TECHNICAL FIELD AND BACKGROUND

The present disclosure relates to an acoustic device, e.g. having or forming an acoustic valve, and method of controlling the device.

In the field of hearing and audio, various acoustic devices exist which can be used, e.g., to protect, enhance and/or enable users to have a normal or better hearing experience. Examples of such acoustic devices may include hearing protection devices, hearing instruments, hearing aids, hearables, et cetera. Depending on the type, the acoustic devices can be placed at different positions in and around the human ear/canal. For example, acoustic devices can take the form of ear buds or head phones.

Typically, an acoustic device comprises one or more channels which can be used to form a connection between the ear canal and external surroundings. In some cases, the channel may help to prevent a feeling of occlusion, e.g. by allowing sound to travel from the ear drum to the external environment or vice versa. In other or further cases, the channel may act as a vent, e.g. to provide ventilation inside the ear canal and/or relieve static pressure in the ear canal.

An acoustic valve can be used to control the sound or air passing in and out of the system. For example, the valve can he installed in the channel or vent. In some cases, the acoustic valve can be switched between different states, e.g. based on one or more control parameters or other conditions. For example, an open state can be uses in situations where the natural sound (including directionality) is preserved thus getting rid of the occlusion to a certain extent by allowing sound to escape from the ear canal. It allows free flow of ear, hence offers ventilation and occlusion reduction. For example, a closed state provides a seal from the external environment to create an enhanced sound quality (in comparison with the open state) for low frequencies from the sound source (for e.g. Balanced Armature Receivers or Dynamic drivers). In addition to this directionality and noise suppression can also be achieved in this state.

As background, various types of acoustic devices, channels, and valves are described, e.g., in US8798304B2, WO2008/086188A1, US8923543B2, WO2011149970A1, US20160255433A1, US20190106416A1, US20190106436A1, US20190106438A1, EP3169290B1, US20190320272A1, US2014/0169603A1, US6549635B1.

For example, US2014/0169603 A1 describes techniques for actuating a valve of a hearing assistance device. In one example, a hearing assistance device comprises a device housing defining a vent structure, a vent valve positioned within the vent, the vent valve having first and second states. The vent valve comprises a magnet, a disk configured to move about an axis, and a magnetic catch. The hearing assistance device further comprises an actuator, and a processor configured to provide at least one signal to the actuator to cause the disk to move to eontrollably adjust the vent structure. In some examples, the actuator can be a coil. In other example implementations, the actuator can be an electroactive polymer, a shape memory alloy, piezoelectric element, or a flexible polymer that comprises magnetic material, for example.

There is a need for further improvement in acoustic devices, e.g. with an acoustic valve that is reliable with low power consumption.

SUMMARY

Aspects of the present disclosure relate to an acoustic device with an acoustic channel for passing sound through its housing. An acoustic valve is arranged in the channel. The valve comprises a valve member configured to determine the passing of sound through the channel depending on its configuration, e.g. with respect to a corresponding valve seat. For example, the valve member can move and/or reshape while the valve seat stays in place, e.g. as part of the channel. At least one SMA wire is configured to actuate the valve member and switch the configuration, A set of electric terminals, e.g. connection points or wires, are configured to supply electric power for the activation of the respective SMA wire section.

In this way the switching of the configuration can be controlled. By providing a relatively soft or elastic material between the interfaces of the valve member and valve seat, this material can be compressed. By compressing the elastic material the valve member can at least partially pass through an opening formed by the valve seat, or vice versa. For example, a first wire section of the SMA wire is activated to switch to a closed configuration and a second SMA wire is activated to switch to an open configuration. Advantageously, the closed configuration can be maintained by a contact force between the valve seat and valve member caused by the compressed elastic material pushing to re-expand there between. The valve member can be released from the closed configuration by overcoming the contact force when the second wire section is activated to switch to the open configuration. Accordingly, the valve can remain reliably closed (or open) without power to the SMA wire.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawing wherein:

FIGs 1A and 1B illustrate an acoustic device wherein a ball shaped valve member cooperates with a ring shaped valve seat to form an acoustic valve;

FIGs 2A and 2B illustrate an acoustic device wherein the valve seat is relatively thin forming flexible washer;

