Apparatus for acoustic loading of a diaphragm

An apparatus acoustic loading of a diaphragm is disclosed. The apparatus comprises a rear face having a shape corresponding to a shape of a compression side of the diaphragm, a front face opposite to the rear face along an axis of symmetry, and a plurality of voids formed between the rear face and the front face. The plurality of voids are arranged so that their intersection with rear face forms a plurality of inlets, the plurality of inlets defining a higher compression ratio in the proximity of a voice coil compared to a compression ratio defined in at least a portion of the remaining area of the diaphragm.

BACKGROUND

The present invention relates to transducers of electrical energy to acoustic energy and vice versa. More particularly, the present invention relates to an apparatus for acoustic loading of a diaphragm in such a transducer.

Transducers have been developed to allow transformation of electrical energy into acoustic energy, as well as to allow transformation of acoustic energy into electrical energy. The former are known as loudspeaker drivers, the latter are known as microphones.

Among the many considerations when designing such transducers is efficiency of energy transformation. To accomplish such an energy transformation, a transducer typically comprises a moving element—a diaphragm. In a driver, the moving element is responsive to the electrical energy. In a microphone, the moving element is responsive to the acoustic energy. Because of different properties, e.g., masses, between the media through which the acoustic energy propagates, e.g., air, and the moving element of a transducer, i.e., the diaphragm, the energy transformation efficiency is small.

To improve the energy transformation efficiency, designers construct the diaphragm from light materials in an attempt to minimize the diaphragm's mass, thus decreasing the mismatch of the properties. However, this design approach is limited by a multiplicity of factors, including, but not being limited to: properties of the diaphragm material, properties of the diaphragm structure, acoustic loading on the diaphragm, and frequency of operation. Even advanced materials used for diaphragm construction reach a point, beyond which further attempts to decrease the diaphragm's mass causes the diaphragm to change its shape during operation, which results in non-pistonic movement and; consequently, an erratic acoustic or electrical energy output. The term pistonic means that each point at the diaphragm keeps constant position relative to the other points at the diaphragm, as the diaphragm moves.

Another technique to improve the energy transformation efficiency used in the art is to increase acoustic loading, or radiation resistance, acting upon the diaphragm. This loading is achieved by placing an acoustic transformer in front of the diaphragm.

One known class of acoustic transformers comprises horns. In general, a horn is a device, which achieves the acoustic transformation by converting large pressure variations in a small amount of air into a low pressure variation in a large amount of air. The conversion is effected by increasing cross-section area in the progression from a throat of the horn to the mouth of the horn, according to a function describing the horn's flare. Modern horn designs typically feature some form of flare, e.g., exponential, tractrix, or conical.

Very often, the loading is further modified by making the cross-section area of the horn throat smaller than the area of the diaphragm of the transducer to which the horn is attached. The relationship of the horn throat cross-section area to the diaphragm area is typically referred to as a compression ratio of the horn.

Although the above-described horn improves efficiency of the energy transformation, the acoustic energy being radiated from different locations on the diaphragm in a form of sound waves, may arrive in the horn throat at different times due to differing path lengths. Such an arrival creates an out of phase condition causing irregularities in the frequency response.

A solution to this problem is to make paths from different parts of the diaphragm as similar as possible, to avoid phase cancellation that results from such an out of phase condition. One such a solution utilizes an acoustic transformer known as a phase—or phasing—plug, which is interposed between the diaphragm and the horn. A phasing plug thus improves loading of a diaphragm, and equalizes path lengths.

Reference is now made toFIG. 1, which illustrates a transducer in accordance with a general construction principles of prior art. AlthoughFIG. 1illustrates a driver, such is for tutorial purposes only because the below described concepts are equally applicable to microphones.

The transducer comprises an acoustic transformer, i.e., a phasing plug102. The acoustic transformer possesses rotation symmetry about an axis100. The axis of rotation is a line such that for every point of the body its distance to the line remains constant under the rotation, and the point remains in the same plane perpendicular to the axis. Thus the point moves in a circle in that plane.

A rear face106of the phasing plug102is shaped in accordance with the shape of a diaphragm108, generally in a shape of a dome, or a portion of a sphere. To enable movement of the diaphragm108, a surround124is attached in vicinity to the circumference of the diaphragm108. The surround124operates like a suspension as well as locating device for the diaphragm108, and is affixed to a first pole piece110, by fasteners112.

