Electroacoustic plasma transducer system

Abstract

The electroacoustic plasma transducer comprises a plasma electrode arrangement 110 including at least one corona electrode 120 and a collector electrode 130 separated by an air gap. An electrically in

Description

The present invention relates to an electroacoustic plasma transducer system, and, particularly, but not exclusively, an electroacoustic plasma transducer system for use as part of an active noise reduction (ANR) system employing acoustic impedance control. Active noise reduction is a technique that aims to reduce unwanted sound either at a specific control location or globally within a space. It uses acoustic sensing means to sense the unwanted sound or sound field, a controller to derive one or more control signals, and one or more electroacoustic drivers (e.g. loudspeakers or earphone drivers) to reproduce the control signals as one or more controlling sounds. Active noise reduction can be implemented using active noise cancellation, which is based on the principle of destructive interference of signals, or active acoustic impedance control. The former has been widely adopted in commercial applications, such as active noise-cancelling earphones, whereas the latter has seen limited use due to the performance limitations of available transducers. Electrodynamic loudspeakers are widely used for active noise reduction due to their simplicity and affordability. They typically comprise a diaphragm suspended in a mounting structure and are driven by an electromagnetic force generated between a permanent magnet, installed in said supporting structure, and a coil attached to the diaphragm. The acoustic response of a dynamic loudspeaker can be represented by a single-degree-of-freedom resonator. This resonator is controlled by its compliance and mass at the frequencies below and above its resonance frequency respectively. That is, the transducer is non-ideal with its inertia reducing the sharpness of its acoustic response. Reducing a dynamic loudspeaker diaphragm’s mass is thus a desired design obj ective, but this can be challenging, especially in applications requiring high acoustic source strength and durability. Furthermore, mitigating the mass-induced performance limitations of the dynamic loudspeaker’s diaphragm through the application of active control methods can add to the complexity of the connected controller, reducing the overall system’s robustness and increasing the possibility of instability. Accordingly, there is a need for an alternative transducer technology for active noise reduction applications which operates without the limitations imposed by mass and compliance. An atmospheric corona discharge-based loudspeaker can produce audible sound if the high voltage across its corona electrode (being that electrode around which ionization of air particles occurs) and collector electrode (being that electrode to which ionized air particles are attracted) is modulated at audible frequencies. This modulation changes the mechanical energy transferred from charged particles to the neutral air, resulting in the propagation of sound waves. While prior art corona discharge loudspeakers can produce sound over an extended surface, they are not well-suited for active noise reduction applications as they are not optimised to produce the controlling sound over a particular frequency range important in active noise reduction applications. Additionally, prior art corona discharge transducers are prone to arcing, limiting the voltage difference that can be supplied to produce sound and thus their sound-producing capabilities. They also generate ozone, which can be harmful in high concentrations. These issues associated with prior art must be addressed for the successful implementation of a corona discharge-based transducer in active noise reduction applications. The present applicant has identified the need for a new electroacoustic plasma transducer apparatus to address or at least alleviate problems associated with the prior art. In accordance with the present invention, there is provided an active noise reduction (ANR) system comprising: an electroacoustic plasma transducer system comprising: electroacoustic plasma transducer apparatus comprising: a plasma electrode arrangement including an inner electrode (e.g. corona electrode) and an outer electrode (e.g. collector electrode) separated from the inner electrode by an air gap; a peripheral support (e.g. frame) operative to support the plasma electrode arrangement (e.g. mechanically couple the collector electrode to the corona electrode); and an enclosure defining a chamber for receiving rearward radiation from the plasma electrode arrangement; and drive means operative to supply a voltage U(t) between the corona electrode and the collector electrode of the electroacoustic plasma transducer apparatus; acoustic sensing means; and ANR circuitry operative to receive a signal from the acoustic sensing means and control an output of the electroacoustic plasma transducer system to reduce noise; wherein the apparatus further comprises an ozone reducing material (e.g. ozone reducing catalyst). In this way, an electroacoustic plasma transducer system is provided that provides improved sound production means for active noise reduction with the elimination of undesirable ozone. In one embodiment, the ozone reducing material comprises manganese (e.g. manganese oxide-based catalyst). In one embodiment, the electroacoustic plasma transducer apparatus comprises a porous structure and the ozone reducing material is provided on (e.g. within or applied as a coating to) the porous structure. In one embodiment, the plasma electrode arrangement defines a longitudinal axis along which the inner and outer electrodes are spaced. In one embodiment, the collector electrode forms the outer electrode and the corona electrode forms the inner electrode. In one embodiment, the porous structure comprises a foraminous plate (e.g. perforated plate) or a mesh structure. In one embodiment, the porous structure comprises an outer ozone filter provided in front of the plasma electrode arrangement (e.g. whereby forward airflow from the plasma electrode arrangement passes through the outer zone filter before exiting the electroacoustic plasma transducer apparatus). In one embodiment, the outer ozone filter has a thickness of less than 15 mm. In one embodiment, the outer ozone filter is longitudinally spaced from the collector electrode by up to 10 mm. In one embodiment, the outer ozone filter abuts the outer electrode of the plasma electrode arrangement (e.g. abuts the outer electrode). In one embodiment, the outer ozone filter is incorporated into at least one electrode of the plasma electrode arrangement (e.g. incorporated into the outer electrode). In one embodiment, the ozone reducing material is incorporated into the outer electrode (e.g. into the collector electrode). In one embodiment, the outer electrode (e.g. collector electrode) defines an inner face facing the inner electrode (e.g. corona electrode), an outer face substantially opposed to the inner face, and internal surfaces defined by apertures in the outer electrode. In one embodiment, the ozone reducing material is applied to the internal surfaces of the outer electrode and / or the outer face of the outer electrode. In one embodiment, the ozone reducing material is substantially not applied to the inner face of the outer electrode. In one embodiment, the outer electrode (e.g. collector electrode) is removably coupled to the peripheral support (e.g. with suitable electrical connections provided to allow the outer electrode to be electrically disconnected). In one embodiment, the outer ozone filter has an average effective aperture diameter of between 0.1-2 mm. In one embodiment, the outer ozone filter has an aperture density of greater than 30 apertures per cm In one embodiment, the outer ozone filter is acoustically sealed around a peripheral edge of the plasma electrode arrangement. In one embodiment, the outer ozone filter is configured to achieve a certain acoustic response. In one embodiment, the outer ozone filter is configured (e.g. its acoustic response is tuned) to have an acoustic power transmission coefficient greater than 50% in the frequency range 0-5000 Hz. In one embodiment, the outer ozone filter is configured (e.g. its acoustic response is tuned) to have an acoustic power reflection coefficient no higher than 10% over a frequency range 0-5000 Hz. In one embodiment, the acoustic response of the outer ozone filter is configured to ensure that the outer ozone filter does not reduce the produced controlling sound level by more than 3 dB at 5000 Hz. In one embodiment, the porous structure (e.g. outer ozone filter) comprises a honeycomb structure formed from a clay ceramic, or a mesh, a grid or a perforated structure. In one embodiment, the outer ozone filter is sealed to the front of the plasma electrode arrangement using a compliant adhesive or suspension. In one embodiment, the enclosure is a substantially sealed enclosure. In one embodiment, the ozone reducing material is provided within the enclosure (e.g. in or on walls of enclosure and / or in or on a porous structure provided within the chamber). In one embodiment, the electroacoustic plasma transducer apparatus further comprises an airflow blocking screen (e.g. airflow suppression screen) provided in front of the plasma electrode arrangement (e.g. in front of the outer electrode), wherein the airflow blocking screen is operative to substantially allow transmission of acoustic pressure waves but substantially prevent transmission of airflow. In one embodiment, the airflow blocking screen is formed from a porous sheet material (e.g. wire mesh or textile material). In one embodiment, the airflow blocking screen has a longitudinal thickness of less than 5mm. In one embodiment, the porous structure is provided within the chamber. In one embodiment, the porous structure is an acoustically absorbent structure. In one embodiment, the acoustically absorbent structure is a honeycomb structure with openings facing the plasma electrode arrangement (e.g. facing the inner electrode). In one embodiment, the ozone reducing material is provided within or applied as a coating to the honeycomb structure. In one embodiment, the peripheral support is formed from an electrically insulating ceramic or a plastic polymer. In one embodiment, the ozone filter is a rectangular porous structure. In one embodiment, at least 10% of a top surface area of the ozone filter is open and coated with an ozone catalyst, such as manganese, to capture ozone from air passing over it. In a set of alternative embodiments of the invention, the ozone filter comprises a honeycomb structure formed from a clay ceramic, or