Device with helmholtz resonator

Abstract

A device, such as a fan, an air conditioner, a space heater, an air cleaning or filtration device, or an air freshener, includes an airflow generator 108 and an elongate plenum 110 configured to receive airflow generated by the airflow generator 108; An outlet (112, Fig. 2) extends along at least part of the plenum 110, such that the airflow passing through the plenum 110 exits the outlet (112, Fig. 2); The device includes at least one Helmholtz resonator 114 having an inlet port 116 and a cavity 118, the inlet port 116 being in fluid communication with the plenum 110 at a position that is on the other side of the outlet (112, Fig. 2) relative to the airflow generator 108; The plenum 110 and Helmholtz resonator 114 are arranged and configured to reduce the noise generated by device around at least one frequency while reducing the effect of the resonator 114 on airflow to the outlet.

Description

BACKGROUND Devices for moving air are known. For example, fans can be used to move air for the purposes of generating air currents, air filtration, heating, cooling, and the like. SUMMARY According to the present invention, there is provided a device comprising: an airflow generator; a plenum configured to receive airflow generated by the airflow generator, the plenum being elongate; an outlet extending along at least part of the plenum, such that the airflow passing through the plenum exits the outlet; and at least one Helmholtz resonator having an inlet port and a cavity, the inlet port being in fluid communication with the plenum at a position that is on the other side of the outlet relative to the airflow generator; the plenum and Helmholtz resonator being arranged and configured to reduce the noise generated by device around at least one frequency. By arranging the inlet port of the Helmholtz resonator on the other side of the outlet relative to the airflow generator, the effects of airflow on operation of the Helmholtz resonator may be reduced, and conversely, the effects of the Helmholtz resonator on airflow may be reduced. The device may comprise a plurality of the Helmholtz resonators, each of the Helmholtz resonators being tuned to a different frequency. Helmholtz resonators tuned to different frequencies may offer the ability to reduce noise across corresponding frequencies. The device may comprise a coupling region disposed between the plenum and the inlet port(s) of the Helmholtz resonator(s), the coupling region being configured to improve impedance matching between the plenum and the Helmholtz resonator(s). Improved impedance matching improves efficiency of operation of the Helmholtz resonators. The coupling region may comprise a coupling volume for the or each Helmholtz resonator, each coupling volume being disposed adjacent the inlet port of its corresponding Helmholtz resonator. The use of a coupling volume for each inlet port may allow impedance matching for each Helmholtz resonator. The inlet port(s) of the Helmholtz resonator(s) may be disposed opposite a wall of the plenum, the coupling volume for each Helmholtz resonator being defined between the inlet port of that Helmholtz resonator and a portion of the wall. This arrangement may provide compact and / or convenient to manufacture Helmholtz resonator(s) and coupling volume(s). The cavities of the Helmholtz resonators may be cylindrical or prismatic, and / or may have a constant cross-section along at least one axis. This arrangement may provide compact and / or convenient to manufacture Helmholtz resonator(s). The cavities of the Helmholtz resonators may be of different relative volumes but share a common linear dimension along at least one axis. This arrangement may provide compact and / or convenient to manufacture Helmholtz resonator(s). The cavities and / or the inlet ports of the Helmholtz resonators may be spaced apart in a direction away from the plenum. This may allow for a relatively compact arrangement, and may also simplify modelling and / or design. A volume of the Helmholtz cavities may decrease monotonically with a distance of their respective inlet ports from the plenum. The inlet port(s) may be in fluid communication with the plenum at a point where airflow would be substantially null in the absence of the resonator(s). This may reduce the impact of moving air on operation of the Helmholtz resonator, and may also position the Helmholtz resonator in a position that reduces an impact on airflow. The device may comprise at least two of the plenums, each of the plenums having at least one of the Helmholtz resonators. The at least two plenums may be provided with airflow from a common airflow generator. The device may take the form of one or more of a fan, an air conditioner, a space heater, an air cleaning or filtration device, or an air freshener. