Patent Grant
8379899
The present invention relates to an electro-acoustical transducer, and a transducer assembly comprising the transducer, which transducer is adapted to operate with a sealed sound port blocked by an air impervious member or fabric. The electro-acoustical transducer and the transducer assembly according to the invention contain first and second gas flow channels to provide necessary static pressure compensation to internal chambers of a transducer housing.
In prior art electro-acoustical transducers, two separate chambers, often termed front and back volumes, exist into one of which sound from the surroundings is conveyed—or from which sound generated therein is transmitted to the surroundings. Static pressure compensation has traditionally been provided to the other chamber by a small vent or aperture between the front volume and the back volume. This vent provides a gas flow channel between interiors of the front and back volume so as to maintain a static pressure difference of substantially zero between two sides of a deflectable diaphragm. This vent or gas flow channel may have dimensions sufficiently small to prevent pressure changes with frequencies much higher than a DC level to pass between the front and back volumes. Consequently, the vent may only serve the purpose of pressure equalization in the inner space, or back volume, not connected directly with the surroundings via the sound port. A transducer of this type is disclosed in U.S. Pat. No. 4,450,930.
In applications where the sound port of the transducer is blocked by the air impervious member it has been found advantageous to provide pressure equalization of both inner spaces independently of the sound port.
Thus, in a first aspect, the invention relates to an electro-acoustical transducer comprising a transducer housing, having an inner space. The electro-acoustical transducer comprises a sound port, a deflectable diaphragm, and a first and second gas flow channel. The sound port is adapted to convey sound between the inner space and the surroundings. The deflectable diaphragm is responsive to received sound or adapted to generate sound. The diaphragm divides the inner space into a first and a second, separate chambers. The first and the second gas flow channels are adapted to facilitate gas transport there through between the surroundings and the first and the second chambers.
A second aspect of the invention relates to a transducer assembly comprising an electro acoustical transducer as described above and a sound transporting member adapted to transport sound between the surroundings and the sound port. The sound is blocked with an element that is adapted to transport sound and is substantially impervious to gas flow.
A third aspect of the invention relates to an electro-acoustical transducer comprising a transducer housing, having one or more side portions and an inner space. The transducer comprises a deflectable diaphragm, a sound port, and a channel. The deflectable diaphragm is responsive to received sound or adapted to generate sound. The diaphragm divides the inner space of the housing into a first and a second, separate chambers. The sound port is adapted to transport sound between the first chamber and the surroundings. The channel extends between a first opening toward one of the first and second chambers and a second opening to the surroundings. The channel is adapted to facilitate gas transport there through between the first or second chamber and the surroundings. The channel has dimensions providing a transfer function between a sound pressure in the first or second chamber and a sound pressure at the second opening of the channel. The transfer function has a cut-off frequency, measured under free field conditions, of less than 100 Hz, such as less than 50 Hz, or such as less than 20 Hz. At least part of the channel extends within a side portion and substantially along a plane thereof.
A fourth aspect of the invention relates to a miniature hearing aid receiver, microphone, or loudspeaker comprising an electro-acoustical transducer or an assembly as described above.
A fifth aspect of the invention relates to an electro-acoustical transducer comprising a transducer housing having an inner space. The electro-acoustical transducer comprises a sound port, a deflectable diaphragm, and a gas flow channel. The sound port is adapted to convey sound between the inner space and the surroundings. The deflectable diaphragm is responsive to received sound or adapted to generate sound. The gas flow channel is adapted to facilitate gas transport there through between the surroundings and the inner space. The gas flow channel comprises an opening through the transducer housing and a porous member covering the opening.
The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. Additional features and benefits of the present invention are apparent from the detailed description, figures, and claims set forth below.
In the following, preferred embodiments will be described with reference to the drawing, wherein:
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
In general, in relation to the invention, a few comments are pertinent:
In general, the gas flow channels are provided for, for example, atmospheric pressure equalization of the chambers, which is desired when the transducer is subjected to changes in ambient pressure, such as from variations in its height over sea level (travelling in elevators, airplanes, or the like). However, it is generally undesirable that any sound is leaked from the inside out or from the outside to the diaphragm through the first and second gas flow channels. One reason for this is seen when the transducer is used in a communication device such as a hearing aid in which a leaking sound pressure from the internal of the transducer housing could cause a disturbing feed back signal to a microphone of the communication device. This type of gas flow channel or channels may, as will be illustrated further below, have any of a large number of shapes and dimensions.
