Compression driver

Abstract

A compression driver comprises a diaphragm having a concave sound radiating surface, a phase plug having a convex surface shaped to match the concave surface of the diaphragm, the diaphragm being connected to a voice coil former along a line forming a closed loop, the diaphragm and former being adapted to reciprocate along an axis, the diaphragm, the phase plug, and the voice coil former are configured together to form: a compression chamber; a surrounding cavity; the abstraction surface can be generated by rotating an abstraction line around the axis, the abstraction line extends perpendicular to the diaphragm and the convex surface from the diaphragm on a closed-loop line to the convex surface of the phase plug, and wherein the abstraction line is arranged on the convex surface of the phase plug. A plurality of apertures are formed in the voice coil former about at least a portion of its periphery and through the voice coil former to connect the compression cavity with the encircling cavity, the apertures having a total area substantially the same as an area of the abstract surface.

Description

Technical Field

[0001] The present invention relates to the field of compression drivers and in particular to compression driver voice coil formers and to compression drivers incorporating such voice coil formers. Background Art

[0002] A compression driver is a diaphragm speaker that generates sound in a horn speaker. The compression driver is attached to the throat of an acoustic horn, which is a widened pipe that is used to efficiently radiate sound into the air. A compression driver generally comprises a diaphragm connected to a voice coil driver, wherein the voice coil driver is placed in a magnetic field usually provided by one or more permanent magnets. Passing an audio signal current through the voice coil induces a force, causing the voice coil driver to reciprocate between the poles of the magnet, and thus the diaphragm vibrates and thus radiates acoustic waves. The voice coil driver usually includes a voice coil former around which a conductive wire is wound; the former and the coil form an integral piece, and they vibrate as a whole. The voice coil former is usually (but not always) cylindrical. The area of ​​the speaker diaphragm is usually significantly larger than the throat aperture of the horn, so that the compression driver provides high acoustic pressure. A compression driver loaded with a horn can be very efficient, with an efficiency of about 10 times that of a direct radiating cone speaker. They are used as midrange drivers and high-frequency and tweeter drivers in high-power sound reinforcement speakers, as well as reflex or folded horn loudspeakers in loudspeakers and public address systems.

[0003] Compression drivers typically use a phase plug that collects sound radiated by the sound-radiating side of the diaphragm; one common arrangement is to use an axisymmetrically curved diaphragm such as a segment of a sphere, with a phase plug configured to conform to the sound-radiating side of the diaphragm (a spherical diaphragm may be adapted to radiate from either its convex surface or its concave surface, in which case the surface of the phase plug will be spherically concave or convex, respectively). The phase plug typically has a channel through it to collect the sound radiated by the diaphragm and direct it toward the horn; the simple spherical geometry allows the channels to be of equal length.

[0004] FIG1 shows a conventional compression driver 1 in partial cross-section in order to illustrate some of the features to which the present invention relates; a diaphragm 3 is shaped as part of a sphere and is adapted to radiate sound from its concave surface (downwards in the figure, in the direction of arrow A), and a phase plug 5 has a convex spherical surface adjacent the diaphragm 3 and a passage 7 therethrough to direct the sound downwards towards a horn (not shown). Between the surfaces of the diaphragm 3 and the phase plug 5 is a compression chamber 9, and a surrounding chamber 11 is defined by the outside of a voice coil former 13, the inside of a magnet 15 and the underside of the outer edge of the diaphragm outside the voice coil former 13. Between the magnet 15 and the voice coil former 13 (on different parts of which a voice coil is wound (not shown in FIG1 for clarity, but shown in FIG1 ), a voice coil is wound.Figure 2b , Figure 2c , Figure 4a and Figure 4b There is a magnetic gap 17 between the outer surfaces of the voice coil former 13 and the magnet 15 at the top, and the magnetic gap 17 leads to a former cavity 19, which extends downward between the outside of the voice coil former 13 and the magnet 15, leading to the magnet cavity 21.

[0005] Ferrofluids are often used in loudspeakers to allow the voice coil to dissipate heat more efficiently, resulting in longer tweeter life; ferrofluids are used to form a seal in the magnetic gap between the magnet and the voice coil former, which prevents sound from passing into the magnetic gap. This seal creates a compression cavity between the diaphragm and the phase plug, which is essentially a spherical cap with a typical thickness of about 0.4mm. Sound radiates into the compression cavity due to the axial movement of the diaphragm, and leaves the cavity through channels in the phase plug, which are often annular. The channels load the cavity in an amount that depends on their area, and each channel will excite a radial mode in the compression cavity depending on its diameter and area. Choosing the correct area and diameter for each of the channels allows the sum of the modal excitations to be close to zero. A method for achieving this is described in our patent GB2437125.