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa vertical section through a device in the form of a fan; Figure 2 is a front elevation of the fan of Figure 1; Figure 3 is a perspective view of the fan of Figure 1; Figure 4 is a vertical section through the upper region of a further fan; Figure 5 is a vertical section through the operation of yet a further fan; Figure 6 is a vertical section through an array of Helmholtz resonators; Figure 7 is a perspective view of a further fan comprising two of the arrays of Figure 6; Figure 8 is a front elevation of the fan of Figure 7; Figure 9 is a side elevation of the fan of Figures 7 and 8; Figure 10 is a sectional view taken through lines X-X of Figure 9; Figure 11 is a graph showing modelled sound pressure levels relating to acoustic resonances with and without Helmholtz resonators, for one plenum of the fan of Figures 7 to 10; and Figure 12 is a graph showing sound pressure levels relating to total acoustic output of one plenum of the fan of Figures 7 to 10. DETAILED DESCRIPTION Referring to Figures 1 to 3, there is shown a device for moving air within a room. In the implementation of the Figures 1 to 3, the device takes the form of a fan 100, which can be used for moving air with a room to cool the room’s occupants, for example. Fan 100 comprises a housing 102 including a base 104 that supports housing 102 on a surface such as a table or floor (not shown) Rase 104 comprises a compartment 106 within which is disposed an airflow generator 108. Airflow generator 108 includes, for example, a motor, an impeller driven by the motor, and control circuitry for driving the motor responsive to user inputs via a user interface. The individual elements of airflow generator 108 and user interface, etc, are known, and so are not described in detail. Housing 102 defines a plenum 110 extending upwards from compartment 106. Plenum 110 is elongate in a generally vertical direction. An outlet 112 in the form of an elongate slot extends along a front surface of plenum 110. Housing 102 defines a Helmholtz resonator 114 having an inlet port 116 that is in fluid communication with plenum 110, and a cavity 118 that is in fluid communication with inlet port 116. Inlet port 116 is in fluid communication with plenum 110 at a position that is on the other side of outlet 112 relative to airflow generator 108. Throughout this description, terminology such as the “other side of the outlet” is to be interpreted in accordance with the skilled person’s understanding of that terminology. One such definition is that a distance from the airflow generator to the inlet port of the Helmholtz resonator (or the start of a coupling region, as described below) is a greater than or equal to a distance from the airflow generator to the furthest point of the outlet from the airflow generator. Such distances can, for example, be measured along the airflow path. In the implementation of Figures 1 to 3, inlet port 116 is in fluid communication with plenum 110 at a position that is above an upper end 120 of outlet 112. In use, airflow generator 108 generates airflow by drawing air into compartment 106 as shown by arrow 122, optionally through one or more filters (not shown), and then driving the air into a lower region 124 of plenum 110 as shown by arrow 126. Air pressure increases within plenum 110, causing air to exit outlet 112 along its length, as shown by arrows 128. The movement of the air as it leaves outlet 112 entrains ambient air to multiply the mass of air moved by fan 100, thereby providing a breeze within the region in front of fan 100. Operation of a fan, such as fan 100, can generate several different types of noise Such noise includes, for example, mechanical noise from airflow generator 108, airflow noise as the air is drawn into compartment 106, airflow noise as the air exits outlet 112, and various resonances within the path taken by the air through the fan. For user comfort, it is generally desirable to reduce noise. The many different sources of noise, and their different characteristics, mean that there may be a wide range of frequencies at which noise is generated. Plenum 110 and Helmholtz resonator 114 are arranged and configured to reduce the noise generated by fan 100 around at least one frequency. In general, Helmholtz resonators include a cavity having a particular volume, and an inlet port through which air can pass. An increase in air pressure at the inlet port causes some air to pass through it into the cavity, causing pressure inside the cavity to increase. A subsequent decrease in air pressure at the inlet port allows some of the air to pass out of the cavity through the inlet port. The air moving in and out of the closed cavity through the inlet port causes a resonance, the frequency of which is based on the volume of the cavity. The length and cross-sectional area of the inlet port can also be selected to achieve a certain resonant frequency and Q factor. Returning to Figures 1 to 3, placing inlet 116 of Helmholtz resonator 114 on the other side of outlet 112 relative to airflow generator 108 means that inlet 116 is placed in a position of relatively still or static air, at least from an airflow point of view. This is because all of the air entering plenum 110 exits outlet 112. The only air movement that will be experienced at inlet 116 will be minor eddies caused by non-laminar airflow within plenum 110, and soundwaves including undesirable noise caused by plenum resonance. This arrangement allows the effects of continuous airflow (as distinct from pressure variations due to soundwaves) on the Helmholtz resonator to be reduced or avoided. The skilled person will appreciate that any required number of Helmholtz resonators can be provided. For example, Figure 4 shows the upper end of an alternative implementation of a fan 500. Fan 500 shares several features with fan 100, and such features are