The present invention relates to a speaker assembly, in particular a sinterless speaker intended for use in a hazardous area, for example one having an explosive atmosphere.

At present, a conventional design for a hazardous area speaker, see for example Fig. 1, includes a compression driver unit, as for example shown in Fig. 2, including a diaphragm which is configured to be moveable to produce compression waves, i.e. sound waves, and an optional re-entrant horn used to provide impedance matching between the external environment, e.g. air, to the speaker and the diaphragm of the compression driver unit. The diaphragm is coupled to one or more voice coils, for generating a time varying magnetic field capable of interacting with e.g. a fixed permanent magnet. This . interaction results in movement of the diaphragm.

The compression driver unit including the diaphragm is often housed in a housing which includes an aperture or throat which allows the compression waves generated by the diaphragm to be communicated from the housing to the external environment, e.g. via the optional re- ^• entrant horn.

When such a speaker is intended for use as a hazardous area speaker, the speaker is required to be "explosion proof", and is sometimes referred to as an anti-explosion speaker. Thus, any sources of ignition within the housing, e.g. the electronics associated with the compression driver unit, must be separated from the hazardous area surrounding the speaker in such a way that an explosion within the housing cannot propagate through the aperture and via the reentrant horn to the external environment. The external environment may be in a hazardous area such as an oil platform, for example, where gaseous hydrocarbons may be present and may be liable to explode when ignited.

So, in conventional hazardous area speakers, a "sintered" element is positioned in the throat of the enclosure, e.g. between the diaphragm and the optional re-entrant horn. The sintered element allows the compression waves generated by the compression driver unit and diaphragm to pass out of the enclosure and into the optional re-entrant horn (if present) to be amplified. However, the sintered element is also designed to "arrest" a flame, e.g. propagating within the housing, to prevent the flame propagating to the external environment surrounding the enclosure by removing one element of the combustion triangle which comprises oxygen, fuel and heat. In effect, the sintered element acts as a heat sink and conducts away heat from the flame thereby arresting the flame.

The sintered element is generally a body formed of a large number of metallic, e.g. spherical, particles which are arranged in a disc- like structure, and subsequently heated and pressed, e.g. sintered, to form a generally porous solid metallic body. A sintered element formed in this way will preferably have a large number of channels through it, which will allow the communication of compression waves, i.e. sound waves. Therefore, the sintered element is able to convey the compression waves generated by the compression driver unit from the internal volume of the housing to the external volume of the housing, e.g. via the re-entrant horn. On the other hand, the solid body of the sintered element provides a large heat sink. Furthermore, the sintered element has a large surface area.

Therefore, the sintered element has the ability to extinguish a flame attempting to travel through the sintered element via the communication channels by absorbing heat energy (enthalpy) from the flame which is at high temperature into the solid matrix of the sintered element.

The rate of heat transfer depends on the temperature gradient between the flame and the sinter, the channel hydraulic diameter (of the communication channels crossing the sinter) and the thermal conduction (diffusivity) properties of the gas in the enclosure and in the external environment in the hazardous area, for example. In this specification, the phrase "sintered element" refers to a porous body that allows compression waves (e.g. sound waves) to pass through it via the porous channels defined in it, but which also has the ability to remove heat from a flame passing through the same channels. It is not necessary that the sintered element is in fact manufactured by sintering.

In order to satisfy the counter-balanced criteria discussed above, it is necessary for the ratio of the length and diameter of a particular channel to be such that sound can be relatively communicated from one side of the sintered element to the other, whilst a flame is unable to completely traverse the sintered element. This is because a channel which is of incorrect dimensions might allow a flame to pass along it, potentially exiting into the hazardous area itself.

However, the current methods of manufacture of such sintered elements are wholly reliant on empirical testing of each sintered element that is made. In other words, there is no way of knowing, without testing a particular sintered element, whether the channel sizes will meet certain predetermined manufacturing tolerances. This can lead to a high percentage of a batch of sintered elements being rejected for having inadequate flame arresting properties or sound communicating properties, or perhaps even both. Therefore, the cost of manufacturing and testing usable sinters is disproportionately high.

