Loudspeaker diaphragm

Abstract

A loudspeaker diaphragm (12) comprising a woven fibre body supports damping material (25), for example PVA polymer, on a rearward-facing surface (24). The woven fibre body may be formed of lengths (14) non-metallic fibre material (for example glass fibre) coating with a thin metal coating (32). The mass of the layer of damping material (25) may be less than the mass of the woven fibre body. An attractive sparkly looking loudspeaker diaphragm (12) may thus be provided which damps undesirable vibration whilst providing a flatter frequency-response curve (50).

Claims

1. A loudspeaker diaphragm configured to have a range of operation over a band of frequencies that makes the diaphragm suitable for use in a midrange drive unit for a hi-fi loudspeaker, comprising: a woven fibre body; and a layer of damping material, wherein the woven fibre body is configured to act as a skeleton structure that supports the damping material, wherein the diaphragm has a stiffness such that without the damping material the diaphragm would not exhibit break-up behaviour at frequencies below 500 Hz, but would exhibit break-up behaviour at certain frequencies above 500 Hz but still within the band of frequencies of the range of operation, and wherein the damping material is configured to damp the break-up behaviour at the certain frequencies; and a mass per unit area of damping material on the diaphragm is in the range of 100 to 500 g/m.sup.2.

2. The loudspeaker diaphragm of claim 1, wherein the diaphragm is configured to behave as a rigid piston up to 1,000 Hz.

3. The loudspeaker diaphragm of claim 1, wherein the band of frequencies of the range of operation comprises 200 Hz to 5,000 Hz.

4. The loudspeaker diaphragm of claim 3, wherein the band of frequencies of the range of operation comprises 20 Hz to 6,000 Hz.

5. The loudspeaker diaphragm of claim 1, wherein the frequency response curve is substantially flat over a 200 Hz to 6,000 Hz range.

6. The loudspeaker diaphragm of claim 1, wherein the loudspeaker diaphragm has a stiffness corresponding to a first frequency at which break-up behaviour first occurs.

7. The loudspeaker diaphragm of claim 6, wherein the stiffness corresponds to Young's modulus of 20-140 GPa.

8. The loudspeaker diaphragm of claim 1, wherein the woven fibre body comprises a metal-coated non-metallic fibre material.

9. The loudspeaker diaphragm of claim 1, wherein the mass of the layer of damping material is more than the mass of the woven fibre body.

10. The loudspeaker diaphragm of claim 1, wherein the damping material has a mechanical loss factor of at least 0.25 at a frequency between 1,000 Hz and 8,000 Hz.

11. The loudspeaker diaphragm according to claim 10, wherein the damping material comprises a synthetic resin elastomeric material.

12. A loudspeaker diaphragm for a midrange drive unit for a hi-fi loudspeaker, wherein the diaphragm has a forward-facing sound-radiating surface and a rearward-facing surface, and wherein the diaphragm comprises: a woven fibre body comprising metal-coated glass fibre material; and damping material disposed on the rearward-facing surface of the diaphragm, wherein the damping material has a mechanical loss factor of at least 0.25 at a frequency between 1,000 Hz and 8,000 Hz, wherein a stiffness of the glass fibre material of the diaphragm results in a cone structure that is compliant that it behaves as a rigid piston at frequencies less than 1,000 Hz, and break-up modes, which occur at frequencies extending up to 6,000 Hz, are damped by the damping material.

13. The loudspeaker diaphragm of claim 12, wherein the stiffness corresponds to Young's modulus of 20-140 GPa.

14. The loudspeaker diaphragm of claim 12, wherein a mass of a layer of the damping material is between 100 and 500 g/m.sup.2 and the mass of the layer of damping material is more than a mass of the woven fibre body.

