect or the head wearable air purifier of the first aspect. Thus, for example, the fan assembly and / or filter of the fifth aspect may be as described above with respect to the first aspect or the third aspect. In a sixth aspect, there is provided a wearable air purifier, comprising: headgear for wearing on a wearer’s head; and a fan assembly operable to generate a filtered airflow, the fan assembly supported by the headgear. The fan assembly comprising: a fan comprising an impeller that is rotatable about a rotational axis; an airflow duct extending from the fan; and a filter extending partway around the fan in a direction about the rotational axis from a first end portion to a second end portion, the end portions spaced apart to define a gap through which the airflow duct extends. The wearable air purifier of the sixth aspect may be as otherwise described above with respect to the first or third aspect. That is, the wearable air purifier of the sixth aspect may include any one or more features of the wearable air purifier of the third aspect or the head wearable air purifier of the first aspect. Thus, for example, the fan assembly and / or filter of the sixth aspect may be as described above with respect to the first aspect or the third aspect. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa perspective view of a wearable air purifier. Figures 2A and 2B are schematic section views of an ear cup assembly of the wearable air purifier. Figure 3 is a perspective view of a fan assembly of the wearable air purifier. Figure 4 is a cutaway perspective view of a fan assembly. Figure 5 is a side section view of the fan assembly. Figure 6 is a cutaway perspective view of a fan of the fan assembly. Figure 7 is a top section view of the fan assembly. Figure 8 is a perspective view of a fan and filter of the fan assembly. DETAILED DESCRIPTION Figure 1 illustrates a wearable air purifier 10 (in particular, a head wearable air purifier) comprising headgear 11 for wearing on a wearer’s head. In the illustrated embodiment the headgear 11 is in the form of a headband (i.e. to extend over the top of the wearer’s head), but in other embodiments alternative headgear may be provided. The wearable air purifier 10 includes first and second ear cup assemblies 12, 13 mounted at opposite ends of the headgear 11 so as to locate over a wearer’s ears when the headgear 11 is worn. Each ear cup assembly 12, 13 includes a speaker housing 14 supported by the headgear 11, and a fan assembly 15 that is releasably mountable to the speaker housing 14. In other embodiments, the fan assembly 15 may not be releasably mountable (i.e. may be fixed with the speaker housing 14 so as to form an integral, non-removable part of the ear cup assembly 12, 13). In use, air is drawn into a circumferential external air inlet 16 of each fan assembly 15 and is discharged from an external outlet 17 of each fan assembly 15 for delivery to a user. In particular, the airflow is delivered towards a lower nasal and mouth region of a wearer’s face. In the illustrated embodiment, the delivery of the airflow to this region of the wearer’s face is provided by a nozzle 18. The nozzle 18 is attachable at left 19 and right 20 ends thereof to a respective one of the ear cup assemblies 12, 13, as shown by respective dashdot arrow lines 70, 71 The nozzle 18 defines nozzle air inlets 21 at the left 19 and right 20 ends (of the nozzle 18) that communicate with the fan assembly outlets 17 of the fan assemblies 15 (i.e. to receive purified airflow from the fan assemblies 15). The nozzle 18 is configured to extend from the left 19 and right 20 ends width-wise across the wearer’s face, and comprises a nozzle air outlet 22 at a central region approximately mid-way between the left 19 and right 20 ends to be positioned approximately directly in front of and facing a wearer’s lower nasal and mouth region, in use. The nozzle 18 thus functions to duct filtered airflow from each of the fan assemblies 15 supported at the sides of the wearer’s head, to the central nozzle outlet 22 of the nozzle 18, so as to discharge the airflow from each fan assembly 15 towards the lower nasal and mouth region of the wearer’s face. Each speaker housing 14 houses a speaker 23 directed towards the wearer’s ear when the wearable air purifier 10 is worn, allowing the wearable air purifier 10 to function as a pair of headphones. An annular cushion 24 is also provided on an inner side (facing the wearer’s head) of each speaker housing 14 so as to seat against the side of the wearer’s head (for comfort) when the wearable air purifier 10 is worn. The annular cushion 24 surrounds the speaker 23 to define a space into which sounds is directed