Because an acoustic capacitance of volume of air between the diaphragm108and the phasing plug102causes loss of high frequency energy, the clearance between the rear face of the phasing plug102and the diaphragm108is generally defined to allow only enough room for the diaphragm to move through the diaphragm's intended range without physical interference with the rear face. Such an arrangement minimizes the volume of air between the diaphragm and the phasing plug and; consequently, the acoustic capacitance.

The side of the diaphragm108facing the rear side of the phasing plug102is a compression side of the diaphragm. The non-compression side of the diaphragm108is protected by a cover126, either sealed or vented.

To enable movement of the diaphragm108and; consequently, a transformation of electrical energy to acoustic energy and vice versa, a voice coil122is affixed in vicinity to the circumference of the compression side of the diaphragm108.

A static magnetic flux is provided so that an alternating input signal causing a current flow through the voice coil122, causes the voice coil122to move back and forth along the axis100. A magnetic circuit, i.e., a closed path containing the static magnetic flux, comprises the first pole piece110, a second pole piece116, and pieces118and119. The materials and methods of mechanical connection of the pieces110,116,118and119, forming the magnetic circuit are designed to provide a low reluctance path for the static magnetic flux through the magnetic circuit.

The static magnetic flux in the magnetic circuit is induced by a magnet120, a coil, or any other suitable means. The static magnetic flux produces a magnetic flux density in the air gap between the first pole piece110and the second pole piece116.

A plurality of voids114between the rear face106and the front face104form air channels, allowing the sound waves to travel through the voids114from rear to front, and generally emerge at the front face104of the phasing plug102as a single air channel.

To improve the loading of the diaphragm108, the total cross-section area of the air channels of the phasing plug102at the rear face108is made smaller than the total area of the diaphragm108. The relationship of the total diaphragm area to the total cross-section area of the air channels is typically referred to as a compression ratio of the transducer. The air between the diaphragm and the phasing plug (i.e., the compression region), can be compressed to relatively high pressures by small motion of the diaphragm. This is what allows such a transducer to output acoustic energy at greater pressure levels than can conventional loudspeakers where the diaphragm radiates directly into the air. The efficiency of the transducer is thus increased. A transducer with such an arrangement is generally referred to as a compression driver.

Further to providing a compression ratio, the path lengths of the air channels within the phasing plug may be equalized so as to bring all portions of the sound wave, propagating through the air channels, into phase coherence when they reach the front face of the phasing plug. Without such path length equalization, sound waves emanating from different air channels would destructively combine so as to cause irregularities in the frequency response as discussed above.

The exit path of the transducer is bored into a second pole piece116. The area of the front face104and the area of a transducer's exit128together with the distance between them define a flare. This flare may affect the useful frequency bandwidth of the transducer.

There are many designs of phasing plugs, accomplishing the compression loading and path length equalization. Perhaps the most frequently used type is a circumferential phasing plug. Such a phasing plug comprises annular cross-sections that usually increase in area as the principal radius of each annulus decreases in moving toward the throat of the transducer. An example of such a phasing plug can be found in U.S. Pat. No. 2,037,187, entitled “Sound Translating Device,” incorporated by reference. An often cited disadvantage of these phasing plugs—difficult and expensive manufacturing—lead to development of a radial phasing plug.

A radial phasing plug comprises a plurality of radial slot-shaped inlets extending from the axis of cylindrical symmetry of the speaker. An example of such a phasing plug can be found in U.S. Pat. No. 4,050,541, entitled “Acoustical Transformer for Horn-type Loudspeaker,” incorporated by reference.

Yet another type is a saltshaker design, so called because holes at the spherical outer surface of the plug that extend through to the throat of the speaker resemble the holes of a saltshaker. An example of a saltshaker phasing plug may be found in Fancher M. Murray: “An Application of Bob Smith's Phasing Plug,” 61stConvention of an Audio Engineering Society (AES), Nov. 3-6, 1978, New York.

However, all the known phasing plug designs suffer from several problems. Considering their design, the magnet120, the first pole piece110, and the second pole piece116are commonly located on the front side of the phasing plug102. The voice coil122is disposed within the air gap between the first pole piece110and the second pole piece116.

