Electroacoustic plasma transducer system

Abstract

An electroacoustic plasma transducer system (50) comprising: electroacoustic plasma transducer apparatus (100) comprising: a plasma electrode arrangement (110) including an inner electrode (120) and an outer electrode (130) separated from the inner electrode by an air gap (140); a peripheral support (150) operative to support the plasma electrode arrangement; and an enclosure (170) housing 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; wherein the enclosure is hermetically sealed and further comprises at least one gas-impermeable membrane (190) operative to allow transmission of acoustic energy from an interior of the enclosure to an exterior of the enclosure.

Description

[0001] TITLE: ELECTROACOUSTIC PLASMA TRANSDUCER SYSTEM

[0002] DESCRIPTION

[0003] 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.

[0004] US2023020879 (Al) discloses an ANR system using an electroacoustic plasma transducer employing acoustic impedance control.

[0005] Whilst the use of electroacoustic plasma transducer technology in ANR has numerous significant advantages, one key issue is the generation of ozone (which can be harmful in high concentrations) as a by-product of the plasma generation.

[0006] 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.

[0007] In accordance with a first aspect of the present invention, there is provided 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 (e.g. gas-filled) enclosure housing the plasma electrode arrangement; and drive means (e.g. drive circuit) operative to supply a voltage U(t) between the corona electrode and the collector electrode of the electroacoustic plasma transducer apparatus; wherein the enclosure is hermetically sealed and further comprises at least one (e.g. passive) gas-impermeable membrane (e.g. substantially acoustically transparent gas-impermeable membrane) operative to allow transmission of acoustic energy from an interior of the enclosure to an exterior of the enclosure.

[0008] The gas-impermeable membrane is configured to passively transmit acoustic energy whilst preventing transmission of gas. In this way, an electroacoustic plasma transducer system is provided that provides improved sound production means for active noise reduction whilst preventing the release of ozone from the electroacoustic plasma transducer apparatus.

[0009] 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.

[0010] In one embodiment, the gas-impermeable membrane is provided in front of the plasma electrode arrangement (e.g. to receive and transmit forward acoustic radiation from the plasma electrode arrangement).

[0011] In one embodiment, the enclosure defines a rear chamber section for receiving rearward acoustic radiation from the plasma electrode arrangement.

[0012] In one embodiment, the enclosure comprises at least one aperture and the at least one gas-impermeable membrane seals the at least one aperture.

[0013] In one embodiment, the at least one aperture is associated with a primary (e.g. front) output of the electroacoustic electrode arrangement (e.g. in front of the plasma electrode arrangement) and / or associated with a secondary (e.g. rear or side) output (e.g. acoustic port or vent).

[0014] In one embodiment, the gas-impermeable membrane has a thickness of less than 400 micrometres.

[0015] In one embodiment, the gas-impermeable membrane has a thickness of 5-400 micrometres.

[0016] In one embodiment, the gas-impermeable membrane has a thickness of 10-200 micrometres.

[0017] In one embodiment, the gas-impermeable membrane has a thickness of 10-100 micrometres.

[0018] In one embodiment, the gas-impermeable membrane has a thickness of 10-40 micrometres.

[0019] In one embodiment, the gas-impermeable membrane has a thickness of 40-100 micrometres.

[0020] In one embodiment, the gas-impermeable membrane is configured to have an acoustic power transmission coefficient greater than 50% in the frequency range 0-5000 Hz.

[0021] In one embodiment, the gas-impermeable membrane is configured to have an acoustic power reflection coefficient no higher than 10% over a frequency range 0-5000 Hz.

[0022] In one embodiment, the gas-impermeable membrane is configured to not reduce the produced sound level by more than 3 dB at 5000 Hz.

[0023] In one embodiment, the gas-impermeable membrane is formed from a flexible (e.g. flexible resilient) gas-impermeable material.

[0024] In one embodiment, the gas-impermeable membrane comprises a polyester film (e.g. Mylar™) or equivalent material.

[0025] In one embodiment, the gas-impermeable membrane is held with substantially zero tension (e.g. in a neutral, non-folded or stretched configuration).

[0026] In one embodiment, the gas-impermeable membrane is supported by (e.g. bonded to) the outer electrode.

[0027] In another embodiment, the gas-impermeable membrane is longitudinally spaced from the outer electrode.

[0028] In one embodiment, the gas-impermeable membrane is optionally supported by (e.g. bonded to) a gas-permeable support (e.g. support frame or backing layer). In one embodiment, the gas-permeable support is positioned such that the gas-impermeable membrane is longitudinally spaced from the outer electrode.

[0029] In one embodiment, the gas-permeable support is a compliant resilient (e.g. compliant elastomeric) gas-permeable support (e.g. compliant elastomeric foam backing layer).