In one embodiment of the first aspect, the first gas flow channel facilitates gas transport between the first chamber and the surroundings and the second gas flow channel facilitates gas transport between the second chamber and the surroundings.
In this situation, the gas flow into or out of each chamber may be controlled separately, by the dimensions of each channel. Also, the channels may be positioned independently of each other to allow separate control of acoustical properties of each channel. This positioning may be optimized in relation to, for example, a sound pressure in the pertaining chamber so that a position may be chosen where the sound pressure is the lowest (maybe also depending on a given frequency range).
In a preferred embodiment, however, the first gas flow channel facilitates gas transport between the first chamber and the surroundings and the second gas flow channel facilitates gas transport between the first chamber and the second chamber. One reason for this is that apertures having openings to the surroundings may be polluted or blocked by dust or debris. Even during manufacturing of the transducer, operations may be performed (e.g., polishing or working/cutting metal or plastics) that may generate dust which may cause problems even before the transducer leaves the production plant. Providing one of the channels inside the transducer housing reduces this problem.
In one situation, this second channel may be provided in any position in the housing, such as in the form of a small aperture in the diaphragm such as a circular vent with a diameter of between 3 and 100 μm such as between 3 and 30 μm or even more preferably between 3 and 20 μm.
When the first channel extends between a chamber and the surroundings, it may advantageously have dimensions providing a transfer function between a sound pressure within the first chamber and a sound pressure at an opening of the first channel toward the surroundings. The transfer function comprises a low pass filter having a cut-off frequency, measured under free field conditions, of less than 100 Hz, such as less than 50 Hz, or less than 20 Hz. Naturally, the other vent may also have this characteristic. In this manner, the desired atmospheric pressure equalization is provided while at the same time, leakage of sound to or from the outside through the channel is prevented or limited to a frequency range below the operational frequency range of the communication device which houses the electro-acoustical transducer.
In the present connection the sound pressure at the opening of the gas flow channel toward the surroundings is normally determined under controlled circumstances, such as at a predetermined distance from that opening. A test of this type may be performed on a receiver or loudspeaker according to the invention by positioning a miniature microphone at a distance of, for example, about 1-10 cm from the opening. The transducer and miniature microphone are located under simulated free field conditions such as in an anechoic test chamber or anechoic room. A predetermined sound pressure is generated inside the housing of the transducer by applying a predetermined electrical stimulus input signal to a motor means operatively connected to the diaphragm so as to excite the diaphragm.
In a receiver, the test may be made by blocking the sound port so as to make it sound impermeable. Then, a predetermined sound pressure is provided outside the receiver, and the sound pressure entering the receiver via the ports is determined using the receiver itself.
Just as well as it may be desirable to be able to select the positions inside the chamber(s) for the opening(s) of the vent(s), it may be desirable to determine where the opening(s) to the surroundings is/are positioned. Thus, the sound port may be provided at a first, predetermined position of the housing and one or both of the gas flow channels can facilitate gas flow between a chamber and the surroundings. The channel(s) may be provided at an opposite position of the housing. In this manner, any sound exiting or impinging on the sound port may have a less significant role at the positions of the channel(s). In this situation, the opposite position may be diametrically opposite to the sound port or the channel opening(s) may be provided in side parts opposite to a side part in which the sound port is provided.
In one embodiment, the transducer further comprises motor means for deflecting the diaphragm in response to an electrical signal and means for providing an electrical signal to the deflecting means. The motor means may be based on a moving coil operation or on moving armature operation to provide a loudspeaker or sound producing transducer. The means for providing the electrical signal may comprise a digital or analogue power amplifier or other signal generator.
In another embodiment, the transducer further comprises means for providing an electrical signal corresponding to a deflection of the diaphragm and means for receiving an electrical signal corresponding to a deflection of the diaphragm. In this manner, a microphone or a sound detecting transducer is provided. The means for receiving the electrical signal may then be an amplifier and/or a circuit for processing and outputting a signal correlated to the sound detected.
