Diaphragms for loudspeaker drive units

Abstract

A diaphragm for a loudspeaker drive unit or for a microphone includes a rigid dome-shaped member having a thickness that varies from a first thicker thickness at a first location at the periphery of the dome-shaped member to a second thinner thickness at a second location, which is nearer to the center of the dome-shaped member. There is a step-wise change in thickness at a location between the first location and the second location. Having greater thickness at the periphery of the dome-shaped member may improve stiffness of the diaphragm and may allow for an increased break-up frequency. Having thinner material elsewhere in the dome-shaped member may allow the mass of the diaphragm to be kept low and may result in better acoustic sensitivity.

Claims

1. A diaphragm for a tweeter loudspeaker drive unit, the diaphragm comprising a rigid dome-shaped member comprising a first, ring-shaped part and a second, dome-shaped part, the first and second parts being joined to each other by an adhesive layer which, together with the first and second parts, form a constrained layer damping system, there being a step-wise change in thickness of the dome-shaped member in the region of the innermost boundary of the first part such that the dome-shaped member has a thickness that varies from a first thickness at a first location at a periphery of the dome-shaped member and located outwardly away from the step-wise change in thickness to a second thickness at a second location, which is located inwardly of the step-wise change in thickness and thus nearer to the center of the dome-shaped member than the first location, wherein the first thickness is more than twice the second thickness, and the mechanical loss factor at 35 KHz of the adhesive is at least 0.5, and the step-wise change in thickness, when viewed in cross-section across a center of the diaphragm, is centered at a location which is between 5% and 25% of a distance measured from the periphery of the dome-shaped member along an external surface of the dome-shaped member to the center of the diaphragm.

2. A diaphragm according to claim 1, wherein the step-wise change in thickness, when viewed in cross-section across the center of the diaphragm, is localised within a distance of 1% of the width of the diaphragm.

3. A diaphragm according to claim 1, wherein the maximum thickness of the dome-shaped member is less than 0.1 mm and the minimum thickness of the dome-shaped member is less than 50 m.

4. A diaphragm according to claim 1, wherein at least 90%, by area, of the thickness of the dome-shaped member has a thickness that is substantially the same as one of five fixed thicknesses.

5. A diaphragm according to claim 1, wherein the first and second parts are made from different materials.

6. A diaphragm according to claim 1, wherein the first and second parts are made from the same material.

7. A diaphragm according to claim 1, wherein the adhesive layer has a thickness of at least 10 m.

8. A method of manufacturing a diaphragm for a tweeter loudspeaker drive unit, wherein the method comprises the steps of: providing a first part in the shape of a truncated dome-shaped member providing a second, dome-shaped, part, joining the first part and the second part to each other with an adhesive layer formed by an adhesive with a mechanical loss factor of at least 0.5 at 35 KHz, the first part, the second part and the adhesive layer forming a rigid dome-shaped member having a constrained layer damping system, the rigid dome-shaped member having a peripheral region of greater thickness than a central region, and wherein, a change in thickness between the peripheral region and the central region is step-wise when viewed in cross-section across a center of the diaphragm, and is centered at a location which is between 5% and 25% of a distance measured from a periphery of the rigid dome-shaped member along an external surface of the rigid dome-shaped member to the center of the diaphragm.

9. A tweeter loudspeaker drive unit including a diaphragm as claimed in claim 1.

10. A tweeter loudspeaker drive unit comprising: a mounting; a diaphragm as claimed in claim 1, the diaphragm being mounted for movement relative to the mounting; and a voice coil and magnet assembly arranged to cause movement of the diaphragm relative to the mounting in response to an electronic signal.

11. A loudspeaker enclosure including a tweeter loudspeaker drive unit according to claim 10.

12. A diaphragm according to claim 1, wherein the rigid dome-shaped member has a diameter and the step-wise change in thickness has a diameter which is between 85% and 95% of the diameter of the dome-shaped member.

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 cross-section through a loudspeaker drive unit including a diaphragm in accordance with a first embodiment of the invention mounted in an enclosure of known form;

(3) FIG. 2 is a perspective view of the diaphragm of FIG. 1;

(4) FIG. 3 is a front elevation view of the diaphragm of the first embodiment of the invention;

(5) FIG. 4 is a sectional view of the diaphragm of the first embodiment of the invention;

(6) FIG. 5 is an enlarged cross-sectional view of a portion of the diaphragm of the first embodiment of the invention corresponding to a portion of FIG. 4; and

(7) FIG. 6 is a graph of break-up frequency plotted against skirt depth.

DETAILED DESCRIPTION

(8) FIG. 1 shows schematically part of a tweeter loudspeaker drive unit 10 for mounting in an enclosure (not shown). The loudspeaker drive unit 10 may have its rear connected to a rearwardly-projecting sound absorbing tube system (not shown). A grill (not shown) may also be provided at the front of the enclosure.

