Patent Description:
Sound output devices such as speakers and sounders are commonly employed within, for example, industrial and/or processing environments comprising hazardous areas and environments. Such areas and environments include in particular those where there is the danger of explosion due for example to the possible presence of explosive gases, and in particular explosive dust. Explosive dust environments are particularly dangerous since dust explosions tend to be much more powerful than gas explosions for a given volume. For such areas classified as hazardous, it can prove essential that an electroacoustic driver unit is present so as to provide for audible sounds/signals such as for communication and/or alarm purposes.

The required use in such hazardous areas dictates that the electroacoustic driver unit is provided in a housing offering a sufficient degree of sealing so as to prevent any potentially explosive event occurring within the housing travelling to the hazardous area/environment within which the housing is located.

It is generally easier to prevent the ingress of dust into an enclosure than the ingress of gas and so the nature of protection is different for each. The protection concept for a gas atmosphere is to allow the gas into the enclosure but then prevent any internal explosion propagating to the outside atmosphere. The protection concept for an explosive dust atmosphere is to prevent any dust ingress into the enclosure since it is not currently possible to construct practical enclosures strong enough to contain dust explosions.

One form of speaker or sounder arranged for use within a hazardous area employs a sintered material to seal the housing and which, while allowing an audible signal to pass through, provides a sufficient degree of isolation to prevent any explosive event within the housing travelling into the hazardous area/environment.

However, various limitations and disadvantages are exhibited by such known arrangements. While allowing the required sound to be output into the area/environment, the sintered element nevertheless serves to attenuate the sound output from the driver thereby limiting the effective volume of its output. Also, such sintered elements commonly allow for water ingress and, if insufficiently dense, can also allow for dust ingress making such known driver units unsuitable for explosive dust-laden atmospheres.

Typically, a balance has to be struck between a requirement for a low density sinter for sound output and as against a minimum density requirement to effectively quench a flame arising from an explosive event within the housing and so as to prevent it propagating into the external explosive-atmosphere.

A further known complication is that for explosive dust environments, the density of the sinter element should be increased further so as to prevent the ingress of dust into the enclosure. As a consequence of this, loud speaker/sounder housings that are certified for use within explosive dust environments will typically offer a lower sound output for any given power rating.

It is known that particularly effective sintered elements for electroacoustic driver housings can comprise a layered structure employing several layers of an industry standard cross-woven metal mesh insofar as this is found to provide a good balance between flame quenching and porosity to sound, particularly insofar as the gas volume behind the sinter element is limited. Typically with such multi-layered standard cross-weave metal mesh sinter elements sound output is attenuated by -1dB relative to the sinter being absent. While the level of attenuation is therefore attractive for such known woven metal mesh sinters, such structures are however not considered suitable for use in explosive dust environments and atmospheres, in particular since they are insufficiently dense to prevent ingress of dust.

The present invention therefore seeks to provide an electroacoustic driver housing element arranged to allow for a sufficient level of sound output while offering the required degree of safety for operation in explosive environments and in particular those where explosive dust might be present.

It is a particular object of the present invention to provide an explosion-proof sounder/speaker having advantages over known such sounders/speakers and employing a sintered element that, while offering safe operation in explosive dust environments in particular, does not overly compromise the level of sound output from the sounder/speaker enclosure.

<CIT> discloses a portable electronic device having an outer case having a planar face in which a microphone associated acoustic port is formed. The device also has a micro-electro-mechanical system (MEMS) microphone positioned within the outer case, the MEMS microphone having a diaphragm facing the microphone associated acoustic port. An acoustic mesh is positioned between the front face of the outer case and the diaphragm, the acoustic mesh having a non-linear acoustic resistance.

<CIT> discloses a portable electronic device having acoustic ports such as microphone and speaker ports. Acoustic devices such as microphones and speakers may be associated with the acoustic ports. An acoustic port may have an opening between an interior and exterior of the portable electronic device. The opening may be covered by a metal mesh. An acoustic fabric may be interposed between the metal mesh and the opening. The opening may be formed from a hole in a glass member having outer and inner chambers. A microphone boot may be provided that forms front and rear radial seals with a housing of the device and a microphone unit respectively. The microphone boot may also form multiple face seals with the microphone unit. A speaker for the speaker port may be enclosed in a sealed speaker enclosure. The speaker enclosure may have a pressure-equalizing vent slit covered with an acoustic mesh.

<CIT> discloses an acoustic apparatus including an acoustic driver, acoustically coupled to a pipe to radiate acoustic energy into the pipe. The pipe includes an elongated opening along at least a portion of the length of the pipe through which acoustic energy is radiated to the environment.

