Articles and whitepapers

The Rise of Flat Panel Loudspeaker Technology (2/12/2005)

By Richard Newlove, Amina Technologies

Flat display technology, the expanding number of TV channels, the ability to store and deliver sound and images locally, and interact at high speed online and wirelessly by mobile telephony, have created a new aspirational economy. The combination of house price rises, high employment levels and sophisticated television programmes, such as Grand Designs, show that building design scope and individuality are within our reach. Indeed over the last five to ten years in the UK, the home entertainment and home automation markets have blossomed into an important and lucrative section within the total consumer electronics industry.

People are increasingly aware of the possibilities of distributing media, be it computer, gaming, audio, video and CCTV, around all living spaces, and have the desire and funds to do it. Just about every form of media has sound attached, so integral to any system, in any space, is the need to reproduce audio. However, if we now want sound in every room of our very design-oriented home, we certainly do not want our beautiful spaces to be cluttered with boxes and wires. What we need are incredibly discrete or completely concealed audio reproduction devices.

The flat panel loudspeaker

Unlike a conventional speaker, which is in essence a highly-controlled piston-like structure, the flat panel speaker goes back to first principles in sound reproduction. Those simple experiments with a tuning fork at school, tell us that the tuning fork itself, when struck, can hardly be heard. Yet rest the fork on a surface e.g. desk, filing cabinet, etc, and the surface comes alive and reproduces a tone essentially at the frequency set by the tuning fork. The loudness of the sound will depend on how stiff and heavy the surface is. These surfaces are creating sound, not by being a highly controlled piston, but by vibrations that are occurring across the entire surface.

The acoustic musical instrument is just the same. The numerous strings in the piano are of specific length and tension to create individual notes. When the string is plucked or hit, the energy established in the string is transferred through the 'bridge' onto the soundboard of the instrument. The soundboard is essentially the whole body of the instrument itself. That body is vibrating, very powerfully in the case of the piano, to create the beautiful sounds we all expect from such an instrument. The frequency range is dependant on the size of the instrument - in effect the size of the vibrating area. The grand piano has an extremely wide frequency range because it has an enormous soundboard. The violin of course does not generate any where near the same low frequencies as the double bass, simply because it has a smaller vibrating surface area.

The flat panel speaker is very simply the concept of making a loudspeaker from a vibrating soundboard, put into practice. At first thought, one might think that sound, particularly the reproduction of voices, might be confused or overlaid, from a vibrating source. In practice of course, this is not the case. Our brain is immensely sophisticated and sorts out all the sounds happening around us into a consistent stream.

The Amina AIW5 in-wall loudspeaker

Composition of the flat panel

Using composite materials, which are incredibly stiff, yet very lightweight, the flat panel speaker is inherently thin, lightweight, can be curved, and can be virtually any shape and size. Yet it produces incredibly powerful sound levels. The final loudness of sound produced is dependent on how light the panel material is and on the energy that can be transferred to the panel by the 'engine' that transfers the electrical energy from the amplifier into the small movements that set up the vibrations in the panel. These engines, or as we call them, exciters, can themselves be a made from a variety of technologies. Electromagnet exciters are by far the most common, but flat panel speakers for special applications are available that use magneto-restrictive (crystalline material that changes size when a magnetic field is passed across them) exciters and piezo electric (crystalline material that changes shape when a electric field is passed across them) exciters.

Flat panel characteristics

Flat panel loudspeaker technology has the generic term 'NXT'. In terms of the audio energy reproduced, the flat panel loudspeaker has three characteristics that are inherently different to that of the conventional loudspeaker. Whilst these are not immediately obvious to the uninitiated, they are important in helping to realise the benefits when this technology is applied to real spaces.

The first is that the flat panel speaker generates sound in all directions. If it helps, think of the piano in the centre of the Royal Albert Hall. Sit anywhere around it and the reproduction is inherently the same. Conversely, the conventional piston-based speaker is directional in nature. Hence the reason we turn our hi-fi speakers to the central listening position. A non-directional device allows both speaker positioning and listener positions to be much less critical.