(9) The loudspeaker drive unit 10 comprises a mounting block 12, a dome-shaped diaphragm 14, and a flexible surround 16 connecting the diaphragm to the mounting 12. A voice coil former 17, on which a voice coil 18 is mounted, is attached to the diaphragm 14 (as shown schematically in FIG. 1). A magnet assembly 20 surrounds the voice coil 18. The general configuration and mounting of the parts that form the tweeter loudspeaker drive unit are known and will not be described further.

(10) The present invention concerns the dome-shaped diaphragm 14. A dome-shaped diaphragm 14 according to a first embodiment of the present invention is shown in perspective in FIG. 2. FIG. 3 shows a front elevation of the same dome-shaped diaphragm 14. FIG. 4 shows the diaphragm 14 in cross-section, taken about the section A-A.

(11) The diaphragm 14 has two distinct regions: a first, peripheral ring-shaped, region 30 (in the shape of a truncated dome) having a first substantially constant thickness of about 90-100 m and a central domed region 40 having a second substantially constant thickness of 30 m (i.e. less than half the thickness of the first region 30). There is a step-wise change in thickness at the ring-shaped boundary 50 between the first peripheral ring-shaped region 30 and the second central domed region 40 (the ring-shaped boundary 50 being the location at which there is the transition between the first thickness and the second thickness). The ring-shaped boundary 50 has a diameter of 23.8 mm as compared to the outer diameter of the diaphragm 14 of 26.4 mm.

(12) FIG. 5 shows in further detail, and as an enlarged view, the portion of the cross-section of FIG. 4 indicated with the circle B. As can be seen from FIG. 5, the two regions 30, 40 of different thickness are formed by two separate parts each of substantially constant thickness. Thus, there is a first part 32 which has the general shape of a ring (or more precisely a truncated dome-shaped member) which is glued to a second, dome-shaped, part 42. The first part 32 is formed by punching a hole out from a circular disc and then forming the part on a suitably shaped forming member (using a punch and die-type arrangement). The second part 42 is similarly made from a circular disc formed on a suitably shaped forming member.

(13) The adhesive used to glue the two parts 32, 42 together is selected to improve acoustic performance. The structure of the diaphragm of the present embodiment is such that it is desirable to reduce resonances at around 36 KHz. An adhesive that provides good damping effects at this frequency is therefore chosen. The damping properties of the adhesive polymer can be defined by the mechanical loss factor, which can be measured by means of a DMTA (dynamic mechanical thermal analysis) test. In the present embodiment, the adhesive is a PVAc glue, namely that sold under the name Cascorez A452, which has a loss factor of about 0.6 at 35 KHz at 25 degrees Celsius. The loss factor of a polymer (and also its Young's modulus) is frequency and temperature dependent, so it is important to measure this property at the frequency and temperature at which the damping effects of the polymer are beneficial (around 36 KHz and room temperature in this case). It will be understood that when choosing a particular adhesive consideration should be given to achieving a relatively high loss factor around the first break-up frequency of the tweeter structure, at the normal operating temperature.

(14) Before the adhesive is applied, the dome part 42 is held in place upside down and a bead of glue is evenly applied to the periphery of the dome part 42 with a glue dispensing machine. In this embodiment about 8 mg of adhesive is applied. The ring-shaped part 32 is brought into contact with the adhesive on the dome part 42, and the parts 32, 42 are gently urged together, until some of the adhesive is squeezed out (indicating that both parts are sufficiently in contact for an effective joint to be made). The excess adhesive is then wiped away. The layer of adhesive between the two parts has a thickness of about 20 m. The adhesive layer provides enhanced mechanical damping of the diaphragm structure by effectively creating a constrained-layer damping system. This enables a decrease in the mechanical Q of some of the resonances.

(15) The first part has a flange 34 and the second part has a corresponding flange 44. Both parts are formed from Aluminium. The two flanges 34, 44 are glued to each other, as a result of the above-mentioned gluing process, and provide a surface that facilitates mounting of the diaphragm relative to a mounting block via a suspension mounting. The same surface may also facilitate connection to a voice-coil assembly.

(16) The first part 32 has a thickness of 50 m and a mass of 35 mg and the second part 42 has a thickness of 30 m and a mass of 54 mg, resulting in a total diaphragm mass of about 90 mg (excepting the mass of the adhesive). A conventional design of diaphragm of the same shape, size and material might have a uniform thickness of 50 m and therefore roughly the same mass. By having a thicker, and therefore stiffer, peripheral region and a thinner central dome region, the mass of the diaphragm may be kept low whilst improving stiffness in the region where stiffness is most beneficial. As a result sensitivity may be maintained whilst improving (increasing) the break-up frequency. There is a steep slope 36 on the innermost diameter of the first part 32 which means that the change in thickness, as measured with increasing distance along the external surface of the diaphragm from the periphery to the centre, from the first region 30 to the second region 40 occurs within much less than 1% of the diameter of the diaphragm.