Furthermore, even when a suitable sintered element is found which provides adequate communication of compression waves and also has suitable flame arresting properties, there is a problem associated with the conventional speaker assemblies incorporating such a sintered element. In particular, as the sinter effectively acts as a heat sink, its own temperature inevitably increases. Eventually, the temperature rise can reach a point where the sinter itself becomes an ignition source. But, because the sintered element has a surface which is in direct contact with the external environment surrounding the hazardous area speaker, such an ignition source is potentially highly dangerous and undesirable.

Furthermore, because the sintered element is permeable to air it may also be permeable to liquid. Therefore, the sinter compromises any fluid ingress protection (for example liquid and/or solid, e.g. dust) offered by the housing surrounding the compression driver unit.

Furthermore, as the channels through the sintered element are by definition more restrictive to the free fluid flow of air (essentially the compression waves, i.e. sound waves) than an aperture without a sinter fitted, the level of sound produced by the compression driver that is capable of exiting the enclosure into the re-entrant horn is restricted to some extent. This reduction in effective sound output is an issue as in order to achieve a given sound level from the speaker as a unit a more powerful driver unit is required to compensate. This is undesirable as this can increase the power consumption of the unit and the heat produced in use which when added to the heat produced by the temperature rise across the sinter can severely restrict the upper ambient temperature it is safe to operate the speaker in without risk of explosion.

The present invention has been devised in view of the above problems. The present invention aims to solve at least some of the above problems by providing a speaker assembly according to claim 1.

Advantageously, a speaker assembly according to claim 1 has no need for a sintered element as present in the conventional speaker assemblies discussed above. In particular, all of the potential sources of ignition associated with the compression driver unit, for example one or more electromagnets, are effectively isolated from the external environment of the housing. On the other hand, the movable body, e.g. a diaphragm, is arranged in the external environment to the housing, and it is coupled to a first magnetic element, which may be a permanent magnetic for example, and which is arranged to interact with a magnetic field generatable by the one or more electromagnets.

In the present specification, by "effectively sealed" it is meant that the interior space of the housing may not be isolated from the external environment - for example it may be in fluid communication with the external environment - but that a flame is prevented from exiting the interior space of the housing and entering into the external environment. For example, a path providing fluid communication between the interior volume of the housing and of the external environment may be configured in such a way as to provide both fluid communication and simultaneously not allow a flame to exit the housing and ignite the external environment. "Effectively sealed" is to be contrasted with the use of the term "sealed", which is intended to convey that the interior space of the housing is not in fluid communication with the external environment, and is therefore isolated from the external environment.

Thus, where it is stated here that an electrical component is isolated from the external environment, it is intended to convey that the electrical component is at least effectively sealed from the external environment, and preferably sealed from the external environment.

Coupling the movable body to the first magnetic element, which may be a permanent magnet for example, may result in a relatively lower frequency response characteristic for the movable body than if it was coupled to an electromagnet such as a voice coil, but this is not of primary concern in a hazardous area speaker assembly. Indeed, in the known speaker assemblies which employ a sintered element, the sintered element itself can severely adversely affect the frequency range generatable by the final speaker assembly. The present inventor has found that a speaker assembly according to the present invention can provide a frequency response which is comparable with a conventional hazardous area speaker assembly employing a sintered element. Advantageously, the present invention has the advantage that there is no sintered element present to act as an ignition source, or to allow the ingress of liquid to the internal space of the housing.

Indeed, the housing may be configurable to seal the internal space from the external environment, and the or each electromagnet may be isolated from the external environment by at least a portion of the housing. Therefore, even if an explosion (an ignition event) occurs within the internal space of the housing, it is practically impossible in ordinary circumstances for a flame resulting from such an explosion to exit the housing and reach the external environment to ignite it.

Preferably, the or each electromagnet is isolated from the external environment by at least a lmm thick portion of the housing. The housing may be formed of a suitable thermoplastic. In any case, the lmm thick portion of the housing should be sufficient to prevent the electromagnet from acting as an ignition source by isolating it from the external environment .

Preferably, the or each electromagnet is at least partially embedded in the wall of the housing so as to isolate the or each electromagnet from the external environment. Preferably, although the or each electromagnet is (electrically) isolated from the external environment the housing allows magnetic flux to pass freely.

The or each electromagnet may be at least partially located in the internal space of the housing.