The second is that energy (sound loudness) tails off rapidly as you put more distance between you and a conventional loudspeaker. This is because it is a very small physical source (we sometimes call this a 'point' source). If you take a large flat panel speaker, the whole surface of which is exciting air molecules, then this is a large area source, and the tail-off in loudness will be much less severe as you move away from it. Again if it helps, think about how the violinist is not deafened, yet is holding the instrument very close to the ear, while the audience hear the violin very well. In practical terms, this helps create an even sound field in a given space.

The third, and perhaps most difficult characteristic to understand, or hear, is that a conventional piston speaker generates energy that is essentially phase oriented. Think of a pebble dropping into still water, and the resulting rings of waves moving outwards from the point of entry. This very consistent wave pattern is 'phase related'. This is essentially what a conventional piston speaker is doing. However a vibrational surface is very complex, generating a very complex waveform, not nice neat waves of energy. This is therefore termed 'non phase correlated'. In practice, this means that reflections from boundaries are simply more amounts of complex energy, rather than phase-related energy which either adds or cancels and all the rest in between. This means that when a flat panel speaker is played in a real room with walls and ceilings, the evenness of sound across the room will be more consistent. Again it may be helpful to think of the piano in a concert hall. The piano fills the hall very consistently, far more so than a conventional piston speaker playing piano music in its place.

Piezo electric exciters

Examples of flat panel speakers using piezo electric exciters are just becoming available in the high-volume consumer electronics industry. At least two mobile phones have already been released that use the transparent plastic protective membrane in front of the LCD display as the flat panel speaker, driven by special edge-mounted piezo electric exciters. This means that the phone can be held at a viewable distance and heard at the same time, without adding weight and keeping battery usage incredibly low.

Market implementation

In residential audio systems, flat panel technology is currently being applied in three main ways. Due to the exceptional wide sound distribution of a flat panel speaker, particularly at higher frequencies, this technology is extremely competent at filling a space - even large open-plan reverberant spaces - with an even sound field.

Brands such as KEF and Mission market NXT technology as standalone, thin and very aesthetically-styled loudspeakers. For high-volume manufacturers, this technology offers much more design flexibility, allowing the creation not just of 'thin' loudspeakers, but beautiful designs that think outside the box, with the end result of challenging our preconceptions of how or what we think a loudspeaker should look like.

Customisation

Other brands have chosen to make flat panel speakers that look like something more acceptable. SoundartUK for example, has designed a speaker that looks exactly like a canvas painting or image mounted on a wall. A major part of SoundartUK's service are the unique mechanisms and relationships with other companies that allow the consumer to choose the images to be used. One such relationship is with Venture, the UK's pre-eminent photographic studio. Upon choosing the photographic image as a result of a studio session, the client has the choice of having this as a piece of art, or as an active loudspeaker that looks exactly the same.

Flat panel speaker disguised as a wall-mounted image

Amina has taken this one stage further in order to achieve an unobvious or invisible source. Using the skills and techniques developed over the last six years from the commercial installation market, Amina has created a true in-wall speaker which is skimmed over with plaster. Both the composite panel structure and the thin layer of plaster and paint or wallpaper on its surface, become the vibrational soundboard. In fact in a stud wall, or a ceiling, the vibrations spread out beyond the boundary of the panel into the surrounding plasterboard. Just like the acoustic musical instrument (e.g. the sides of the piano), these vibrations are not as active as the main panel area, but they still make a valuable contribution in generating sound energy in the room, thus making an even more consistent sound field in the space. And just like the piano in the concert hall, such a surface can fill a very large space.

Installing a plaster in-wall speaker

Compared to a conventional piston-based in-wall speaker, which has a grille and limitations in terms of its position and directionality, the plaster in-wall speaker is truly flexible in terms of positioning, and truly invisible to the eye.