indicated with like reference signs in both implementations Fan 500 includes a first Helmholtz resonator 136 having a first inlet port 138 that is in fluid communication with plenum 110 and a first cavity 140 that is in fluid communication with first inlet port 138. Fan 500 also has a second Helmholtz resonator 142 having a second inlet port 144 that is in fluid communication with plenum 110 and a second cavity 146 that is in fluid communication with second inlet port 144. First inlet port 138 and second inlet port 144 are each in fluid communication with plenum 110 at a position that is on the other side of outlet 112 relative to airflow generator 108. In particular, in this implementation, first inlet port 138 and second inlet port 144 are both in fluid communication with plenum 110 at positions that are above an upper end 120 of outlet 112. Figure 5 shows the upper end of an alternative implementation of a fan 600. Fan 600 shares several features with fans 100 and 500, and such features are indicated with like reference signs in all three implementations. Fan 600 includes a coupling region 130 (bounded by the dashed line) disposed between plenum 110 and inlet port 116. The coupling region 130 comprises a coupling volume 132 for Helmholtz resonator 114, coupling volume 132 being disposed adjacent inlet port 116. In fan 600, inlet port 116 of Helmholtz resonator 114 is disposed opposite a wall 134 of plenum 110, wall 134 forming part of housing 102 in the implementation of Figure 4. Coupling volume 132 is defined between inlet port 116 of Helmholtz resonator 114 and a portion of wall 134. Coupling region 130 is configured to improve impedance matching between plenum 110 and the Helmholtz resonator 114. The configuration of coupling region 130 generally includes its volume. While it is possible in certain circumstances to match the impedance between a Helmholtz resonator and a plenum by directly connecting them, this can be challenging, especially when a cross-sectional area of the inlet port is small compared to that of the plenum. The use of a coupling region allows a transition from the cross-sectional area of the plenum to that of inlet port, which can make impedance matching easi er Impedance matching can be achieved by making use of open Helmholtz resonators. A resonator is considered “open” when it is connected to another system (such as the plenums described herein) and the acoustic energy in both systems can interact across the interface (which includes a coupling region, when used) between them. Acoustic energy can therefore enter the resonator but also leave it through the interface if no mechanisms are used to trap the energy into the resonator. The amount of energy that enters and then leaves the resonator is generally called energy leakage. The lossy property of a resonator comes from the friction of the air against the inner walls of the resonator. This friction is mainly driven by the viscosity of the air and converts the acoustic energy into heat. The quantity of converted energy is also called the inherent losses of the resonator. Impedance matching involves configuring components of the system such that losses within the resonator equal the energy leakage. In this context, impedance matching is also known as the critical coupling condition. This occurs at the resonance frequency of the resonator and leads to perfect absorption of the wave energy by the resonator at this specific frequency. Perfect absorption at broadband frequency can also be achieved by using a set of several resonators that are critically coupled at different frequencies. It will be understood that “perfect absorption” in this context does not mean that the energy is totally dissipated by the absorber, but that the energy is totally trapped in the absorber. Dissipation mechanisms are part of the trapping effect but do not constitute the main phenomenon obtained. Figure 6 shows an array of Helmholtz resonators 700 that can be attached to, or form part of, a device (such as any of the devices described herein). Array 700 shares several features with fans 100, 500 and 600, and such features are indicated with like reference signs in all four implementations. Array 700 includes ten Helmholtz resonators 150, 152, 154, 156, 158, 160, 162, 164, 166, 168, each of which includes a cavity 170, 172, 174, 176, 178, 180, 182, 184, 186, 188 and an inlet port 190, 192, 194, 196, 198, 200, 202, 204, 206, 208. Each Helmholtz resonator 150, 152, 154, 156, 158, 160, 162, 164, 166, 168 is tuned to a different frequency. As described above, the resonant frequency of a Helmholtz resonator is related to the volume of its cavity, and the length and cross-sectional area of its inlet port. The cavities of Helmholtz resonators 150, 152, 154, 156, 158, 160, 162, 164, 166, 168 are rectangular prisms, although the cavities may be defined by any suitable three dimensional volume. The use of a rectangular prism may help simplify design and / or manufacture of Helmholtz resonators 150, 152, 154, 156, 158, 160, 162, 164, 166, 168. For example, the walls of all the Helmholtz resonators can be extruded in a single process, requiring only the forming of inlet ports 176, 178, 180, 182, 184, 186, 188, 190, 192, 194 and end walls (not shown) to enclose them. Cavities 170, 172, 174, 176, 178, 180, 182, 184, 186, 188 are of different relative volumes