Ideally, the air gap should be made as narrow as is practicable since reluctance in the magnetic circuit increases as a square function of the width of the gap, lowering the magnetic flux density in the air gap rapidly as the dimension is increased. Nevertheless, there is a region, comprising a considerable volume of air in the air gap surrounding the voice coil122as well as in the spaces along the inner circumference of the surround124and outer circumference of the diaphragm108. Because this region is far from the inlets of the phasing plug air channels, the variations of air pressure in that region are coupled negligibly, i.e., little or not at all, to the phasing plug102and; consequently the transducer's exit128. As such the pressure variations do not contribute to the generation of sound output, and cause energy losses in the form of heat.

In addition, the uncoupled region also causes cavity resonance effects which distort the overall sound output of the speaker due to anomalies in its frequency response. The problem is treated in, e.g., Kinoshita, et al.: “The Influence of Parasitic Resonances on Compression Driver Loudspeaker Performance”, 61st Convention of the Audio Engineering Society in 1978.

The ideal behavior of a diaphragm movement would be a purely pistonic motion over the entire area of the diaphragm in response to forces imposed upon it by the input signal, over the entire range of audio frequencies being reproduced by the transducer. However, this is and ideal, and can not be achieved in practice. In general, above a certain audio frequency, the diaphragm begins to deform, and portions of the diaphragm move non-pistonically. This deformation results in creation of signals not present in the input signal (a distortion).

As the frequency increases, the properties of the diaphragm, e.g., the mass and lack of stiffness, cause region(s) of the diaphragm to decouple, i.e., fail to follow the motion of the voice coil. Only the region of the diaphragm in proximity to the voice coil is coupled, and follows the motion of the voice coil faithfully. This results in a decline in a power response of the transducer, especially at higher frequencies.

The decoupling effect has been extensively studied in the art; e.g., William F. Boyce: Hi Fi Stereo Handbook, second edition, October 1964; Abraham B. Cohen: Hi Fi Loudspeakers and Enclosures, revised second edition, 1978. Hence, determining the region of the diaphragm that is not decoupled is a routine engineering task once required design criteria, including, but not being limited to highest frequency of operation, properties of the diaphragm, i.e., shape, size and construction, have been established.

An increased compression ratio may compensate for the decline in the power response; however, the bandwidth of uniform power response is narrowed and generally moved higher in frequency. Furthermore, the above-described decoupling effects are increased.

At least one design attempt, a U.S. Pat. No. 2,832,844, entitled “Speaker Driver”, addressed the above-identified problems of minimizing the air gap and cavity resonances of air gap volume by re-locating the magnetic circuit and the voice coil at the rear side of the phasing plug. However, the effects of uncoupled region and decoupling effects were not solved.

A U.S. Pat. No. 5,177,462, entitled “Phasing Plug for Compression Driver”, expressly rejected the approach taken by U.S. Pat. No. 2,832,844, and instead addressed some of the above-identified problems by creating an “auxiliary air passage” from the air gap. However, adding the auxiliary air passage in the vicinity of the voice coil causes the necessity to replace part of the magnetic circuit with a magnet embedded in the phasing plug. As such, the magnet is necessarily small, resulting in a weaker magnetic field. Furthermore, the air passage itself adds reluctance to the magnetic circuit. To minimize this added reluctance, the auxiliary air passage should take up no more volume than necessary, which compromises the optimal shape and size of the auxiliary air passage.

SUMMARY OF THE INVENTION

The invention aims to address the at least some of the above-identified and related problems. The features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings.

DETAILED DESCRIPTION

Turning now toFIG. 2of the accompanying drawings, there is shown a conceptual design of a transducer202.

A structure providing a magnetic circuit for the transducer202comprises a cylindrically-shaped inner pole piece204, attached at one end205ato a base plate206. The other end205bof the inner pole piece204has a diameter reduced below an inner diameter of a voice coil assembly326. An annular outer pole piece210with an inner diameter greater that the outer diameter of the voice coil assembly326is positioned in such a spatial relationship to the other end205bof the inner pole piece204that an air gap211is created by the difference between the reduced outer diameter of the inner pole piece204and the inner diameter of the outer pole piece210. The air gap211is illustrated in greater detail in the detailed drawing A.