In a particularly preferred embodiment, the sound port is substantially impervious to gas. This may be obtained by covering the sound port with a sound transporting member, such as a thin compliant membrane, which is substantially impervious to the flow of gas. Sound may be transported through the member in a substantially transparent manner by choosing a suitable flexible or deflectable fabric for the thin compliant membrane.
This sound transporting member has the advantage that debris, dust, sweat, or other liquids are unable to block the sound port or actually enter into an interior of the transducer and damage the sensitive elements contained therein. A problem encountered with prior art electro-acoustical transducers when blocking the sound port with a sound transporting member that is impervious to gas flow, is the lack of pressure equalization in one of the chambers. Prior art transducers have traditionally used the sound port for static pressure compensation. However, due to the provision of gas flow channels for both chambers, this problem has been circumvented and solved in transducers according to the present invention.
It should be noted that the housing may comprise an extension, such as a pipe stub, for easy attachment and positioning of the transducer to other elements of a larger assembly, such as a portable communication device, comprising the transducer. The sound port may then be provided in the distal end of the stub and the blocking or sealing member at any position from the sound port at the distal end to the chambers in the housing. In fact, one of the channels may be provided from a chamber to the internal space of the pipe stub so as to interconnect the two chambers.
As to the second aspect, the sound transporting member may be provided for a number of reasons such as to transport sound to or from another part of the assembly to the sound port. The member may be detachably attached to the housing of the transducer and may be provided with the blocking member before or after this attachment. In addition, the member may comprise a thin compliant membrane or diaphragm in order to facilitate a more free positioning of the transducer in relation to the sound port. Preferably, the member is airtight between the blocking member and the transducer. The blocking member may be positioned at an end of the member opposite to an end connected to the transducer.
In one embodiment particularly suitable for hearing aid applications, the blocking member comprises a wax-barrier or wax-filter. In other embodiments, the blocking member is a membrane or a diaphragm. Alternative materials may be metals, plastics, polymers, silicone, or rubbers. Such membranes may be, for example, a 10-100 μm film or foils of Polyethylene or Teflon.
As to the third aspect, a plane of a side portion is a plane co-extending with that side portion. Naturally, this plane may be flat or bent in any direction or manner. In this connection, the channel extends along the plane when a general axis of the channel is in the plane. This is in order to facilitate a channel having a length longer than the thickness of the side portion. The part of the channel extends within the side portion and may be defined by part of the material defining the side portion. Then, the at least part of the channel may have a meandering shape in order to be able to increase a length of the channel without having to change overall dimensions of the transducer. The meandering shape may be a flat spring-shape or a serpent/sine-shape.
The length, cross section, and shape of the channel defines its acoustic properties. Another manner of controlling the acoustic properties of the channel is to provide one or more cavities therein along the length thereof. Such cavities also affect the acoustic properties and may be used for providing a more effective (higher order) low pass filtering of sound travelling in the channel. These cavities will normally have a larger cross section than neighbouring parts of the channel.
The cross section of the channel and/or cavities may be predetermined and may, for example, be semicircular or have any particular shape. The side portion may be defined by two elements coextending in the plane, the channel being defined between the two elements. In that manner, the provision of the channel and the cross section of the channel are made easy in that the channel may be provided simply by providing a groove in one of the elements. In fact, a corresponding (in shape and position) ridge may be provided in the other element so that these two elements (the ridge being positioned in the groove) will cooperate in defining the cross section of the channel.
Punching, embossing, or injection moulding processes may be made with an impressing precision, so that the above method may be used for actually providing very narrow channels with well-defined cross sections. Also, in order to prevent or make difficult the blocking of the opening of the channel toward the surroundings with, for example, dust or debris, it may be desired that the channel at the second opening widens toward the surroundings. Thus, a larger exit opening out of the transducer is provided.
As to the fourth aspect, normally, the term miniature transducer designates a small or sub-miniature transducer such as one having an extension, in the plane of the diaphragm, of less than 7.0×5.0 mm or less than 5.0 mm×4.0 mm, such as 3.5 mm×3.5 mm, or even more preferably less than 3.0 mm×3.0 mm. Alternatively or additionally, a miniature transducer may comprise a so-called MEMS based transducer element which is a transducer element wholly or at least partly fabricated by application of Micro Mechanical System Technology. The miniature transducer element may comprise a semiconductor material such as Silicon or Gallium Arsenide in combination with conductive and/or isolating materials such as silicon nitride, polycrystalline silicon, silicon oxide and glass. Alternatively the miniature transducer element may comprise solely conductive materials such as aluminium, copper, etc., optionally in combination with isolating materials like glass and/or silicon oxide.