15. The loudspeaker diaphragm according to claim 12, wherein the damping material comprises polyvinyl-acetate (PVA).

16. A loudspeaker diaphragm configured to have a range of operation over a band of frequencies that makes the diaphragm suitable for use in a midrange drive unit for a hi-fi loudspeaker, comprising: a woven fibre body; and a damping material, wherein the woven fibre body is configured to act as a skeleton structure that supports the damping material, wherein the diaphragm has a stiffness such that without the damping material the diaphragm would not exhibit break-up behaviour at frequencies below 500 Hz, but would exhibit break-up behaviour at certain frequencies above 500 Hz but still within the band of frequencies of the range of operation, and wherein the damping material is configured to damp the break-up behaviour at the certain frequencies; and the damping material has a mechanical loss factor of at least 0.25 at a frequency between 1,000 Hz and 8,000 Hz.

17. A loudspeaker drive unit comprising a loudspeaker diaphragm according to claim 1.

18. A loudspeaker drive unit comprising a loudspeaker diaphragm according to claim 12.

19. A loudspeaker drive unit comprising a loudspeaker diaphragm according to claim 16.

Description

DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

(2) FIG. 1 is a perspective view of a loudspeaker enclosure incorporating a woven fibre cone according to a first embodiment of the invention;

(3) FIG. 2 shows the directions of the fibres of the woven fibre cone of FIG. 1;

(4) FIG. 3 shows a side view of the cone of FIG. 1;

(5) FIG. 4 includes a close-up view of a portion of the woven fibre cone of FIG. 1;

(6) FIG. 5 is a cross-sectional view of the portion of the woven fibre cone shown in FIG. 4 taken across the plane represented by line A-A in FIG. 4;

(7) FIG. 6 is a close up cross-sectional view of one of the length of material of FIG. 5;

(8) FIGS. 7 and 8 show frequency response curves comparing the acoustic performance of the loudspeaker of FIG. 1 with a comparable loudspeaker of the prior art; and

(9) FIG. 9 is a flow chart illustrating a manufacturing method according to a second embodiment of the invention.

DETAILED DESCRIPTION

(10) FIG. 1 shows a hi-fi loudspeaker enclosure 2 in the form of a generally cuboidal cabinet 4. The cabinet 4 accommodates a mid-range/bass drive unit 6, and a tweeter 8. The loudspeaker is vented by means of a forward-facing port 10. The drive unit 6 comprises a cone-shaped diaphragm 12, having a generally concave shape as viewed front-on (as shown in FIG. 1). The diaphragm has a diameter of about 150 mm (a 6 inch drive unit) and operates over frequencies ranging from 20 Hz to 6 kHz. The diaphragm is formed from a woven fibre cone, as shown schematically in FIGS. 2 and 3, which show respectively the cone front-on and as a side view. Thus, there are adjacent lengths 14 of fibre running approximately parallel to each other that weave in and out of other corresponding adjacent lengths of fibre running transverse thereto, to form a woven mat. The lengths 14 of fibre material are curved and cross at different angles to each other in order to define the desired (concave) conical shape of the diaphragm. The diaphragm 12 defines a forward-facing sound-radiating surface and a rearward-facing surface which supports a damping material. FIG. 2 shows the lengthwise extent of some only of the lengths 14 of fibre, illustrating the non-linear shape that the lengths of fibres of the diaphragm 12 have.

(11) It will be seen from FIG. 3 that the generally concave shape of the cone-shaped diaphragm 12 is formed by a wall extending 360 degrees around a central axis 12a, the wall 16 having a shape which when viewed in cross-section has a gently curving convex shape. FIG. 3 also shows the forward-facing sound-radiating 22 (as visible also in FIG. 1) surface and the rearward-facing surface 24 of the diaphragm.

(12) FIG. 4 shows the cone 12 and a magnified view 18 of a section thereof. As will be seen from FIG. 4, the respective lengths 14 of fibre are woven together, with a relatively open weave such that there are spaces 20 between the adjacent general-parallel lengths 14 of fibre running in a given direction. FIG. 5 shows, highly schematically, three parallel lengths 14 of the fibre material in cross-section, the cross-section being taken along the line A-A as shown in FIG. 4. The forward-facing sound-radiating 22 surface is at top of FIG. 5 whereas the rearward-facing surface 24 is at the bottom of FIG. 5. The layer of the woven glass fibre material has a thickness Tf of about 0.2 mm to 0.3 mm. The rearward-facing surface 24 of the diaphragm supports a layer of damping material 25, which fills the spaces 20 between the woven lengths 14 of fibre. The damping material is in the form of a cured PVA polymer and has a mass of about 240 g/m.sup.2. It has an average thickness Td which is not very different from the thickness Tf of the glass fibre layer, being about 0.2 mm to 0.3 mm. The cured PVA layer 25 fills the gaps 20 between the lengths 14 of fibre material and thus acts as a sealant (the cone would be porous without it).

