TECHNICAL FIELD AND BACKGROUND

The present disclosure relates to an acoustic filter for attenuating sound in an earplug, a method for manufacturing the acoustic filter, and use of the acoustic filter.

Various products like hearables, hearing aids, earphones, and headphones can provide an acoustical seal to the ear. Wearing such product may give the user a feeling of being occluded and cut off from their surroundings. Furthermore, prolonged use may lead to irritated ears, as the ears are not ventilated. Hence some of these products may include an ambient channel or sound passage to provide an open (acoustical) pathway through the otherwise sealed housing. Typically, such channel provides two modes. In an open mode it preserves natural hearing, and in a closed mode the environment is blocked to protect the ear from undesired ambient sound and/or to provide enhanced sound quality (compared to open mode sound) when an acoustic generator is present.

For example, WO 2005/041831 Al describes an earplug comprising a plug member for blocking a person's ear canal, said plug member comprising at least one acoustic channel for channeling incoming acoustic energy into said person's ear, said earplug further comprising a detector for detecting an acoustic energy level or for detecting a control signal that is indicative for an acoustic energy level to be received, an acoustic valve positioned in said channel, and a control unit that, in response to said detector, controls said valve so as to attenuate the acoustic energy channeled through said acoustic channel.

There remains a desire to improve control over attenuation of sound using acoustic valves. SUMMARY

Aspects of the present disclosure relate to an acoustic filter and earplug comprising the filter. In preferred embodiments, the acoustic filter comprises a filter housing. Typically, the acoustic filter or housing is placeable or integrated in an earplug. In this way a seal is formed between an auditory canal of an ear and an external surroundings. A sound passage may extend through the filter for receiving sound from the external surroundings and transmitting the sound into the auditory canal. The sound passage can have one or more apertures. The passage and apertures can provide an acoustic resistance for the sound received and transmitted there through. In preferred embodiments, the filter comprises a valve system with one or more acoustic valves arranged in the sound passage. The valves may control the acoustic resistance of the one or more apertures e.g. based on a control signal. In this way an attenuation of a sound pressure level of the transmitted sound can be selected.

Advantageously, the acoustic filter can be adapted to provide the selection from a sequence comprising a number of predetermined

attenuations. For example the selection can be made from a discrete number of programmed sound levels. The selection may typically involve sending the corresponding control signal which can also be preselected, e.g. programmed into a controller or hardwired in circuitry. Each attenuation in the sequence may correspond to one or more acoustic valves providing one or more respective apertures with predetermined acoustic resistance. In some preferred embodiments, the acoustic resistances of subsequent attenuations in the sequence are adapted to correspond with a (near) linear increase or decrease in the attenuation of the sound as measured in decibels. This may correspond to logarithmic variation of the relative sound pressure level.

It is recognized that attenuating the sound pressure level on a logarithmic scale, may better conform to human hearing and thus improve desired control over the sound level. While conventional volume control may be realized by an electronic sound generator, the inventors find that improved control of ambient volume can be achieved using a preselected range of acoustic resistances which may not require electronic conversion of the sound. In some embodiments, the attenuation control may be improved by using multiple valves and/or adapted aperture shapes. For example, the tuning of acoustic resistances can be improved by combining predetermined aperture sizes that are selectively opened or closed. Additionally, or alternatively, partially opened valves can be used to provide intermediate acoustic properties. By shaping the aperture and/or valve, the open area can be more accurately determined.

Some aspects of the present disclosure may relate to methods of manufacturing acoustic filters, e.g. as described herein. For example, an acoustic filter can be specifically designed to provide a discrete number of attenuations which linearly increase or decrease the transmitted sound in decibels. For example, the corresponding sequence of control signals for control of the acoustic filter may be stored in software or hardware implementations. Some aspects of the present disclosure may also relate to the use of an acoustic filter, e.g. for controlling attenuation of the external surroundings into the auditory canal. For example, the use may comprise changing the attenuation in discrete steps, wherein in each step a subsequent attenuation in the sequence of predetermined attenuations is selected to linear increase or decrease in the attenuation of the sound in decibels.