The spatial relationship between the inner pole piece204and the outer pole piece210is defined by a plurality of cylindrically-shaped stand-offs208(1)-208(n), attached to the outer periphery of the base plate206. In one embodiment, there are eight standoffs (n=8). The members of the magnetic circuit structure are attached by means of fasteners, e.g., screws. However, any other means for attachment, facilitating a low magnetic reluctance between the different members are contemplated and are; therefore, within the scope of the disclosed concepts.

A coil216wound around the inner pole piece204, is secured between the base plate206and the annular outer pole piece210by means of annulus-shaped compressible spacers218(1) and218(2). When connected to a supply (not shown) of electric energy, the coil216generates a magnetic flux in the magnetic circuit. However, other techniques for generating the magnetic flux, e.g., by means of permanent magnets, are contemplated and are; therefore, within the scope of the disclosed concepts. The magnetic flux produces a magnetic flux density in the air gap211between the inner pole piece204and the annular outer pole piece210.

Referring to the detailed drawing A, the inner diameter area of the outer pole piece210is tapered to form a reduced outer pole tip212to concentrate the magnetic flux into the gap. The voice coil assembly326comprises a voice coil326awound around a former326b. The outer section of the former326bis attached to a non-compression side of the diaphragm318; the inner section of the former326bwith the voice coil326ais disposed into the air gap211.

A sub-assembly, comprising an acoustic transformer302with an attached diaphragm318/voice coil326assembly, is affixed to the above described magnetic circuit structure. A cross-sectional view of a conceptual design of such a sub-assembly is shown inFIG. 3of the accompanying drawings.

Referring now toFIG. 3, as illustrated, the sub-assembly possesses rotation symmetry about an axis303. A rear face of the acoustic transformer302is formed by a first conically shaped surface304with a first included angle. As illustrated, the first included angle is 103 deg; however, other angles may be used as a matter of design choice.

The first conically shaped surface304is truncated by a second conically shaped surface306with a second included angle, wherein the second include angle is greater than the first included angle. As shown, the second included angle is 172 deg; however, other angles may be used as a matter of design choice.

The rear face of the acoustic transformer302is shaped correspondingly to the shape of a compression side of the diaphragm318, with a clearance between the rear face and the diaphragm318being defined to allow only enough room for the diaphragm to move through the diaphragm's intended range without physical interference with the rear face. Therefore, should another shape of the compression side of a diaphragm than a first conical shape truncated by a second conical shape be desired, the rear face of the acoustic transformer302will be formed accordingly. The first shaped surface and the second shaped surface do not need to have the same shape, but may comprise any combination of concentrically arranged shapes. Furthermore, the rear face of the acoustic transformer302may comprise other than two surfaces, including only one surface. Exemplary shapes for diaphragms or surfaces of diaphragms are curvilinear, dome, planar, and any other shapes known in the art.

The second conically shaped surface306is further optionally truncated by a flat surface308formed by an insert310, which is threaded into a first passage311with threaded walls formed in the acoustic transformer302. The clearance between a diaphragm318and the rear face of the acoustic transformer302is controlled by threading the insert310in or out of the first passage311. In the center of the insert310is formed a second threaded passage312. The second threaded passage312accepts a first fastener314, which prevents movement of the insert310relative to the acoustic transformer302, once the required clearance was established. A second fastener316affixes the diaphragm318to the insert310.

Alternatively, the insert310need not be used and the flat surface308with the threaded passage312is instead machined as a part of the second conically shaped surface306.

The diaphragm318is shaped substantially to follow the shape of the rear face of the acoustic transformer302. Accordingly, the diaphragm318comprises a first conically shaped section320with a first included angle. As shown, the first included angle is 103 deg; however, other angles may be used as a matter of design choice. The first conically shaped section320may optionally comprise a flared lip adapted to be affixed to the acoustic transformer302.

The first conically shaped section320is truncated by a second conically shaped section322with a second included angle, wherein the second include angle is greater than the first included angle. As shown, the second included angle is 180 deg; however, other angles may be used as a matter of design choice. The second conically shaped section322of the diaphragm318may optionally contain several annular corrugations324, which function as hinges to provide adequate freedom of movement to the diaphragm.

