GB2474853A

GB2474853A - Gap seal for a compound loudspeaker

Info

Publication number: GB2474853A
Application number: GB0918851A
Authority: GB
Inventors: Jack Oclee-Brown; Julia Davidson
Original assignee: GP Acoustics UK Ltd
Current assignee: GP Acoustics UK Ltd
Priority date: 2009-10-27
Filing date: 2009-10-27
Publication date: 2011-05-04
Anticipated expiration: 2029-10-27
Also published as: WO2011051659A3; WO2011051659A2; GB2474853B; GB0918851D0

Abstract

A compound loudspeaker is described, comprising an inner driver 130 and an outer driver having a cone 112 that surrounds the inner driver. The cone has an annular air seal 10 that is affixed to a rear face of the cone adjacent its inner diameter and which extends rearwardly therefrom to adjacent a side face 128 of the inner driver. Such a seal closes the gap neatly, in an unobtrusive manner, without affecting the movement of the cone. The air seal is affixed to a rear face of cone, on a moulded section that provides a planar surface. The air seal is not affixed to inner driver, but slides over the outer diameter of the inner driver. The air seal can be of rubber or another elastomeric material, and is preferably foamed to reduce both stiffness and weight.

Description

Loudspeakers

FIELD OF THE INVENTION

The present invention relates to loudspeakers.

BACKGROUND ART

Compound loudspeakers position a high frequency transducer at the apex of a cone mid or low frequency driver resulting in a very wide bandwidth transducer with consistent directivity over its entire operating range. An example is disclosed in EP-A-0341926. In operation the midrange driver cone acts as a waveguicie for the tweeter, controlling its dispersion and providing some advantageous acoustical loading. It is a well known shortcoming of this kind of design that the physical irregularities surrounding the tweeter result in frequency response irregularities in the working range of the tweeter.

There are a number of patents which discuss various arrangements which reduce the extent of these physical irregularities in order to improve the tweeter * response of such driver units. For example, US2003051940 describes the use of a smooth flat outer air seal (or surround) on the midrange driver, in order to avoid frequency response irregularities caused by the non smooth front surface that is found on a conventional surround.

W02006/092609 describes, in some detail, the optimal conditions for the geometry around the tweeter to avoid any irregularities in the frequency response by ensuring that a wave with a coherent spherical wavefront is propagated down the cone waveguide.

A welt-known source of irregularities in the tweeter frequency response is the presence of the annular gap between the midrange voice coil and the tweeter assembly. This gap represents a significant physical irregularity when the acoustics of the waveguide is considered. The result of the presence of this gap can often be a very significant irregularity in the frequency response of the tweeter. This frequency response irregularity is normally most obvious when measured on the axis of the driver and is typically in the order of a few dB of magnitude in the region between 1,000Hz and 15,000Hz. It is preferable to cover this gap in order to remove this frequency response irregularity. However, it is problematic to cover this gap as one edge of the annular gap is stationary, i.e. the outside diameter of the tweeter assembly, whilst the other must be free to move, i.e. the midrange voice coil and the inside diameter of the midrange cone.

One such method to cover the gap is disclosed in W02009109228. This patent describes the use of a seat which smoothly continues the shape of the cone and bridges the gap to the tweeter. The seal is in this case made from a soft material, preferably foam, and used in conjunction with a similarly shaped outer air seal such that the restoring force provided by the two seals on the moving part of the midrange driver is "tinearised". However, there are a number of difficulties with the outlined approach. Considering first the acoustical performance of such a device, it is apparent that the restoring stiffness is only tinearised over a small range of driver excursions as the combined inner and outer air seal increase in stiffness rapidly with forward and rearward cone excursion respective'y, thus severely limiting the linear range of movement. In addition, even with the very softest material possible the stiffness at rest position of the combined inner and outer air seal arrangement is much higher than on a conventional driver surround. These two factors limit the device to working at a higher frequency, and may also require a steeper sloped high-pass filter than would be expected for a conventional driver of the same size. There are also some difficulties in manufacturing; firstly the soft material required for the inner surround is typically hard to handle and cosmetically poor in finish compared to the surrounding parts, such as the midrange cone. Secondly, it is difficult to attach the soft air seal to the components on either side of the gap as the gluing surface is i) conical in shape and ii) on the cosmetically-critical visible front surface of the driver.

SUMMARY OF THE INVENTION

The present invention therefore provides a compound loudspeaker, comprising an inner driver and an outer driver having a cone that surrounds the inner driver, the cone having an annular air seal that is affixed to a rear face of the cone adjacent its inner diameter and which extends rearwardly therefrom to adjacent a side face of the inner driver. Such a seal closes the gap neatly, in an unobtrusive manner, without affecting the movement of the cone.

