BACKGROUND TO THE INVENTION Many forms of equipment need to be tested in acoustically anechoic chambers. The conventional way of creating such a chamber is to line all the bounding walls of a room with cone shaped or wedge shaped elements. A variety of materials are used for this purpose. Open-cell foamed synthetic plastics material is possibly the most widely used material. The elements are shaped, and/or their density varies, so as progressively to change the impedance from that of air to that of the walls without creating reflections.

It is also desirable to be able to test some equipment in an environment which is free of radio frequency (RF) emissions and reflections. This is conventionally achieved by lining a room with sheet metal panels thereby to create a Faraday cage which excludes emissions of a background nature. Carbon or another electrically conductive material usually in powdered form is mixed into foamed synthetic plastics material and the cage is lined with of this material. The conductive material creates the multiplicity of

electrical flow paths that are required to absorb radio frequency signals. Such a material, however, has two major problems. Firstly, the open-cell foam required is expensive and secondly the quantity of carbon or other conductive powder that must be used to be effective in absorbing RF emissions is such that the mechanical strength of the foam is impaired.

The main object of the present invention is to provide a new acoustically anechoic material. A subsidiary object is to provide a new acoustically anechoic material which also absorbs RF signals.

BRIEF DESCRIPTION OF THE INVENTION According to a first aspect of the present invention there is provided a method of manufacturing an acoustically anechoic material which comprises compressing a mass of synthetic plastics material fibres into a mould, and bonding the fibres to one another where they are touching.

According to a second aspect of the present invention there is provided a method of manufacturing an acoustically anechoic material which comprises compressing a mass of synthetic plastics material fibres into a mould, at least the surfaces of the fibres being of a material which welds to itself when heated, and heating the mould to a temperature at which the surfaces of the fibres soften and fibres are welded together where they are touching.

According to a third aspect of the present invention there is provided a method of manufacturing an acoustically anechoic material which comprises compressing a mass of synthetic plastics material fibres into a mould, the fibres being a mixture of higher melting point fibres and lower melting point fibres and heating the mass of fibres to a temperature between said melting points so as to cause the lower melting point fibres to melt and bond the higher melting point fibres together.

The method can include the step of dispersing electrically conductive fibres in the synthetic plastics material fibres before the mixed fibres are compressed into the mould thereby to provide an acoustically anechoic material which also absorbs RF signals.

Said fibres are preferably carbon fibres, and can be carbon fibres with a metallic coating.

Bonding can be achieved by means of an adhesive.

According to a fourth aspect of the present invention there is provided an acoustically anechoic element which comprises a compressed mass of fibres of a synthetic plastics material, the fibres being bonded to one another where they touch.

In one form at least the surfaces of the fibres are of heat weldable material, the fibres in the mass being heat welded to one another where they touch. In another form the mass includes fibres of a higher melting point mixed with fibres of a lower melting point,

the lower melting point fibres bonding the higher melting point fibres together. In yet another form said fibres are bonded to one another by an adhesive.

Electrically conductive fibres can be dispersed in the synthetic plastic material fibres to obtain an element which absorbs RF signals. Preferably said fibres are carbon fibres which can have a metallic coating. Said metallic coating can be of nickel.

Preferably the conductive fibres are evenly dispersed in the plastics fibres but there can be circumstances where distribution of the conductive fibres in strata is desirable to create zones of varying conductive fibre concentration.

DETAILED DESCRIPTION OF THE INVENTION Fibres of materials such as polyester are widely available commercially.

Solid fibres are available as also are hollow fibres. This latter form of fibre is often referred to as"Hollowfil"and is used in pillows, duvets etc. The solid fibres available are usually referred to as monofilament or coarse spun.

Coarse spun fibres are of composite construction and have a core and a coating on the core, the core and coating being of different materials. The coating melts at a lower temperature than the core. A monofilament is of unitary construction and is composed of a single material. To achieve the requisite bonding the nature of the fibre must be such that its surface softens when heated and will then weld to another fibre at a

zone where the two fibres are touching. The types of fibre which are packed in layers to form thermally insulating sheets, padding for upholstered furniture etc can be used.

A mould is filled with fibres having the described characteristics and the fibres are compressed into the mould until the desired density is achieved. The mould is then heated. In Applicant's experimental work it has been found that the temperatures which must be achieved to bond the fibres into the form of a block are such that simple wooden moulds can be used. These can be heated in an electrical oven at a temperature such that the moulds are not damaged but adequate fibre bonding is achieved.

The mould can be shaped to produce cones, wedges, blocks or panels depending on the intended use of the final product.

The product resulting from the method of production described has sound absorbing qualities which suit it for use in acoustical anechoic chambers and other places where sound absorption is required. However, it has no ability to absorb RF signals. By mixing electrically conductive fibres such as stainless steel or carbon into the synthetic plastics material fibres before moulding, RF absorbing properties are achieved.

A product comprising a carbon fibre core with a nickel coating which is applied by chemical vapour deposition is available commercially. Such fibres have properties which suit them for use mixed with synthetic plastics materials to produce RF

emission absorbing material. The use of nickel coated carbon fibre has the further advantage that it also has magnetic properties not displayed by uncoated carbon fibres.

It is also possible to mix fibres which have different melting points, and then compress the mixed fibres into a mould. A mixture of 60% by mass high melting point fibres and 40% by mass of low melting point fibres has been found to be suitable. As a specific example, polyester is available in grades which will melt at temperatures of 130°C and 160°C. By heating the mould to a temperature between these two temperatures the low melting point fibres can be softened and whilst the high melting point fibres remain unaffected. It is the low melting point fibres which bond the high melting point fibres together to give the block its structural integrity.

It is also possible to use adhesives to bond the compressed fibres into the form of a block. The adhesive can be heat activated after being sprayed on the fibres.

Alternatively the adhesive can be solvent based, the solvent being driven off, or allowed to evaporate off, so as to bond the fibres. Adhesives that cure as a result of catalytic action can also be used. These are applied before the fibres are compressed into the mould, and then set as the components in the adhesive react.