by the speaker 23. It should be appreciated that both ear cup assemblies 12, 13 are the substantially the same, except that one ear cup assembly 12, 13 is a mirror image of the other. The internal components of the second ear cup assembly 13 are shown schematically in Figure 2A (in which the fan assembly 15 is separated from the speaker housing 14) and Figure 2B (in which the fan assembly 15 is mounted to the speaker housing 14). It should be appreciated that the below description is also equally applicable to the first ear cup assembly 12. As apparent from Figures 2A and 2B, the fan assembly 15 comprises a radial fan 16 that has a rotational axis 26 (see Figure 2B) oriented such that when the ear cup assembly 13 is worn, the rotational axis 26 is substantially normal to the side of the wearer’s head (i.e. it extends laterally in a direction away from / towards the wearer’s ear). As is particularly apparent from Figure 2A, to facilitate mounting of the fan assembly 15 to the speaker housing 14, the fan assembly 15 includes an intruding portion 27 and the speaker housing 14 includes a complementary protruding portion 28. The protruding portion 28 is provided on a first (mating) wall 29 forming part of the speaker housing 14 and the intruding portion 27 is provided on a second (mating) wall 30 that forms part of the fan assembly 15. The first wall 29 faces outwardly, in a direction away from the wearer’s head when the ear cup assembly 13 is worn. The second wall 30 faces inwardly in a direction towards the wearer’s head when the ear cup assembly 13 is worn. When the fan assembly 15 is mounted to the speaker housing 14, in the direction shown by the dash-dot arrow 74 the first 29 and second 30 walls are brought into abutment with one another. The protruding 28 and intruding 27 portions include sloped side surfaces 31 (i.e. so as to be tapered), which aid in guiding the fan assembly 15 onto the speaker housing 14. The intruding 27 and protruding 28 portions also help to ensure that the fan assembly 15 and speaker housing 14 are correctly aligned. The fan assembly 15 is retained on the speaker housing 14 by a mounting portion 76 of the fan assembly 15, in the form of a magnet. The speaker housing 14 comprises a complementary mounting portion 77, again in the form of a magnet, for detachable mounting of the fan assembly 15 to the speaker housing 14. The mounting portions 76, 77 of the fan assembly 15 and speaker housing 14, could otherwise be in the form of, or may otherwise comprise, e.g. a bayonet connection or a screw connection. The speaker housing 14 is operatively (i.e. electrically) connected to the fan assembly 15 by a first connector 32 (forming part of the speaker housing 14) that connects with a second connector 33 (forming part of the fan assembly 15). The first connector 32 is operatively connected to a power supply 34 (e.g. a rechargeable battery) for supply of power to the first connector 32. When connected to the second connector 33 (i.e. when the fan assembly 15 is mounted to the speaker housing 14), the first connector 32 is able to supply power to the second connector 33, and thus to the fan 25 (to which the second connector 33 is operatively connected). The speaker housing 14 also houses a controller 35 for controlling the supply of power to the first connector 32 (and thus, ultimately, to the fan 25). Likewise, the controller 35 controls the supply of power to the speaker 23 (although in other embodiments, separate controllers may be provided for each of the speaker 23 and the fan 25). In use, the fan 25 is operated to draw an airflow 72 into the external inlet 16 of the ear cup assembly 13. The airflow 72 is drawn through a filter 36 that is upstream of the fan 25, such that the air received by the fan 25 is filtered. The fan 25 than discharges a filtered airflow 73 from the fan assembly 15 via the external outlet 17. Figures 3 to 8 further illustrate the fan assembly 15. As best seen in Figure 3, the fan assembly 15 includes a substantially cylindrical fan assembly housing 37 that comprises a plenum housing 38 and a C-shaped filter cover 39 that is concentrically arranged around the plenum housing 38. The fan assembly housing 37 has a distal side 40 (away from the head of the wearer when worn) that is substantially planar, and an opposite proximal side 41 (near to the head of the wearer when wom). A circumferential side wall 42 of the fan assembly housing (provided by the filter cover 39) is sloped between the proximal 41 and distal sides 40, such that the fan assembly housing 37 has a generally frustoconical shape (tapering towards the distal side 40). The filter cover 39 includes the circumferential external inlet 