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-1C schematically illustrate various dimensions of example apertures;

FIG 2A schematically illustrates a general trend of acoustic resistance as a function of one or more apertures being closed;

FIG 2B schematically illustrates a selection of attenuations;

FIG 3A illustrates an embodiment of an acoustic filter;

FIG 3B illustrates an embodiment of an earplug;

FIGs 4A-4C illustrate an embodiment of a sound passage with multiple apertures;

FIGs 5A-5D schematically illustrate top and side views of an embodiment comprising a non-linear shaped aperture;

FIGs 6A-6C schematically illustrate side views of a valve system comprising stacked valves;

FIGs 7A-7C schematically illustrate side views of apertures including an acoustic element.

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.

It is recognized that human senses such as hearing typically work in accordance with Weber-Fetcher law, i.e. wherein a perceived change such as volume or loudness of sound is logarithmically related to an actual change of the corresponding physical stimulus such as the sound pressure level or intensity entering the ear canal. So it may better conform to human experience to use a logarithmic scale for expressing attenuation of sound. For example, the decibel dB or dB SPL (sound pressure level) in acoustics can be used to quantify sound pressure levels and intensities relative to a reference on a logarithmic scale. For example, an intensity level“IL” of a sound intensity“I” can be expressed as

IL = lO logio

where“L-ef” is a reference intensity. Alternatively or additionally, since the intensity carried by a traveling wave is typically proportional to the square of the pressure amplitude, the intensity level can be expressed by the sound pressure level (SPL) as

where“P” is the measured effective pressure amplitude of the sound wave and“Pref is the reference effective pressure amplitude. For example, the effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. For example for air, the pressure reference is usually taken to be

P _ref = 20 · 10 ^~6 [Pa] (3) the lowest sound pressure a human can hear, also called the threshold of hearing. As described herein, sound attenuation“AS” through a filter is expressed in decibel e.g. based on sound pressure levels of the sound“So” entering the filter and the sound“SN” exiting the filter:

The factor 20 and base 10 of the logarithm are typically used in audiology for the expression in decibel. However, it will be recognized that the formula can be linearly scaled to other pre-factors and/or other based logarithms e.g. using mathematical conversion such as:

logdO)

logi,(x) = (5) log _d O)

Accordingly, the statement that the attenuation changes linearly (or in constant steps) on a decibel scale may be equivalent to any exponential changes or steps of the relative sound pressure level or intensity. Generally the difference between linear and exponential steps may best be recognized when a sequence of at least three, preferably more different values are compared.

As described herein, attenuation of sound waves through an acoustic filter can be realized by blocking part of a canal and only allowing sound to enter through one or more apertures having relatively restricted dimensions e.g. compared to the rest of the filter. Without being bound by theory, it is found that the acoustic properties of filters can be described by parameters such as the acoustic resistance which may be a measure of the energy dissipation or non-transmittance of sound through the filter. For example, acoustic resistance can be defined as the real component of acoustic impedance e.g. expressed in acoustic ohms [W] This quantity may be analogous in some ways to the quantity of electric resistance.

Without being bound by theory, it is found that the relative attenuation“AS” of sound waves (in decibel) through an acoustic filter may be correlated with the relative acoustic resistance“RN” of the filter or restricted sound passage according to a proportionality relation such as: AS _{N CX} log(R _N ) [dB] (6)

Generally, it will thus be appreciated that an exponential increase of the acoustic resistance“RN” may lead to a corresponding decrease in the sound pressure level of the transmitted sound“SN” compared to the received sound “So” and/or a linear increase of the attenuation“AS” as expressed in decibel.

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.