Naturally, the transducer may be used in larger applications such as in mobile telephones or PDA's.
As to the fifth aspect, in this manner, the specific sound characteristics of the gas flow channel may be defined by the porous member and not the opening in the housing. Normally, very small and well-defined gas flow channels are desired that are difficult to provide in this type of housing.
As mentioned above, the gas flow channel may have any desired transfer function between a sound pressure within the housing and a sound pressure at an opening of the gas flow channel toward the surroundings. This transfer function may be that of a low pass filter having a cut-off frequency, measured under free field conditions, of less than 100 Hz, such as less than 50 Hz, or such as less than 20 Hz.
The porous member may be positioned inside the housing or on an outer surface thereof. Preferably, the porous member is attached to the housing, such as by gluing, welding, soldering or clamping. Normally, all of the opening in the housing is covered by the porous member in order to have the porous member define the sound characteristics of the gas flow channel.
Porous materials, in general, may have a plurality of holes or channels extending in parallel or more stochastically or uncontrolled. The holes or channels will facilitate gas transport from one surface of the member to the other. The holes or channels may have the same cross section along their lengths, or the cross section thereof may vary.
In one embodiment, the porous member comprises a foil comprising a plurality of through-going holes positioned in gas flow connection with the opening. The holes may be parallel and directed perpendicularly (or at least at an angle to) to a surface of the foil. Alternatively, the holes may be meandering and at least partly extend also in the plane of the foil. Any number of holes may be provided. The number of holes, the thickness of the foil (the length of the holes), as well as the size (normally diameter) of the holes will take part in the definition of the sound characteristics of the gas flow channel.
It may be desired that the foil has a porosity between 0.05% and 3% in a volume comprising the holes. This volume normally is defined as a volume delimited by a boundary rather closely encircling the holes.
In another embodiment, the porous member comprises a grid comprising a plurality of through-going holes positioned in gas flow connection with the opening. A grid normally is a pattern of holes or openings. These holes may, as in the first embodiment, be parallel and to an angle to a surface of the grid or may be meandering. Preferably, the grid has between 2 and 50 holes having a radius between 1.8 μm and 30 μm. One grid has 2 holes with a radius of 3.5-21 μm. Another grid has 4 holes with a radius of 2.9-17.7 μm. An alternative grid has 8 holes with a radius of 2.5-14.9 μm, and yet another grid has 25 holes with a radius of 1.8-11.2 μm.
A third embodiment is one wherein the porous member comprises a foam, a web, or a ceramic comprising a plurality of through going holes positioned in gas flow connection with the opening. Such types of materials define a plurality of meandering channels there through.
As is described above, both the sizes of the holes, the meandering shape, the porosity, as well as the thickness take part in the definition of the sound characteristics of the porous member. In one embodiment, the porous member has a porosity between 0.02% and 15% and a thickness, in a direction away from the opening, of between 10 μm and 300 μm.
Naturally, the fifth aspect may be combined with any of the other aspects in order to provide the particular venting using this porous member.
Returning now to the figures, in
The drive coil 22 is electrically connected to externally accessible terminals 26 to which an electrical signal may be provided which is subsequently transformed to a corresponding sound pressure and emitted via the sound port 14. The sound port 14 is closed or blocked by a thin compliant membrane 28 which is sound transmissive but gas impermeable.
The front volume 15 and back volume 17 are pressure equalized to the surroundings by two vents, or gas flow channels, positioned at the positions identified by numbers 34 and 36. It is seen that the volume 17 is vented via the vent 36, the volume 15, and the vent 34. Alternatively, the vent 36 may be provided in the diaphragm 16 or directly between the volume 17 and the surroundings 11.
A sound recipient device or microphone in accordance with the present invention would also have a front and back volume divided by a deflectable diaphragm 16. However, the driving mechanism or motor 18 of the receiver 10 would then be replaced with means for sensing the movement of the diaphragm 16.