[0006] Not all compression drivers use ferrofluid, and in those compression drivers, the compression chamber is also loaded by a narrow channel within the magnetic gap between the voice coil and the pole piece (i.e., the former chamber 19 in Figure 1). The acoustic impedance of the voice coil gap and the former chamber is highly irregular due to the volume of air in the magnet cavity and the surround chamber, which leads to significant Helmholtz-type resonances. In addition, the surround radiates additional sound through the magnetic gap. Both of these effects result in frequency-dependent modal excitations, making methods for minimizing modal excitations ineffective. The paper "Boundary conditions of the dome compression chamber in horn drivers", A. Voishvillo, AES Express Paper 46, (October 2022) concludes that for drivers with a voice coil cavity on the outer diameter of the compression chamber, it is impossible to balance the modal excitations, and therefore minimize the modal excitations.

[0007] The sound radiated from the concave side of the diaphragm in the compression cavity leaves through a phase plug channel which leads to a short flare and an outlet where the horn is connected. However, the gap between the center pole and the inner diameter of the voice coil acts as an additional outlet and eliminates the possibility of suppressing resonances by excitation of balanced cavity modes (A. Voishvillo, supra). The magnet cavity and the surround cavity and the acoustic mass formed by the narrow gap between the coil and the pole coil give rise to several resonances. In a further complication, the surround sound radiates into the cavity behind it and the sound is transmitted through an acoustic filter formed by the mass and compliance. The irregular input of the compression cavity further exacerbates the response irregularities.

[0008] Conventional design approaches require the former to be sealed to contain the sound within the compression chamber, and this is always the case for compression drivers that radiate from the concave side of the diaphragm to which the voice coil is attached. Summary of the invention

[0009] The present invention arises from the recognition that an alternative approach to the conventional design of a compression driver can produce advantageous results. Thus, the present invention provides a compression driver for connection to the throat of an acoustic horn, the compression driver comprising a diaphragm having a concave sound radiating surface, a phase plug having a convex surface complementarily shaped to match the concave surface of the diaphragm, and a magnet, the diaphragm being connected to a voice coil former along a line forming a closed loop and lying in a plane, the diaphragm and former being adapted for reciprocating motion along an axis, the diaphragm, phase plug, voice coil former being configured together to form: a compression chamber between the concave surface of the diaphragm and the convex surface of the phase plug; a surround chamber defined by the outer surface of the voice coil former, the inner surface of the magnet, and the edge side of the diaphragm outside the voice coil former; a magnetic gap adjacent the diaphragm transverse to the axis between the outer and inner portions of the magnet, the voice coil former reciprocating through the magnetic gap, the magnetic gap leading to a voice coil former cavity, the voice coil former cavity extending away from the magnetic gap along the axis, the voice coil former cavity having an outer portion extending between an exterior of the voice coil former and the magnet and an inner portion extending between an interior of the voice coil former and the phase plug, the voice coil former cavity leading from the magnetic gap to the magnet cavity, wherein an abstract surface can be generated by rotating an abstract line about the axis, the abstract line extending perpendicular to the diaphragm and the convex surface from the diaphragm on a line of a closed loop to the convex surface of the phase plug, and wherein a plurality of holes are formed in the voice coil former around at least a portion of the periphery of the voice coil former and through the voice coil former to connect the compression cavity with the surround cavity, the holes having a total area substantially the same as or greater than the area of ​​the abstract surface.

[0010] In the case of a circular flexure diaphragm, it will be understood that the abstract surface will be conical (i.e., shaped as a frustum of a cone), and the term "perpendicular thereto" means perpendicular to the two flexure surfaces when these two flexure surfaces are shaped complementarily, but if the two surfaces are not exactly the same shape at the junction of the diaphragm and the former, the term means perpendicular only to the convex surface of the phase plug.

[0011] By introducing holes or perforations through the voice coil former having an area similar to the cross-sectional area of the cavity adjacent to the diaphragm, the compression cavity can be extended and the path through the magnetic gap is bypassed with lower impedance, thus greatly reducing gap excitation. Suppressing the modes in the extended cavity is now possible, depending on the geometry surrounding the cavity and the surround / cavity width. Adjacent to the coil, the surrounding portion of the extended cavity should have a spacing similar to the diaphragm-phase plug spacing. The axial distance between the diaphragm and the phase plug should be equal to the displacement of the diaphragm when the diaphragm is displaced a nominal displacement from the phase plug. The extended cavity should preferably thin from this thickness adjacent to the former to as close to zero as possible to minimize the excitation of cavity modes. Preferably, the total area of the holes is the same as the area of the magnetic gap, but the total area of the holes can be 5%, 10% or even 15% larger or smaller than the area of the magnetic gap without significantly affecting performance.

[0012] As stated above, the length of the surrounding cavity in the axial direction can be substantially the same as the length of the compression cavity in the axial direction, but the length of the surrounding cavity can be 5%, 10% or even 15% larger or smaller than the length of the compression cavity.