FIG 1A illustrates an example, wherein acoustic resistance is provided by a circular duct forming an aperture 12a. Without being bound by theory, it is found that the resistance of such restricted passage may be modelled or approximated at least in relative terms using formulas such as:

where“h” is the viscosity of air (typically about 18.4 gPa-s),“L” is the depth or length of the duct,“r” is the radius of the duct, and“A” is the

corresponding cross-section area of the duct (transverse to the main sound direction). It will be noted that the acoustic resistance of the aperture may increase very rapidly (with the fourth power of 1/r) as the radius of the duct or aperture decreases. A typical aperture for acoustic attenuation in an earplug may e.g. have a depth between 10 - 1000 mih, preferably between 50 - 500 mih, more preferably between 100 - 200 pm. The aperture may e.g. have a diameter between 10 - 2000 mhi, preferably between 20 - 1000 pm, between 50 - 500 mhi, or between 100-300 mih.

Without being bound by theory it may be observed that a circular aperture wherein half of the circle is closed off, e.g. by a valve 13 shding along the direction“X” as shown, may not be have the same acoustic resistance as another circular aperture with half the cross-section area A (or the radius divided by square root of two). This can be explained e.g. by the predominant effect which may be played by the smallest restricting dimension of the aperture on the acoustic resistance. So the half closed aperture may exhibit an acoustic resistance which is more similar to a pair of parallel circular apertures each with half the radius rh=r ₀ /2, and the relative increase in resistance can in some case be modelled or

approximated at least in relative terms by:

Thus closing half the duct from one side may increase the acoustic resistance as much as a factor eight instead of a factor four which would be the case if only the area should play a role. And the effect of restricting the passage can be even higher as the aperture is being further closed wherein the resulting attenuation may increase exponentially. This behavior is one reason which can make it relatively difficult to accurately control the acoustic resistance for setting a desired attenuation by means of a valve partially covering an aperture.

FIG IB illustrates an example wherein a valve 13 is added hovering at some distance above the exit of the duct or aperture 12a. It is found that the close presence of a valve may cause an additional acoustic resistance, e.g. due to a restriction formed by the slit between the valve 13 and the ri 12r forming the aperture 12a. For example, the additional acoustic resistance may be modelled or approximated at least in relative terms using formulas such as:

where“h” and“r” have the same meaning as before,“t” is a thickness of the rim 12r,“h” is a distance or height between the valve and an exit of the duct (rim 12r), as shown, and“b” is a circumference length of the rim (not shown in the figure).

When acoustic resistances are encountered in series, e.g. as in the resistance of the duct and valve as shown, the total resistance“Rmtaf may be modelled or approximated at least in relative terms by addition, e.g. using:

Rtotal Rduct Ί Rvalve (10)

It will be noted that the resistance of the valve slit may quickly vanish for larger distances“h” and the total resistance approaches the value of“Rduci”, which is logical. On the other hand, the acoustic resistance can increase very rapidly (this time with the third power of 1/h) as the valve 13 closes the aperture 12a by decreasing the distance“h”. Again, this can make it relatively difficult to accurately control the acoustic resistance for setting a desired attenuation. FIG 1C illustrates yet another or further example wherein, the sound passage 12 comprises a plurality of apertures 12a, 12b through which the sound may travel. Each aperture may provide a respective acoustic resistance, which may be individually calculated e.g. using the formula for “ Rduci” above. When multiple apertures are arranged as parallel channels for the sound to travel through, their combined total resistance may be modelled or approximated at least in relative terms using formulas such as:

1/R _t0 cal = /Ra + /R _b (H) where“R<i” and“Rb“ are the respective acoustic resistances of the individual apertures. It will thus be appreciated that a combination of multiple apertures each having a different radius or cross-section (or different depth “L”) may be used to control a desired acoustic resistance in a more

reproducible way than by e.g. half closing a valve. For example, a total resistance of two open apertures with the same depth“L” and different radii “r _a “ and“n“, as shown, can be modelled using:

Additional resistances can be determined by simply closing the respective apertures 12a, 12b completely. For example in this way the following sequence of relative acoustic resistances (from small to big) can be realized:

FIG 2A schematically illustrates a general trend of acoustic resistance“RN” as a function of one or more apertures being closed between zero percent (fully open) and hundred percent (fully closed). In this case the graph is calculated by taking the combined acoustic resistance of a hundred parallel apertures with the same dimensions and closing them one by one. It will be noted that the acoustic resistance increases dramatically, especially when the one or more apertures are almost completely closed.