(13) An individual length 14 of fibre material is shown in cross-section in FIG. 6. The length of fibre material comprises a collection of individual glass fibres 26 (not shown individually in FIG. 6) arranged in parallel to form a thread 28. The woven glass fibres have an open weave with a mass density of about 120 g/m.sup.2 (when dry).

(14) The gaps 20 between the lengths 14 of fibres have a width of about 400 to 500 μm. The fibres 26 forming the thread 28 are embedded in a resin matrix 30, which on its outer surface is coated in a thin layer of Aluminium 32 which in turn is protected by a layer of lacquer 34. The amount of resin used per unit area is by itself less than ideally required to provide the preferred amount of stiffness in the glass fibre layer. The layer of lacquer 34 however contributes to the stiffness of the woven fibre material and has a mass that whilst lower per unit area than the resin is still of the same order of magnitude. The mass per unit area of the resin and lacquer together will typically be in the range of 20 to 60 g/m.sup.2 depending on the particular application. (The woven glass fibres including the resin and lacquer thus have a mass density of the order of about 160 g/m.sup.2±20 g/m.sup.2). The layer 32 of Aluminium is about 0.1 μm thick and therefore has a mass that is negligible compared to the mass of the other component materials of the diaphragm. The presence of the layer 32 of Aluminium provides opacity without which the PVA layer 25 behind and/or the resin matrix 30 around the glass fibre threads could be exposed to more light and/or be more visible than would be desirable. The Aluminium layer 32 has a silver appearance and provides a shiny highly reflective outer surface to the threads. With the weave of the threads, incoming light is reflected in various different directions, giving the diaphragm a sparkly or twinkly appearance. The warp and weft catch the light in different ways, which also contributes to the visually striking appearance. Furthermore it may be that a slight shift in viewing angle has a noticeable effect on the way in which light is reflected, which also results in the diaphragm having unusual optical properties and appearance for a speaker diaphragm particularly when viewed with two eyes and/or with slight relative movement between viewer and diaphragm.

(15) The amount of PVA damping material used in the embodiments described herein provides improved performance of the diaphragm in relation to mechanical resonances (also described as break-ups). Dealing appropriately with mechanical resonances is very important to the performance of the loudspeaker diaphragm. For lower frequency units, operating at frequencies up to about 500 Hz, one can design a cone with mechanical resonance out of band by selecting the correct shape and material. The material specific modulus (Young's modulus divided by density) is a good metric to quantify the stiffness of a structure. By choosing a high specific modulus material (like aluminium or carbon fibre), the cone break-ups are pushed well above 500 Hz and the unit therefore behaves only in a piston-like manner. In the case of midrange or bass-midrange drive units, the problem is not so easily dealt with, as these units have to cover a wide range of frequencies, from 20 Hz to 6 kHz for example, which makes it more difficult to design a cone which does not exhibit break-ups in this (wide) band. The non-isotropic nature and other mechanical properties of Kevlar weave of the prior art diaphragms have been used to reduce the problems associated with break-up modes in the frequency range of operation.