a mesh, a grid or a perforated structure. In one embodiment, the ozone filter is sealed to the front of the plasma electrode arrangement using a compliant adhesive or suspension. In one embodiment, the plasma electrode arrangement, peripheral support and enclosure form part of a single mechanical structure. In an alternative embodiment, the enclosure and / or ozone filter are separate parts, and may attach and detach to the plasma electrode arrangement, or a frame or structure encapsulating such using coupling means (e.g. at least one coupling device). In on embodiment, the coupling means might be mechanical, electrical, and / or magnetic. In one embodiment, the electroacoustic plasma transducer system is an electroacoustic plasma transducer system in accordance with the first aspect of the present invention (e.g. any embodiment of the first aspect of the present invention). In one embodiment, the acoustic sensing means comprises at least one sensing microphone. In one embodiment, the ANR circuitry is operative to perform active acoustic impedance control. In the case of electroacoustic plasma transducer apparatus including an outer structure (outer ozone filter or airflow blocking screen), the acoustic sensing means comprises an outer sensing microphone (e.g. positioned immediately in front of the outer structure) for monitoring a pressure pi in front of the outer structure. In one embodiment, the acoustic sensing means comprises an inner sensing microphone for monitoring a pressure p2 between the outer structure and the plasma electrode arrangement (e.g. between the outer structure and the front collector electrode, e.g. immediately in front of the plasma electrode arrangement). In one embodiment, the outer structure (outer ozone filter or front airflow blocking screen, or additional acoustical screen) has an acoustic resistance (e.g. frequency dependent acoustic resistance) Res, and the part of ANR circuitry is operative to determine an estimate of acoustic velocity Vest, wherein Vest = p^2. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1A is a schematic illustration of an ANR system including electroacoustic plasma transducer apparatus in accordance with a first embodiment of the present invention; Figure IB is an exploded schematic view of the ANR system of Figure 1A showing details of the electroacoustic plasma transducer apparatus; Figure IC is a block diagram illustrating operation of the ANR system of Figure 1A; Figures 2A and 2B are schematic views of the electroacoustic plasma transducer apparatus of Figure IB; Figure 2C is a schematic cross-sectional view BB of Figure 2B; Figure 2D is a schematic cross-sectional view AA of Figure 2B; Figures 3 A-C illustrate the principle of minimum path separation at the periphery of the plasma transducer apparatus geometry design; Figure 4A is a schematic view of electroacoustic plasma transducer apparatus in accordance with a second embodiment of the present invention for use in the ANR system of Figure 1A; Figure 4B is an exploded schematic view of the electroacoustic plasma transducer apparatus of Figure 4 A; Figure 4C is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 4 A; Figure 4D is a schematic cross-sectional view BB of Figure 4C; Figure 4E is a schematic cross-sectional view AA of Figure 4C; Figure 5A is a schematic view of electroacoustic plasma transducer apparatus in accordance with a third embodiment of the present invention for use in the ANR system of Figure 1A; Figure 5B is an exploded schematic view of the electroacoustic plasma transducer apparatus of Figure 5 A; Figure 5C is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 5A; Figure 5D is a schematic cross-sectional view BB of Figure 5C; Figure 5E is a schematic cross-sectional view AA of Figure 5C; Figure 6A is a schematic view of electroacoustic plasma transducer apparatus in accordance with a fourth embodiment of the present invention for use in the ANR system of Figure 1A; Figure 6B is an exploded schematic view of the electroacoustic plasma transducer apparatus of Figure 6 A; Figure 6C is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 6 A; Figure 6D is a schematic cross-sectional view BB of Figure 6C; Figure 6E is a schematic cross-sectional view AA of Figure 6C; Figure 7A is a schematic view of electroacoustic plasma transducer apparatus in accordance with a fifth embodiment of the present invention for use in the ANR system of Figure 1A; Figure 7B is an exploded schematic view of the electroacoustic plasma transducer apparatus of Figure 7 A; Figure 7C is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 7A; Figure 7D is a schematic cross-sectional view BB of Figure 7C; Figure 7E is a schematic cross-sectional view AA of Figure 7C; Figure 8A is a schematic illustration of an ANR system including electroacoustic plasma transducer apparatus in accordance with a further embodiment of the present invention; Figure 8B is an exploded schematic view of the ANR system of Figure 8 A showing details of the electroacoustic plasma transducer apparatus; Figure 8C is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 8B; Figure 8D is a schematic cross-sectional view BB of Figure 8C; Figure 8E is a schematic cross-sectional view AA of Figure 8C; Figure 9A is a schematic part cross-sectional illustration of an ANR system including electroacoustic plasma transducer apparatus in accordance with a yet further