FIG 2B schematically illustrates a relative attenuation ASN (in decibel) corresponding to a logarithm of the acoustic resistances RN at a sequence of discrete steps N selected from values in the graph of FIG 2A as indicated. In this case, the acoustic values for the acoustic resistances RN are selected such that the relative attenuation ASN in decibel has a near linear increase with each subsequent step.

For example, a selected sequence of relative attenuations ASN in decibel may be described by the dashed trend-line in as shown in the graph, e.g. obtained by least squares fitting the points. Preferably, the relative attenuations in the sequence can be described at least qualitatively e.g. using a formula such as

AS _N = S _Q + SS N ^x ± e [dB] (14) wherein“So” is an attenuation offset e.g. minimum or maximum attenuation for extrapolated step 0,“N” is the step number in the sequence e.g. an integer number,“6S” is a step size indicating the relative increase of the attenuation for each step, and“x” is an exponent which is preferably as close as possible to 1 for the desired linear behavior (in decibel), e.g. 0.8 < x < 1.2, preferably 0.9 < x < 1.1, more preferably 0.95 < x < 1.05, most preferably 0.99 < x < 1.01.

It will be noted that the linear approximation may be more difficult for larger attenuations, i.e. less open area, e.g. at N=1 to N=4. For these or other reasons, points may deviate from the trend-line, e.g. within an acceptable error margin“e”. For example, the error margin“e” may be defined relative to the step size 6S, e.g. e < 0.5· 5S, preferably e < 0.1· 6S, more preferably e < 0.05· 6S, most preferably, e < 0.01· 6S, or smaller.

Alternatively, or in addition, the error margin“z” may be defined simply in decibel, e.g. less than one decibel, preferably less than half a decibel, more preferably less than a tenth of a decibel, or less, e.g. 0.0001 dB < e < 1 dB. In some cases it may also be acceptable that not all possible selections for the attenuation are at or near the line. For example, there may be one or more outliers, e.g. when using a relatively large aperture and valve for completely opening the acoustic filter in a setting with minimal attenuation.

Alternatively, or in addition, when all valves are closed, the attenuation may be arbitrarily high and limited by other factors such as the seal of the earplug.

FIG 3A illustrates an embodiment of an acoustic filter 10. FIG 3B illustrates an embodiment of the acoustic filter 10 in an earplug 100. For example, a customized ear mould or universal earplug may be used. In the embodiment shown, the acoustic filter 10 comprises a filter housing 11 which may be placeable or integrated in the earplug 100. The housing or earplug may form a seal between an auditory canal Pi of an ear and an external surroundings Po. A sound passage 12 may extend through the filter housing 11 for receiving sound So from the external surroundings Pi and transmitting the sound Si into the auditory canal Pi. The sound passage 12 may have, e.g. form or define, one or more apertures 12a (one shown here). The apertures may determine in large part an acoustic resistance RN for the sound received and transmitted through the sound passage 12.

In the embodiment shown, the filter comprises a valve system 13. The valve system 13 may comprise one or more acoustic valves 13a (one shown here). The valves can be arranged in the sound passage 12 for controlling the acoustic resistance RN of the one or more apertures 12a. For example, the valves are controlled based on an electrical or other control signal EN.