In any case, the vent(s) preferably have their outlets toward the surroundings as far from the output of the sound port 14 as possible in order to minimize any sound leakage or cross talk of sound along that path. One position of the vent(s) is beneath an elastomeric rubber suspension or “boot” normally used in hearing aids for decoupling receiver vibrations from the shell of the hearing aid. An effective pipe diameter of less than 100 μm such as about 50 μm will be sufficient to compensate for normally encountered pressure variations.
In the following, a number of manners will be described for providing a vent or gas flow channel in transducer housings. Naturally, one or more such vents or different vents may be provided in a transducer, and a vent may still be provided interconnecting the front and back volumes so that both these volumes may be vented through a single vent to the outside of the transducer.
It is seen that the channel 46 has a cross section and a shape defined by a groove formed in the element 44, such as by punching or embossing. It is also seen that any shape of the groove and channel may be obtained, such as a U-shaped, a spiral shaped, a sine-shaped channel, or the like.
In these embodiments, the channel 46/46′ preferably has a 0.1 mm diameter and a length of 6 mm. Since the acoustical resistance is proportional to the pipe length and inversely proportional to the 4th power of the pipe diameter, increasing the diameter rapidly leads to a very long pipe for a given acoustical resistance.
As to the cross section of the channel, a semicircular shape is desirable in that it provides a lowest possible cut-off frequency for a maximum given dimension.
The channel 46 may alternatively be provided by providing a flat member, such as a flat plate, a film, or the like, having the channel cut out therein. This flat member is provided between the first and second elements in which the holes are provided at the inlet and outlet of the channel, whereby the channel is formed.
In order to avoid glue entering into the channel, a Teflon spray may be sprayed through a shadow mask to an area corresponding to that of the channel 46 so that, when the flat member and the first/second element are glued together, the glue will not stick in the channel 46. Either the glue will not travel into the channel, or it may be removed from the channel such as by pressurized air. Alternatively, the glue may be prevented from entering the channel by a film line of polyether-urethane in much the same manner as the above Teflon spray.
In one embodiment, a substantially plane plate is used on the inside of the receiver top cover, so that the pipe is created by the concave shape of the cover towards the outside of the receiver due to the metal drawing process applied during manufacturing. Thus, by simply providing this plate inside the cover a thin channel is provided. More such channels may be provided by providing plates at multiple surfaces of the cover. A plurality of channels in parallel has more resistance against clogging of one or more individual channels.
In
In
The cover 49′ of
Gas venting from the hole 48 or aperture now firstly enters the inner cavity 51, and travels through the inner channel 46′ by entering the opening 47 at the right side of the inner channel 46′. The gas then travels around in the inner channel 46′ to the left side thereof and enters the next cavity (the middle cavity 51′) through the corresponding opening 47′. The gas then travels around in the middle cavity 51′ to enter the next channel (the middle channel 46″) at the corresponding opening 47. This is repeated until the gas exits the outer channel 46″′ and is vented to the outside.
In this manner, the gas is exposed to a number of narrower channels (e.g., the inner channel 46′) and a number of wider cavities (e.g., the inner cavity 51, the middle cavity 51′, and the outer cavity 51″) which all provide this venting with acoustical properties closely linked with the dimensions thereof. Thus, in this manner, a wide range of desired acoustical properties may be obtained by a suitable interconnection of narrower and wider elements in the gas flow path from the (inner) hole 48 to the opening 50. The illustrated embodiment provides a fourth order low-pass filter by virtue of the cascade of acoustical inductances, capacitances, and resistances, but clearly higher or lower order low-pass filters may be constructed in suitable modification of the disclosed embodiment.
In
In the upper illustration of
In the lower illustration of
Another embodiment is seen in
Annex 1
In this annex, formulas are given for determining the acoustic impedance for a pipe with very small diameter:
Note: the factor 4/3 is caused by the parabolic flow profile in a long thin pipe.
The acoustic impedance of the pipe is:
Za=Ra+jω·Ma
For low frequencies the resistive term is dominant. The corner frequency is:
where η=18.6·10−6 Pa-s (air viscosity coefficient @ 293 K, 105 Pa)
ρ=1.29 kg/m3 (mass density of air @ 293K)
Annex 2
In this annex, it is described how to determine the venting properties of porous materials, such as grids, foams or the like. The present annex describes the calculations for a representative product. Other products having other pore sizes, wall thicknesses, or the like or with different requirements as to high frequency cut-off, naturally, should be calculated using other parameters.