FIGs 3A and 3B illustrate a valve member having an outer rim cooperating with a ridged valve seat; FIGs 4A and 4B illustrates multiple SMA wires to actuate the valve member;

FIGs 5A and 5B illustrate a ring shaped valve member actuated by SMA wires to cooperate with a ball shaped valve seat;

FIGs 6A and 6B illustrate a deformable cup shaped valve member being actuated with respect to the valve seat forming a rim around the cup shape;

FIGs 7A and 7B illustrate photographs of a deformable cup shaped valve member and corresponding valve seat;

FIGs 8A and 8B illustrate acoustic measurements of an acoustic device according to FIGs 1A and 1B.

DESCRIPTION OF EMBODIMENTS

Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and/or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and/or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.

In a preferred embodiment, a combination of a SMA (micro) springs forms a mechanical valve e.g. comprising a hard polymer type of a sphere being locked in a soft polymer material type of ring. This type of construction with combination of soft and hard material may provide a good sealing of the valve in a closed state and allow relatively free sound passage in an open state. It may also prevents leakage due to relaxation of the SMA (micro) spring. The whole module can be light-weight and miniaturized so that it can fit in an ear tip.

The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross- section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

FIGs 1-6 illustrate various acoustic devices 100 in respective open and closed configurations “Co” and “Cc”.

Typically, the acoustic device 100 comprises a housing 11 with an acoustic channel 12 for passing sound “S” there through. As described herein, a valve seat 13 is arranged in the acoustic channel 12 and a valve member 14 is configured to determine the passing of sound through the channel 12 depending on the relative configuration. Preferably, at least one SMA wire 15 configured to actuate the valve member 14, For example, the wire can actuate valve member (14) to switch the configuration when a respective SMA wire section 15o,15c of the at least one SMA wire 15 is activated. A set of electric terminals 17 can be used to supply electric power for the activation of the respective SMA wire section 15o,15c to control the switching of the configuration “Co” and/or ”Cc”.

In some embodiments, at least one of the valve member 14 and valve seat 13 comprises an elastic material. In other or further embodiments, the valve member 14 is dimensioned to at least partially pass through an opening formed by the valve seat 13, or vice versa. Most preferably the valve member passed through or past the valve seat by compressing the elastic material. For example, this can happen when a first wire section 15c of the SMA wire 15 is activated to switch to a closed configuration “Cc”. Preferably, the closed configuration “Cc” is maintained by a contact force Fc between the valve seat 13 and valve member 14 caused by the compressed elastic material pushing to re -expand there between. For example, the valve member 14 is released from the closed configuration “Cc” by overcoming the contact force Fc when a second wire section 15o is activated to switch to an open configuration “Co”,

As described herein, the SMA wire comprises or essentially consists of a shape-memory alloy SMA material, SMA material also referred to as smart metal, memory metal, memory alloy, muscle wire, smart alloy is an alloy that "remembers" its original shape and that when deformed may return to its pre-deformed shape when activated. For example, the SMA wire comprises or essentially consists of a Ni-Ti alloy, Cu-Zn-Al alloy, Cu-Al- Ni alloy, or any other material acting as an SMA. SMA material may provide a lightweight, solid-state actuator as an alternative to conventional actuators such as piezo, hydraulic, pneumatic, and motor -based systems.

Typically, the SMA wire is affected by temperature. For example, a relatively high temperature may cause the wire to contract. When the element cools down it may reach a relatively low stabilization temperature, e.g. at ambient temperature. In some cases, this may cause at least some extension of the wire length. Accordingly, a length of the SMA wire may be related to its temperature. The SMA wire may be provided with heat according to some embodiments. This may cause the wire to attain an elevated temperature. For example, the SMA wire can be heated to at least fifty degrees Celsius, preferably in a range between sixty and hundred twenty degrees Celsius, e.g. ninety degrees Celsius, The elevated temperature may cause the wire to contract to a heated contraction length, e.g. regain its original (“remembered”) shape after having previously been extended. Accordingly, mechanical movement can be provided to the valve member 14 to another position.

In a preferred embodiment, the heated contraction length of a respective wire section is shorter than its stabilized extended length by at least one percent, at least two percent, or at least three percent, or more.