In some embodiments, the acoustic filter 10 is adapted to provide the selection of the attenuation ASN. For example, the attenuation is selected from a sequence comprising a discrete number“N” of

predetermined attenuations. Preferably, at least three predetermined attenuations are provided, preferably more, e.g. at least four, at least five, at least ten, e.g. between twenty and hundred, or more. Instead of a discrete number it may also be envisaged to provide as many different settings as possible, e.g. depending on digital circuitry.

In some embodiments, the selection of the attenuation may comprise selection of a control signal EN to the valve system. For example, a control signal is selected from a corresponding sequence of predetermined control signals E _ί ,Eί,E;^ Each attenuation ASN in the sequence may correspond to one or more acoustic valves 13a providing one or more apertures 12a with a respective predetermined acoustic resistance RN _. Typically, the selection of an attenuation may take place via a volume control, e.g. integrated in a controller or coupled there with. Also other types of control are possible, e.g. based on measurements. In some preferred embodiments, the acoustic resistances of subsequent attenuations in the sequence are adapted to correspond with a linear increase or decrease in the attenuation ASN of the sound in decibels. Preferably, the attenuation is relatively constant over different frequencies at least in an audible frequency range. Alternatively, or in addition, the attenuation may be defined at a specific one or more frequencies of the sound, e.g. at 200 Hz or any other audible frequency.

In one embodiment, the valve system 13 is configured to control a total opening cross-section“A” through the sound passage 12 by actuating the one or more acoustic valves 13a relative to the one or more apertures 12a. For example, each acoustic valve 13a comprises a respective flap or other passage restricting element configured to at least partially close or open the respective aperture 12a by sliding, pivoting, lifting, bending, or other relative movement of the restricting element relative to the aperture. In some embodiments, the acoustic valves 13a comprise an actuator, e.g. electromechanical transducer or material which may provide electrically controllable movement of the valve, e.g. flap. Preferably, the acoustic valves 13a comprise a piezoelectric material for actuating the valves. For example, the acoustic valves 13a comprise ceramic piezoelectric benders which may form both actuator and flap. Alternatively, or additionally the acoustic valves 13a may comprise active polymer benders which can have the further advantage of providing a smaller bending radius B, in some embodiments. Advantageously, these or other acoustic valves may be constructed in a MEMS process out of one plate of piezoelectric material, in some

embodiments.

In some embodiments, the system comprises or communicates with a controller 14 which may be integrated with the filter housing or earplug, or provided in a separate dedicated or general control unit. In one embodiment, the controller is configured and/or programmed to provide the selection of the control signal EN for linearly varying the attenuation ASN in decibels. For example, the controller 14 provides a predetermined sequence of volume settings wherein an increase or decrease of the volume results in a fixed change of the attenuation D8N in decibels and/or logarithmic change of the sound pressure level. The volume settings may e.g. be stored in a table linked with corresponding control signals, and/or hardwired into circuitry.

In some embodiments, a controller can thus be used for selecting an attenuation ASN of a sound pressure level of the transmitted sound Si compared to that of the received sound So. Manual control over the sound level preferably lets the acoustic filter react linearly in decibel (i.e.

logarithmically in SPL) to user input, e.g. via volume knob or volume input from another device, e.g. smartphone. In other or further embodiments (not shown), the acoustic filter 10 comprises a microphone or pressure sensor for measuring the internal sound pressure level in the auditory canal. In some cases, the control signal EN can based, at least in part, on an output of the microphone. For example, alternatively, or in addition to controlling the attenuation manually, the controller may automatically increase

attenuation in response to excessive ambient sound. There may also be an external microphone e.g. for measuring ambient sound such as human speech and automatically decrease the attenuation in response thereto. Also combinations of control are possible based on internal/external microphones and/or other sensors. The acoustic filter 10 may also provide further functionality, e.g. by integrating or cooperating with an acoustic generator. For example, the generator may be arranged between the valve and the auditory canal arranged for generating an acoustic signal into the auditory canal. The acoustic filter 10 as described herein can be manufactured e.g. by designing the filter to provide a discrete number“N” of attenuations AS _l ,AS2,AS3 which linearly increase or decrease the transmitted sound in decibels. Some embodiments may comprise storing the corresponding sequence of control signals E _l ,E2,E; for control of the acoustic filter 10, e.g. in software or hardware. Also other methods of manufacturing may be envisaged.