Needed Pressure Compensation
External influences like, for example, temperature changes can cause an over- or under pressure in the receiver. This pressure difference between internal chambers of the receiver and outside world causes increases in distortion and needs to be compensated for through a gas flow channel or vent.
In some cases there can be quite rapid increase or decrease of environmental pressure. This could for example happen when a person is in an elevator in a very high building and travels from the ground level to the top level. The same holds when traveling by plane during take off and landing. Other situations can be getting in and out of an air conditioned car during a hot day. In all these cases, it is desirable to compensate for this change of environmental pressure within a certain time.
Say, 10% pressure increase max, dissipates to 10% in 10 seconds.
Minimum Dimensions of Holes
Say, 10% pressure increase max, dissipates to 10% in 10 seconds.
Simply like a RC network discharging:
Pc(t)=(Pressure_difference)*e−t/RC
where
V=pressure difference
R=acoustic resistance
C=acoustic compliance
After 10 seconds to 10%:
C is given: acoustical compliance of the combined volume of front volume and tubing up to a sound penetratable, air impervious barrier.
Front volume: about 7.82e−9 m3 (as in the Sonion 2300 series receiver manufactured by Sonion Netherlands BV)
Tubing: 10 mm (length)×1.4 mm (inner diameter)=10e−3*3.14*(1.4e−3/2)2=15.4e−9 m3
Acoustical compliance:
C=V/(ρ_air*c_air2), ρ_air=1.2 kg/m3 (density of air), c_air=344 m/s (speed of sound in air)→C=(7.82e−9+15.4e−9)/(1.2*3442)=1.64e−13 m5/N
So the resistance has to be less than:
R<(10/2.30)/C=4.35/1.64e−13=2.66e13 Ns/m5
Thus, the dimensions of the hole (assuming that a channel has a length equal to the receiver cover thickness and a width equal to the whole diameter) may be calculated.
r—a=(8*μ_air*L)/(π*R^4),
where
r_a=acoustic resistance
R=radius of channel
L=length of channel
μ_air=1.81e−5 (viscosity of air)
So, R=[(8*1.81e−5*0.18e−3)/(3.14*r_a)]1/4=[(8*1.81e−5*0.18e−3)/(3.14*2.66e13)]1/4=4.2 μm.
The diameter of the hole therefore should be at least 8.4 μm to dissipate a 10% change in pressure within 10 s to 10%.
Low Acoustical Influence: Maximum Dimension of Holes
We want the influence of the vent on the higher frequencies in which the receiver normally operates to be as small as possible. From simulation it appears that a hole in the cover of 50 μm already has a noticeably influence on higher frequencies. This is an acoustic resistance of:
r—a=(8*μ_air*L)/(π*R4)=(8*1.81e−5*0.18e−3)/(3.14*(50e−6/2)4)=21.24e9 Ns/m5
The hole in the receiver is about 0.3 mm diameter and will be covered with porous material. Since we want that the effective hole is no bigger than 50 μm diameter this will require a porosity of:
Ahole=πR2=3.14*(0.3e−3/2)2=70.6e−9 m2
Aeffective,desired=πR2=3.14*(50e−6/2)2=1.96e−9 m2
Porosity<1.96e−9/70.6e−9=2.77%
since porosity is defined as the surface of free flowing air/total surface.
This is of course also depending upon the size of the hole in the cover. For a 0.2 mm hole this will be:
Porosity<1.96e−9/3.14*(0.2e−312)2=6.25%
A general formula is:
Porosity=1.96e−9/(3.14*Rhole2)
This comes down to a porosity of less than 25% for holes bigger than 0.1 mm diameter.
Smaller holes in the cover will require higher porosity but are difficult to make.
Since the effective hole diameter is less than 50 μm in diameter, the pore size of the porous material should be considerably less than that, generally <5 μm.
Thickness of the porous material will be less than 2 mm to keep dimensions of the receiver small. Note that the thickness of the porous material together with the pore size defines the porosity.