The housing itself may include a member projecting from a wall of the housing, the or each electromagnet may be embedded in said projecting member. The projecting member may therefore project from an external surface of the housing into the external atmosphere. Preferably, the projecting member is removably coupled to the wall of the housing to form a portion thereof. Therefore, if any of the or each electromagnets fails, the projecting member may be removed from the housing and replaced with another projecting member. The or each electromagnet being embedded in said projecting member can thereby be replaced.

Alternatively, the projecting member may be formed integrally with the wall of the housing if desired.

The first magnetic element may be formed of one or more magnetic elements other than an electromagnet. Therefore, the second magnetic element may be formed to include an electromagnet. Preferably, the first magnetic element is located in the external environment and is arranged to be capable of interacting with the second magnetic element through at least a portion of a wall of the housing. Preferably, the first magnetic element includes a neodymium magnetic body. However, it will be appreciated by those skilled in the art that the first magnetic element may be formed of any magnetic material which, when subjected to a suitable magnetic field generated by e.g. an electromagnet, is capable of moving the movable body.

A speaker assembly according to the present invention may further include a drive source for supplying an electric current to the or each electromagnet, and the drive source may be housed within said housing.

The or each electromagnet may include a voice coil driver.

The present invention will now be described by way of example only, with reference to the following figures in which:

Fig. 1 shows a conventional hazardous area speaker assembly; Fig. 2 shows a conventional driver unit, suitable for use in the conventional hazardous area speaker shown in Fig. 1, for example; Fig. 3 shows a first hazardous area speaker assembly according to the present invention; Fig. 4 shows a second hazardous area speaker assembly according to the present invention;

Fig. 5 shows an example of a speaker assembly 10 according to the present invention.

A speaker assembly according to the present invention, e.g. as shown in Fig. 3, is intended for use in a hazardous area, such as an area in which the environment is combustible and/or explosive, for example. Examples of such an environment might be that surrounding an oil platform, or that in a mine shaft.

The hazardous area may be populated by one or more dangerous gases and/or dust types from the tables below:

Gases are classified according to the ignitability of gas-air mixture. Refer to IEC/EN 60079- 20 for classification of common gases and vapours .

Accordingly, turning to Fig. 3, the speaker assembly 10 includes a housing 12 suitable for containing within its interior driving means for supplying an electrical current to at least one or more of the voice coils 14 which are capable of interacting with a magnetic element 16, e.g. a permanent magnet, coupled to a movable body 18, e.g. a diaphragm 18, which itself is arranged to be capable of generating sound waves when moved appropriately. A re-entrant horn 20 may be provided e.g. as a part or portion of the housing 12. The re-entrant horn 20 may be detachable from the housing 12, or it may be formed integrally with the housing.

The diaphragm 18 may be movably coupled to the housing 12 by means of one or more hinges (not shown) .

The diaphragm 18 is arranged in the environment external to the interior of the housing 12. In other words, the diaphragm is intended to be arranged on the outside of the housing 12, in a hazardous environment, for example. Preferably, the or each magnetic element 16 coupled to the diaphragm 18 is also arranged in the external environment. Thus, the or each magnetic element 16 should be a passive magnetic element, such as a permanent magnet for example, incapable of acting as an ignition source in the hazardous area.

Thus, preferably, the or each voice coil 14 is isolated from the external environment by at least a portion of the housing 12; the or each voice coil 14 therefore being incapable of acting as an ignition source in the hazardous area. However, when supplied with suitable electrical current, each voice coil 14 is able to interact with a magnetic element 16 coupled to the diaphragm 18 to cause it to move; the interaction preferably taking place through at least a portion of the housing 12.

Therefore, all sources of ignition associated with the present inventive speaker assembly are isolated, e.g. effectively sealed or sealed, from the external environment, thereby ensuring that such sources cannot contribute to ignition of the external environment.

The driving means (not shown) contained within the housing 12 might include, for example, an electrical transformer (configured as an audio line transformer) or an electrical power supply in electrical communication with electronics suitably configured to drive the or each voice coil 14.

Preferably, the housing 10 is sealable to isolate completely the interior of the housing from the external environment. Accordingly, when sealed, for example in use, the housing preferably does not provide fluid communication between the interior of the housing and the external environment. The housing may be selected from the list of enclosures given in the following table, which lists the type of protection allowable in the 60079- series of standards together with the part of the standard which relates to each type of protection:

Preferably, the housing 12 is a flameproof enclosure of the d-type. This type of enclosure can withstand the pressure developed during an internal explosion of an explosive mixture, and can prevent the transmission of the explosion to e.g. an explosive atmosphere surrounding the enclosure.