The more the relative contraction, the less limited the mechanical stroke which can be provided. In some embodiments, the absolute contraction may also be improved by lengthening the SMA wires, most preferably by coiling the wire. In some embodiments, the combination of the SMA wire length and relative contraction provides a mechanical stroke of at least hundred micrometer, preferably at least half a millimeter or even more than one millimeter.

In some embodiments, the at least one SMA wire 15 is coiled. When coiled, the length (distance between end points) can be much shorter than its uncoiled length , e.g. by at least a factor two, three, four, five, or more. For example, the coiling may increase stroke. Alternatively, or additionally the SMA wire can form a (micro) spring capable of exerting resilient force. Typically, the total SMA wire, or the respective SMA wire section 15o,15c each, has an uncoiled length between ten and fifty millimeter, e.g, twenty five millimeter. For example, the total SMA wire, or the respective SMA wire section 15o,15c each, has a coiled length between four and twelve millimeter, e.g, six millimeter. When activated, e.g, by heat, wherein the SMA wire (distance between points) may typically shorten by at least one, two, or three percent of the uncoiled length and/or by at least five, ten, fifteen, or even twenty percent of the coiled length (distance).

After heating the wire may lose at least part of its heat so it may cool down to a stabilization temperature, which may be the same or different from the initial temperature. For example, cooling can be effected by radiation, convection, or conduction, which may cause partial relaxation of the contraction, i.e. re-extension of the respective wire section. As described herein, the valve member 14 may be kept in (closed) position against the valve seat 13 by the elastic material there between, despite the relaxation of the SMA wire.

In some embodiments, the SMA wire, or parts thereof, are activated electrically, e.g. wherein an electric current results in Joule heating. The heat can e.g, be supplied by an electrical current through the SMA wire, electric wire contacting the SMA wire, or any other heating and/or cooling element, e.g, proximate to, adjacent, or contacting the SMA wire (not shown). Deactivation typically occurs by free convective heat transfer to the ambient environment. In some cases, SMA material may exhibit hysteresis, i.e. a dependence of the state of the system on its history. In a preferred embodiment, the at least one SMA wire 15 is electrically conductive and the electric terminals 17 are connected to selectively pass an electric current through a respective wire section 15o,15c of the SMA wire 15.

In some embodiments, the acoustic device 100 comprises or is coupled to a controller (not shown). For example, the at least one SMA wire 15 is activated by a controller supplying an electric current via a respective set of terminals 17a, 17o; 17a, 17c for actuating the valve member 14, wherein the respective SMA wire section 15o,15c is configured to contract or extend depending on its temperature determined by the electrical current to exert an actuation force Fa by its connection to the valve member 14, Aspects described herein can also be embodied as a method for controlling an acoustic device 100 as described herein. In some embodiments, a first control signal is generated to activate the first wire section 15c to pass the valve member 14 at least partially through an opening formed by the valve seat 13, or vice versa, by compressing elastic material there between. Preferably, the closed configuration “Cc” is maintained by a contact force Fc between the valve seat 13 and valve member 14 caused by the compressed elastic material pushing to re -expand there between. In other or further embodiments, a second control signal is generated to activate the second wire section 15o to release the valve member 14 from the closed configuration “Cc” by overcoming the contact force Fc when a second wire section 15o is activated to switch to an open configuration “Co”. For example, the control signals are generated by the controller. Alternatively or additionally, aspects can be embodied as a non- transitory computer-readable medium storing instructions that, when executed by one or more processors, cause a device to perform the method as described herein.

In some embodiments, the SMA wire 15 is configured to change, by pulling on the valve member 14, the configuration of the acoustic device 100 between at least an open configuration “Co” wherein the channel 12 for the passage of sound “S” through the housing 11 is relatively open and a closed configuration “Cc” wherein the channel 12 is relatively restricted (partially or fully closed). Also other or further states can be defined, e.g. one or more intermediate states allowing various degrees of attenuation. Alternatively, or in addition to varying attenuation, the set of states may also include different filtering of sound, e.g. wherein a first state has a different sound transmission characteristic than a second state. For example, the moving valve member 14 can open or close different passages with different filters. In some embodiments, a bi- stable (or multi- stable) actuator is desired, that is, an actuator that can move between two or more positions and remain at any of those without consuming power. In some embodiments, this is achieved by creating an actuator based on antagonist respective SMA wire sections 15o,15c which act against each other. When one contracts, the other one elongates and is ready to be contracted. The contracting SMA wire can be used to pull on the valve member 14 in either direction. Then, the elongated wire section is ready to contract as it is activated e.g. by passing electricity there through, thus moving the valve member 14 to the one side and elongating the other wire section. This mechanism can be bi-stable and used cyclically.