but share a common linear dimension in view of having the same depth (as measured normal to the page in Figure 6). Optionally, other types of prism, cylinder, or other volume may be employed in the design of the cavities. Coupling regions 210, 212, 214, 216, 218, 220, 222, 224, 226, 228 (bounded by the dashed lines) are disposed between plenum 110 and inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208. The coupling regions 210, 212, 214, 216, 218, 220, 222, 224, 226, 228 comprise corresponding coupling volumes for their respective Helmholtz resonators 150, 152, 154, 156, 158, 160, 162, 164, 166, 168, each coupling volume being disposed adjacent the inlet port 190, 192, 194, 196, 198, 200, 202, 204, 206, 208 of its respective Helmholtz resonator 150, 152, 154, 156, 158, 160, 162, 164, 166, 168. Tn array 700, inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208 are disposed opposite wall 134 of plenum 110, wall 134 forming part of housing 102 in the implementation of Figure 6. The coupling volumes are defined between inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208 and a portion of wall 134. In array 700, cavities 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, and inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, are respectively spaced apart from each other in a direction away from plenum 110. That is, Helmholtz resonators 150, 152, 154, 156, 158, 160, 162, 164, 166, 168 are disposed in a linear array. In addition, the respective volumes of cavities 170, 172, 174, 176, 178, 180, 182, 184, 186, 188 decrease monotonically with a distance of their respective inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208 from plenum 110. This arrangement may simplify design and / or modelling especially where a large number of resonators is to be employed. For example, if the intention is to attenuate a range of frequencies from Fmin to Fmax, one resonator cavity can be tuned at Fmin and another resonator cavity can be tuned at Fmax. The volumes of the other cavities must lie between those of the two cavities for which Fmin and Fmax were calculated. By monotonically adjusting the volumes of the other resonator cavities between the calculated volumes for Fmin and Fmax, the attenuation will cover frequencies from Fmin to Fmax. In addition, geometry of the resonators can be improved or optimised by using a numerical optimisation program. Having the depth of the cavities decrease as they get further from the plenum can makes this optimisation easier to program. Examples of dimensions and volumes of cavities 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, and coupling regions 210, 212, 214, 216, 218, 220, 222, 224, 226, 228 will now be set out. It will be appreciated that these values are exemplary only. All values are based on the dimensions of rectangular prisms. Dimensions and volumes of cavities 170, 172, 174, 176, 178, 180, 182, 184, 186, 188 are as follows (noting that length for each cavity is the dimension extending away from the inlet port, and width 1 and width 2 are orthogonal to each other and the length dimension): Cavity Vol (m3) Width 1 Length Width 2 170 4.4265E-05 0.0185 0.0337 0.071 172 2.1570E-05 0.014 0.0217 0.071 174 1.2013E-05 0.009 0.0188 0.071 176 1.2177E-05 0.005 0.0343 0.071 178 7.5970E-06 0.005 0.0214 0.071 180 5.7936E-06 0.004 0.0204 0.071 182 4.4588E-06 0.004 0.0157 0.071 184 3.4364E-06 0.004 0.0121 0.071 186 2.6128E-06 0.004 0.0092 0.071 188 2.0448E-06 0.003 0.0096 0.071 Dimensions and volumes of inlet ports 190, 192, 194, 196, 198, 200, 202, 204, 206, 208 are as follows (noting that length for each inlet port is the dimension parallel to the length dimension for its corresponding cavity, and width 1 and width 2 are orthogonal to each other and the length dimension): Inlet port Volume (m3) Width 1 Length Width 2 190 4.860E-09 0.0018 0.0015 0.0018 192 3.375E-09 0.0015 0.0015 0.0015 194 3.375E-09 0.0015 0.0015 0.0015 196 4.950E-09 0.0015 0.0022 0.0015 198 4.500E-09 0.0015 0.002 0.0015 200 4.500E-09 0.0015 0.002 0.0015 202 4.500E-09 0.0015 0.002 0.0015 204 4.725E-09 0.0015 0.0021 0.0015 206 5.175E-09 0.0015 0.0023 0.0015 208 5.400E-09 0.0015 0.0024 0.0015 5 Dimensions and volumes of coupling regions 210, 212, 214, 216, 218, 220, 222, 224, 226, 228 are as follows (noting that length for each coupling region is the dimension parallel to the length dimension for its corresponding cavity and inlet port, and width 1 and width 2 are orthogonal to each other and the length dimension): Coupling Region Volume (m3) Width 1 Length Width 2 210 1.9596E-05 0.0138 0.071 0.02 212 2.7477E-05 0.0258 0.071 0.015 214 2.0448E-05 0.0288 0.071 0.01 216 5.2824E-06 0.0124 0.071 0.006 218 1.0906E-05 0.0256 0.071 0.006 220 9.4430E-06 0.0266 0.071 0.005 222 1.1076E-05 0.0312 0.071 0.005 224 1.2354E-05 0.0348 0.071 0.005 226 1.3348E-05 0.0376 0.071 0.005 228 1.0508E-05 0.037 0.071 0.004 10 Turning to Figures 7 to 10, there is shown an alternative implementation in the form of a fan 800. Fan 800 shares several features with fans 100, 500, and 600, and array 700, and such features are indicated with like reference signs in all implementations. Fan 800 comprises a first plenum 230 and a second plenum 232. Both plenums 230, 232 are elongate in a generally vertical direction. A first array 234 of Helmholtz resonators is disposed at an upper end of first plenum 230 and a