embodiment of the present invention; Figure 9B is a schematic part cross-sectional illustration of the ANR system of Figure 9A with the front airflow blocking screen removed; Figure 9C is an exploded schematic view of the ANR system of Figure 9A showing details of the electroacoustic plasma transducer apparatus; Figure 9D is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 9C; Figure 9E is a schematic cross-sectional view BB of Figure 9D; Figure 9F is a schematic cross-sectional view AA of Figure 9D; Figure 10 is a schematic exploded schematic view of a modified version of the ANR system of Figure 1A including inner and outer sensing microphones; Figure 11A is a schematic cross-sectional view of a first alternative enclosure for electroacoustic plasma transducer apparatus of Figure 1 A; and Figure 1 IB is a schematic cross-sectional view of a second alternative enclosure for electroacoustic plasma transducer apparatus of Figure 1 A. Figures 1A-C show an ANR system 10 comprising: an electroacoustic plasma transducer system 50 including electroacoustic plasma transducer apparatus 100 and drive means 160; at least one sensing microphone 200; and ANR circuitry 300 operative to receive a signal from the at least one sensing microphone 200 and control an output of the electroacoustic plasma transducer system 50 to reduce noise. As shown in Figures IB and IC, electroacoustic plasma transducer apparatus 100 comprises: a plasma electrode arrangement 110 including a corona electrode 120 and a panelform collector electrode 130 separated from the corona electrode by an air gap 140; an electrically insulative peripheral frame 150 operative to support the plasma electrode arrangement 110; an enclosure 170; and an outer ozone filter 180 provided in front of the plasma electrode arrangement 110. As also shown, electroacoustic plasma transducer system 50 comprises drive means 160 operative to supply a voltage U(t) between corona electrode 120 and collector electrode 130. Plasma electrode arrangement 110 defines a longitudinal axis “L” along which the corona electrode 120 and collector electrode 130 are spaced (e.g. with the collector electrode 130 forming the front or outer electrode and the higher voltage and more fragile corona electrode 120 forming the rear or inner electrode). With reference to Figure 2B, plasma electrode arrangement 110 further defines a first transverse axis AA extending perpendicular to longitudinal axis “L” and a second transverse axis BB extending in the same plane as first transverse axis AA and perpendicular thereto. Corona electrode 120 comprises a plurality parallel elongate corona electrode wires 122 coupled to the peripheral frame 150 at their respective ends; collector electrode 130 has a panelform and substantially comprises a foraminous collector electrode plate 132. Corona electrode wires 122 are longitudinally spaced from collector electrode 130 by distance d and laterally spaced from one another by an equal distance a. Each corona electrode wire has an effective diameter in the range 100-1000 pm. With reference to Figures 3 A-C, the geometry of plasma electrode arrangement 110 is designed to satisfy the optimised condition that: a central active area of the corona electrode is spaced longitudinally from the collector electrode by a distance d\ and the periphery (indicated by a dashed rectangle) of the corona electrode is spaced from the collector electrode and / or peripheral support such that for any path from the corona electrode to the collector electrode via the peripheral support, the path has a path length c, wherein 1,085t / <c<2d. In this example, this condition is satisfied along the second transverse axis BB by spacing the laterally outermost corona electrode wires 122 from an inner surface 152 of the peripheral frame 150 by a lateral distance b, wherein b = 0.45 d. Along the first transverse axis AA, the condition is satisfied by providing collector electrode plate 132 with curved end profiles 134’ configured such that the longitudinal distance between corona electrode wires 122 and the collector electrode 130 in the central active area of the corona electrode is d and the path between any point of corona electrode wires 122 and any point of the collector electrode 130 via the peripheral frame at the region where the corona electrode wires 122 meet the peripheral frame 150 is equal to c. As illustrated, the curvature of the curved end profiles 134 start at a lateral distance of around b from the inner 152 surface of peripheral frame 150, wherein b = 0.45 d. In use, the geometry of plasma electrode arrangement 110 allows the applied voltage U(t) to be maximised for a given value of dto achieve highly efficient operation without risk of arcing, which enables higher performance ANR. Outer ozone filter 180 comprises a foraminous filter plate 182 with a fine aperture structure 184 (e.g. with an average effective aperture diameter of between 0.2-1 mm and an aperture density of greater than 30 apertures per cm2). An ozone reducing catalyst (e.g. manganese oxide-based catalyst) is provided on (e.g. within or applied as a coating to) the porous structure. In one embodiment, filter plate 182 is a foraminous metal plate (e.g. steel or aluminium plate) and the ozone reducing catalyst is applied as a coating to outer surfaces of the plate (e.g. including opposed inner and outer surfaces of the plate and internal