[0013] Preferably, when the surrounding cavity extends outward from the voice coil former in a direction transverse to the axis, the length of the surrounding cavity in the axial direction decreases; this minimizes the excitation of cavity modes.

[0014] The holes can be axially extended a sufficient distance such that at least a portion of the holes is contained within the voice coil cavity. Extending the holes a short distance into the voice coil gap allows the ideal area to be achieved close enough to allow the compression cavity and the surrounding cavity to act as a single cavity. The surrounding cavity should be narrow enough to allow all the holes to be within the voice coil cavity when the voice coil former reciprocates.

[0015] A hollow channel can be provided which allows the magnet cavity to communicate directly with the throat of the horn. For example, plugging the magnet cavity and adding holes to transmit sound through the gap to the throat such that the magnetic gap enters another exit channel for the sound. In this case, it is preferable to match the flare rate of the other channels in the phase plug. This method is similar to using the gap as a phase correction channel in US 5,117,462, but has the benefit of allowing the magnetic flux to pass through the ferromagnetic material of the driver rather than through the air in the channel extending from the magnetic gap.

[0016] The channel may have an inlet located in the compression chamber and an outlet located in the throat of the horn, the inlet being located at a node of a selected mode in the compression chamber. The compression driver may also include a bead that may be positioned within the surround chamber and is effective to modify the axial extent of the surround chamber adjacent the magnetic gap and / or to change the radial area of ​​the surround chamber so that it decreases in an outward direction.

[0017] The present invention may be combined with the features set out in our co-pending patent application GB_________, which provide axial mechanical compliance to the voice coil driver as a means of adapting the frequency response of the loudspeaker. In a compression driver loudspeaker, a diaphragm is attached to a voice coil driver, and the voice coil driver is placed in a magnetic field, typically provided by one or more permanent magnets. By passing an alternating current through the voice coil, a force is induced, causing the voice coil driver to reciprocate, and thus causing the diaphragm to vibrate, thereby radiating acoustic waves. The voice coil driver includes a voice coil former around which an electrically conductive wire is wound; the former and the coil form an integral piece, and they vibrate as a unit. The voice coil former is typically (but not always) cylindrical. In some applications where mass is critical and / or space is limited, the voice coil former is made of a material such as titanium or Nomex (Nomex is a trademark of DuPont Safety & Construction, inc., Delaware, USA). Titanium voice coil formers are typically formed from a flat strip of material rolled into a cylindrical shape; typically, the axial ends of the rolled strip are not joined together, which leaves a narrow axial gap running along the length of the voice coil former across which the circumferential forces cannot be balanced by symmetry. Thus, the "hoop" stiffness acting on the circumferential forces due to the axisymmetry is greatly reduced near the gap in the former.

[0018] Our co-pending application GB_________ discloses an arrangement for introducing mechanical axial compliance into a voice coil former which is relatively simple, easy to manufacture and easy to "tune", particularly (but not exclusively) for use in mass critical and / or space limited loudspeakers such as in compression drivers. It describes a mechanical axial compliance arrangement which can be relatively easily tuned to take account of a voice coil former which has been roll formed and has a narrow axial gap extending along the length of the voice coil former. Compression drivers benefit from the introduction of resonances where the mass results in a 6dB / octave low pass filter of typically 2-3kHz. In many cases the output level in the upper part of the response is lower than desired and the introduction of resonances by making the former axially compliant produces a more desirable response.

[0019] By exploiting the relatively easily calculated effects of the cantilever, a relatively simple mechanical compliance arrangement can be provided, and certain arrangements of the cantilever can be used to form a voice coil driver having significantly improved overall performance compared to conventional systems.

[0020] A voice coil former for a compression driver may have at least two axially spaced rows of holes extending circumferentially or at least partially circumferentially about the axis, adjacent rows being rotated relative to one another so that adjacent holes overlap circumferentially to form between them an arcuate spar disposed circumferentially about the voice coil former, each arcuate spar being adapted to flex in a cantilevered manner in an axial direction and to allow the axial length of the voice coil former to vary in response to the voice coil former being axially driven. The amount of overlap between adjacent holes in adjacent rows may be such that the length of the arcuate spar is at least 25% of the circumferential length of the adjacent holes. One or more of the at least two rows of holes may comprise holes through the voice coil former having an area similar to an abstract surface, or they may be in addition to holes through the voice coil former. Where the one or more rows of holes are in addition to holes through the voice coil former having an area similar to an abstract surface, the one or more rows of holes are preferably located axially within the voice coil cavity.

[0021] The curved spars form a structural connection that transmits forces between the portion of the former on which the voice coil is wound and the portion of the former attached to the diaphragm. The spars flex in a spring-like manner and generate a restoring force when deflected so that the arrangement behaves as a spring linking the coil and diaphragm in a manner similar to the corrugations on the former. The circumferential alignment of the spars provides increased flexibility when compared to axial spars.