(16) FIG. 7 shows a frequency response curve 50 as a graph of the sound pressure level (along the y-axis) measured by a microphone position along the axis of the diaphragm of the first embodiment at a distance of 1 meter from the plane of the outer diameter of the diaphragm, with increasing frequency of sinusoidal input signal (along the x-axis). To allow comparison, a corresponding frequency response curve 52 is also shown on the graph for a loudspeaker using B&W's Kevlar cone of an equivalent diameter, the loudspeaker otherwise being identical in all respects. A portion 54 of the graph of FIG. 7 is shown the enlarged view of FIG. 8. It will be seen from FIGS. 7 and 8 that whilst the frequency response curve 52 of B&W's Kevlar cone is relatively flat, over the 200 Hz to 6 kHz range, there is room for further improvement. PVA-based damping material is used already in the (prior art) Kevlar diaphragm to provide damping, but the present embodiment proposes a much higher amount, and in conjunction with a glass-fibre woven cone rather than one made from Kevlar. Perhaps surprisingly, the use of glass fibre instead of Kevlar fibre, when coupled with use of much greater amounts of PVA material, is able to yield better results. Thus, it will be seen that the frequency response of the diaphragm of the first embodiment (see curve 50 in FIG. 8) compares favourably with the frequency response of the Kevlar diaphragm (see curve 52 in FIG. 8). The frequency response of the Kevlar diaphragm has two peaks 56 at around 3.5 kHz and 5 kHz, whereas the frequency response of the diaphragm of the first embodiment is flatter at such frequencies. It will also be seen from FIG. 7 that the frequency response of the diaphragm of the first embodiment (see curve 50 in FIG. 8) is as flat as the frequency response of the Kevlar diaphragm at lower frequencies (see curve 52 in FIG. 8).

(17) The type of highly damped polymer material to be used, such as PVA material, may exhibit a high mechanical loss factor (above 0.5) in the frequency bands of interest (in the above-described first embodiment at around 3.5 kHz and at around 5 kHz). The mechanical loss factor can be measured by means of a DMTA (dynamic mechanical thermal analysis) test. Such a test is conveniently conducted at 25 degrees Celsius.

(18) FIG. 9 shows a flow-chart illustrating the method according to a second embodiment of the invention. Thus, as a first step 162 a woven disc-shaped glass fibre mat is provided, in which lengths 114 of bundles of aligned glass fibres are woven to form the fibre material mat. As the next step 164, this fibre material is then coated with resin, so that the fibres are coated with (and partially pre-impregnated with) with an uncured resin 130 (thus forming a “pre-preg” mat). The resin-coated mat is then heat-treated in a vacuum-forming mould apparatus, using a mould that causes the shape of the resulting resin-infused glass fibre mat to take on the cone-shape required of the diaphragm. Gaps 120 remain between the lengths of the resin-infused bundles of glass fibres, in the product once the resin is cured. During the next step (box 166 in FIG. 9), a metal-vapour deposition system is then used to apply an Aluminium coating 132 to the lengths of fibres. The metal coating then has a lacquer 134 applied, using a lacquer spraying system (step 168). A thick layer of PVA material 125 is then applied to the rear surface of the cone of material using a cone-spinning application system, which is described in further detail below (step 170). The cone is then trimmed, and integrated into a loudspeaker drive unit in a manner that is conventional in the art.

(19) The result of the cone-spinning PVA application step 170 is the deposition of a large amount of PVA in liquid form (PVA held in a water-based emulsion) on the back of an inverted cone, using the centrifugal force to spread the liquid over the cone surface. This is achieved as follows. A continuous bead of liquid (PVA) is extruded and deposited in a spiral path on the rear surface of the cone of material, which is rotating at a slow speed (less than 100 revolutions per minute). An air flow is used to disperse the liquid onto the cone surface creating a continuous unbroken coverage of liquid on the cone. The air flow used also urges the PVA into the gaps in the weave of the woven fibre material. The cone is then spun at high speed in a two stage process as follows. The 1st phase of the spin is to try and smooth out the PVA across the cone prior to the 2nd phase. The 1st phase of spinning aim to remove any islands of non PVA, in order for the 2nd phase to spin properly. The speed of rotation of the 1st phase is about 150 rpm and lasts for approximately 5 seconds. The 2nd phase of the spin is at 750 rpm for about 5 seconds (but might need to be longer in duration for larger diameter cones). These high speed rotation stages have the surprising effect of smoothing out the PVA over the surface of the cone and providing a clean finish with a relatively constant thickness of PVA across the whole area of the cone. The PVA is then promptly cured at about 65 degrees Centigrade to dry the liquid such that it can be handled and to reduce the risk of the PVA flowing and losing its shape. A relatively low air temperature) (<100 C.°) is used to cure the PVA so as to reduce the risk of the water in the emulsion from boiling. In the present embodiment, the PVA polymer used has a loss factor of over 0.5 at 5 kHz at 25 degrees Celsius. The PVA layer is deposited so that it forms ⅔ (two thirds) of the total mass of the cone. Having a cone in which the PVA layer forms significantly more than half the mass of the cone provides a particularly beneficial level of damping, as mentioned above. The PVA layer acts like a free-layer damping system but also acts to seal the diaphragm (the cone would be porous without it).