surfaces of the apertures). Outer ozone filter 180 generally has a thickness of less than 15 mm and is spaced from the central part of collector electrode 130 by a distance of at least 10 mm. Ozone filter 180 preferably has low or zero flammability. Outer ozone filter 180 is acoustically sealed around the edge of plasma electrode arrangement 110. To avoid interfering with the controlling sound from the plasma transducer apparatus, which may include frequency components between 0-5 kHz, the filter's geometry, including its depth and perforations, is configured such that its acoustic response is tuned. Specifically, such tuning includes, but is not limited to, calibrating the geometry of the ozone filter such that its acoustic power transmission coefficient is no less than 50%, and its reflected power coefficient is no more than 10%. More generally, such tuning aims to ensure that the ozone filter does not reduce the produced controlling sound’s level by more than 3 dB at 5 kHz. The ozone filter according to this description is able to capture ozone from the air while minimising its impact on the sound production of plasma transducer apparatus. Its rectangular shape and perforations, along with the use of an ozone catalyst and acoustic calibration, make it a highly efficient and precise component within the apparatus. Enclosure 170 is a sealed enclosure and defines a chamber 172 for receiving rearward radiation from the plasma electrode arrangement 110. Chamber 172 has a longitudinal length / , typically less than 20 mm, and houses acoustic response tuning means 190. Sound produced by electroacoustic plasma transducer apparatus 100 can be represented as the combination of two sound sources caused by different physical phenomena. First, sound is produced due to the heat transfer from the ionisation zone to the neutral air particles around the corona electrode 120 and behaves as a monopolar sound source. The heat power H produced at each frequency can be estimated as: H = C(3U2c - 2UdMuac Here, Uo is the critical voltage at which the corona discharge initiates, C is a dimensional constant. Uo and C can be identified by measuring the transducer’s voltagecurrent characteristics and approximating it with formula I = CU(U — Uo) , where I and U are total current and voltage respectively. The second dipolar source is linked to the electromechanical force F and is transmitted from accelerated ions to the neutral air particles and directed from the corona electrode 120 to the collector electrode 130: F = ^(2Udc-Uq)uac Pi where pi is the effective mobility of ions. With these sound sources, the corresponding pressures close in front of the electroacoustic plasma transducer apparatus 100 at distance x from the centre of the plasma electrode arrangement 110 can be calculated: p^x) = e^%(l - Re~2jkl)F pH(x) = —^eikx(l + Re^^H In the formulas above, S is the total cross-section of the electroacoustic plasma transducer apparatus’s plasma electrode arrangement 110 that has a longitudinal separation of distance d, CP is the capacity of heat per unit mass, To is the ambient temperature, R is the complex reflection coefficient from the enclosure, c is the sound speed in the air, k is the wavenumber. In a majority of the active noise reduction applications envisaged for the electroacoustic plasma transducer apparatus, its compact construction is desirable and thus a small distance I is needed, preferably not greater than 20 mm. In this case, with rigid termination of the enclosure (reflection coefficient R is close to 1), mostly the monopolar pressure pH controls the acoustic field at low frequencies since the pressure pF is attenuated due to its dipolar nature. In the preferred embodiment of the electroacoustic plasma transducer apparatus 100, the magnitude of acoustic pressure created by the source F is 3-4 times stronger than pressure from the source H. The acoustic response of the electroacoustic plasma transducer apparatus 100 can be optimised in the target frequency range for improving the performance of the active noise reduction system 10, more specifically using an impedance control technique in accordance with the methods described in patent application US2023020879 (Al). The approach of tuning the combination of the enclosure’s back wall distance I and back wall properties defined in general by reflection coefficient R allows modifying the electroacoustic plasma transducer apparatus acoustic response. For example, the total produced sound pressure pH + pF can be maximized in a particular frequency range. Fixing back wall distance I defines the frequency-dependent behaviour of the reflection coefficient R. Such an approach of desired enclosure property identification can serve as an approximate guideline for the construction of the electroacoustic plasma transducer apparatus’ enclosure design. It can be combined with an experimental approach. With reference to this acoustic response tuning means can be provided in the form of an acoustically absorbent structure 192 formed from porous material. Acoustically absorbent structure 192 may be operative to absorb sound above a low-frequency threshold (e.g. above around 300 Hz). With reference to Figures 11A and 11B, acoustic response tuning means 190 may alternatively be configured to absorb sound at low frequency (e.g. under around 300 Hz). In one embodiment, the acoustic response tuning means may comprise a