[0022] The rib is fabricated by making a plurality of perforations or holes in the former by removing material from the former, such that a portion of the former is linked to another portion of the former by a circumferential array of flexing ribs, and the former is coupled to the diaphragm. By varying the length, axial depth, position, orientation, or number of the ribs, the axial compliance can be varied over a wide range of values, thereby allowing the desired axial compliance to be achieved. The length of the ribs can be 30%, 35%, or 40% of the circumferential length of adjacent holes; the longer the ribs, the more they bend under a given axial load, and more compliance is introduced into the voice coil former. The overlap must be less than 50%, otherwise the continuous slots will merge with each other and complete separation will occur in the former; a maximum overlap of 40% is preferred such that the circumferential dimension of the axial extension between adjacent holes is sufficiently rigid. Depending on the type of material to be removed, stamping tool forming, laser cutting, precision lithography, high-precision micro water jet cutting, plasma cutting, or micro milling are possible manufacturing methods. Additionally, these ribs can vary (e.g., in position, size, shape, or orientation) in order to easily compensate for the varying circumferential effects caused by axial gaps in the case where the former is formed by rolling, and / or to vary the axial stiffness of the former at different points around its periphery. Generally speaking, the longer the ribs, the greater the manufacturing tolerances for achieving acceptable response variations; this allows for the economical manufacture of the voice coil former.

[0023] The holes / perforations can extend circumferentially or at least partially circumferentially, or at least have portions with a circumferentially directed component about the axis, and arcuate ribs are formed along at least a portion of each hole. In this case, a single row of holes can provide the ribs to impart the desired axial compliance to the former. There can be one, two, or any number of circumferential rows of holes extending about the axis, which are oriented and / or shaped to form arcuate ribs adapted to flex in a cantilever manner.

[0024] There can be two circumferential rows of holes extending about the axis and axially spaced apart such that the voice coil former forms arcuate ribs between the holes in adjacent rows. Such an arrangement with two rows of holes is both easy to manufacture and provides ribs that impart axial mechanical compliance, which is relatively easy to calculate using finite element method (FEM) analysis; it is also most easily tuned to accommodate non-axisymmetry (presence of axial gaps) or to provide axial compliance that is itself non-axisymmetric.

[0025] Holes axially located between the compression chamber and the surround chamber provide for air exhaust; depending on the specific application, in the case where some of the holes or perforations are axially located within the voice coil chamber, at least some of these holes or perforations can be filled with a damping material that is sound-absorbing and more flexible than the material of the voice coil former to provide damping of the air flow through the perforations, and / or at least some of the perforations can be covered with an air-impermeable flexible material (and more flexible than the material of the voice coil former) to prevent air from flowing through the holes.

[0026] The holes may have substantially the same shape, which ensures that all spars are similar, allowing for ease of manufacture and allowing the axial compliance effects of the spars to be relatively easily calculated. Alternatively, the holes may have different shapes, which may help to circumferentially tune the axial compliance, and / or to distinguish holes that allow air flow and acoustic communication between the compression and surround cavities from holes within the voice coil cavity, and may or may not allow air flow and acoustic communication between the outer and inner portions of the voice coil former cavity.

[0027] The holes may be of substantially the same size, which ensures that all spars are similar, facilitating ease of manufacture and allowing the axial compliance effects of the spars to be calculated relatively easily. Alternatively, the holes may be of different sizes and / or have different circumferential lengths, which may help to circumferentially tune the axial compliance.

[0028] The holes may be spaced substantially the same distance circumferentially and / or axially, which ensures that all spars are similar, facilitating ease of manufacture and allowing the axial compliance effects of the spars to be calculated relatively easily. Alternatively, the holes may be spaced different distances, which may help to circumferentially tune the axial compliance.

[0029] The holes may be oriented similarly, which ensures that all spars are similar, facilitating ease of manufacture and allowing the axial compliance effects of the spars to be calculated relatively easily. Alternatively, the holes may be oriented differently, which may help to circumferentially tune the axial compliance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The invention will now be described by way of example and with reference to the accompanying drawings, in which: FIG1 is a view in cross section of a portion of a known compression driver; Figure 2a is a schematic illustration, partially in cross-section and partially broken away, of one embodiment of a compression driver according to the present invention, Figure 2b yes Figure 2a An enlarged schematic diagram of a portion of a compression driver showing the voice coil former and portions of the cavity within the compression driver, Figure 2c is similar to Figure 2b , but the voice coil former is shown only in outline, and Figure 2d yes Figure 2a A partially cutaway schematic diagram of a diaphragm and voice coil former; Figure 3 is a schematic illustration, partially in section and partially broken away, of a compression driver according to the present invention, similar to Figure 2a But with different diaphragm components; Figure 4a and Figure 4b is a schematic diagram, partly in cross section, of a second embodiment of a compression driver according to the invention, and Figure 4c yes Figure 4a and Figure 4b an enlarged schematic diagram of a portion of a compression driver showing a portion of a voice coil former and contours of a press bar and a cavity within the compression driver; Figure 5a yes Figure 2a to Figure 2d A schematic diagram of a voice coil former, and Figure 5b yes Figure 5a A magnified view of a portion of the voice coil bobbin, Figure 5b is an enlarged schematic diagram of a portion of an alternative voice coil former; Figure 6a and Figure 6b is an enlarged schematic diagram of a portion of an alternative type of voice coil former, Figure 7 yes Figure 2a to Figure 2d an enlarged view of a portion of a voice coil former showing an axial gap extending along the former; and Figure 8 A plane wave tube simulated SPL response curve of a compression driver according to the present invention (curve with asterisks) and a plane wave tube simulated SPL response curve of a conventional compression driver (simple curve) are shown. DETAILED DESCRIPTION