The air seat is. preferably affixed to a rear face of cone, to assist with manufacture and to provide a better cosmetic appearance. The air seal can be affixed via a double sided adhesive tape. The cone will ideally have a moulded area on its rear face adjacent its inner diameter, which provides a planar surface to which the air seal can be affixed easily. To provide room for attachment of the seal, the cone can be driven by a voice coil attached to a rear face thereof at a location spaced outwardly of the inner diameter of the cone.

The air seal is preferably not affixed to inner driver, but s'ides over the outer diameter of the inner driver.

The inside diameter of the cone is ideally greater than the outside diameter of the inner driver, to provide a margin between the two which will prevent the cone striking the inner driver, and to prevent the seal "catching" and interrupting the movement of the cone.

A rear portion of the air seal can be affixed to a stationary part of the loudspeaker. This stationary part is preferably located such that the air seal is subject to expansion and/or compression as the cone moves. It is also preferably that the stationary part of the loudspeaker is of a smaller diameter than the diameter at which the air seal is affixed to the cone. This places the seal at least partly in shear mode, making it more compliant. A rear suspension can also be provided, to support the cone and provide a restoring force thereto.

The air seal is preferably of rubber or another elastomeric material. It is preferably foamed to reduce both stiffness and weight.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figure 1 shows an axial view of an air seal according to the present invention; Figure 2 shows a transverse section through the air seal of figure 1; Figure 3 shows an enlarged portion of figure 2; Figure 4 shows an front axial view of a driver incorporating an air seal according to the present invention; Figure 5 shows a section on V-V of figure 4; and Figure 6 shows an enlarged portion of figure 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

We describe a new acoustical seal for the gap which overcomes these problems. An embodiment of the new invention is shown in figures 1 to 3, in which figure 1 shows the generally circular seal 10 viewed along an axial direction 8, figure 2 shows an axial section through the seal 10, and figure 3 shows the cross-section of the seal 10. This inner air seal 10 is formed by, in section, a block of material 12 which is compressed when the cone moves towards the magnet, and expands when it moves away. This block then extends around a complete circular path so as to sit at the inner throat of the midrange cone of a compound driver unit, around the inner driver. The material should be a soft material with little resistance to compression or extension and have low density. For example, a foamed rubber or thermoplastic elastomer or other compliant elastomer could be used.

The block 12 has a chamfer 14 on one inner edge, matched to the angle of the midrange cone at its inner throat. The surface 16 immediately outside and adjacent the chamfer 14 is flat, i.e. transverse to the axial axis 8 of the circular seal 10, to provide for a straightforward adhesive attachment of the seal to the rear of the cone.

The inner surface 18 of the seal 10 is cylindrical, i.e. parallel to the axial axis 8. This allows a smooth abutment (or near-abutment) against a like outer surface of the inner driver.

The precise shape of the other surfaces of the seal 10 are not crucial and may (as illustrated) be simply parallel to the surfaces 16, 18 to for a chamfered rectangular cross-section, or may be shaped differently to control the weight (and hence the inertia) of the seal or to assist in its moulding.

Figures 4 to 6 show the new inner air seal located in a compound driver assembly 100. Figure 4 shows a front axial view of the compound loudspeaker, whilst figure 5 shows a partial axial section on figure 4, and figure 6 shows the interface between the inner driver and the midrange cone, enlarged.

Referring to all figures 4 and 5, the loudspeaker has a frame 102 which supports an outer magnet 104 for the midrange driver. This is sandwiched between magnetic conductors 106, 108 which define a gap 110 into which a midrange voice coil (not shown) is placed. This voice coil supports a midrange cone 112, consisting of a thin lightweight layer, reinforced with suitable ribs 114 on its rear face to provide added rigidity.

The central throat 116 of the midrange cone 112 is relatively wide in order to accommodate an inner driver 118 which will usually be a high frequency driver or tweeter. This has (likewise) a magnet and magnetic conductors 122 to define a gap in which sits the tweeter voice coil that drives a tweeter dome 124.

Referring to figure 6, the inner air seal 10 is attached by means of a suitable adhesive such as double sided tape on its front surface 16 to the rear of the cone 112 adjacent to the neck 116. In the illustrated design the cone 112 has a small plastic moulding 126 at its neck 116 so that this adhesive surface is planar. The inside diameter 18 of the air seal 10 is in contact with, or abuts, the outside diameter 128 of the tweeter assembly 130 (which is preferab'y a cylindrical surface). The two surfaces 18, 128 are not glued together (at least not in the vicinity of the front face), so the surfaces are free to slide over one another. The inside diameter 116 of the cone 112 is larger than the outside diameter 128 of the tweeter assembly 130 such that the two parts do not come into contact or catch the soft air seal 10 when the midrange cone 112 moves rearwards.