combination of an acoustically absorbent structure and a passive resonator 196. Figure 11A shows an example of an electroacoustic plasma transducer apparatus with enclosure which comprises a passive (radiator or) resonator 196 damped by a porous material 194 with two vents 198. Figure 1 IB shows an example of an electroacoustic plasma transducer apparatus with the enclosure which comprises a passive radiator 196 mounted instead of a back wall. Figures 4A-E show an alternative plasma electrode arrangement 110’ for use in the electroacoustic plasma transducer apparatus 100 of Figure 1. Plasma electrode arrangement 110’ is based on plasma electrode arrangement 110 (features in common are labelled according) with corona electrode wires 122 replaced by elongate corona electrode rods 122’. As shown, each corona electrode rod 122’ supports a row of longitudinally extending pins 124 having tapered tips 124A spaced from collector electrode 130’ by distance d. In this example, the 1.085t / <c <2d condition is satisfied along the second transverse axis BB by spacing the laterally outermost corona electrode rods 122’ from an inner surface 152’ of the peripheral frame 150’ by a lateral distance b. wherein b = 0.45 d. Along the first transverse axis AA, the condition is satisfied by spacing the tapered tips 124A of the longitudinally extending pins 124 that are closest to the inner surface 152’ of the peripheral frame 150’ (i.e. the opposed outermost pins of each row) by the same distance b. As a consequence, collector electrode 130’ is permitted a uniformly planar surface all the way up to the peripheral frame 150’ along both axes AA and BB. Figures 5A-E show an alternative plasma electrode arrangement 110” for use in the electroacoustic plasma transducer apparatus 100 of Figure 1. Plasma electrode arrangement 110’ is based on plasma electrode arrangement 110 (features in common are labelled according) with corona electrode wires 122 replaced by elongate corona electrode rods 122” each supporting a single a longitudinally extending elongate blade 124’ (e.g. elongate in the first transverse axis AA) having an extended blade tip 124A’ spaced longitudinally from collector electrode 130’ by distance d. As illustrated, each longitudinally extending elongate blade 124’ has a tapered end profile 124B such that the extended blade tip 124A’ is spaced inwardly relative to the base of the longitudinally extending elongate blade 124’. In this example, the 1.085t / <c <2d condition is satisfied along the second transverse axis BB by again spacing the laterally outermost corona electrode rods 122” from an inner surface 152” of the peripheral frame 150” by a lateral distance b, wherein b = 0.45d Along the first transverse axis AA, the condition is satisfied by spacing the opposed ends of the longitudinally extending elongate blade 124’ from the peripheral frame 150”. Specifically, the opposed ends of the longitudinally extending elongate blade 124’ from the peripheral frame 150” are positioned such that ends of the extended blade tip 124A’ are spaced from the inner surface 152” of the peripheral frame 150” by the same distance b. By virtue of the spacing of the longitudinally extending elongate blades 124’ along the first transverse axis AA, collector electrode 130” is permitted a uniformly planar surface all the way up to the peripheral frame 150” along both axes AA and BB. Figures 6A-E show an alternative plasma electrode arrangement 110’” for use in the electroacoustic plasma transducer apparatus 100 of Figure 1. Plasma electrode arrangement 110’” is based on plasma electrode arrangement 110 (features in common are labelled according) with corona electrode wires 122’” retained and collector electrode plate 132 replaced by a plurality of parallel elongate collector electrode rods 136. Elongate collector electrode rods 136 are equally spaced laterally by distance a (i.e. the same lateral spacing as the corona electrode wires 122’”). In addition, elongate collector electrode rods 136 are laterally offset relative to corona electrode wires 122’” by all whereby spacing d between corona electrode 120’” and collector electrode 130’” is achieved with a longitudinal spacing less than d. In this example, the 1.085t / <c <Id condition is satisfied along the second transverse axis BB by spacing the laterally outermost corona electrode wires 122’” from the inner surface 152’” of the peripheral frame 150’” by a lateral distance b. Laterally outermost elongate collector electrode rods 136 are also spaced from the inner surface 152’” ofthe peripheral frame 150’” thereby allowing values ofb to be lower than in the previously illustrated embodiments. Along the first transverse axis AA, the condition is satisfied by providing collector elongate collector electrode rods 136 with curved end profiles 138 configured such that the distance between corona electrode wires 122’” and elongate collector electrode rods 136 in the central active area of the corona electrode is d and the path between any point of corona electrode wires 122’ ’ ’ and any point of the collector electrode 130’” via the peripheral frame at the region where the corona electrode wires 122’” meet the peripheral frame 150’” is equal to c. As illustrated, the curvature of the curved end profiles 138 start at a lateral distance of around b from the inner 152’” surface of peripheral frame 150’”. Figures 7A-E show an alternative