HK1170881B - Cone loudspeaker - Google Patents

Description

Conical loudspeaker

Technical Field

The present invention relates to a loudspeaker having an acoustically radiating diaphragm comprising a substantially frusto-conical membrane. The diaphragm is commonly referred to as the loudspeaker "cone".

Background

The tapered geometry is inherently robust because the axisymmetric external forces applied thereto exhibit tensile stresses in the material. Advantageously, this allows very thin film materials to be successfully used.

There is an increasing demand in the competitive market for improved performance of cone loudspeakers. Fig. 1(a) shows a view of the cone of a cone loudspeaker, and fig. 1(b) shows the pressure response when propagating into a 2pi steradian infinite acoustic region using a 93mm diameter mouth end neck drive in the prior art. The pressure is plotted at 46 positions 1m from the loudspeaker and in 2 degree angle increments. It can be seen from fig. 1(b) that above about 1.5kHz, the pressure response becomes irregular and resonance phenomena occur as the taper is driven beyond its rigid limit and exhibits non-rigid behavior. The non-rigid characteristic is undesirable because it leads to non-uniformities in both the pressure and directional responses of the microphone.

It has long been known that the rigid bandwidth of a loudspeaker diaphragm can be extended by driving the diaphragm at a node of the first mode of vibration ("node drive"). Nodal driving is disclosed in JP57068993 which shows a planar diaphragm being driven at a node of a first mode of vibration, which for a circular diaphragm is a circle around the diaphragm. This method, although already known, has not been applied to cone loudspeakers. The geometry of the cone naturally places its first vibration mode node towards its mouth end, which necessitates the use of a large voice coil. The use of large diameter coils negatively impacts efficiency and increases the cost of the associated magnet system and coil assembly, which significantly limits the utility of nodal drives. To date, it has been common practice in the art to drive the taper from its neck.

GB308,318 discloses a loudspeaker having a frusto-conical diaphragm driven at concentrically spaced locations both in the neck of the diaphragm and outside the diaphragm. The purpose of this is to deliver a high frequency signal to the inner (neck) drive and a low frequency signal to the outer drive, which is then placed at the node for the high frequency signal. Thus, it does not actually suggest a nodal drive, since the drive is not located at the node of the diaphragm for the signal provided by the drive in question. In practice, the diaphragm is also not driven at the node of the first vibration mode. The node at which it is driven is the node of the higher mode corresponding to the higher frequency drive. Furthermore, there is no disclosure of the stiffening of the diaphragm, whereby the external drive is spaced by a considerable diameter, causing the problems described above.

US5,323,469 discloses a loudspeaker having a conical diaphragm driven by a rearwardly extending voice coil former attached to the diaphragm throat. Additional stability is provided to the diaphragm in the form of a second cone extending radially outward from the shelf at the rear side of the diaphragm and attached to the diaphragm at a first node. Thus, the interface region through which the diaphragm is driven by the voice coil former extends between and includes both the throat and the nodal point, and the diaphragm is therefore not driven by the nodal point. The additional stability also unnecessarily extends the thickness of the loudspeaker, provides little significant stiffening of the diaphragm away from the nodal point, and is not adapted to allow modification of the stiffness characteristics of the diaphragm.

Disclosure of Invention

According to a first aspect, the invention may provide a loudspeaker comprising:

a sound radiating diaphragm forming part of a moving diaphragm assembly and comprising: a generally frustoconical membrane having a narrow neck end and a wide mouth end; a reinforcing structure for reinforcing a joint area of the radiation thin film and the diaphragm adapted to be driven; and

a transducer comprising a voice coil mounted to drive a diaphragm through a bonding region thereof;

wherein the junction region is located at a node of a first vibration mode of the moving diaphragm assembly.

By suitably arranging the stiffening structure, the node position of the first mode of vibration can be moved up the membrane closer to its neck end (relative to a similar unsupported/unreinforced structure) to allow the diaphragm to be node driven using a transducer having a voice coil with a smaller diameter. Whereby the throat end of the diaphragm may not be driven or free to rock. In fact, the neck or throat of the diaphragm is always the anti-nodal point and thus cannot be a possible location for nodal actuation as described in the present invention.