second array 236 of Helmholtz resonators is disposed at an upper end of second plenum 230. First and second arrays 234, 236 are each internally configured similarly to array 700, and so will not be described separately. In use, airflow generator 108 generates airflow by drawing air into compartment 106 as shown by arrow 122, optionally through one or more filters (not shown), and then driving the air into a common region 238 at the entry to first and second plenums 230, 232. The air splits as indicated by arrows 240, with around half of the air entering each of first and second plenum 230, 232 Air pressure increases within first and second plenums 230, 232, causing air to exit outlets 112 along their lengths, as shown by arrows 128. The movement of the air as it leaves outlet 112 provides a breeze as described above in relation to fan 100. First and second plenums 230, 232, and corresponding first and second arrays 234, 236, are arranged and configured to reduce the noise generated by fan 100 around certain frequencies. Turning to Figure 11, a graph shows the acoustic gain due to the plenum, with and without a resonator array, of fan 800 (for simplicity, only one half was modelled). A first line 242 shows the acoustic output of the outlet without any Helmholtz resonator(s), and a second line 244 shows the acoustic output with array 234, 700 attached as shown in Figures 7-11. Peaks 246, 248, 250, 252, located respectively at about 200 Hz, 486 Hz, 950 Hz, and 1400 Hz, represent resonances within the plenums due to reflections and the physical internal dimensions of the plenums. The 486 Hz and 950 Hz resonances are of particular concern due to them falling within a relatively sensitive part of the audible spectrum. While acoustically absorbent materials could be used for sound absorption, such materials tend to be less effective at lower frequencies such as 486 Hz and 950 Hz. Second line 244 shows the impact of array 234, 700. The resonance at 486 Hz is substantially removed. Overall gain between around 400 Hz and 900 Hz is significantly flatter, which may be perceived as less noisy. The 950 Hz peak is moved up to around 1050 Hz and significantly reduced in magnitude. The first peak at 200 Hz is largely unchanged, but as this frequency lies outside the most sensitive part of the audible spectrum and would require a relatively large resonator, it has not been corrected for. Turning to Figure 12, there is shown a graph of the self-noise (also known as flow-induced noise) generated by the plenum with (line 260) and without (line 262) the resonators, for a 50 L / s flowrate. This shows that the airflow is not significantly disturbed by the Helmholtz resonators, since they do not generate additional flow-induced noise in the plenum. In addition, flow-induced noise is reduced overall with the resonators, particularly within the 300-700 Hz region. In general, placing inlet port(s) of the Helmholtz resonators such that they are in fluid communication with the plenum at a point where airflow would be substantially null in the absence of the resonator(s) can improve performance of the system as a whole. This is particularly the case where this point is also an area of high acoustic reflection, for example due to its proximity to the relatively rigid end wall of the plenum. This acoustic reflection generates acoustic resonances in the plenum, which amplify the acoustic sound emitted by the outlet slots. By placing the inlet port at or near this area and matching its impedance to that of the plenum, the reflection is reduced or eliminated, which correspondingly reduces or eliminates the resonances and therefore the acoustic sound emitted by the outlet slots. This reduction or elimination would not be the same if the resonator inlet(s) were positioned adjacent or opposite to the outlet slot, for example. Optionally, at least part of the interior of any of the Helmholtz resonators can be filled with an acoustic absorbing material, such as acoustic foam. This can change the resonance characteristics of the Helmholtz resonator and contribute further to energy absorption. Similarly, at least some of the internal surface of any of the Helmholtz resonators can be covered with an acoustic absorbing material. Although all described implementations use a single airflow generator, it will be appreciated that more than one airflow generator may be employed. For example, where two or more plenums are used, each may optionally have its own airflow generator. The device may be manufactured in any suitable way. For example, the device can be assembled from components that are moulded, machined, additively manufactured, extruded, or otherwise manufactured. Any suitable materials can be used, such as polymers, metals, ceramics, composite materials, or any combination thereof. While various fans have been described, it will be appreciated that the invention can be implemented in other types of air-moving devices. A non-exhaustive list of such devices includes air conditioners, space heaters, air cleaning or filtration devices, and air fresheners. The additional components needed for such devices compared to a fan are well known to the skilled person and so will not be described in detail. Although various implementations have been described, the skilled person will appreciate that the invention can be embodied in many other forms.