Requirements for Different Manners of Providing Porosity or Small Holes
Only a Hole in the Cover
To obtain a vent with the desired specifications (pressure compensation within a certain time, and no acoustical influence on the normal operation of the receiver), there preferably is a hole in the cover of more than 8 Ξm and less than 50 μm. Hereby it is assumed that the cover has a thickness of 180 μm.
Porous Foil
With porous material this means (for a 0.3 mm hole in the receiver cover):
Aeff,min=3.14*(8e−6/2)2=5.03e−11 m2
Aeff,max=3.14*(50e−6/2)2=1.96e−9 m2
Aeff,holeincover=3.14*(0.3e−3/2)2=70.6e−9 m2
So for porosity:
Porous min=5.03e−11/70.6e−9=0.07%
Porous max=1.96e−9/70.6e−9=2.77%
These values are very dependant on the diameter of the hole in the cover. Maximum pore size was already given, it should be less than 50 μm. Minimum pore size is difficult to give, the porosity lower limit should at least be realized for that given pore size. Thus, a large hole may be provided in the actual transducer cover, when this is further limited (as in
Grid
If a grid is created in the cover with multiple holes (e.g., n holes):
r—a=r—a_single/n
where r_a_single=acoustic resistance of a single hole
where n=number of holes
r—a<2.66e13 (from 3)
r—a>21.24e9 (from 4)
Then, radius of the hole should be minimal:
R=[(8*1.81e−5*0.18e−3)/(3.14*r—a*n)](1/4)=[(8*1.81e−5*0.18e−3)/(3.14*2.66e13*n)](1/4)
And maximal:
R=[(8*1.81e−5*0.18e−3)/(3.14*21.24e9*n)]1/4
Thus, R for 2 holes: minimal 3.5 μm, maximal 21.0 μm
R defines the radius of each of the holes, not a diameter.
Foam
The calculation relating to the grid is also valid for foams in that the overall porosity of a foam also corresponds to a number of channels with determined pore sizes. That these channels in the foam will be meandering and interacting does not affect the throughput of air to any significant degree and does also not affect the acoustic properties of the venting.
While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the scope of the claimed invention, which is set forth in the following claims.
This application claims priority to U.S. provisional application Ser. No. 60/623,278, filed on Nov. 1, 2004, and to U.S. Provisional application Ser. No. 60/696,595, filed on Jul. 5, 2005. The disclosures of the aforementioned provisional applications are incorporated by reference in their entirety herein.
| Number | Name | Date | Kind |
|---|---|---|---|
| 388069 | Thuras | Aug 1888 | A |
| 2958739 | Schoeps et al. | Nov 1960 | A |
| 4450930 | Killion | May 1984 | A |
| 4815560 | Madaffari | Mar 1989 | A |
| 4837833 | Madaffari | Jun 1989 | A |
| 6549634 | Vonlanthen | Apr 2003 | B1 |
| 6813364 | Vonlanthen | Nov 2004 | B1 |
| 7076074 | Gebert et al. | Jul 2006 | B2 |
| 7088839 | Geschiere et al. | Aug 2006 | B2 |
| 7106878 | Saiki et al. | Sep 2006 | B2 |
| 7110565 | Engbert et al. | Sep 2006 | B1 |
| 7305098 | Vonlanthen et al. | Dec 2007 | B2 |
| 7366317 | Miller et al. | Apr 2008 | B2 |
| 7492919 | Engbert et al. | Feb 2009 | B2 |
| 20020136425 | Warren | Sep 2002 | A1 |
| 20050002542 | Warren et al. | Jan 2005 | A1 |
| Number | Date | Country |
|---|---|---|
| 3540106 | May 1987 | DE |
| 1257147 | Nov 2002 | EP |
| 1 341 395 | Sep 2003 | EP |
| 1 562 400 | Aug 2005 | EP |
| 388069 | May 1931 | GB |
| WO 2005000158 | Jan 2005 | WO |
| Entry |
|---|
| European Search Report for European Patent Application No. 05023742.9, dated Oct. 20, 2008. |
| Number | Date | Country | |
|---|---|---|---|
| 20060109999 A1 | May 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60623278 | Nov 2004 | US | |
| 60696595 | Jul 2005 | US |