Preferably, the reinforcing structure provides a reinforcing effect sufficient to govern the vibration characteristics of the diaphragm. The reinforcing structure may be arranged in such a manner that a node of the first vibration mode of the diaphragm is located at a predetermined position. Since the nodal position is influenced by these elements, the moving diaphragm assembly includes elements such as cones (and stiffening elements), shelves, coils and (to some extent) components such as surrounds or suspensions which have a minor effect on nodal position.

Preferably, the predetermined location of the node of the first mode and hence the location of the land area is designed to provide compatibility with transducers having voice coils of standard diameter. By using standard components in this way, the loudspeaker according to the invention can be manufactured cost-effectively.

Preferably, the diaphragm includes a connection protrusion at the bonding region, through which the diaphragm is connected to the transducer.

Preferably, the transducer includes a frame on which a voice coil is mounted, the frame being connected to the connection protrusion to drive the diaphragm. Alternative combinations may be cylindrical or other suitable shapes, but the use of protrusions is preferred as this allows the volume of the moving structure to be reduced whilst venting the air chamber inside the voice coil.

Preferably, the reinforcing structure comprises ribs. In one embodiment, the stiffening member comprises a plurality of longitudinal ribs, each longitudinal rib extending between the neck end and the mouth end of the radiation film, and wherein each longitudinal rib is thinner in thickness in a direction towards the neck end and/or the mouth end. Thinning the ribs in this manner reduces the mass of the ends of the radiating film. The stiffening member may comprise a circumferential rib at the neck and/or mouth end of the radiation film. The circumferential end ribs help prevent a chime mode.

In a preferred embodiment, the diaphragm is formed as part of a composite loudspeaker; in one embodiment, the loudspeaker further comprises a hemispherical diaphragm mounted at the neck end of the membrane, such that in use the membrane acts as a waveguide for acoustic radiation emitted by the hemispherical diaphragm.

The moving diaphragm assembly will typically also include an air seal, a shelf and a voice coil at the tapered neck and mouth ends. In fact, we have found that the best results can be obtained by modeling not only the cone alone, but also any air seals located on the inner or outer edges of the cone, as they can have an effect on the location of the node positions. Finally, the shelves and the voice coil itself may also be included in the calculation of the position of the node of the first resonance mode when the end result is approached.

According to a second aspect, the invention may comprise a loudspeaker diaphragm for sound radiation comprising a generally frustoconical membrane having a narrow neck end and a wide mouth end; a reinforcing structure for reinforcing a joint area of the radiation thin film and the diaphragm adapted to be driven; wherein the reinforcing structure is arranged such that a node of the first vibration mode of the diaphragm is located substantially coincident with the bonding region.

Preferably, the diaphragm includes a connection protrusion at the bonding region, through which the diaphragm is coupled to the transducer.

According to a third aspect, the invention may comprise a method of designing an acoustically radiating loudspeaker diaphragm comprising a substantially frusto-conical membrane, by computer modelling various arrangements of reinforcing structures applied to the membrane to locate a node of a first mode of vibration of the diaphragm at a location substantially coincident with a desired location of a junction region through which the diaphragm is intended to be coupled to a transducer.

A loudspeaker diaphragm designed according to this aspect of the invention may advantageously be node driven using a transducer with a voice coil of standard or usual diameter.

Drawings

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1(a) shows a view of a simple conical diaphragm of a loudspeaker;

FIG. 1(b) shows a simulated pressure response of the conical diaphragm of FIG. 1(a) when neck-driven;

FIG. 2 illustrates a front view of a conical diaphragm according to an embodiment of the present invention;

FIG. 3 shows a rear view of the conical diaphragm of FIG. 2;

FIG. 4 shows a cross-sectional view along the B-B axis shown in FIG. 2;

FIG. 5 shows an enlarged close-up view of detail C from FIG. 4;

FIG. 6 shows simulated pressure responses of the conical diaphragms depicted in FIGS. 2-5;

FIG. 7 shows a cross-sectional view of a composite loudspeaker comprising a diaphragm according to an embodiment of the invention; and

fig. 8 shows an enlarged close-up view of the specific mark B in fig. 8.

Detailed Description

A conical diaphragm 10 according to a preferred embodiment of the present invention is shown in fig. 2-5.