The acoustic filter 10 as described herein can be used in various application, e.g. for controlling attenuation of the external surroundings Po into the auditory canal Pi. For example, such use may comprise changing the attenuation ASN in discrete steps, wherein in each step a subsequent attenuation in the sequence of predetermined attenuations ASI,AS2,AS:J is selected to linear increase or decrease in the attenuation ASN of the sound in decibels. Also other uses may be envisaged.

FIGs 4A-4C illustrate an embodiment of a sound passage 12 with multiple apertures 12a, 12b.

In one embodiment one or more acoustic valves are configured to open or close the respective one or more apertures. In the embodiment shown, the sound passage 12 comprises a plurality of apertures 12a, 12b. In some preferred embodiments, at least some of the apertures 12a, 12b individually have a different predetermined respective acoustic resistance Ra, Rb. For example, the different apertures have different opening cross- sections Al,A2 and/or different depths (not shown).

In some embodiments, e.g. as shown, the valve system 13 comprises a plurality of acoustic valves 13a, 13b. Preferably, the valve system 13 is configured to provide the predetermined attenuations

AS _l AS2,AS3 by combining respective acoustic resistances R of the plurality of apertures 12a, 12b. For example, each acoustic valve 13a is configured to selectively switch between at least two predetermined sub-states“O” or“C” changing the respective apertures 12a to provide one of a predetermined selection of acoustic resistances Ra.R .

Preferably, the sub-states of a respective valve 13a include at least a fully open state Ό”, e.g. wherein the respective aperture is open with negligible or no restriction of the valve to provide a well-controlled predetermined acoustic resistance Ra depending predominantly on the dimensions of the aperture 12a. Preferably, the sub-states of a respective valve 13a include at least a fully closed state“C” wherein the respective aperture is closed off providing a very high acoustic resistance R which may be virtually independent of the dimensions of the aperture 12a e.g. with negligible or no sound being transmitted there through. In some

embodiments, one or more valves are adapted to optionally provide more than two sub-states, e.g. one or more intermediate state (not shown here) wherein the respective aperture is partially closed off between a fully open state“O” and a fully closed state“C” to provide an intermediate acoustic resistance. The intermediate acoustic resistance may e.g. be higher than the fully open state“O” and lower than the fully closed state“C”. The

intermediate resistance may be dependent on both the dimensions of the aperture 12a and a relative position and/or shape of the respective valve and therefore relatively difficult to control for some embodiments.

In some embodiments, combinations of sub-states“O” or“C” of multiple valves 13a, 13b, controlling combinations of different individual acoustic resistances Ra,Rb are adapted to provide a number“N” of predetermined attenuations ASpASs ASs as describe herein. Preferably the states linearly increase or decrease the attenuation AS of the sound in decibels.

In some embodiments, the number“N” of predetermined attenuations provided by the combination of valves is greater than the number of valves itself. In other words, at least some attenuations are provided by different combinations of valves, e.g. at least two open valves. For example, in the embodiment shown combinations of sub-states Ό” or “C” of two of the valves 13a, 13b, controlling combinations of different individual acoustic resistances Ra,Rb, are configured to provide at least three of the predetermined attenuations AS _I ,AS2,AS3.