[0031] The prior art arrangement shown in FIG. 1 has been described above.

[0032] Figure 2a 1 shows an embodiment of a compression driver 1' according to the invention having a spherically curved diaphragm 3' driven by a cylindrical voice coil former 13' having a plurality of holes 25' arranged around its periphery; Figure 2d The diaphragm 3' is adapted to radiate acoustic waves from its concave surface in the direction of arrow A towards a horn (not shown) in a conventional manner.

[0033] Figure 2b and Figure 2c Only one tangible object is shown, namely part of the voice coil former 13' of Figure 1 together with the hole 25' therethrough and part of the voice coil 23; Figure 2c In the drawings, these tangible features are shown only in outline. Figure 2b and Figure 2c The other features shown in the diagram are primarily volumes or acoustic paths that allow acoustic waves to pass through and are defined by the tangible parts of the compression driver—these tangible parts are derived from the Figure 2b1 , the tangible parts are omitted in order to more clearly illustrate the invention (these tangible parts are the diaphragm, phase plug and magnet shown in FIG1 ). These empty volumes are the compression chamber 9′, the surround chamber 11′, the voice coil chamber 19′ and the magnet chamber 21′. The holes 25′ are arranged to allow the surround chamber 11′ and the compression chamber 9′ to communicate freely when the compression chamber 9′ is at its maximum volume (i.e. when the diaphragm is at its maximum positive excursion, i.e. when the voice coil former 13′ is displaced upwards to its maximum extent in the figure).

[0034] Reference now Figure 2c , the dotted line 33 indicates that the radius line 31 (hereinafter Figure 2d Description) the shortest distance between the diaphragm and the phase plug; Figure 2a In the case of a spherically curved diaphragm and phase plug, line 33 is perpendicular to both the concave sound radiating surface of the former and the convex surface of the other. Line 33 produces an abstract conical surface in the form of a frustum when rotated 360° around the axis A of the compression driver. The total area of ​​all holes 25' in the voice coil former 13' is substantially the same as the area of ​​the abstract conical surface (i.e., the curved surface of the frustum). This arrangement effectively extends the compression cavity by providing an alternative acoustic path (through the holes in the voice coil former and the voice coil cavity to the magnet cavity), which significantly reduces acoustic excitation in the gap.

[0035] Figure 2d Show Figure 2a The compression driver diaphragm 3' and the voice coil former 13', except for the hole 25' (hereinafter referred to as Figure 5a and Figure 5b The diaphragm 3' has an inner spherical portion 27 and an outer annular portion 29; the transition between the spherical portion and the annular portion is marked by a radius line 31 (which is a circle in the arrangement shown). The diaphragm 3' and the voice coil former 13' are manufactured separately and are joined together along the line of the radius line 31 on the side of the concave surface of the diaphragm to form an integral or single-piece product. A conductive voice coil 23 (only partially shown) is wound around the outer peripheral surface of the voice coil former. There are a plurality of holes 25' extending circumferentially around the voice coil former 13' between the voice coil 23 and the radius line 31.

[0036] Figure 3 A compression driver 1' is shown with a different diaphragm assembly 3", wherein Figure 2d The two rows of slotted holes 25' in the voice coil former 3' are replaced by a single row of circular holes 25" in the former 13", and again the total area of ​​all holes added up to the same area as the surface of the abstract cone. Figure 2a , Figure 2b , Figure 2c and Figure 2dThe holes in the Figure 3 The hole in the does not provide axial mechanical compliance (compression and extension); refer to Figure 5a and Figure 5b Describes axial compliance.