With particular reference to fig. 3, the diaphragm 10 comprises a generally frusto-conical membrane 12 having its narrow neck end, indicated at 14, its wide mouth, indicated at 16, and its central longitudinal axis/axis of rotation extending in a direction perpendicular to the axes indicated at X and Y. The diaphragm 10 further includes a plurality of ribs 20 on the rear surface of the film 12 extending longitudinally along the entire length from the neck end 14 to the mouth end 12. The ribs 20 may be said to be radial in that the hypothetical extension of the longitudinal ribs 20 converges at a single point on its central longitudinal axis. The diaphragm 10 also includes a circumferential rib 25 at the mouth end 16 of the membrane 12. The ribs 20, 25 function to enhance the rigidity of the diaphragm 10, i.e., to increase the bending resistance thereof. The diaphragm 10 also includes a plurality of projections 30 located between each pair of adjacent ribs 20, the projections 30 being shaped and positioned so that they together define a circumferential wall at a location intermediate the back of the diaphragm 12 and between the neck and mouth ends 12, 14 thereof. The purpose of the protrusions 30 is to provide a means of engaging/connecting the diaphragm to the voice coil assembly of the drive transducer as described below.

The conical vibrating piece 10 is designed by the following method. First, the appropriate dimensions of the membrane 12 are selected to meet design specifications. One or more target areas are then defined on the back side of the membrane 12, by which it is advantageous to engage/couple to the sound driving the transducer. The choice of target area is governed by the need to keep the voice coil diameter as small as possible and compatibility with standard or already available voice coil dimensions. Although no industry standard specifies such dimensions, voice coil diameters are typically or standard practice at half inch intervals, such as 12.7mm, 25.4mm, 50.8mm, 76.2mm, etc. After these parameter settings are complete, computer-aided nodal analysis of the diaphragm 10 is performed at various applied rib settings. The arrangement of the ribs is repeatedly adjusted until the node of the first vibration mode coincides with the target joining area. The adjustment of the rib arrangement can take various forms, including adjusting the number of ribs, the pattern of the ribs and the shape of the individual ribs themselves. It can be seen from figure 4 that the ribs 20 do not have a constant thickness along their length but taper towards their ends. After establishing the arrangement of the ribs required to locate the node of the first vibration mode of the diaphragm 10 at a desired position, the diaphragm 10 of this specification and the connecting protrusions disposed at the first mode/joining region are molded in one piece.

The frusto-conical membrane 12 of the diaphragm 10 is of the same size as the conventional unsupported/unreinforced diaphragm shown in figure 1 (a). However, nodal analysis shows that the unsupported diaphragm of fig. 1(a) has its node of the first mode of vibration at a position along the membrane 0.879 times its diameter (at the mouth end), whereas the node of the first mode of vibration of the diaphragm 10 occurs at a position along the membrane 0.78 times its diameter (at the mouth end). The provision of ribs 20 as applied in accordance with a preferred embodiment of the present invention will therefore be understood to serve to shift the nodes of the first mode of vibration towards the neck end of the membrane thereby allowing the use of a smaller diameter voice coil. Figure 6 shows the simulated pressure response of the diaphragm 10 which is a significant improvement over the simulated pressure response of the prior unsupported/unreinforced diaphragm shown in figure 1 (a). Further, the frequency of the second vibration mode becomes a limit of the stiffness characteristic by node-driving the diaphragm, and this is substantially improved by the use of the ribs as shown in the following table.

	Supportless cone	Ribbed cone
			First mode	2268Hz	2995Hz(+32％)
Second mode	3414Hz	6069Hz(+77％)

In the practice of the invention, the ribs are preferably of relatively strong construction providing significant stiffening, for example, the ribs are preferably at least 2mm thick. As the ribs are made more robust, they dominate the vibration characteristics of the diaphragm. Such a rib arrangement is preferred because in practice this means that the vibration characteristics of the diaphragm can be effectively tuned by merely adjusting the arrangement of the ribs individually.

Figure 7 shows that the conical diaphragm 10 forms part of a compound loudspeaker generally designated 50. The conical diaphragm 10 is used to emit low frequency acoustic radiation and also acts as a waveguide for the high frequency radiation emitted by the hemispherical diaphragm 52. The hemispherical diaphragm 52 is located just outside the neck of the diaphragm 10, behind the phase plug 53. The diaphragms 10, 52 are mounted in such a way as to show a uniform sound source to a listener. The geometry and arrangement of the hemispherical diaphragm 52 and conical diaphragm 10 is within the preferred ranges given in GB 2423908. The phase plug 53 is as described in GB 2437126.