In the embodiment shown in FIG 4A, a first attenuation ASt may be provided by the valve system 13 opening two valves 12a, 12b. This may provide a combined acoustic resistance which may be calculated with the law of parallel resistances using R1 = 1/(1/Ra+1/Rb). In the embodiment shown in FIG 4B, a second attenuation AS2 may be provided by the valve system 13 closing the second valve 13b associated with the smaller aperture 12b while keeping the first valve 13a open. Because the acoustic resistance through the second valve is thus very high (R¥), the combined acoustic resistance may be very close to that of the larger aperture 12a alone, i.e. R2 = Ra. In the embodiment shown in FIG 4C, a third attenuation ASa may be provided by the valve system 13 closing the first valve 13a associated with the bigger aperture 12a whde keeping the second valve 13b open. Because the acoustic resistance through the first valve is thus very high Rx, the combined acoustic resistance may be very close to that of the smaller aperture 12b alone, i.e. R3 = Rb. In this way the two different apertures may provide a series of at least three different acoustic resistances. For example, this may have the advantage of saving the limited space inside the earplug

In some embodiments, it may be preferred to provide the same linear increase in decibel between the attenuations ASi and ASa as between the attenuations AS2 and AS: , wherein the following relations may hold:

AS ₂ - AS _l = AS ₃ - AS ₂ (15) log(Ra) - log (

Interestingly, this equation may be solved by choosing relative values for the acoustic resistances Ra and Rb in accordance with the golden ratio:

where f ~ 1.618033... Of course also deviations from the golden ratio may be acceptable, e.g. f ±0.2, preferably f ±0.1, more preferably f ±0.05, most preferably as close to f as possible or practical.

In some embodiments, at least some of the predetermined attenuations are provided by combinations of multiple open valves. For example, a combination of valves may be used to provide a combined acoustic resistance that is an interpolation or extrapolation of two or more acoustics resistances provided by individual valves or other combinations of valves on the linear decibel scale of the attenuation ASN. While the present embodiment shows only two valves, in other or further embodiments, the valve system 13 may comprise at least three valves for opening or closing respective apertures each having different acoustic resistances, preferably at least four, at least five, or more, e.g. between ten and forty valves, e.g. thirty-two valves.

FIGs 5A-5D schematically illustrate top and side views of an embodiment comprising a valve in various states of opening.

In one embodiment, at least one of the acoustic valves 13a is configured to set a partially opened apertures 12a providing one or more of the predetermined acoustic resistances RN. In another or further

embodiment, a sequence of predetermined control signals E _I ,E2,E3 causes, a total cross-section“A” of the at least one aperture 12a to sub-linearly decrease between a maximally opened state“O” and a maximally closed state“C”. In other words, the cross-section changes more slowly when the acoustic valve is nearly closed than when it is nearly open. This may compensate, at least in part, for the fact that the acoustic resistance may rapidly increase when an aperture is almost closed. In other or further embodiments (not shown), the sequence of predetermined control signals E _I ,E2,E: _S causes, a height of at the least one acoustic valve 13a to sub- linearly lower between a maximally opened state Ό” and a maximally closed state“C”. In other words, the height of the valve above the aperture may change more slowly when the acoustic valve is nearly closed than when it is nearly open. Again, this may be related to the observation that the acoustic resistance may rapidly increase when the valve is almost closed.

In some embodiments, at least one aperture 12a and/or acoustic valve 13a is adapted to provide a non-linear change of its acoustic resistance RN in responds to a linear change of a voltage or current in the control signal EN. For example, at least one aperture 12a and/or acoustic valve 13a is adapted to provide a logarithmically or exponentially changing open area for a linear motion of the acoustic valve 13a with respect to the aperture 12a.