16 of the fan assembly 15, which comprises an array of apertures arranged in a regular grid-like pattern. A duct 43 (of rectangular cross-sectional shape) extends laterally from the plenum housing 38 on a slope towards the proximal side 41 of the fan assembly housing 37. The duct 43 defines the external outlet 17 of the fan assembly (which is rectangular shaped). The interior of the fan assembly 15 is shown in Figures 4 and 5. As is apparent from these figures, the fan assembly housing 37 houses the radial fan 25 that is operable to generate an airflow. The fan 25 is shown in detail in Figure 6 and includes a fan housing 44 and an impeller 45 housed within the fan housing 44 and that is rotatable about a rotational axis 26. The impeller 45 includes a hub 46 (housing a motor) and blades 47 that extend radially from the hub 46. The fan housing 44 (and thus the fan 25) includes a distal side 48 that is distal from the wearer’s ear (and head) when the wearable air purifier 10 is wom, and an opposite proximal side 49 that is proximal to the wearer’s ear (and head) when worn. Each of the proximal 49 and distal 48 sides are substantially normal to the rotational axis 26 (i.e. the rotational axis 26 extends in a direction from the proximal side 49 to the distal side 48). A circular distal inlet 50 is provided on the distal side 48 of the fan housing 44 (so as to face towards the wearer’s ear when worn), and an annular proximal inlet 51 is provided on the proximal side 49 of the fan housing 44 (so as to face away from the wearer’s ear when wom). The cross-sectional area of the distal inlet 50 is greater than the cross-sectional area of the proximal inlet 51. The provision of a larger distal inlet 50 means that most air enters the fan 25 at a point that is distal from the wearer’s ear when the wearable air purifier 10 is worn. This can aid in reducing the transmission of noise (generated as a result of airflow into the fan 25) to the wearer’s ear. The fan housing 44 is further provided with an outlet 52, which extends laterally (i.e. perpendicular to the rotational axis 26. The fan 25 (and, indeed, the impeller 45) is configured to draw air into the proximal 51 and distal 50 inlets and to discharge air from the outlet 52. As may be appreciated, air is drawn into the proximal 51 and distal 50 inlets in a generally axial direction, parallel to the rotational axis 26 (although, air flows into the proximal inlet 51 in an opposing direction to that of the distal inlet 50). Air is then driven by the rotating impeller 45 within the fan housing 44 for discharge in a radial direction through the outlet 52 (i.e. perpendicularly to the rotational axis). As may be apparent from Figure 5, in particular, because of the radial nature of the fan 25 (e.g. the blades 47 of the impeller 45 extend radially), the fan 25 is relatively compact, especially in the height dimension (i.e. taken parallel to the rotational axis 26). This helps to limit the overall height of the fan assembly 15 (again, height being taken in the direction of the rotational axis 26). Use of the radial fan 25 minimises the amount the fan assembly 15 protrudes from the wearer’s head when the fan assembly 15 is mounted to the speaker housing 14 and the wearable air purifier 10 is worn. This can increase comfort for the wearer. The fan 25 is mounted within the fan assembly housing 37, in particular within an airflow plenum 53 defined by the plenum housing 38. The fan 25 is mounted such that the proximal 49 and distal 48 sides of the fan 25 are substantially parallel to the respective proximal 41 and distal 40 sides of the fan assembly housing 37. In particular, the fan 25 is soft mounted by resilient elements 54 (see Figure 5) to the plenum housing 38 (i.e. to the proximal side 41 of the fan assembly housing 37). In other embodiments, the fan 25 may of course be soft mounted to other portions of the fan assembly 15. Soft mounting the fan 25 allows the fan 25 to move relative to the remaining parts of the fan assembly 15 and can help to avoid damage to the fan under shock loading (for example, caused by the fan assembly 15 being dropped). To allow such relative movement (at least in a direction along the rotational axis), the fan 25 is spaced (along the rotational axis) from both the proximal 41 and distal 40 sides of the fan assembly housing 37. This forms a proximal space 55 on the proximal side 49 of the fan 25 (within the airflow plenum 53) and a distal space 56 on the distal side 48 of the fan 25 (again, within the airflow plenum 53). In use, air in the airflow plenum 53 is drawn into the proximal 55 and distal 56 spaces and flows into the respective proximal 51 and distal 50 inlets of the