The conical diaphragm 10 is suspended between inner and outer surrounding seals 56, 58 and is driven by a transducer 60. The transducer 60 includes a hub 62 having a main body portion 62a and a top planar portion 62b, and a magnet 64 disposed in a magnetic circuit having a gap 65 in which a voice coil assembly including a voice coil 66 mounted on a frame is disposed. The shelf 68 comprises a first portion 68a carrying the voice coil 66 and located substantially in the magnetic gap 65 and a second portion 68b extending therefrom to provide a connection to the connection protrusion 30 on the diaphragm 10. Transducer 60 operates in a conventional manner such that when a drive current is applied to coil 66, coil 66 and magnet 64 magnetically interact to produce a force that moves frame 68 and thus diaphragm 10 back and forth along the axis labeled Z in fig. 7.

A diaphragm designed according to the above method offers the further advantage that the ribs can be arranged in such a way as to compensate for other effects in the loudspeaker that result from its actual configuration, such as the vibration effect of the flexible seal added to the outer edge of the driver.

In other embodiments of the invention, reinforcing structures other than ribs may be used. Other shapes than ribs in general may be used, such as a honeycomb pattern protruding from the tapered surface. In one embodiment, a sandwich-type structure may provide the required reinforcement.

It will of course be appreciated that many variations may be made to the above-described embodiments without departing from the scope of the present invention.

Claims (16)

1. A loudspeaker, comprising:

a sound radiating diaphragm forming part of a moving diaphragm assembly and comprising: a generally frustoconical membrane having a narrow neck end and a wide mouth end, a stiffening structure for stiffening the radiating membrane and a bonding area adapted to drive the diaphragm therethrough; and

a transducer comprising a voice coil mounted to drive the diaphragm through its engagement region;

wherein the junction region is located at a node of a first mode of vibration of the moving diaphragm assembly.

2. A loudspeaker as in claim 1, wherein the stiffening structure dominates the vibrational characteristics of the diaphragm.

3. A loudspeaker as claimed in any preceding claim, wherein the location of the bonding region provides compatibility with transducers having voice coils of standard diameter.

4. The loudspeaker of claim 1, further comprising a connecting protrusion at the junction region through which the diaphragm is coupled to the transducer.

5. A loudspeaker as in claim 1, wherein the transducer comprises a shelf on which a voice coil is mounted, the shelf being connected to the connecting protrusion to drive the diaphragm.

6. A loudspeaker as in claim 5, wherein the shelves are cylindrical.

7. The loudspeaker of claim 1, wherein the stiffening structure comprises ribs.

8. A loudspeaker as in claim 7, wherein said stiffening structures comprise longitudinal ribs, each longitudinal rib extending between a neck end and a mouth end of said radiating film, and each longitudinal rib being thinner in thickness towards said neck end and/or mouth end.

9. A loudspeaker as claimed in claim 7 or 8, wherein the stiffening formations comprise circumferential ribs at the neck and/or mouth end.

10. A loudspeaker as in claim 1, further comprising a hemispherical diaphragm mounted at the neck end of the membrane such that, in use, the membrane acts as a waveguide for acoustic radiation emitted by the hemispherical diaphragm.

11. The loudspeaker as in claim 5 wherein said moving diaphragm assembly further comprises at least one air seal at said neck end of said cone, an air seal at said mouth end of said cone, said shelf and said voice coil.

12. A loudspeaker diaphragm for sound radiation, comprising: a generally frustoconical membrane having a narrow neck end and a wide mouth end, a stiffening structure for stiffening the radiating membrane and a bonding area adapted to drive the diaphragm therethrough; wherein the reinforcing structure is arranged such that a node of the first vibration mode of the diaphragm is located at a position substantially coincident with the joint region.

13. A loudspeaker diaphragm as in claim 12, wherein the stiffening structure dominates the vibrational characteristics of the diaphragm.

14. A loudspeaker diaphragm as claimed in claim 12 or 13, wherein the location of the first vibration mode/junction region provides compatibility with transducers having voice coils of standard diameter.

15. A loudspeaker diaphragm as claimed in claim 12 or 13, further comprising a connection protrusion at the junction region through which the diaphragm may be coupled to a transducer.

16. A method of designing an acoustically radiating loudspeaker diaphragm comprising a generally frusto-conical membrane, the diaphragm being intended to be coupled to a transducer through a joint region by computer modelling various arrangements of stiffening structures applied to the membrane such that a node of a first mode of vibration of the diaphragm is located at a position substantially coincident with a desired position of the joint region.