In another embodiment of the present invention, as shown in Fig. 4, the housing 10 may be modified to include a projecting member 30. The projecting member 30 preferably projects from the housing 10 in the direction of the external environment. The projecting member 30 preferably includes one or more electromagnets, e.g. voice coils 32, embedded in it. The or each electromagnet may be embedded in the projecting member 30 by being formed in a recess in the projecting member and subsequently being covered over, e.g. potted over, with a potting layer 34, e.g. with an epoxy resin, a silicone based material or a rubberised plastic to provide suitable isolation from the external explosive atmosphere surrounding the enclosure.

The projecting member 30 may also include one or more pole pieces 36 for locally enhancing the magnetic field generatable by the or each electromagnet 32. The or each pole pieces are also preferably incapable of acting as an ignition source to the external explosive atmosphere. For example, if the pole pieces are formed of a magnetic metal, they may be cover over, e.g. potted over, as above, to isolate them from the external environment, i.e. the external explosive atmosphere.

Each voice coil is preferably suppliable with electric current via feedwires 38, which are preferably isolated from the external environment, e.g. by passing along a lumen formed within the projecting member 30, and which enter the interior space of the housing 10 to be connectable to a drive source via one or more conduits formed in the housing wall.

The diaphragm 18 may be connected to the housing 10, which may include a re-entrant horn (not shown) , by one or more hinges 40 to permit the diaphragm 18 to move to generate sound waves. The diaphragm 18 is coupled to one or more magnetic elements 16, which may be positioned between a pair of voice coils 32 proximate to a pole piece 36 associated with this pair of voice coils 32. As the or each magnetic element is likely to be exposed to an external environment in a hazardous area, the or each magnetic element preferably includes a permanent magnet, e.g. a neodymium body.

The projecting member may be detachable from the rest of the housing 10. This should help repair and maintenance work, for example when a voice coil breaks. Alternatively, the projecting member may be formed to be an integral portion of the housing 10, and thus would generally be considered not detachable.

In a further embodiment, as shown in Fig. 5, a housing 50 may be formed with one or more apertures in it to permit a diaphragm 18 located in the external environment to be coupled by one or more coupling members 52 to one or more associated magnetic elements 54, 56 housed within the internal space of the housing 50. The or each coupling member may be coupled to a respective permanent magnet 54 or to a voice coil type electromagnet 56 having one or more feed wires 58. Preferably each coupling member 52 associated with a particular diaphragm is connected to the same type of magnetic element, e.g. a permanent magnet 54 or an electromagnet 56.

Fig. 5 shows that the diaphragm may be coupled to one or more permanent magnet type magnetic elements 54 and/or to one or more electromagnet type magnetic elements 56. Depending on the selection of the magnetic element to which the diaphragm is coupled, the interior space of the housing will also house one or complementary magnetic elements 54' and/or 56' . The electromagnet type magnetic elements 56 and 54' are connectable to a drive source (not shown) for supplying a time varying electric current via feed wires 58.

The interaction between the magnetic elements coupled to the diaphragm and the complementary magnetic elements is capable of causing the diaphragm to move to generate a sound wave. The diaphragm may be coupled to the housing 50 by one or more hinges 60.

This particular embodiment of the present invention permits fluid communication between the external environment and the internal space of the speaker assembly via one or more apertures through which the or each coupling member 52 passes, e.g. as shown. However, the aperture (s) through which the or each coupling member 52 passes is configured to arrest a flame attempting to propagate from the interior of the housing to the exterior of the housing (and preferably vice versa) . To do this, the width x of the or each aperture and the length y of the or each aperture must be selected appropriately. The typical dimensions for (width x) and (length y) (i.e. the "flamepath dimensions") are related to the internal volume of the enclosure and the intended external explosive atmospheres classification. The generic dimensional requirements and type and routine test requirements and QC requirements are all detailed in the relevant section of the 60079-* series of standards. Using the information and data in the 60079-* standards as a guide and experimentation and testing to prove it is possible for a person skilled in the appropriate art to determine the required "flamepath dimensions".