In a preferred embodiment, the acoustic device 100 comprises at least two SMA wire sections 15o,15c with respective sets of control terminals 17a, 17o; 17a, 17c configured to the selectively heat either one of the SMA wire sections 15o,15c to cause contraction in the heated wire section, wherein the contraction causes an actuation force Fa by pulling the valve member 14 in one of at least two different directions towards the closed configuration “Cc” or the open configuration “Co” depending on which wire section is heated. For example, the actuation mechanism of the acoustic valve, e.g. valve member 14, respective controllers or switches connected to the respective terminals and configured to the selectively activate (e.g. heat by electricity) either one of the respective SMA wire sections 15o,15c.

In some embodiments, a first electric terminal 17a is connected, e.g, by a respective electric wire 16, to a respective wire section of the at least one SMA wire 15 at the valve member 14 and/or between the first and second sections 15o,15c. In other or further embodiments, a second electric terminal 17c is connected to an end of the first wire section 15c forming a first electrical path between the first electric terminal 17a and the second electric terminal 17c through the first wire section 15c. In other or further embodiments, a third electric terminal 17o is connected to an end of the second wire section 15o forming a second electrical path between the first electric terminal 17a and the third electric terminal 17o,

In some embodiments, the respective SMA wire sections 15o,15c may comprise separate SMA wires, e.g. each connected with a respective electric wire 16. In other or further embodiments, different parts of a single SMA wire 15 form the first and second wire sections 15c on either side of the valve member 14. For example, the same electric wire 16 may be connected to a middle of the single SMA wire 15 and conduct a current in either directions of the respective SMA wire sections 15o,15c, e.g. depending on a switching or voltage at the other terminal 17o,17c.

In other or further embodiments, the (single) SMA wire passes through the valve member 14. Typically, the at least one SMA wire may also pass through the valve seat 13 and/or opening there through. For example, the valve member 14 is disposed in the channel 12 hanging by the at least one SMA wire 15. Also other or further support structures can be envisaged. For example, the valve member 14 can be attached to a guidance structure, e.g. one or more further support wires (not shown). In one embodiment, the guidance structure can be used to guide the valve member 14 along a specific trajectory as it is actuated by the respective SMA wire sections 15o,15c. For example, the valve member 14 may slide along a guide wire (not shown) through the valve member 14 and/or through the opening in the valve seat 13.

With reference now to material properties, preferably the elastic material is configured to recover a specific form in the absence of external forces. A temporary shape change that is self- reversing after the force or stress is removed, so that the object returns to its original shape, can be referred to as elastic deformation e.g. as opposed to plastic deformation. In other words, elastic deformation may refer to a change in shape of a material that is recoverable after the force is removed. So the one or both of

configuration. On the other hand, the force to release the configuration should not be excessive. For example, the contact surfaces between the valve seat 13 and the valve member 14 typically have a coefficient of (static) friction between 0,2 and 0,8 or between 0,3 and 0,6 (lower is less friction).

In some embodiments, both the valve seat 13 and valve member 14 comprise elastic material. Preferably though, at least one of the valve member 14 or valve seat 13 comprises a relatively hard material, e.g. relatively rigid or non-elastic material, at least disposed at the respective contact interface. In one embodiment, the valve seat 13 comprises a relatively elastic material, and the valve member 14 comprises a relatively rigid material. In another or further embodiment, the valve seat 13 comprises a relatively rigid material, and the valve member 14 comprises a relatively elastic material. Using a combination of elastic and hard contact interfaces may improve performance.