<CIT> discloses an acoustic sensor system including a housing structure, and a miniaturized acoustic transducer mounted in the housing structure. A flame arrestor structure is mounted on or within the housing structure between the acoustic transducer and the external environment, so that ambient acoustic energy passes through the flame arrestor structure before reaching the acoustic transducer.

<CIT> discloses a speaker assembly including a housing for enclosing an internal space from an external environment. A movable body is arranged in the external environment and configured to generate audible sound waves when moved suitably. A first magnetic element is coupled to the movable body and a second magnetic element is capable of interacting with the first magnetic element to cause the movable body to move, wherein at least one of the first and second magnetic elements including an electromagnet. Each electromagnet is sealed from the external environment by at least a portion of the housing. The housing is configurable to seal the internal space from the external environment, and each electromagnet is sealed from the external environment by at least a portion of the housing. <CIT> discloses a method of fabricating a gas-permeable, metallic cover for a pressure-resistant casing intended for accommodating electrical operating media. A multiplicity of slots through which the gas can penetrate are made in the cover.

<CIT> discloses a multilayer knitted wire mesh component for acoustic attenuation applications and made of sintered Dutch twill.

<CIT> discloses a protective mesh for sound ports made of polymeric material formed as monofilament Dutch-type closed mesh.

According to one aspect of the present invention, there is provided a woven mesh element for an electroacoustic driver enclosure, the element comprising a plurality of mesh layers, and wherein the said plurality includes a layer of Dutch weave mesh, said woven mesh element being configured to cover a sound opening of said enclosure.

The present invention is advantageous insofar as it allows for an increase in sound output from electroacoustic drivers employed within hazardous environments and atmospheres, such as hazardous area horns and speakers, and which can be certified for use in hazardous dust environments but while providing improved sound attenuation figures as compared with the current art.

An advantage of the present invention is that it covers the aspects required by both gas and dust protection concepts, and so can provide a standard product that can meet both requirements with little compromise on sound output.

Through the provision of a multi-layer woven mesh element with one of the layers comprising a Dutch weave mesh, the present invention allows for the provision of a combination of wire mesh types, sintered together to form a flame arrester which can readily comply with the requirements of hazardous dust environment standards, without the elements porosity to sound being unduly compromised.

In one particularly advantageous configuration, the layer of Dutch weave mesh can comprise a layer of Dutch twill weave mesh.

Of course, it will be appreciated that the mesh layers best comprise woven wire mesh layers. In one particular example, at least one of the plurality of mesh layers comprises a layer of cross woven mesh.

A particularly advantageous configuration of the present invention can comprise a multi-layered element comprising a plurality of layers of cross weave mesh and a single layer of Dutch weave mesh.

In one arrangement, the layer of Dutch weave mesh can be provided as an outer layer of the element, whereas in another example, the layer of Dutch weave mesh can comprise an inner layer of the said element.

Of course, it will be appreciated that aspects of the present invention provide for a sintered metal element employing the structure as defined above and also an electroacoustic driver housing including an element such as defined above.

In particular, the housing can advantageously be arranged for providing use in flame-proof and/or explosion-proof characteristics.

Yet further, a loud speaker or sounder can be provided including such a housing as defined above.

As will therefore by appreciated, the present invention provides for an electroacoustic-driver-enclosure element comprising a plurality of mesh layers, and wherein the said plurality includes a layer of Dutch weave mesh. The provision of a single layer of, for example industry standard Dutch weave mesh, it is found to be effective at preventing the ingress of dust due, in particular, to the shape of the pores presented by the Dutch weave mesh.

With the combined use of different meshes that is at the heart of the present invention would advantageously provide a sintered element suitable for use in both explosive gas and explosive dust environments but without adversely affecting the level of attenuation of the sound produced by the electroacoustic driver. For example, it is found that the invention can, while being suitable for use in both explosive gas and explosive dust environments, attenuate the sound output only by -2dB relative the sinter being absent.

It is noted that typical dust/certified sinters formed according to conventional methods typically attenuate sound by in the region of -6dB.

It should also be appreciated that, while being focused in particular on explosive dust environments, the housing element of the present invention would be suitable for use in relation to all hazardous area gas groups. This has the particular advantage that a common driver/sounder/loud-speaker device can be provided meet both requirements.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which:.