[0037] Figure 4a and Figure 4b is a compression driver 1", which differs from Figure 2a Firstly, a plurality of channels 37 extending from the inlet 41 through the phase plug 5 to the outlet 43 are provided to allow the magnet cavity 21" to communicate directly with the throat T of the phase plug 5, and secondly, a pressure strip 39 (see Figure 4b and Figure 4c ) to fit in the surrounding cavity (11 in FIG. 1, Figure 2c bead 39 forms a cavity as an extension of the voice coil cavity 19 ". The magnet cavity 21 "is still shown in the drawings, but is now isolated from the acoustic path. Figure 4c Only two tangible objects are shown, namely Figure 2a The portion of the voice coil former 13' together with the hole 25' passing therethrough, the portion of the voice coil 23 and the pressure strip 39; Figure 4c In the drawings, these tangible features are shown only in outline. Figure 4c The other features shown in the diagram are primarily volumes or acoustic paths that allow acoustic waves to pass through and are defined by the tangible parts of the compression driver—these tangible parts are derived from the Figure 2b 1 in order to more clearly illustrate these two aspects of the invention (these tangible parts are the diaphragm, phase plug and magnet shown in FIG. 1 ).

[0038] The addition of the channel 37 allows sound to be transmitted through the magnetic gap 17" so that the magnetic gap 17" enters another acoustic outlet channel. Preferably, the expansion rate of the path through the channel matches the expansion rate of the other phase plug channels 7", which means that the volume and / or shape of the magnet cavity 21" and the volume of the channel 37 are adjusted to match the expansion rate of the flow path through all other phase plug channels 7" (as will be seen from the figure, Figure 4c The magnet cavity 21" in the axial direction is Figure 2aIn this case, the acoustic waves generated by the diaphragm propagate downwardly along two parts of the voice coil former cavity 19" (an outer part extending between the outside of the voice coil former 13' and the magnet 15, and an inner part extending between the inside of the voice coil former 13' and the phase plug 5). In practice, the radius of the inner part of the voice coil is in the range of about 0.15mm to about 0.25mm, and the radius of the outer part of the voice coil cavity is between about 0.25mm and 0.35mm. This approach is similar to the use of a gap as a phase corrector channel in US 5,117,462, but has the benefit of allowing the magnetic flux to pass through iron rather than air in the channel extending from the magnetic gap, and the benefit of allowing the compression driver to emit an increased amount of HF acoustic energy. The channel outlet 43 is located at the node to allow simple modal balancing, or can be located on alternating axial sides of the node, and can also be made larger or smaller (so that the channel tapers) depending on the application requirements.

[0039] The bead 39 has two separate functions that can be combined or incorporated separately in all embodiments: making the axial length of the surround cavity (11 in FIG. 1) adjacent to the magnetic gap (17 in FIG. 1) the same as the radial length of the magnetic gap, and making the surround cavity taper in a gradual taper to provide equal excitation of the acoustic pressure across the diaphragm surface. In some applications, the magnet cavity can be made equal to the area of ​​the voice coil cavity.

[0040] Figure 5a Shown is similar to the embodiment of the invention in that it has two axially spaced rows 4a, 4b of holes 6. Figure 2a to Figure 2c In the voice coil former 2a, each hole 6 has the shape of a slot formed by two semicircles joined by straight edges, where the straight edges extend circumferentially. In this example, the former is 0.025 mm thick titanium, rolled into a cylinder with a diameter of approximately 34 mm, and there are 28 holes / slots in each row; each slot is approximately 2.2 mm long and 0.2 mm wide, with a radius of 0.1 mm at each end, and is spaced approximately 1.1 mm from the next slot in the row. Figure 4b As more clearly shown in FIG. 1 , between adjacent holes 6 in each row are axial extensions 8, and the rows 4a, 4b are rotated relative to each other so that each axial extension 8 is aligned with the middle of the nearest slot; this forms a circumferentially extending spar 10 on either side of each axial extension 8, the spar extending between the ends of the slots in the two rows where the slots in the two rows overlap (at the end of the spar). Figure 5a The spars 10 are also shown in dark shading, although for clarity these spars are not shown with the rounded ends extending into the holes, as is the case with Figure 5b). Each spar is arcuate because it is formed on the surface of a cylinder. In the embodiment shown, there are 56 circumferential spars in total (two spars per slot); the circumferential length of each spar is 0.7 mm, and the axial depth (i.e. the axial distance between the two rows 4a, 4b (vertical in the figure)) is 0.3 mm. In this case, the overlap (i.e. the length of each circumferentially extending arcuate spar) is about 27% of the circumferential length of each hole.

[0041] By varying the size of the grooves, their circumferential spacing and / or the distance between rows, it is possible to vary the axial compliance of this arrangement to suit particular requirements / applications, and this axial compliance can be calculated relatively easily.