(20) Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

(21) It is stated above that having a cone in which the PVA layer forms significantly more than half the mass of the cone provides a particularly beneficial level of damping. It will be understood that the PVA layer forming 62.5% or more of the mass of the cone would be judged as significantly more than half the mass of the cone provides a particularly beneficial.

(22) The constant thickness of the PVA coating is not necessary. Indeed there may be advantages in providing a PVA coating having a varying thickness.

(23) Materials other than PVA, such as other synthetic resin elastomeric materials having high mechanical loss, may be used provided they yield appropriately high losses at relevant frequencies. Materials having a high viscosity and high hysteresis may be suitable alternatives. The vinyl resin-based thermoplastic material sold as Cone Edge Dampener E-5525 by the Barrett Varnish Co may be a suitable alternative. Another potential candidate is PVB (Polyvinyl Butyl) which is also available as an emulsion and exhibits good damping properties.

(24) Rather than using the PVA application method that utilises a spinning cone, polymer could be applied by brushing, sponging, or otherwise adding, successive layers of polymer. Many layers may be required to achieve the required thickness.

(25) The term “woven material” (for example in the context of “woven fibre material”) is used herein to include any material which is formed from threads or lengths of material which are woven, knitted, or otherwise arranged in an interlinking fashion to form a fabric having a mesh-like structure with spaces between, the threads (or lengths of material) forming the main sub-structure of the material. Whilst in the described embodiments, the material used is in the form of a woven glass-fibre fabric, other woven or knitted materials may be used. For example, embodiments of the invention may have application wherein the fibre material is made from an aramid (aromatic polyamide) fibre or similar materials, such as Kevlar, for example.

(26) The resin with which the woven fibre material is impregnated (that resin used as the stiffening material) may be a synthetic resin, for example, a phenolic, epoxy or melamine resin. However, any other flexible heat-resistant thermo-setting resin or high-temperature thermo-plastic resin material may be used. In some embodiments, the mass per unit are of the resin may be between 15 to 40 g/m.sup.2 or 2.5 to 40% of the mass of the damping material.

(27) Before describing additional variations on the loudspeaker diaphragm, a discussion of the design constraints is provided. Electrodynamic transducers, such as the loudspeaker diaphragm, rely on the concept of rigid piston to convert an electrical signal into an acoustic pressure, partially because the acoustic radiation of a vibrating piston can be described using analytical equations. However, because a practical implementation of this concept typically uses materials with finite stiffness, mechanical resonances naturally occur in the assembly (which are sometimes referred to as ‘break-up modes’).

(28) At the mechanical resonance frequencies, the acceleration of the loudspeaker diaphragm is not uniform across the cone surface (i.e., the points on the cone surface are no longer all moving in phase). Instead, the cone surface may have nodes and antinodes, such as in the case of the modes of vibrations of a circular membrane. Therefore, these mechanical resonances create peaks and dips in the acoustic responses, both on-axis and off-axis (so the transducer power response is also affected). Moreover, because most materials have very little inherent mechanical damping, the pressure magnitude is often high at the break-up frequencies.

(29) Typically, a transducer may be designed to move or put the mechanical resonances out of band (such as below 100 Hz and above 10 kHz) by making the assembly very stiff, in the hope that the high-Q mechanical resonance(s) will not be audible.