The diaphragm318is affixed to the acoustic transformer302at its outer circumference. As illustrated, the outer circumference of the diaphragm318(comprising optionally the flared lip), is glued to an annulus328, which is clamped about the annulus' outer circumference by a clamping annulus330by means of a plurality of fasteners332. The annulus328functions as a suspension as well as locating device for the diaphragm318. Accordingly, a desired behavior of the suspension may be achieved by a proper selection of the material for the annulus328, e.g., a cloth, a felt, leather, or any other material, which provides a compliance of the diaphragm required by particular design criteria. This type of suspension is known as a flat suspension.

However, other types of suspension known in the art, e.g., a half-roll suspension, a tangential suspension, a diamond pattern suspension, may be used when required by design criteria.

As described in reference toFIG. 2, the voice coil assembly326(comprising a voice coil326a, wound around a former326b), is attached via the former326bto the non-compression side of the diaphragm318, so that the section of the voice coil assembly326that is not attached to the diaphragm318is facing away from the acoustic transformer302. Furthermore, the voice coil assembly326is attached between the outer circumference of the diaphragm318and the border defined by intersection of the diaphragm318with the flat portion308. In other words, a circumference of the voice coil assembly326, defined by its diameter, is smaller than the outer circumference of the diaphragm318. As shown, the voice coil326is attached at the boundary between the first conically shaped section320and the second conically shaped section322.

The center of the diaphragm318/voice coil326assembly is affixed by a fastener316to the flat section308of the acoustic transformer302thus providing a reliable means for centering the diaphragm318/voice coil assembly326in the air gap211(shown inFIG. 2).

By means of this arrangement, the voice coil assembly326does not interfere with the acoustic transformer302. Consequently, inlets of air channels may be arranged to intersect any region of the rear face of the acoustic transformer302, thus minimizing or preventing occurrence of uncoupled region(s) and preventing distortion due to undesirable effects of cavity resonances. Additionally, by appropriate arrangement of the inlets of the air channels as described in detail below, the decoupling effects can be at least mitigated.

As can be seen inFIG. 3, a plurality of voids334is formed between the rear face and a front face335of the acoustic transformer302to provide a plurality of the air channels. A plurality of inlets336(not shown) of the air channels is created by intersections of the plurality of voids334with the rear face of the acoustic transformer302. Because the voice coil assembly326does not interfere with the acoustic transformer302, the plurality of voids334may be arranged to intersect any region of the rear face of the acoustic transformer302, including the region of or even past the outer circumference of the diaphragm318, thus minimizing or preventing occurrence of uncoupled region.

Furthermore, the plurality of voids334is arranged to provide different compression ratios over different areas of the diaphragm318. Due to decoupling effects at high frequencies, only a region of the diaphragm318in a proximity to the voice coil326, i.e., the area defined by the coupled region, is coupled, and follows the motion of the voice coil faithfully. By increasing the compression ratio in the proximity to the voice coil326, the decoupling effects (a decline in a power response of the transducer), are mitigated. Because the compression ratio in the remaining area of diaphragm318is not altered from its optimum, the bandwidth of uniform power response is not affected, and the decoupling effects are not increased.

One suitable arrangement of the plurality of voids334is illustrated inFIG. 4. InFIG. 4, the reference XX indicates a plane of the cross-sectional view of the acoustic transformer302illustrated inFIG. 3. As shown, the plurality of voids334is arranged as radii about the axis of symmetry, said arrangement providing an acoustic transformer302known as a radial phasing plug.

As illustrated, the plurality of voids334intersects both the first conically shaped surface304and the second conically shaped surface306; the intersection forming a plurality of inlets336. Because the voice coil326(not shown), is located at or about the intersection of the first conically shaped surface304and the second conically shaped surface306, the inlets336, traverse the surface defined by the diameter of the voice coil326, thus providing a plurality of air channels in this area.

To minimize or prevent occurrence of uncoupled region, the plurality of inlets336is extended to a region of the rear face of the acoustic transformer302defined by a diameter338. As shown, the diameter338is smaller than a diameter defining the outer circumference of the diaphragm318(not shown). Alternatively, the plurality of inlets336may be extended to or even past the outer circumference of the diaphragm318(not shown).