[0030] In the case that the gas-permeable support is bonded to either the outer electrode or to a gas-permeable support, the bonding may occur over substantially a full contact area between the outer electrode / gas-permeable support and the gas-impermeable membrane (e.g. so that the gas-impermeable membrane is non-bonded only in areas directly in front of apertures in the outer electrode / gas-permeable support).

[0031] In one series of particularly advantageous embodiments, the enclosure is substantially filled with a gas other than air (e.g. a mixture of gases other than air). The use of a gas (or mixture of gases) allows for optimisation of the properties of the gas-filled enclosure as discussed in more detail below.

[0032] In one embodiment, the gas other than air (e.g. a mixture of gases other than air) makes up at least 50% by volume of the gas contained in the enclosure (e.g. at least 60% by volume of the gas contained in the enclosure, e.g. at least 70% by volume of the gas contained in the enclosure, e.g. at least 80% by volume of the gas contained in the enclosure, e.g. at least 90% by volume of the gas contained in the enclosure, e.g. substantially 100% by volume of the gas contained in the enclosure).

[0033] In one embodiment, the gas other than air (e.g. a mixture of gases other than air) is selected to provide one or more of: a) reduced oxidation of the plasma electrode arrangement relative to air and / or reduced ozone production relative to air; b) reduced effective breakdown voltage relative to air and / or reduced Corona Inception Voltage (CIS) relative to air; c) increased ion production efficiency (e.g. more ions released (i.e. higher ionisation current) for the same electrical input voltage) relative to air.

[0034] In one embodiment, the comparison with air in a)-c) above is determined at 20°C and 760 Torr (e.g. Standard Atmospheric Conditions).

[0035] The property a) offers the potential to significantly reduce or even eliminate oxidation of the electrodes of the plasma electrode arrange thereby increasing longevity of the apparatus.

[0036] The properties b) and c) offer the potential to significantly simplify amplifier design by reducing power and voltage levels whilst achieving the same output to a high voltage air-based plasma transducer.

[0037] In one embodiment, the gas other than air comprises at least one inert gas (e.g. comprises a single inert gas or a mixture of inert gases).

[0038] In one embodiment, the at least one inert gas is a non-oxidising gas or non-ozone producing gas.

[0039] In one embodiment, the at least one inert gas contains one or more of: nitrogen; a noble gas (e.g. neon, argon, krypton, xenon, etc.); and an inert gas compound (e.g. carbon dioxide).

[0040] In one particularly advantageous embodiment, the gas other than air comprises a mixture of inert gases (e.g. a mixture of the gases defined immediately above).

[0041] In one embodiment, the gas other than air comprises a mixture of noble gases.

[0042] Advantageously, a mixture of inert gases may allow use of significantly lower ionisation voltages whilst avoiding both oxidation of the electrodes and generation of ozone.

[0043] Whilst in practice the inclusion of air within the enclosure may be undesirable, in some embodiments the enclosure may contain a mixture of gas other than air (e.g. mixture of gases other than air) and air (e.g. with the air being present in a smaller volume that the gas other than air. By including at least a component of air inside the enclosure any mismatch in acoustic impedance across the membrane may be reduced (which may be sufficiently important in some applications to outweigh the downsides of the presence of air).

[0044] In one embodiment, the electroacoustic plasma transducer apparatus further comprises a pressure-equalisation system operative to balance static pressure inside the enclosure with exterior pressure (e.g. to adjust for changes in atmospheric pressure).

[0045] In one embodiment, the pressure-equalisation system comprises an electrically (e.g. electronically) controlled valve and pump connected to a sealed auxiliary gas chamber.

[0046] In another embodiment, the pressure equalisation system comprises a variable volume chamber.

[0047] In one embodiment, the variable volume chamber is achieved by providing a pressure balancing diaphragm (e.g. housed in a passageway in a wall of the enclosure) operative to deflect inwardly to reduce the volume of the chamber when atmospheric pressure is higher than the internal pressure of the chamber and / or to deflect outwardly to increase the volume of the chamber when atmospheric pressure is lower than the internal pressure of the chamber.

[0048] In accordance with a second aspect of the present invention, there is provided an active noise reduction (ANR) system comprising: electroacoustic plasma transducer system in accordance with the first aspect of the invention (e.g. any embodiment of the first aspect of the invention); acoustic sensing means (e.g. at least one acoustic sensor); 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.

[0049] In one embodiment, the acoustic sensing means comprises at least one sensing microphone.

[0050] In one embodiment, the ANR circuitry is operative to perform active acoustic impedance control.