Claims

1. A device comprising:an airflow generator;a plenum configured to receive airflow generated by the airflow generator, the plenum being elongate;an outlet extending along at least part of the plenum, such that the airflow passing through the plenum exits the outlet; andat least one Helmholtz resonator having an inlet port and a cavity, the inlet port being in fluid communication with the plenum at a position that is on the other side of the outlet relative to the airflow generator;the plenum and Helmholtz resonator being arranged and configured to reduce the noise generated by device around at least one frequency.

2. The device of claim 1, comprising a plurality of the Helmholtz resonators, each of the Helmholtz resonators being tuned to a different frequency.

3. The device of claim 1 or 2, comprising a coupling region disposed between the plenum and the inlet port(s) of the Helmholtz resonator(s), the coupling region being configured to improve impedance matching between the plenum and the Helmholtz resonator(s).

4. The device of claim 3, wherein the coupling region comprises a coupling volume for the or each Helmholtz resonator, each coupling volume being disposed adjacent the inlet port of its corresponding Helmholtz resonator.

5. The device of claim 4, wherein the inlet port(s) of the Helmholtz resonator(s) are disposed opposite a wall of the plenum, the coupling volume for each Helmholtz resonator being defined between the inlet port of that Helmholtz resonator and a portion of the wall.

6. The device of any preceding claim, wherein the cavity of the or each Helmholtz resonator is cylindrical or prismatic, and / or has a constant cross-section along at least one axis.

7. The device of claim 2, or any one of claims 3 to 6 when dependent on claim 2, wherein the cavities of the Helmholtz resonators are of different relative volumes but share a common linear dimension along at least one axis.

8. The device of claim 2, or any one of claims 3 to 7 when dependent on claim 2, wherein the cavities and / or the inlet ports of the Helmholtz resonators are spaced apart in a direction away from the plenum.

9. The device of claim 2, or any one of claims 3 to 8 when dependent on claim 2, wherein a volume of the Helmholtz cavities decreases monotonically with a distance of their respective inlet ports from the plenum.

10. The device of any preceding claim, wherein the inlet port(s) are in fluid communication with the plenum at a point where airflow would be null in the absence of the resonator(s)11. The device of any preceding claim, comprising at least two of the plenums, each of the plenums having at least one of the Helmholtz resonators.

12. The device of claim 11, wherein the at least two plenums are provided with airflow from a common airflow generator.

13. The device of any preceding claim, wherein the device is one or more of a fan, an air conditioner, a space heater, an air cleaning or filtration device, or an air freshener.

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