fan. Thus, by providing the fan 25 with both proximal 51 and distal 50 inlets, the proximal 55 and distal 56 spaces serve two functions: airflow paths to supply the fan inlets 50, 51 and space to accommodate movement of the fan 25 due to the soft mounting of the fan 25. The duct 43 extends from the outlet 52 of the fan 25. A downstream section 57 of the duct 43 is integral with a circumferential side wall 58 of the plenum housing 38. An upstream section 59 of the duct 43, which connects the downstream section 57 to the fan 25, is separate from the plenum housing 38 and is configured to accommodate movement of the fan 25 (and thus the fan outlet 53) relative to the plenum housing 38. The circumferential sidewall 58 of the plenum housing 38 comprises a plurality of (obround-shaped) slots 60 that are elongate in a direction that is substantially parallel to the rotational axis 26. The slots 60 are spaced evenly along the circumferential side wall 58 and function to allow airflow into the airflow plenum 53. As is apparent from Figure 7 and Figure 8 (which shows only the filter 36 and the fan 25), the filter 36 is C-shaped (i.e. in a plane that is normal to the rotational axis 26 of the fan 25) and extends circumferentially around the plenum housing 38 between first 61 and second 62 end portions that provide opposite extremities (i.e. ends) of the filter 36. The filter 36 includes a rigid filter frame 63 and a filter media 64 that is formed of pleated porous material and that is housed within the filter frame 63. The first 61 and second 62 end portions, which are provided by respective end surfaces 65 of the filter frame 63, seal against opposite sidewalls of the duct 43 (see Figure 7). The filter media 64 extends substantially for the entire length of the filter frame 63 (in other embodiments, the filter 36 may instead include a plurality of separate filter media). The shape of the filter 36 provides for compact arrangement of the fan assembly 15, so as to aid in minimising the height (taken along the rotational axis 26) of the fan assembly 15. As shown in Figure 8 (in which only the fan 26 and the filter 36 are visible), the filter 26 defines a central void 66 in which the fan 26 is located (i.e. nested). The central void 66 has a circular cross-sectional shape (i.e. taken in a plane that is normal to the rotational axis 26), so as to be generally disc shaped. The filter 26 also defines, between the first 61 and second 62 end portions, a gap 67 formed along a thickness direction of the filter 36 (i.e. parallel to the rotational axis 26) in which the duct 43 is located (i.e. nested). In other words, the filter 36 includes diametrically opposed inner surfaces 68 (in this case, regions of a single internal circumferential surface) that are separated by the void 66 and which are also separated by the gap 67 (formed along a thickness direction of the filter 36) through which the airflow duct 43 extends. The thickness of the filter 36 (and thus the depth of the void 66) is greater than the height of the radial fan 25. This means the height of the radial fan 25 is able to be fully accommodated within the central void 66, helping to minimise the overall height of the fan assembly 15. The diameter of the void 66 is also greater than that of the fan 25, meaning that the lateral (or radial) extent of the fan 25 is also able to be fully accommodated within the void 66, again aiding in providing a compact assembly. The depth of the gap 67 (dictated by the thickness of the filter 36) is greater than the height of the duct 43. Thus, again, the duct 43 is able to be fully accommodated within the gap 67. Once again, this helps to minimise the overall height of the fan assembly 15. In use, when the impeller 45 of the fan 25 is rotated (i.e. driven by the motor of the fan 25), an airflow 72 is drawn into the external inlet 16 of the fan assembly 15 and through the filter media 64 of the filter 36 (which is upstream of the fan 25). The airflow 72 passes from the filter 36 into the airflow plenum 53 (through the slots 60 formed in the plenum housing 38). In the airflow plenum 53, the airflow 72 splits into two branches 75a, 75b, which respectively flow into the proximal 55 and distal 56 spaces and then into the proximal 51 and distal 50 inlets of the fan 25. The airflow 72 (i.e. each air flow branch 75a, 75b) enters the fan 25 and is then driven by the impeller 45 within fan housing 44 such that an airflow 73 is discharged from the fan outlet 52 and into the duct 43. When the nozzle 18 (as in Figure 1) is mounted to the fan assembly 15, air in the duct 43 passes through the external outlet 17 of the fan assembly 15 and into the nozzle 18 for supply to the wearer.