[0042] Figure 6a An enlarged portion of the former 2a is shown, which is similar to Figure 5a The former is a different shape of hole, but with a different shaped hole; provided the sides of the hole adjacent to the spar remain constant / straight, the shape of the hole has little effect on the stiffness of the spar. In this example, the "D" shaped hole is shaped to Figure 4a and Figure 4b The racetrack-shaped holes of the spar behave in almost exactly the same manner, and the amount of overlap between adjacent holes in adjacent rows is such that the length of the curved spar is approximately 27% of the circumferential length of the adjacent holes. The holes may be of any shape (e.g., semicircular, semi-oval, semi-elliptical) as long as the shape of the perforated edges forming the circumferential spar remains substantially constant / straight.

[0043] Figure 6b A portion of another voice coil former 2' is shown, which is similar to Figure 5b A voice coil former in the embodiment of the present invention but having three rows 4a, 4b, 4c of similarly sized and shaped holes and having a greater degree of circumferential overlap between holes in adjacent rows of about 33% of the circumferential length of the holes (and longer circumferentially extending arcuate spars) which provides a voice coil former with a greater degree of circumferential overlap between holes in adjacent rows of about 33% of the circumferential length of the holes than the voice coil former. Figure 5b The arrangement provides a greater amount of axial compliance.

[0044] Figure 7 Show Figure 5b An enlarged view of a portion of a voice coil former of FIG. 1 (but here the amount of overlap between the perforations is such that the length of the curved spar is approximately 25% of the circumferential length of adjacent perforations), this time showing an axial gap 12 extending along the former. By ensuring that the gap 12 is between the slots, preferably equidistant and bisecting the axial extension 8 in one of the rows 4a and the slot 6' in another of the rows 4b, a former with flexing spars can be designed so that there is little variation in the local axial stiffness around the circumference of the former. If necessary, the length and thickness of the spars adjacent the gaps in the former can be adjusted to correct for any reduction in stiffness due to changes in geometry.

[0045] exist Figure 8 In the example, the improved response below 1.7kHz is due to the modified cavity, and the enhancement of the response above 5kHz is due to the flex former design shown in FIG4 and the patent application. In this example, the phase plug is not modified from the original design, and no surround cavity ring is used. Reducing the surround cavity volume and adjusting the phase plug channel position and area to minimize modal excitation of the compression cavity will further improve the response.

[0046] Of course, it will be appreciated that many variations may be made to the above described embodiments without departing from the scope of the invention. For example, the invention is described herein primarily with reference to a cylindrical voice coil (in the form of a substantially planar ring having a central hole extending along an axis perpendicular to the plane); however, the invention is equally applicable to non-circular arrangements, such as oval, elliptical or racetrack (figure 8, or triangular / square / polygonal with rounded corners) voice coils, or any shape that is symmetrical in one or two orthogonal directions lying in a generally plane perpendicular to the voice coil axis and having a central hole - it will be appreciated that in any such non-circular arrangement, the shape of the abstract surface will not be that of a frustum. The diaphragm is described herein as being spherical, but the invention is applicable to any curved non-spherical diaphragm, such as an elliptical, parabolic or hyperbolic diaphragm. In any of the illustrated embodiments, damping material may be provided in some or all of the holes, and / or an airtight flexible material may be provided to cover the inner or outer surface of any hole located within the voice coil cavity. The described embodiments of the voice coil former and / or diaphragm are all titanium, but they may be formed of thermoset or polyimide composite materials. The holes connecting the compression chamber to the surrounding chamber can also be configured to provide a flexural cambered spar that increases axial mechanical compliance (as in FIGS. 2, 4, 5, and 6), or they can be configured to connect only the compression chamber to the surrounding chamber (as in Figure 3 in the latter case, although the only configuration of the pores described is Figure 3 A single row of circular holes is shown in the drawings, but it will be understood that holes of different shapes may be used (square, diamond, oval, elliptical or racetrack (figure 8, or triangle / square / polygon with rounded corners), and / or the holes may be arranged regularly, such as in one row, in two or more rows and / or in a matrix, and / or irregularly. The sides of the spar are described above as being straight, however, provided there is still a spar that can flex with an axial component to provide axial compliance, the spar may have sides of any shape, and / or they may be inclined away from the circumferential direction.

[0047] Where different variations or alternative arrangements have been described above, it will be appreciated that embodiments of the invention may incorporate such variations and / or alternatives in any combination for different applications, such that features of different embodiments may be combined to form further embodiments. For example, each circumferential row of holes may comprise holes that are all of the same size, shape and orientation, or any of these features may vary within a row; additionally or alternatively, the holes or perforations in a row may be regularly spaced, or they may be irregularly spaced, and in either case the holes or perforations may be of the same length or of different lengths. Any or all of these combinations may be equally applicable to voice coil formers having three or more rows of holes and / or perforations. Reference Figure 4c The described phase plug channels and beading strips may be used individually in embodiments of the present invention and need not be used in combination. Where axial mechanical compliance is desired, two or more rows of perforations providing flexure arc spars as in FIGS. 2, 4, 5 and 6 may be located within the voice coil cavity, while one, two or more circumferential rows of holes may be provided that pass through the voice coil former to connect the compression cavity to the surround cavity, and these holes may also be perforations of a type that increases axial mechanical compliance (as in FIGS. 2, 4, 5 and 6), or they may be of another type (such as in Figure 3 In short, all variations described in this paragraph and the previous paragraph may be combined in any combination in any embodiment described in detail herein, and for the sake of brevity, not every possible combination is described, but these combinations are understandable to those skilled in the art.