Turning first to <FIG>, there is illustrated a sounder/speaker assembly <NUM> comprising a housing <NUM> enclosing an internal volume <NUM> in which there is located an electroacoustic driver <NUM> comprising a relatively large permanent magnetic <NUM>, a voice coil <NUM> mounted to a diaphragm <NUM>. The driver and diaphragm are arranged to supply, in response to an audio signal at the positive/negative terminals <NUM>, audible sound waves arranged to exit the housing <NUM> by way of an opening <NUM>. Beyond the opening <NUM> on the outside of the housing <NUM> is a frusto-conical outer horn <NUM> serving to acoustically couple the output audible signal.

In order to provide sufficient sealing of the housing <NUM>, so as to effectively isolate it from the external environment and so prevent any internal explosion event within the internal volume <NUM> from travelling to the external environment, a flat disk-like sintered metal element <NUM> is provided and serving to close the opening <NUM> and offer the required isolation between the enclosure internal volume <NUM> and the hazardous environment external to the housing <NUM>. In particular, the sintered metal element <NUM> is provided to quench a flame of an internal explosion and comprises a metal mesh sinter.

The sound waves created by the driver <NUM> can however pass through this sintered element <NUM> and onward via the outer horn <NUM> although a degree of attenuation occurs at the sintered element <NUM>. Such attenuation can, to some extent, be compensated for by an overly large and expensive driver <NUM> and associated magnet <NUM>.

Within the illustrated example of <FIG>, the sintered element <NUM> comprises a plurality of layers of wire mesh 28A sintered together so as to form the sintered element <NUM> and, as required by the present invention, employing as one of the layers, a layer of Dutch weave mesh.

Further details of the sintered element <NUM> of the present invention according to the embodiment of <FIG> are illustrated with reference to <FIG> and <FIG>.

Turning therefore to <FIG>, there is provided a sectional view of the sintered element <NUM> employed within the housing <NUM> of <FIG> and which, as noted, comprises a plurality of wire mesh layers 28A sintered together.

Although in no way limited to the present invention, within the illustrated embodiment the multi-layered sintered element <NUM> includes a single layer <NUM> of Dutch weave mesh which in the illustrated example comprises a single layer of industry standard Dutch twill weave <NUM> × <NUM> mesh, <NUM>' wire diameter, which can also be referred to as micromesh. The remaining (<NUM> in the illustrated example) layers are each formed of industry standard cross weave wire mesh, which in the illustrated example can comprise <NUM> × <NUM> mesh, <NUM>' wire diameter.

As illustrated with reference to <FIG>, in this particular example, the layer of Dutch twill weave mesh is provided within the body, and generally within a central region, of the multi-layered sintered element <NUM>.

However, other locations for the layer of Dutch twill weave mesh are available such as illustrated with reference to <FIG>.

Here, it can be seen that the single layer <NUM> of Dutch twill weave mesh is provided on, and forming part of an outer surface of the sintered element <NUM>.

As will be appreciated from reference to the illustrated examples of the present invention in particular, the element of the present invention proves advantageous insofar as it can prevent the ingress of dust due to the presence of the layer of Dutch twill weave mesh thereby meeting the required standards for explosive dust hazardous areas. However, insofar as the remainder of the sinter element is provided by layers of industry standard cross weave wire mesh the overall porosity to sound of the sintered element <NUM> is limited only to a minor, and readily acceptable, degree.

The overall layered structure therefore acts an effective flame arrester for explosive gas atmospheres, while being sufficiently porous to sound, but with the added feature of preventing the ingress of dust as noted above.

The combination of mesh types as employed within the present invention therefore provides for a sintered element offering sufficient porosity to sound along with effective flame arrester capabilities and while preventing the ingress of dust when located in explosive dust environments.

It should be appreciated that the embodiments illustrated with reference to the accompanying Figures are only some of the possible examples of the present invention and which is therefore not limited to the details of the illustrated embodiments. For example, the sintered element can be formed of any required number of layers of mesh material which could comprise two or more different types of mesh. Also, if required, more than one layer of Dutch twill woven mesh could be provided within the sintered element <NUM> and the overall shape and configuration of the sintered element is in no way restricted to that as illustrated in the accompanying drawings.

Claim 1:
A woven mesh element (<NUM>) for an enclosure (<NUM>) for an electroacoustic driver (<NUM>), the woven mesh element (<NUM>) comprising a plurality of mesh layers sintered together, wherein the plurality of mesh layers includes a layer of Dutch weave mesh, said woven mesh element (<NUM>) being characterized in that it is configured to cover a sound opening (<NUM>) of said enclosure (<NUM>).