To mitigate the decoupling effects, a higher compression ratio is provided in the proximity of the voice coil326, i.e., in an area defined by the coupled region, compared to a compression ratio provided in at least a portion of the remaining area of the diaphragm318(not shown). Because a compression ratio is defined as a ratio of the total diaphragm area to the total cross-section area of the air channels; once a shape and dimensions of a diaphragm are determined; therefore, becoming constant, the desired compression ratio is defined by the cross-section area of the inlets336. Consequently, once the areas (the proximity of the voice coil326and at least a portion of the remaining area of the diaphragm318(not shown)), and associated desired compression ratios are designed, the cross-section areas of the inlets336in the respective areas defines the desired compression ratios. The shape of the inlets336, determining their cross-section area, may be constant or may vary uniformly or non-uniformly over the respective areas of the diaphragm318(not shown).

InFIG. 4two areas with two associated compression ratios may be defined. The first compression ratio is provided in the proximity of the voice coil326(not shown), i.e., within the first area formed by a diameter338, greater than the diameter of the voice coil326(not shown). The first compression ratio is provided by determining the desired cross-section area of the inlets336in that area. As shown, the desired cross-section area is implemented by constant width inlets336.

The second compression ratio is provided in the second—remaining—area of the diaphragm defined as a belt between the diameters338and340. The second compression ratio is provided by determining the desired cross-section area of the inlets336in that area. As shown, the desired cross-section area is implemented by inlets336, with non-uniformly shaped cross-section areas.

Because the design goal is to provide a higher compression ratio in the proximity of the voice coil326(not shown), the first compression ratio is greater than the second compression ratio.

Alternatively, there may be more than two areas, each such an area having an associated compression ratio. To mitigate the decoupling effects, the area in the proximity of a voice coil has a higher compression ratio than at least one of the remaining areas of the diaphragm. Thus, referring again toFIG. 4, a first area is a first band between diameters338and340, a second area is a second band between diameters340and342, and third area is defined to be within diameter342. Because the diameter of the voice coil326(not shown) is smaller than the diameter338, but greater than diameter342, the second area is in the proximity of the voice coil326(not shown). Therefore, the second area has higher compression ratio than at least one of the two remaining areas, e.g., the first area.

The compression ratio in the remaining area, i.e., the third area is a design choice. For example, as illustrated, the compression ratio in this third area is approximately equal to the compression ratio in the second area. However, since the movement of the diaphragm318(not shown) in this area is limited, another design choice could set this compression ratio equal to zero.

As the voids334progress from the rear face to the front face of the acoustic transformer302, the voids expand from the shapes defined by the inlets336at the rear face to outlets342at the front face. As illustrated, the outlets342having a shape of a truncated isosceles triangle with a vertex of said triangle located about the axis303. As illustratedFIG. 5, the truncating portion is a line. However, other truncating shapes, e.g., arch, opposite isosceles triangle, and other shapes may be used.

The above-described concepts may be equally applied to a circumferential phasing plug. As well known, in a circumferential phasing plug, the plurality of voids334is arranged to intersect the rear face of the acoustic transformer302as a plurality of inlets (not shown) formed by a plurality of circles. The plurality of circles is defined by radii about the axis of symmetry; the radii increasing in the direction away from the flat portion308.

Because the voice coil assembly326does not interfere with the acoustic transformer302, the plurality of voids may be arranged to intersect any region of the rear face of the acoustic transformer302, including the region of or even past the outer circumference of the diaphragm318, thus minimizing or preventing occurrence of uncoupled region.

Furthermore, to mitigate the decoupling effects, the area in the proximity of a voice coil has a higher compression ratio than at least one of the remaining areas of the diaphragm. Consequently, the plurality of voids334is arranged to provide different compression ratios over different areas of the diaphragm318(not shown).

InFIG. 4two areas with two associated compression ratios may be defined. A first area in the proximity of the voice coil326(not shown), i.e., within the area formed by a diameter338, greater than the diameter of the voice coil326(not shown). A second—remaining—area of the diaphragm318(not shown) is defined as a band between the diameters338and340. Because in circumferential phasing plug the plurality of inlets is formed by a plurality of coaxial circles, the width of the inlets determines the cross-section area. Therefore, the width of the inlets is selected to define a first compression ratio within the first area, and a second compression ratio in the second area. Because the design goal is to provide a higher compression ratio in the proximity of the voice coil326(not shown), the first compression ratio is greater than the second compression ratio.