(30) The loudspeaker diaphragm (which is sometimes referred to as a ‘continuum cone’) may address these challenges by using a cone structure is unusually compliant. From a mechanical point of view, this means that the loudspeaker diaphragm may only behave as a rigid piston at relatively low frequency (e.g., up to about 1 kHz for 5 or 6 inch diameter cones). Above this frequency range, the loudspeaker diaphragm may exhibit break-up modes that are controlled by adding mechanical damping (as quantified by the structure loss factor) to the structure or assembly. For example, in some embodiments the base structure in the loudspeaker diaphragm may be made of a low stiffness (such as a Yong's modulus of 20-140 GPa), open glass fibre weave. Moreover, the loudspeaker diaphragm may include a thick layer of high-damping material (such as a polymer, e.g., PVA) applied to the glass weave. Furthermore, the material mass ratio in the loudspeaker diaphragm may be 20-40% glass and 60-80% PVA. In some embodiments, the material to mass ratio is around 33% glass and 66% PVA.

(31) The compliance of the base glass structure may enhance the performance of the loudspeaker diaphragm because a compliant structure may be easier to damp than a stiff one. For example, for a sandwich material, as is the case in several embodiments, the composite loss factor may be a function of the mechanical modulus ratio of both of the layers. In other words, for a given damping layer, the composite loss factor may be reduced as the base layer stiffness is increased.

(32) Moreover, the stiffness of the composite structure in the loudspeaker diaphragm may be chosen to minimise the radiation of the break-up modes. This may be related to the relationship between the mechanical wavelength in the material (which, in turn, may be related to the composite modulus and density) and the acoustic wavelength at the same frequency. The composite modulus and density may be chosen to minimise the resonance radiation and the added damping may further dampen the amplitude.

(33) In variations on the loudspeaker diaphragm, a variety of materials may be used for the base material, including: EO823 glass weave, glass, Kevlar, quartz fibre, a woven carbon fibre composite, etc.

(34) Moreover, a variety of materials may be used for the high-damping material, including a PVA composite based on glass microspheres (GMS). This PVA-GMS composite may include PVA heavily loaded with hollow GMS (e.g., a volume ratio of 35-55% PVA to 45-65% GMS, such as 45% PVA to 55% GMS) to form a tight network of GMS connected together with PVA. The GMS may have an average diameter between 20-60 μm, such as 40 μm. Furthermore, the PVA-GMS composite may have: a composite modulus similar to the low stiffness open glass fibre weave impregnated with PVA; and a low volume density of 0.6-0.8, e.g., 0.7 (which corresponds to a sphere packing density around 60%). Note that the PVA-GMS composite may have a topology leading to a mix of shear and tensile strain for the PVA during deformation (because the PVA is deforming between the stiff spheres). This may lead to higher levels of damping than designs in which tensile deformation dominates. Additionally, note that the PVA-GMS composite may be fabricated by mixing the PVA emulsion with the GMS, and then brushing or spraying the mix on a substrate. In some embodiments, the thickness of the PVA-GMS composite is between 0.2-0.4 mm. The PVA-GMS composite may have very low harmonic distortion (such as a third harmonic distortion of less than −50 dB, e.g., −60 dB or 0.1%) because the damping may reduce strain amplitudes and, this, nonlinearities, and may facilitate a reduced cone mass.

(35) In some embodiments, the PVA-GMS composite may use a wider range or distribution of microsphere diameters in order to increase the packing ratio. This may further decrease the density. Moreover, the microspheres may include a stiff material, such as ceramic or diamond. For example, the microspheres may include: silicon chromium, diamond SP3, aluminium oxide (Al.sub.2O.sub.3), boron carbide (B.sub.4C), etc. This may increase the microsphere to PVA stiffness ratio, which may result in more strain concentration in the PVA and, thus, in more damping. Furthermore, the microsphere surface may be chemically functionalised to improve the PVA-to-microsphere interface (and, thus, to improve the cone strength).

(36) Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.