(17) FIG. 6 shows the acoustic response of a tweeter loudspeaker drive unit in which the diaphragm of the first embodiment of the invention is installed as compared to a tweeter loudspeaker drive unit in which a control diaphragm is installed. The control diaphragm is also made from aluminium, but is of one piece construction with a constant thickness of 50 m and a mass of 90 mg. It has a shape and form otherwise very similar to the diaphragm of the first embodiment of the invention. The acoustic response graph shown in FIG. 6 shows the on-axis acoustic response, at 1 meter with a 2.84V RMs input voltage driving the speaker, in each case, with the y-axis of the graph showing the acoustic response as measured and the x-axis the frequency of the input drive signal. The response exhibited by the control diaphragm is shown in the light grey line 60 and the response exhibited by the diaphragm of the first embodiment of the invention is shown in the black line 62.

(18) To a first approximation, one can consider the frequency response of a tweeter to be relatively flat until the first break-up frequency, that is, the frequency at which the tweeter stops moving as a rigid piston, that is, with all points on the surface moving with the same phase. At the break-up frequency, a peak occurs in the frequency response and the peak can be large for materials with low damping (which usually happen also to be desirable, stiff materials). Beyond the first break-up frequency a series of peaks and dips are apparent in the frequency response. Though resonance peaks in the frequency response in stiff, low damped materials are usually of high Q and are centred on a well defined frequency, the leading edge of the resonance can reach down by two or more octaves below the resonant peak. Thus, for instance, a break-up frequency occurring at 30 kHz, can result in performance degradation at 7.5 kHz and below. For this reason it is desirable to have break-up frequencies as high as possible. A second reason for having the first break-up frequency as high as possible, and thus a flat response to as high a frequency as possible, arises from the advent of audio formats with bandwidths beyond the 22 kHz of the ordinary compact disc, effectively up to 192 kHz. If large peaks occur in the frequency response, the inherent non-linearity of the tweeter (arising from primarily the motor system and suspension) will be greatly increased, owing to the relatively high voice-coil displacement, and thus signals with more than one frequency component will provoke inter-modulation distortion, which will result in spurious signals at many frequencies, including the directly audible, sub 20 kHz range.

(19) The commonly accepted upper frequency limit for human hearing is approximately 20 kHz but it is desirable that tweeter drive units have a frequency response that extends, and is relatively smooth and flat, well beyond this limit.

(20) As can be seen from FIG. 6, the main dome breakup can be identified as the first peak in the response: 29 kHz for the control diaphragm and 37 kHz for the diaphragm of the first embodiment. It will be observed that the two responses overlay at lower frequencies, showing that the sensitivity of the two designs, both having the same mass, are substantially the same as each other at lower frequencies. Thus the diaphragm of the embodiment has an improved acoustic response with a higher breakup frequency than a diaphragm of same mass not incorporating the features of the diaphragm of the first embodiment. It is also believed that the adhesive layer provides improved mechanical damping properties, which results in the overall level being lower above 50 kHz for this design, as compared to the tweeter unit in which the control diaphragm is installed, as shown on the acoustic plot of FIG. 6.

(21) In a second embodiment, not separately illustrated, the first part is made from Aluminium and the second part is made from Magnesium. The diaphragm is otherwise substantially identical to that of the first embodiment.

(22) In a third embodiment, the diaphragm is of one-piece construction and formed by means of etching away an inner circular region from a circular disc of Aluminium of 75 m thickness, the inner circular region having a diameter of about 90% that of the diameter of the disc. About 50 m is etched away leaving an inner circular region having a thickness of about 25 m. The etching could be via laser etching, or chemical etching (for example by means of a suitable acid). The disc is then formed into the desired shape by means of forming the disc on a suitably shaped forming member. The diaphragm is then coated with a synthetic diamond layer to provide enhanced stiffness. The diaphragm is otherwise substantially identical to that of the first embodiment.

(23) 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.

(24) Different thicknesses and dimensions of dome-shaped member could be utilised. There may be more than one step-wise change in thickness. There may be more than two regions of different thicknesses. Different materials for the diaphragm may be used. A different glue may be used to join the two parts of the diaphragm together when the diaphragm is made by gluing a ring-shaped member to the periphery of a dome-shaped member. One such example glue is Loctite's Instant CA 382 (a Cyanoacrylate adhesive). Manufacturing methods other than those described could be utilised to produce a diaphragm having the advantages and benefits of the diaphragm of the first embodiment. A microphone could readily be made using a diaphragm as illustrated herein.

(25) 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.