It is notable that the point of attachment of the seal 10 to the midrange * cone 112 would normally be the point at which the voice coil is connected to the cone. In the disclosed arrangement, therefore, we attach the voice coil to the cone at a point outside the inner throat of the cone, which provides the necessary space in which to attach the seal. This can be as (for example) to drive the cone at a node thereof, as is for example disclosed in our earlier application No. GB0908542.4 filed on 19 May 2009 (the content of which is hereby incorporated by reference and to which the reader is referred in order to gain a full understanding of the present invention), or otherwise.

The inner air seal 10 is also attached to a stationary part 132 of the driver assembly 100 on its rear face. When the midrange cone 112 moves during operation, the soft foam air seal 10 is compressed or stretched depending on the direction of the motion. Essentially, the seal 10 allows motion of the midrange cone 112 by compression and expansion of the whole body of the seal 10. rather than by the bending and surface stretching of a thin membrane -as is the case with a conventional half roll or multiple roll rear suspension. As the seal 10 is compressed or expanded, the inside diameter 18 of the air seal 10 slides against the outer diameter 128 of the tweeter assembly 130. Provided that the material of the inner air seal 10 is suitably chosen, that the outer diameter 128 of the tweeter assembly 130 is continuous and smooth, and that there is a suitably large gap left between the cone neck inside diameter 116 and the tweeter outside diameter 128 it has been discovered that there is no distortion or noise generated by the two sliding surfaces.

The (circular) location on the stationary part 132 at which the seallO is affixed is set at a slightly lesser diameter than the plastic moulding 126 on the cone 112. This means that the air seal diverges slightly toward the cone 112, so is (partly) in shear mode as it expands and compresses to take up movement of the cone 112. This assists by making the seal 10 slightly more compliant than would be the case if its deformation was purely compressive or expansive.

The soft inner air seal 10 provides little restoring force to the midrange cone 112, so the restoring force required can be achieved (and controlled) conventionally using a rear suspension attached to the midrange voice coil, thus avoiding undesirable suspension non-linearity. The gap between cone inside diameter 116 and tweeter assembly outside diameter 128 is acoustically sealed by the presence of the inner air seal 10, and correspondingly the frequency response irregularities caused by the presence of the gap are eliminated.

The new design of air seal has several advantages over the current state of the art. First, the inner seal 10 is almost completely hidden, which removes any problematic cosmetic issues with the surface finish of the surround or the neatness of the. adhesive application. The two adhesion surfaces on the inner air seal are planar, which makes assembly much more straightforward -for example, double sided adhesive tape can be used in many cases. The soft inner air seal provides little restoring force on the midrange cone, so the optimal stiffness may be set using conventional components. For the same width of gap between the cone and the tweeter assembly, the new air seal allows a much higher range of midrange cone excursion and correspondingly bandwidth and/or maximum output from the midrange driver is increased. The seal also provides superior acoustical isolation between the rear of the cone and the front of the cone.

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

Claims

CLAIMS1. A compound loudspeaker, comprising an inner driver and an outer driver having a cone that surrounds the inner driver, the cone having an annular air seal that is affixed to a rear face of the cone adjacent its inner diameter and which extends rearwardly therefrom to adjacent a side face of the inner driver.
2. A compound loudspeaker according to claim 1, in which the air seal is affixed to the rear face of cone via a double sided adhesive tape.
3. A compound loudspeaker according to any one of the preceding claims, in which the cone has a moulded area on its rear face adjacent its inner diameter, providing a planar surface to which the air seal is affixed.
4. A compound loudspeaker according to any one of the preceding claims, in which the air seal is not affixed to inner driver.
5. A compound loudspeaker according to any one of the preceding claims, in which the inside diameter of the cone is greater than the outside diameter of the inner driver.
6. A compound loudspeaker according to any one of the preceding claims, in which a rear portion of the air seal is affixed to a stationary part of the loudspeaker,
7. A compound loudspeaker according to claim 6 in which the stationary part is located such that the air seal is subject to expansion and/or compression as the cone moves.
8. A compound loudspeaker according to claim 6 or claim 7, in which the stationary part of the loudspeaker to which the air seal is affixed is of a smaller diameter than the diameter at which the air seal is affixed to the cone.
9. A compound loudspeaker according to any one of the preceding claims, further comprising a rear suspension to support the cone and provide a restoring force thereto.
10. A compound loudspeaker according to any one of the preceding claims, in which the air seal is of an elastomeriC material.
11. A compound loudspeaker according to any one of the preceding claims, in which the air seal is rubber.12; A compound loudspeaker according to claim 10 or claim 11, in which the air seal has a foam structure.13. A compound loudspeaker according to any one of the preceding claims, in which the cone is driven by a voice coil attached to a rear face thereof at a location spaced outwardly of the inner diameter of the cone.14. A compound loudspeaker according to any one of the preceding claims, in which the inner driver has a circular outer shape.15. A compound loudspeaker substantially as herein described with reference to and/or as illustrated in the accompanying figures.