plasma electrode arrangement 110”” for use in the electroacoustic plasma transducer apparatus 100 of Figure 1. Plasma electrode arrangement 110”” is based on plasma electrode arrangement 110 (features in common are labelled according) with a modified form of corona electrode wires 122”” retained and collector electrode plate 132”” retained in a uniformly planar form. In this example, the 1.085t / <c <2d condition is satisfied along the second transverse axis BB by spacing the laterally outermost corona electrode wires 122” ” from the inner surface 152”” of the peripheral frame 150”” by a lateral distance b. Along the first transverse axis AA, the condition is satisfied by providing corona electrode wires 122”” with curved end profiles 126 supported by opposed outer rods 128. Curved end profiles 126 are configured such that the distance between corona electrode wires 122” ” and collector electrode plate 132”” in the central active area of the corona electrode is d and the path between any point of corona electrode wires 122”” and any point of the collector electrode 130”’ via the peripheral frame at the region where the corona electrode wires 122”” meet the peripheral frame 150”” is equal to c. As illustrated, the curvature of the curved end profiles 126 start at a lateral distance of around b from the inner 152”” surface of peripheral frame 150””. Figure 8A-E show an alternative ANR system 10’ based on ANR system 10 (features in common are labelled accordingly) in which collector electrode 130 and outer ozone filter 180 are replaced by collector electrode 130’”” including ozone reducing material. Collector electrode 130’”” comprises a foraminous filter / electrode plate 132’”” with honeycomb structure with openings facing the plasma electrode arrangement. Foraminous filter / electrode plate 132’”” has a similar inner profile to collector electrode 130 (including curved end profiles 134’) but a greater longitudinal thickness (e.g. 5-15 mm) and smaller aperture size / increase aperture density (e.g. with an average effective aperture diameter of between 0.1 and 2 mm and an aperture density of greater than 30 apertures per cm2). An ozone reducing catalyst (e.g. manganese oxide-based catalyst) is provided on (e.g. within or applied as a coating to) the porous structure. In one embodiment, filter / electrode plate 132’”” is a foraminous metal plate (e.g. steel or aluminium plate) and the ozone reducing catalyst is applied as a coating to outer surfaces of the plate other than the inner (electrode) surface of the plate (e.g. external outer surface of the plate and internal surfaces of the apertures). The inner surface of the plate is generally not coated in order to avoid any reduction in electrical conductivity of the collector electrode. Collector electrode 130’”” is removably coupled to peripheral frame 150’”” to allow replacement of the collector electrode (e.g. with suitable electrical connections provided to allow the outer electrode to be electrically disconnected). Figures 9A-E show another alternative ANR system 10” based on ANR system 10 (features in common are labelled accordingly) in which acoustic response tuning means 190 is replaced by internal porous structure 190” and outer ozone filter 180 is replaced by airflow blocking screen 250 provided in front of the plasma electrode arrangement 110”””. Internal porous structure 190” has a similar honeycomb aperture structure to foraminous filter / electrode plate 132”” of Figures 8A-E and is configured to absorb sound above a low-frequency threshold (e.g. above around 300 Hz). An ozone reducing catalyst (e.g. manganese oxide-based catalyst) is provided on (e.g. within or applied as a coating to) the internal porous structure 190”. In one embodiment, internal porous structure 190” is a foraminous metal plate (e.g. steel or aluminium plate) and the ozone reducing catalyst is applied as a coating to outer surfaces of the plate (e.g. outer surface of the plate and internal surfaces of the apertures). Internal porous structure 190” may removably coupled to peripheral frame 150””” to allow replacement of the internal porous structure. Airflow blocking screen 250 is operative to substantially allow transmission of acoustic pressure waves but substantially prevent transmission of airflow. Airflow blocking screen 250 is formed from a porous sheet material 252 (e.g. wire mesh or textile material) and has a longitudinal thickness of less than 5mm. In one embodiment, the airflow blocking screen 250 has an acoustic power transmission coefficient greater than 50% in the frequency range 0-5000 Hz. In one embodiment, the airflow blocking screen 250 has an acoustic power reflection coefficient no higher than 10% over a frequency range 0-5000 Hz. Figure 10 shows a further ANR system 10”’ based on ANR system 10 (features in common are labelled accordingly) in which sensing microphone 200 is replaced by a pair of sensing microphones 200A, 200B. Sensing microphone 200A operates as an outer sensing microphone for monitoring a pressure pi in front of the outer ozone filter 180. Sensing microphone 200B operates as an inner sensing microphone for monitoring a pressure p2 between the outer ozone filter 180 and the plasma electrode arrangement 110. In one embodiment, outer ozone filter has a resistance Res and the ANR circuitry is operative to determine an estimate of airflow velocity Vest, wherein Vest = p2.