[0051] An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0052] Figure 1A is a schematic illustration of an ANR system including sealed electroacoustic plasma transducer apparatus in accordance with an embodiment of the present invention;

[0053] Figure IB is an exploded schematic view of the ANR system of Figure 1A showing details of the sealed electroacoustic plasma transducer apparatus;

[0054] Figure 1C is schematic plan view of the electroacoustic plasma transducer apparatus of Figure 1 A with the sealed front membrane omitted for ease of viewing;

[0055] Figure ID is a schematic cross-sectional view AA of Figure 1C;

[0056] Figure IE is a schematic cross-sectional view BB of Figure 1C;

[0057] Figure IF is a block diagram illustrating operation of the ANR system of Figure 1A; and

[0058] Figure 2 is a graph showing Paschen curves for candidate gases for use in the sealed electroacoustic plasma transducer apparatus of Figure 1A.

[0059] Figures 1A-E show an ANR system 10 comprising: an electroacoustic plasma transducer system 50 including sealed 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.

[0060] As shown in Figures 1B-1E, 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 defining a chamber 170 A housing plasma electrode arrangement 110 with an aperture 170B and a gas-impermeable acoustically transparent membrane 190 sealing the aperture 170B. 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.

[0061] 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 1C, 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 rods 122 coupled to the peripheral frame 150 at their respective ends. As shown, each corona electrode rod 122 supports a row of longitudinally extending pins 124 having tapered tips 124 A spaced from collector electrode 130 by distance d and spaced from an inner surface 152 of the peripheral frame 150 by a lateral distance b.

[0062] Collector electrode 130 has a panel-form and substantially comprises a foraminous collector electrode plate 132 with an array of circular apertures 135. Corona electrode rods 122 are longitudinally spaced from collector electrode 130 by distance d and laterally spaced from one another by an equal distance a.

[0063] Gas-impermeable acoustically transparent membrane 190 comprises a membrane (or film) with low mass and low tension and has a thickness of 50-200 micrometres (e.g. in this specific example, a thickness of 200 micrometres).

[0064] As illustrated, gas-impermeable acoustically transparent membrane 190 provided in front of the plasma electrode arrangement 110 and coupled to the plasma electrode arrangement 110 via a an optional compliant foam middle layer (or backing layer) 180. Gas-impermeable acoustically transparent membrane 190 is longitudinally spaced from collector electrode 130 by a short distance corresponding to the thickness of compliant foam middle layer 180 and is hermetically sealed around the edge of enclosure 170 to form a hermetically sealed chamber 170A within the enclosure.

[0065] Compliant foam middle layer 180 comprises a 2mm thick foraminous sheet 132 with an array of circular apertures 185 substantially registered with apertures 135 provided in collector electrode plate 132. As shown, gas-impermeable acoustically transparent membrane 190 and compliant foam middle layer 180 substantially span the full area of the collector electrode 130. Gas-impermeable acoustically transparent membrane 190 is bonded to compliant foam middle layer 180 over substantially a full contact area between the compliant foam middle layer 180 and the gas-impermeable acoustically transparent membrane 190 (e.g. so that the gas- impermeable acoustically transparent membrane 190 is non-bonded only in areas directly in front of apertures 185 in the compliant foam middle layer 180).

[0066] Unlike the membrane of an electrostatic loudspeaker, gas-impermeable acoustically transparent membrane 190 is configured to operate in a substantially zero tension configuration (i.e. non-stretched and non-folded configuration). This is because the gas-impermeable acoustically transparent membrane 190 is operating as a passive element for sound transmission rather than an active element of the transducer.

[0067] Chamber 170A is a sealed enclosure and defines a rear chamber section 172 for receiving rearward radiation from the plasma electrode arrangement 110. Rear chamber section 172 has a longitudinal length / , typically less than 20 mm, and may house acoustic response tuning means (not shown).

[0068] As illustrated in Figure 1C, enclosure 170 includes a passageway 174 extending between rear chamber section 172 of chamber 170A to ambient air an exterior of the enclosure 170. Passageway 174 houses a pressure balancing diaphragm 400 operative to deflect inwardly to reduce the volume of the chamber 170 A when atmospheric pressure is higher than the internal pressure of the chamber and / or to deflect outwardly to increase the volume of the chamber when atmospheric pressure is lower than the internal pressure of the chamber. Typically the pressure balancing diaphragm 400 is provided by non-acoustically transparent durable elastic membrane which is capable of straining under a static pressure differential formed across its surface to either increase or decrease the volume of the gas-filled enclosure until a static pressure equilibrium is reached. This mostly non-acoustically transparent elastic membrane may, for example, be comprised of natural or synthetic rubber.

[0069] 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).

[0070] Whilst the sealed enclosure 170 may be filled with air, the sealed nature of the design enables the use of gases other than air (or mixtures of air and gases other than air).

[0071] In one embodiment, the gas other than air is selected to provide one or more of: a) reduced oxidation and / or reduced ozone production relative to air; b) reduced effective breakdown voltage and / or reduced Corona Inception Voltage (CIS) relative to air; c) increased ion production efficiency (e.g. more ions released (i.e. higher ionisation current) for the same electrical input voltage) relative to air.