[0048] A person skilled in the art will understand that, where properties, advantages and / or applications are described above with respect to only one embodiment, these properties, advantages and applications are equally applicable to other embodiments sharing the same or similar features as the one described, even if this is not explicitly stated herein for reasons of brevity.

Claims

1. A compression driver for connection to the throat of an acoustic horn, the compression driver comprising a diaphragm having a concave sound radiating surface, a phase plug having a convex surface complementarily shaped to match the concave surface of the diaphragm, and a magnet, the diaphragm being connected to a voice coil former along a line forming a closed loop and lying in a plane, the diaphragm and the former being adapted for reciprocating motion along an axis, the diaphragm, the phase plug, the voice coil former being configured together to form: a compression chamber between the concave surface of the diaphragm and the convex surface of the phase plug; a surround cavity defined by an outer surface of the voice coil former, an inner surface of the magnet, and an edge side of the diaphragm outside the voice coil former; A magnetic gap is provided adjacent to the diaphragm and transverse to the axis between the outer and inner portions of the magnet, through which the voice coil former reciprocates, the magnetic gap leading to a voice coil former cavity extending along the axis away from the magnetic gap, the voice coil former cavity having an outer portion extending between an exterior of the voice coil former and the magnet and an inner portion extending between an interior of the voice coil former and the phase plug, the voice coil former cavity leading from the magnetic gap to the phase plug; Magnet cavity, wherein an abstract surface can be generated by rotating an abstract line about the axis, the abstract line extending perpendicularly to the diaphragm and the convex surface from the diaphragm on the line of the closed loop to the convex surface of the phase plug, and wherein a plurality of holes are formed in the voice coil former around at least a portion of the periphery of the voice coil former and through the voice coil former to connect the compression chamber with the surround chamber, the holes having a total area substantially the same as or greater than the area of ​​the abstract surface.

2. The compression driver according to claim 1, wherein: The length of the surrounding chamber in the axial direction is substantially the same as the length of the compression chamber in the axial direction.

3. A compression driver according to claim 1 or claim 2, wherein: The length of the surround cavity in the axial direction decreases as the surround cavity extends outwardly from the voice coil former in a direction transverse to the axis.

4. A compression driver according to claim 1, 2 or 3, wherein: The bore extends axially a sufficient distance such that at least a portion of the bore is contained within the voice coil cavity.

5. A compression driver according to any one of the preceding claims, wherein: A passage is provided that allows the magnet cavity to communicate directly with the throat of the horn.

6. The compression driver according to claim 5, wherein: The passage has an inlet in the compression chamber and an outlet in the throat of the horn, the inlet being located at a node of a selected mode in the compression chamber.

7. A compression driver according to any of the preceding claims, further comprising a bead which is positionable within the surrounding cavity and effectively modifies the axial extent of the surrounding cavity adjacent the magnetic gap and / or changes the radial area of ​​the surrounding cavity so that it decreases in an outward direction.

8. A voice coil former for a compression driver according to any one of the preceding claims, wherein: The voice coil former is configured to provide at least one row of arcuate spars disposed circumferentially around the voice coil former, each arcuate spar being adapted to flex in a cantilever manner in an axial direction and allow the axial length of the voice coil former to vary in response to the voice coil former being axially driven.

9. The voice coil former according to claim 7, wherein: There are at least two axially spaced rows of holes extending circumferentially or at least partially circumferentially about the axis, adjacent rows being rotated relative to each other so that adjacent holes overlap circumferentially to form therebetween a curved spar disposed circumferentially about the voice coil former, each curved spar being adapted to flex in a cantilever manner in an axial direction and to allow the axial length of the voice coil former to vary in response to the voice coil former being axially driven, wherein the amount of overlap between adjacent holes in adjacent rows is such that the length of the curved spar is at least 25% of the circumferential length of the adjacent holes.

10. The voice coil former according to claim 8, wherein: The amount of overlap between adjacent holes in adjacent rows is such that the length of the curved spar is at least 30%, or at least 35%, or at least 40% of the circumferential length of the adjacent holes.

11. The voice coil former according to any one of claims 7 to 9, wherein: The holes have substantially the same shape.

12. The voice coil former according to any one of claims 7 to 10, wherein: The holes have substantially the same size.

13. The voice coil former according to any one of claims 7 to 10, wherein: The holes are spaced circumferentially and / or axially at substantially the same distance.

14. A compression driver comprising a voice coil former according to any one of claims 7 to 12.

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