Preferably, (at least) a contact surface of one of the valve member 14 and valve seat 13 comprises the elastic material, and a contact surface of the other of the valve member 14 and valve seat 13 comprises a relatively hard or rigid material with a Young’s modulus of more than four hundred Mega Pascal, preferably more than one Giga Pascal, e.g. up to two or three Giga Pascal, or more. Examples of such hard materials may include e.g. Polyvinylchloride (PVC), Polycarbonate (PC), Polyethylene terephthalate (PET) and other type of hard plastics or polymers.

Preferably, the valve member 14 is relatively light, e.g. to hang and move the valve member 14 from the respective SMA wire sections 15o,15c alone, or in combination with other support structures. For example, the valve member 14 has a mass of less than twenty grams, preferably less than ten grams. The weight may also depend on the type of material. For example, when the valve member comprises a relatively soft or elastic material, the mass is typically between one and eight grams, preferably between two and six grams. For example, when the valve member comprises a hard or inelastic material, the mass can be a hit higher, e.g. between two and ten grams, preferably between three and eight grams. To fit into a miniature design, the valve member 14 typically has a diameter (Db) less than five millimeter, preferably less than four or even less than three millimeter. Most preferably, the diameter (Db) of the valve member 14 is between one and two-and-half millimeter.

In some embodiments, the valve member 14 is formed as a ball. While the ball shape is preferable for either the valve member 14 or the valve seat 13 (shown e.g. in FIG 5) also other or further shapes can be envisaged, e.g. preferably having rotation symmetry and/or an overall convex contour. In other or further embodiments, the valve seat 13 forms an (round) opening that is part of the acoustic channel 12. For example, the round (or oval) valve member has a diameter (in the cross- diameter direction of the hole) in the aforementioned range. Preferably, the opening through the valve seat 13 has an inner diameter Dh approximately equal an outer diameter Db of the valve member 14, or slightly smaller, e.g, between one and ten percent smaller. This may allow the valve member 14 to pass at least partially through the valve seat 13 while also compressing the elastic material by a few percent.

While the valve seat 13 is preferably dimensioned to trap or hold the valve member 14, the rest of the acoustic channel 12 can be relatively wider to allow more free movement of the valve member 14, at least on a side where the valve member 14 is disposed when in the open configuration “Co”, In a preferred embodiment, the channel 12 has an inner cross-section diameter Dc larger than an outer diameter of the valve member 14 and/or inner diameter of the valve seat 13 by at least a factor 1.1, preferably at least a factor 1,2 (twenty percent), 1,3, 1,5, or more. For example, the acoustic channel 12 has a typical diameter Dc between two and four millimeter. The channel length can be larger, e.g, by at least a factor two. For example, the channel has typical length between eight and twelve millimeter. In one embodiment, the acoustic device as described herein forms an ear plug fitting at least partially in an ear canal. For example, an outer diameter of the housing is less than two centimeters, preferably less than one and half centimeter, most preferably less than one centimeter.

Reference will now be made to some specific and advantages illustrated by the respective figures. Of course, it will be understood that these features can be combined as desired to provide even further advantages.

FIGs 1A and 1B illustrate an acoustic device wherein a ball shaped valve member 14 cooperates with a ring shaped valve seat 13 to form an acoustic valve. In some embodiments, the valve seat 13 comprises a ring having an inner diameter Dh and a thickness T configured to cause the valve member 14 to remain stuck therein when the first wire section 15c is activated to pull the valve member 14 towards the closed configuration “Cc”. Preferably, the configuration get unstuck (only) when the second wire section 15o is activated to pull the valve member 14 towards the open configuration “Co”. For example, the elastic material in one or both of the valve seat 13 or valve member 14 is compressed by pulling the valve member 14 into the valve seat 13. In some embodiments, the valve seat 13 has a thickness T on the order of the diameter Db of the valve seat 13, e.g. with a thickness T at least one quarter, or more than half the diameter Db. In this range, typically the valve member 14, e.g. ball, may get stuck somewhere at the start of the valve seat 13 (e.g. the center of the valve member 14 does not pass the start of the valve seat 13).