[0072] Figure 2 shows a series of Paschen curves for different gases[1]illustrating the relationship between breakdown voltage and the product of pressure and plate electrode gap distance (i.e. an indication of the conditions required for an electrical discharge to transpire between two plate electrodes separated by a defined distance at a given pressure).

[0073] As illustrated, at standard atmospheric pressure, specifically at 760 mm Hg, the Paschen curves indicate the following breakdown voltages for air and other noble gases:

[0074] • Breakdown Voltage (kV at 1 cm)

[0075] . Air: ~ 30 kV

[0076] • Xenon (Xe): ~ 10 kV

[0077] • Krypton (Kr): ~ 7.5 kV

[0078] • Argon (Ar): ~ 7 kV

[0079] . Neon (Ne): ~ 3 kV

[0080] . Helium (He): ~ 2.5 kV

[0081] It is noteworthy that the breakdown voltage of a gaseous mixture does not necessarily conform to the arithmetic average of its components. For example, a mixture of Neon and 0.1% Argon manifests a breakdown voltage of approximately 1.5 kV, despite Neon and Argon individually having breakdown voltages of ~3 kV and ~7 kV, respectively.

[0082] While the Paschen Curve serves as a practical tool for predicting breakdown voltage under specific conditions, it remains an empirical model and is subject to variations due to influencing factors such as plate electrode material, surface roughness, and gas purity.

[0083] The breakdown voltage of a gas is intrinsically linked to its Corona Inception Voltage (CIV), which denotes the minimum voltage required for ionization and electrical conduction in standard atmospheric conditions when in close proximity to a conductor such as a Corona electrode. Factors that affect the breakdown voltage also influence CIV, in addition to the conductor’ s shape and size, and the frequency of the applied voltage. Accordingly, variations to these plots are expected to be observed depending on the setup used.

[0084] In summary, gases with lower breakdown voltages generally exhibit lower Corona Inception Voltages. Therefore, many noble gases and their mixtures present an opportunity for significantly lowering the voltage threshold at which Corona or plasma formation occurs in a plasma transducer.

[0085] References:

[0086] [1] Gas Tube Design, by H H Wittenberg - Electron Tube Design, RCA Electron Tube

[0087] Division, 1962, pages 792 - 817

Claims

Claims:

1. 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; and an enclosure housing 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; wherein the enclosure is hermetically sealed and further comprises at least one gas- impermeable membrane operative to allow transmission of acoustic energy from an interior of the enclosure to an exterior of the enclosure.

2. An electroacoustic plasma transducer system according to claim 1, wherein the enclosure comprises at least one aperture and the at least one gas-impermeable membrane seals the at least one aperture.

3. An electroacoustic plasma transducer system according to claim 2, wherein the at least one aperture is associated with a primary output of the electroacoustic electrode arrangement.

4. An electroacoustic plasma transducer system according to any of the preceding claims, wherein the gas-impermeable membrane has a thickness of less than 400 micrometres.

5. An electroacoustic plasma transducer system according to claim 4, wherein the gas- impermeable membrane has a thickness of 10-200 micrometres.

6. An electroacoustic plasma transducer system according to any of the preceding claims, wherein the enclosure is substantially filled with a gas other than air.

7. An electroacoustic plasma transducer system according to claim 6, wherein the gas other than air is selected to provide one or more of: a) reduced oxidation of the plasma electrode arrangement relative to air and / or reducedozone production relative to air; b) reduced effective breakdown voltage relative to air and / or reduced Corona Inception Voltage (CIS) relative to air; c) increased ion production efficiency relative to air.

8. An electroacoustic plasma transducer system according to claim 6 or claim 7, wherein the gas other than air comprises at least one inert gas.

9. An electroacoustic plasma transducer system according to claim 8, wherein the at least one inert gas is a non-oxidising gas or non-ozone producing gas.

10. An electroacoustic plasma transducer system according to claim 8 or claim 9, wherein the at least one inert gas contains one or more of nitrogen; a noble gas; and an inert gas compound.

11. An electroacoustic plasma transducer system according to any of claims 8-10, wherein the gas other than air comprises a mixture of inert gases.

12. An electroacoustic plasma transducer system according to any of claims 6-11, wherein the enclosure contains a mixture of gas other than air and air.

13. An electroacoustic plasma transducer system according to any of the preceding claims, wherein the electroacoustic plasma transducer apparatus further comprises a pressureequalisation system operative to balance static pressure inside the enclosure with exterior pressure.

14. An active noise reduction (ANR) system comprising: electroacoustic plasma transducer system in accordance with in accordance with any of claims 1-13; 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.

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