Alternatively, there may be more than two areas, each such an area having an associated compression ratio. As an example, the width of the inlets is selected to define a first compression ratio within a first area defined as a first band between diameters338and342; a second compression ratio in a second area defined as a second band between the diameters340and342; and third compression ratio in a third area defined within diameter342. Because the diameter of the voice coil326(not shown) is smaller than the diameter340, but greater than the diameter342, the second area is in the proximity of the voice coil326(not shown). Therefore, the second area has higher compression ratio than at least one of the two remaining areas, e.g., the first area. The compression ratio in the remaining area, i.e., the third area is again a design choice.

Similarly, the above-described concepts may be equally applied to a salt shaker acoustic transformer.FIG. 6illustrates a conceptual design of such an acoustic transformer of the compression driver ofFIG. 2. A plurality of voids602is formed between a rear face604and a front face606of the acoustic transformer600to provide a plurality of the air channels.

As illustrated, the plurality of voids602intersects the rear face604; the intersection forming a plurality of inlets608. The plurality of inlets608is arranged to provide a first total cross-section area in the proximity of the voice coil326(not shown), i.e., in the area defined by the coupled region; and to provide a second total cross-section area in at least a portion of the remaining area of the rear face604.

In one suitable arrangement, the first area is defined to be within a diameter614, greater than the diameter610of the voice coil326(not shown). The first compression ratio is determined as a ratio of total cross-section area of those of the plurality of inlets608in the proximity of the voice coil to the total area in the proximity of the voice coil. Consequently, the first compression ratio is provided in the proximity of the voice coil326(not shown).

However, other possible arrangements of the plurality of voids602providing the required first compression ratio are within the scope of the disclosed concepts. As an example, one such arrangement of voids602creates an alternating pattern of inlets608in the proximity of the voice coil326(not shown), e.g., one inlet608within a circle610the next inlet608without the circle610but within the circle614and so forth:

The second—remaining—area of the diaphragm is a band between the diameters614and616. The second compression ratio is determined as a ratio of those of the plurality of inlets608in the area between the diameters614and616to the total area between the diameters614and616.

However, other possible arrangements of the plurality of voids602providing the required second compression ratio are within the scope of the disclosed concepts. As an example, the plurality of voids602is arranged to intersect said rear face as a plurality of inlets608arranged on a multitude of radii about the axis of symmetry618. In another example, the plurality of voids602is arranged to intersect said rear face as a plurality of inlets608arranged on a multitude of circles defined by radii about the axis of symmetry618.

Because the design goal is to provide a higher compression ratio in the proximity of the voice coil326(not shown), the first compression ratio is greater than the second compression ratio.

Alternatively, there may be more than two areas, each such an area having an associated compression ratio. To mitigate the decoupling effects, the area in the proximity of a voice coil has a higher compression ratio than at least one of the remaining areas of the diaphragm. Thus, a first area is a first belt between the diameters614and616, a second area is a second belt between diameters614and612, and third area is defined to be within diameter612. Because the diameter of the voice coil326(not shown) is smaller than the diameter614, but greater than diameter612, the second area is in the proximity of the voice coil326(not shown). Therefore, the second area has higher compression ratio than at least one of the two remaining areas, e.g., the first area.

The compression ratio in the remaining area, i.e., the third area is a design choice.

The plurality of voids602intersects the front face606in a plurality of outlets619. As illustrated the plurality of voids602is substantially conically shaped, with an included angle of those of the plurality of inlets608in the proximity of the voice coil being greater than an included angle of those of the plurality of inlets608without the proximity of the voice coil. Furthermore, the front face606of the acoustic transformer600is recessed to provide path length equalization. The term “substantially” includes shapes like cone, polyhedron pyramid, and wedge. However, other shapes, e.g., cylinder, may be used as long as the required compression ratios and path equalization in accordance with the disclosed concepts is achieved.

Having thus described the invention by reference to preferred embodiments it is to be well understood that the embodiments in question are exemplary only and that modifications and variations such as will occur to those possessed of appropriate knowledge and skills may be made without departure from the spirit and scope of the invention as set forth in the appended claims and equivalents thereof. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.