Claims

1. An active noise reduction (ANR) system comprising:an electroacoustic plasma transducer system comprising:electroacoustic plasma transducer apparatus comprising:a plasma electrode arrangement including an inner electrode and an outer electrode separated from the inner electrode by an air gap;a peripheral support operative to support the plasma electrode arrangement; andan enclosure defining a chamber for receiving rearward radiation from the plasma electrode arrangement; anddrive means operative to supply a voltage U(t) between the inner electrode and the outer electrode of the electroacoustic plasma transducer apparatus;acoustic sensing means; andANR circuitry operative to receive a signal from the acoustic sensing means and control an output of the electroacoustic plasma transducer system to reduce noise;wherein the electroacoustic plasma transducer apparatus further comprises an ozone reducing material.

2. An ANR system according to claim 1, wherein the electroacoustic plasma transducer apparatus comprises a porous structure and the ozone reducing material is provided on the porous structure.

3. An ANR system according to claim 1 or claim 2, wherein the porous structure comprises an outer ozone filter provided in front of the plasma electrode arrangement.

4. An ANR system according to claim 3, wherein the outer electrode defines an inner face facing the inner electrode, an outer face substantially opposed to the inner face, and internal surfaces defined by apertures in the outer electrode, wherein the ozone reducing material is applied to the internal surfaces of the outer electrode and / or the outer face of the outer electrode.

5. An ANR system according to claim 4, wherein the ozone reducing material is substantially not applied to the inner face of the outer electrode.

6. An ANR system according to claim 4 or claim 5, wherein the outer electrode is removably coupled to the peripheral support.

7. An ANR system according to any of claims 3-6, wherein the outer ozone filter has an average effective aperture diameter of between 0.1-2 mm.

8. An ANR system according to any of claims 3-7, wherein the outer ozone filter has an aperture density of greater than 30 apertures per cm2.

9. An ANR system according to any of claims 3-8, wherein the outer ozone filter is configured to have an acoustic power transmission coefficient greater than 50% in the frequency range 0-5000 Hz.

10. An ANR system according to any of claims 3-9 wherein the outer ozone filter is configured to have an acoustic power reflection coefficient no higher than 10% over a frequency range 0-5000 Hz.

11. An ANR system according to any of claims 3-10, wherein the acoustic response of the outer ozone filter is configured to ensure that the outer ozone filter does not reduce the produced controlling sound level by more than 3 dB at 5000 Hz.

12. An ANR system according to any of claims 3-11, wherein the acoustic sensing means comprises:an outer sensing microphone for monitoring a pressure pi in front of the outer ozone filter; andan inner sensing microphone for monitoring a pressure p2 between the outer ozone filter and the plasma electrode arrangement.

13. An ANR system according to claim 12, wherein the outer ozone filter has an acoustic resistance Res, and the ANR circuitry is operative to determine an estimate of acoustic velocity Vest, wherein Vest = 21—E2.

14. An ANR system according to claim 1 or claim 2, wherein the ozone reducing material is provided within the enclosure.

15. An ANR system according to claim 14, wherein the electroacoustic plasma transducer apparatus further comprises an airflow blocking screen provided in front of the plasma electrode arrangement, wherein the airflow blocking screen is operative to substantially allow transmission of acoustic pressure waves but substantially prevent transmission of airflow.

16. An ANR system according to claim 15, wherein the airflow blocking screen is formed from a porous sheet material and has a longitudinal thickness of less than 5mm.

17. An ANR system according to claim 15 or claim 16, wherein the airflow blocking screen 250 has an acoustic power transmission coefficient greater than 50% in the frequency range 0-5000 Hz.

18. An ANR system according to any of claims 15-17, wherein the airflow blocking screen 250 has an acoustic power reflection coefficient no higher than 10% over a frequency range 0-5000 Hz.

19. An ANR system according to any of claims 15-18, wherein the acoustic sensing means comprises:an outer sensing microphone for monitoring a pressure pi in front of the airflow blocking screen; andan inner sensing microphone for monitoring a pressure p2 between the airflow blocking screen and the plasma electrode arrangement.

20. An ANR system according to claim 19, wherein the airflow blocking screen has an acoustic resistance Res, and the ANR circuitry is operative to determine an estimate of acoustic velocity Vest, wherein Vest = 21—E2.

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