WO2000070910A2

WO2000070910A2 - Bending wave panel loudspeakers

Info

Publication number: WO2000070910A2
Application number: PCT/GB2000/001665
Authority: WO
Inventors: Charles Bream
Original assignee: New Transducers Limited
Priority date: 1999-05-14
Filing date: 2000-05-15
Publication date: 2000-11-23
Also published as: TW488184B; AU4767000A; WO2000070910A3

Abstract

A method of making a structural or semi-structural object e.g. a car interior trim panel, comprising an integral bending-wave panel loudspeaker, characterised by thermoforming the object from a thermoformable panel material.

Description

TITLE : LOUDSPEAKERS

DESCRIPTION

TECHNICAL FIELD

The invention relates to loudspeakers and more particularly to bending-wave panel loudspeakers, e.g. of the kind described in International patent application O97/09842 and to methods of making such loudspeakers. Loudspeakers as described in WO97/09842 are known as

'distributed mode¹ or DM loudspeakers or simply as DML.

BACKGROUND ART The majority of bending wave loudspeakers applications utilise non-structural flat panels. Although these panels are optimised to produce the highest acoustic performance, there is a demand for panels with dual-functionality, where the loudspeaker panel is formed into a complex structure. A typical example would be a car door interior trim panel that also acts as a loudspeaker. Complex shaped sandwich panels are usually manufactured by hand lay-up of thermoset-based faceskins and adhesives. However, the cost and time penalties associated with hand lay-up means that this route is only viable for high-value added and low-volume industries, such as aerospace and Formula 1 racing. As a result this technique is not normally suitable for high volume production of bending wave panel loudspeakers.

DISCLOSURE OF INVENTION

According to the invention, there is provided a method of making a structural or semi -structural object, e.g. a car interior trim panel, comprising an integral bending- wave panel loudspeaker, characterised by thermoforming the object from a thermoformable panel material. By semi- structural' object is meant an object which has some structural significance although having a significant non- structure role. Thus for example an automobile interior trim panel is primarily decorative in nature but nevertheless may be used as a carrier for controls such as door and window opening mechanisms, handles etc and may carry electrical wiring and sound-proofing. The thermoformable panel material may be a sandwich panel, e.g. having face skins separated by a core or maybe a solid laminate. In this way it is possible readily to shape the panel material as desired by the application of heat and/or pressure. If desired the panel may be tensioned during the application of heat and/or pressure. The use of thermoplastic materials has the added benefit of reducing the panel manufacturing cycle time from hours (thermosets require 30-60 minutes at high temperature) to minutes (thermoplastics usually require 5-25 seconds) , making these panels more amenable to high volume production.

The main requirement for thermoformable panels is the specification of materials that can be reformed under heat and pressure. Generally these are:

1. Thermoplastic-based faceskins e.g. polymer films or fibre- reinforced polymers.

2. Thermoplastic adhesives.

3. Thermoplastic cores e.g. foam or honeycomb. The main benefit of this type of panel is that a panel manufacturer could produce a range of flat-panels. These could then be used directly as flat panels or formed into structures . The complexity of the formed component could vary from local forming, e.g. the formation of domes at the exciter position to complete forming of the panel e.g. to form a door trim. The main requirements are to heat the faceskins to above their softening point (Tg) and to heat the core to just below its softening point (Tg) . The panel can then be shaped by pressing in a cold matched tool, using a tensioning frame to prevent faceskin wrinkling.

The complexity of final shape that can be formed is dependent on the formability of the faceskin. This is primarily dependent on the type of reinforcement used (see Table 1) and in particular the amount of fibre- fibre 'slippage' that can occur. It is worth noting that this applies to all fibre types (e.g. carbon, glass, aramid, natural fibres etc.) Table 1 Formability of Faceskin Materials

Currently the majority, if not all, of fibre reinforced thermoplastics, suitable for use as faceskins on bending-wave panels are manufactured using continuous fibre fabrics. Although these readily conform to single curvatures, their formability over double curvatures is severely limited. This is caused by the fibre tows 'locking' as the fabric weave is distorted. This problem can be reduced by using short fibre tows which 'stretch' as the fabric is deformed.

The other type of formable reinforced faceskin would be the use of random short fibre veils. The advantage of this material, over short fibre fabrics, is that veils are available in very low weights (_> 25 gsm) , making them more suitable for small panels.

The main requirement of the core material is that the softening point (Tg) of the core is greater than or equal to that of the faceskin. This means that, under the correct processing conditions, the core can be formed' without collapse of the cell structure resulting in a uniform thickness throughout the shaped component. However, in certain applications collapse/compression of the core may be used to 'isolate' the bending-wave panel from the main structure. A list of several potential thermoplastic core materials is listed in Table 2. Thermoplastic film adhesives are available in two main forms, i.e. films and webs. These are frequently used in the manufacture of interior car trims to bond the fabric lining to the main panel. The main requirement, when shaping sandwich panels, is for the adhesive to soften at the forming temperature. This is particularly important when using faceskins with limited formability (such as continuous fibre fabrics) as this allows movement between the faceskin and core. Experience with polycarbonate cored panels has identified that the weight and type of thermoplastic adhesive alters the level of acoustic damping in the panel and hence is a means of controlling the sound characteristics of the panel. Table 2 - Potential Thermoplastic Core Materials

The main benefits of using thermoplastic-based panels are : 1. Shaped panels can be produced from flat 'blanks' (the level of double-curvature achievable is dependent on the reinforcement form) .

2. Lower cost alternative to thermoset -based panels (applies to flat panels as well as complex shapes) .

3. The level of acoustic damping in the panel can be controlled by altering the weight and type of thermoplastic adhesive.

4. Rapid manufacturing cycle time, making these panels amenable to high-volume production.

5. The ability to locally compress the core material enables a non-structural bending-wave loudspeaker panel to be incorporated into a structural component, without changing the basic configuration of the structural panel.

Claims

1. A method of making a structural or semi-structural object e.g. a car interior trim panel, comprising an integral bending-wave panel loudspeaker, characterised by thermoforming the object from a thermoformable panel material .

2. A method according to claim 1, characterised in that the thermoformable panel material is a sandwich panel having face skins separated by a core.

3. A method according to claim 1 or claim 2, characterised by shaping the panel material by the application of heat and pressure.

4. A method according to claim 3, characterised by tensioning the panel during the application of heat and pressure .

5. A method according to claim 2 or to claims 3 or 4 when dependent on claim 2, characterised by heating the faceskins to above their softening point and to heating the core to just below its softening point.

6. A method according to claim 5, characterised by shaping the panel by pressing while tensioning the faceskins .

7. A method according to claim 2, or any of claims 3 to 6 when dependent in claim 2, characterised in that the face skins comprise random short fibre veils.

8. A method according to claim 2, or any of claims 3 to 7 when dependent on claim 2, characterised in that the softening point of the core is greater than or equal to the softening point of the faceskin.