An advantage of this particular embodiment is that the diaphragm can be located directly in the external environment, and the diaphragm can be coupled to voice coils, rather than potentially heavier permanent magnet type magnetic elements. Thus, the frequency response of the diaphragm can be made to be similar to that of a conventional speaker assembly, but this embodiment of the present invention will not suffer from the adverse frequency dependent attenuation associated with the sinter in the conventional speaker assembly. Furthermore, once the diameter and length of the or each aperture is selected and tested appropriately to determine that a specific configuration is suitable for purpose, the housing can be manufactured on a large scale without the need for laborious repeated empirical testing as is required for the conventional sintered elements. Furthermore, the manufacture of such housings will be more time and cost efficient than the manufacture of conventional sintered elements, as a high percentage of the housings should not need to be discarded.

Examples of permanent magnets that may be suitable for use in the present invention are shown in the following table:

However, a magnetic element formed of other materials which can interact with a varying magnetic field thereby to cause the movable body described above to move could be used.

The foregoing description of the preferred embodiments of the invention have been presented for purposes of illustration and description, it is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings.

The present invention may also be implemented according to any of the following paragraphs:

A. A speaker assembly including a housing for enclosing an internal space from an external environment; a movable body arranged in the external environment and configured to generate audible sound waves when moved suitably; and a first magnetic element coupled to the movable body and a second magnetic element capable of interacting with the first magnetic element to cause the movable body to move, at least one of the first and second magnetic elements including an electromagnet; wherein the or each electromagnet is effectively sealable from the external environment by at least a portion of the housing.

B. A speaker assembly according to paragraph A wherein the housing is configurable to seal the internal space from the external environment, and the or each electromagnet is sealed from the external environment by at least a portion of the housing.

C. A speaker assembly according to paragraph B wherein the or each electromagnet is sealable from the external environment by at least a lmm thick portion of the housing.

D. A speaker assembly according to any one of paragraphs A to C wherein the or each electromagnet is at least partially embedded in the wall of the housing.

E. A speaker assembly according to any one of paragraphs A to D wherein the or each electromagnet is at least partially located in the internal space.

F. A speaker assembly according to any one of paragraphs A to D wherein the housing includes a member projecting from a wall of the housing into the external environment, the or each electromagnet being embedded in said projecting member.

G. A speaker assembly according to paragraph F wherein the projecting member is detachably couplable to the wall of the housing to form a portion thereof.

H. A speaker assembly according to paragraph F wherein the projecting member is formed integrally with the wall of the housing to form a portion thereof.

I. A speaker assembly according to any one of the preceding paragraphs wherein the first magnetic element is a magnetic element other than an electromagnet.

J. A speaker assembly according to paragraph I wherein the first magnetic element is located in the external environment and interacts with the electromagnet through at least a portion of a wall of the housing.

K. A speaker assembly according to paragraph I or J wherein the first magnetic element includes a neodymium magnetic body.

L. A speaker assembly according to any one of the preceding paragraphs wherein the housing includes a d-type enclosure.

M. A speaker assembly according to any one of the preceding paragraphs wherein the housing includes an e-type enclosure.

N. A speaker assembly according to any one of the preceding paragraphs further including a drive source for supplying an electric current to the or each electromagnet, the drive source being housed within said housing.

0. A speaker assembly according to any one of the preceding paragraphs wherein the or each electromagnet includes a voice coil driver, drivable by the drive source to generate a varying magnetic field.

P. A speaker assembly according to any one of the preceding paragraphs wherein the movable body is coupled to the housing by a hinge which allows the movable body to move to generate audible sound waves .

Q. A speaker assembly according to any one of the preceding paragraphs wherein the housing includes a horn member suitable for providing impedance matching between the movable body and the atmosphere of the external space.

R. A hazardous area speaker assembly according to any one of the preceding paragraphs .

S. A method of manufacture of a speaker assembly according to any one of the preceding paragraphs, the method including the steps of locating the or each electromagnet with respect to the housing to be effectively sealed from the external environment by at least a portion of the housing.

T. A method of manufacture of a speaker assembly according to paragraph S, the method including the step of locating with respect to the housing to be isolatable from the external environment by at least a lmm thick portion of the housing.