Claims

1. A head wearable air purifier comprising an ear cup assembly arranged to be worn over a wearer’s ear, the ear cup assembly comprising a fan assembly having a radial fan operable to generate airflow for discharge to the wearer.

2. The head wearable air purifier according to claim 1, wherein the radial fan comprises:opposite proximal and distal sides, the proximal side being closer than the distal side to a wearer’s ear when the head wearable air purifier is worn; andwherein the radial fan comprises a distal airflow inlet on the distal side of the radial fan.

3. The head wearable air purifier according to claim 2, wherein the distal inlet is arranged to face away from the wearer’s ear when the head wearable air purifier is worn.

4. The head wearable air purifier according to claim 2 or 3, wherein the radial fan comprises a proximal airflow inlet provided on the proximal side of the radial fan.

5. The head wearable air purifier according to claim 4, wherein the ear cup assembly comprises an airflow plenum within which the radial fan is received such that the distal and / or proximal airflow inlet is arranged to draw airflow from the airflow plenum.

6. The head wearable air purifier according to claim 5, wherein the airflow plenum has a height of between 8 mm and 16 mm.

7. The head wearable air purifier according to any one of claims 4 to 6, wherein the distal inlet has a greater cross-sectional area than the proximal inlet.

8. The head wearable air purifier according to claim 7, wherein a ratio of the cross-sectional area of the distal inlet to the proximal inlet is between 1.5:1 and 3:1.

9. The head wearable air purifier according to any preceding claim, wherein the radial fan is mounted to a wall of the airflow plenum by a resilient mounting element.

10. The head wearable air purifier according to claim 1, wherein the radial fan comprises:opposite proximal and distal sides, the proximal side being closer than the distal side to a wearer’s ear when ear cup assembly is worn; andwherein the radial fan comprises a proximal airflow inlet on the proximal side of the radial fan.

11. The head wearable air purifier according to any one of the preceding claims, wherein the ear cup assembly comprises an external inlet formed in an external wall of the ear cup assembly, the external inlet arranged for airflow into the ear cup assembly in a direction that is angled with respect to a rotational axis of the radial fan.

12. The head wearable air purifier according to claim 11, wherein the external inlet is arranged for airflow into the ear cup assembly in a direction that is substantially perpendicular to the rotational axis.

13. The head wearable air purifier according to claim 11 or 12, wherein the external inletis arranged for airflow into the ear cup assembly in a substantially radially inward direction with respect to the rotational axis.

14. The head wearable air purifier according to any one of claim 11 to 13, wherein the external inlet extends in a substantially circumferential direction around the radial fan.

15. The head wearable air purifier according to any one of the preceding claims, wherein the ear cup assembly comprises a speaker.

16. The head wearable air purifier according to claim 15, wherein the speaker is arranged so as to be between the radial fan and the wearer’s ear when ear cup assembly is worn.

17. The head wearable air purifier according to any one of the preceding claims, comprising a filter upstream of the radial fan.

18. The head wearable air purifier according to claim 17, wherein the filter defines a central void in which the radial fan is located.

19. The head wearable air purifier according to claim 18, wherein the fan assembly comprises an airflow duct extending from an outlet of the radial fan, and the filter comprises a gap through which the airflow duct extends, the gap extending at least partway along a thickness of the filter.

20. The head wearable air purifier according to any one of claims 17 to 19, wherein the radial fan is arranged concentrically with the filter.

21. The head wearable air purifier according to any one of the preceding claims comprising an external outlet configured to deliver an airflow towards a lower nasal and mouth region of the wearer’s face, wherein:the external outlet forms part of the ear cup assembly; orthe external outlet forms part of a nozzle mountable to the ear cup assembly.

22. The head wearable air purifier according to any one of the preceding claims, comprising headgear for wearing on a wearer’s head, the ear cup assembly mounted to the headgear so as to locatable over a wearer’ ear when the header is worn.

23. The head wearable air purifier according to claim 22 wherein the fan assembly is releasably mountable to the headgear.

24. The head wearable air purifier according to claim 22 or 23, wherein the ear cup assembly is a first ear cup assembly and the head wearable air purifier comprises a second ear cup assembly mounted at an opposite side of the headgear to the first ear cup assembly.

25. A detachable fan assembly for a head wearable air purifier, the detachable fan assembly comprising:a mounting portion for detachable mounting to headgear of a head wearable air purifier; and5 a radial fan operable to generate airflow for discharge to a wearer of the headgearwhen the detachable fan assembly is mounted to the headgear.33

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