In some embodiments, e.g. as shown, at least one aperture 12a has a tapered shape. This means a cross-section of the aperture decreases towards one side, preferably the side opposite a direction from which the respective acoustic valves 13a closes the aperture. For example, the tapered shape comprises a triangular shape, as shown, or a trapezoid shape, exponential shape, or any shape wherein the cross-section transverse to the direction of closing gets narrower towards the end in which the acoustic valves closes the aperture. It will be appreciated that the tapered shape may improve control over the cross-section area, especially over small changes of the area near the position where the valve is almost closed. Alternatively, or in addition to shaping the aperture, also the valve may be shaped to provide similar effect. Also other embodiments may be envisaged to provide a non linear change of cross-section in response to a linear control of moving elements. For example, instead of rolling, the closing may also shde over the non-linear, e.g. tapered opening. Instead of linearly moving closing means, they may also rotate. For example, one ore more rotating disc may rotate with a constant radial velocity while closing off a non -linearly varying opening.

It will be appreciated that the constructional features as described herein may provide specific benefit in digital control e.g. where a discrete number of control signals and corresponding attenuation steps may be available. But the constructional features as described herein may also provide advantages in analog control e.g. wherein the precision of the control may be improved especially at high attenuations (where the aperture is almost closed). Also it may still better conform even to

continuous changes in attenuation as perceived in natural hearing. So in some embodiments, the acoustic filter may be adapted to provide the selection of the attenuation from a continuous sequence of predetermined attenuations based on a selection of the control signal from a corresponding continuous sequence of predetermined control signals. Similarly, each attenuation in the continuous sequence may correspond to the one or more acoustic valves providing the one or more apertures with a respective predetermined acoustic resistance which in some cases may be varied continuously e.g. by an analog signal. The acoustic resistances of subsequent attenuations in the continuous sequence (with arbitrarily small but fixed step size) can be adapted to correspond with a linear increase or decrease in the attenuation of the sound in decibels. So for example, a valve which is controlled by linearly varying analogue control signals, may vary its cross- section non-linearly in order to linearly increase or decrease the attenuation of the sound in decibels.

FIGs 6A-6C schematically illustrate side views of a valve system 13 comprising stacked valves. In some embodiments, e.g. as shown or otherwise, at least one valve 13b comprises an aperture. In the embodiment shown, the aperture in valve 13b is smaller than an aperture 12a which the valve is configured to cover, e.g. the aperture of the sound passage 12 or an aperture of another valve underneath (not shown). In some embodiments, e.g. as shown, the valve system 13 comprises one or more stacked valves 13a, 13b. Preferably, the valves in the stack are individually controllable. In the embodiment shown, a first valve 13a of the stacked valves is configured to at least partially close an aperture in a second valve 13b.

Also more than two valves may be stacked, e.g. three, four, or more (not shown). Preferably, a cross-section area of the one or more apertures in the stacked valves increases from top to bottom towards the respective aperture 12a in the sound passage 12. In this way different effective apertures may be selected depending on which aperture is on top. Alternatively, or in addition to providing the stacked valves with different open areas, the thickness or length of the stacked apertures may already provide a different acoustic resistance.

FIGs 7A-7C schematically illustrate side views of apertures including an acoustic element 15. In some embodiments, one or more of the apertures 12a comprise an acoustic element 15 such as a mesh, flexible membrane, et cetera. For example, a woven or injection molded mesh can be used. In some embodiments, the acoustic element 15 is configured to shape a desired acoustic response e.g. across different frequencies, for one or more of the predetermined attenuations ASN. For example, a membrane may provide a better sound experience from an acoustic generator and a more flat sound transfer from ambient sound into the auditory canal. The acoustic element 15 may alternatively, or additionally be integrated in an aperture of the acoustic valve 13a as shown in FIG 7B, and/or in a stack of valves as shown in FIG 7C. Alternatively to being placed in an aperture with a valve, the acoustic element 15 may also be placed in series with one or more valves ancl/or in a parallel channel (not shown).

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features described. Of course, it is to be appreciated that any one of the above embodiments or processes may be combined with one or more other embodiments or processes to provide even further improvements in finding and matching designs and advantages. It is appreciated that this disclosure offers particular advantages to attenuation of ambient sound, and in general can be applied for any application wherein sound is filtered between two places.

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 be 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.