FIGs 2A and 2B illustrate an acoustic device wherein the valve seat 13 is relatively thin forming flexible washer. In some embodiments, the valve seat 13 comprises a washer (thin ring) configured to deform and allow a center of the valve member 14 to pass there through (and get stuck when trying to pass back). For example, the elastic material of the valve seat 13 bends in one direction while passing the valve member 14 and resists bending back to keep the valve member 14 stuck (until it is released),

FIGs 3A and 3B illustrate a valve member 14 having an outer rim cooperating with a ridged valve seat 13. In some embodiments, the valve seat 13 comprises a set of ridges and the valve member 14 comprises a rim (e.g. Saturn ring) that gets stuck in a respective ridge when the valve member 14 is at least partially passed through the valve seat 13, or vice versa. For example, the rim can be hard and the ridges of the valve seat 13 relatively flexible (elastic), or vice versa. Also the ridges can be disposed on the valve member 14 and the rim (or ridges) disposed on the valve seat 13. Also other inversions or variations can be envisaged.

FIGs 4A and 4B illustrates multiple SMA wires to actuate the valve member 14. In some embodiments, multiple SMA wire sections 15o,15c are disposed on each side of the valve member 14. Providing multiple wires or coils may further improve stroke. For example, a first SMA wire can be loop back from one side of the acoustic channel 12 to the valve member 14 , and a (separate) second SMA wire can loop back from the opposite side of the acoustic channel 12 and the valve member 14. Accordingly, at least two lengths of SMA wire sections can be provided on either side. The SMA wire may also loop back and forth multiple times, or there can be provided a set of separate SMA wires on each side. In this and other embodiments, each SMA wire or section can have its own set of terminals 17a, 17o; 17b, 17c; or some terminals can be shared.

FIGs 5A and 5B illustrate a ring shaped valve member 14 actuated by SMA wires to cooperate with a ball shaped valve seat 13, This may be considered as a sort of inversion of the acoustic device 100 shown in FIGs 1A and 1B. In general, it will be understood that various aspects of the embodiments described herein can be varied, e.g, by inverting the roles/shapes of valve seat 13 and valve member 14 and/or inverting which side has the elastic material. FIGs 6A and 6B illustrate a deformable cup shaped valve member 14 being actuated with respect to the valve seat 13 forming a rim around the cup shape. In some embodiments, the valve member 14 is formed by a cup which is connected to the valve seat 13. For example, the cup folding inward or outward changes the configuration of the acoustic channel 12 between the closed configuration “Cc” and open configuration “Co”. Preferably, the cup, or at least the edges are of an elastic/resilient material. When the cup is folded inward, this may affect openings through the edge of the cup and thereby the passage of sound “S”. When folded in one direction, the flexible cup may get stuck in that configuration until it is pulled to other direction.

FIGs 7A and 7B illustrate photographs of an embodiment for the deformable cup shaped valve member 14 and valve seat 13. In one embodiment, e.g. as shown, the valve seat 13 forms an aperture 13a which can be open or closed depending on a configuration of the cup shaped valve member 14. When the cup is in the open configuration, sound may pass through the aperture 13a, e.g. via holes 13h in a ring connected to a rim of the cup. For example, the ring can form a unit with the valve seat 13 and/or be part of the housing. In some embodiments, the unit comprising the valve seat 13 forming the aperture 13a and/or ring with holes 13a is of relatively hard material. Of course also other structures forming acoustic paths can be envisaged. In other or further embodiments, the cup shaped valve member 14 is of a relatively soft and/or elastic material as described herein.

FIGs 8A and 8B illustrate acoustic measurements of an acoustic device according to FIGs 1A and 1B. The graphs show the attenuation “A”

(in decibel) as function of frequency “f” (in Hertz), The top and figures illustrate the measurement in the open and closed configurations (“Co”, “Cc”), respectively. For example, at 200 Hz the closed configuration provides an attenuation of 25 decibel. Of course also other attenuations can be envisaged. In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit their scope; several "means" may he represented by the same or different item(s) or implemented structure or function; any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise. Where one claim refers to another claim, this may indicate synergetic advantage achieved by the combination of their respective features. But the mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot also be used to advantage. The present embodiments may thus include all working combinations of the claims wherein each claim can in principle refer to any preceding claim unless clearly excluded by context.