Given that audio has now spread into all rooms of the house, to have very visible conventional speakers in each room soon becomes overpowering. With flat panel speakers, be it designer objects (your bath or hot tub can even be turned into a speaker), artworks, or plastered in-wall devices, the interior design can be as creative as your imagination.

Sound quality

Apart from the usual subjective arguments when comparing one speaker against another, the sound reproduction from a well-designed flat panel speaker is not greatly different to that from a well-designed conventional one. Where the panels are relatively small - which tends to be the case in order to keep costs to a realistic level - they will not generate a great deal of low-frequency sound. This is generally overcome in home theatre and serious listening environments by adding a sub-bass discretely into the system design. What is different is the flat panel speaker's greatly enhanced ability to fill a room very consistently with sound, without having the usual issues of critical loudspeaker and listener positioning.

Installation

The interface to a flat panel loudspeaker is exactly the same as any other conventional piston loudspeaker. It is generally a low-impedance device in the region of 4 or 8 ohms, and it connects to a standard amplifier with the same type of loudspeaker cable. How aesthetic you want to be with this depends on your budget. If you are planning to put artwork-based devices on a wall for example, you will need to put speaker cable from the amplifier to that position. For those not keen to channel-out wall surfaces to accommodate the cable, some excellent-quality flexible flat cable (the type of thing used in a photocopier or scanner) is now available. This offers minimal intrusion on the wall surface and can be skimmed, painted or wallpapered over.

Home entertainment room in private barn with in-wall sound panels

Reliability

Reliability is absolutely paramount - no one wants to have to re-plaster a wall if something were to go wrong. NXT technology is inherently maintenance free. As the exciter is only moving fractions of a millimetre, the mechanical stresses are far less than with the drive unit of a conventional speaker. Equally, the technology has no cone surround or suspension that can fail due to material fatigue. The unit does contain a moving coil, and it is possible to overdrive this like any other speaker by applying too much power for too long. Provided the coil is not abused however, in wall products should last a lifetime.

The future

The uptake of flat panel display technology has increased enormously in the last couple of years, and will eventually make CRT-based devices obsolete. Many flat panel displays use conventional speakers either side of the display, resulting in a wide and bulky border. Manufacturers are therefore already beginning to integrate flat panel loudspeaker technology for very narrow, homogenously-coloured borders. One manufacturer even uses the transparent plastic membrane in front of the display as the loudspeaker. Indeed the two technologies are an obvious and perfect match.

Component manufacturers in the laptop computer industry are supplying touchpad systems, which now double as flat panel speakers, therefore reducing the total number of components in the computer.

Using glass as the vibrating soundboard and combining it with touch-sensing systems, a projection display and sophisticated online database, retail shop fronts can become completely interactive giant media points. This allows clients to search for, and receive information, and even order goods or services such as holidays, whilst standing in the middle of the high street, at any time of day.

A couple of prestige automotive brands have already adopted the technology for in-cabin entertainment systems. Apart from the weight saving aspects, the technology is more effective at filling the complicated cabin space with accurate sound. As mass-market auto manufacturers adopt surround sound in-car entertainment systems, the likelihood is that flat panel speaker technology will be an important component within the system.

Coming back to the home and work environments, due to the enormous design flexibility of the technology, it is quite likely that we will see it combined within fashion lighting products and lighting bulkheads as just one example of a number of dual-use applications that could be available in future. They can already be combined within wooden furniture such as cupboard doors, and this again may become more widespread if volume furniture manufacturers pick up on the concept of dual-use products.

Amina SoundUnseen AIW in-wall loudspeakers providing 5.1 surround sound

One more interesting jewel lies at the audiophile end of the market. There are developments that combine the sound energy created from this form of vibrational loudspeaker with that from a conventional piston speaker. The results are quite astonishing in the realism that is reproduced. This could have enormous ramifications in the high-end residential market, and in professional concert halls and theatres.

Conclusion

Flat panel loudspeakers are inherently thin, lightweight, do not need to be in a box, and positioning of the speaker and the listener is far less critical. The ability to fill spaces effectively provides fantastic freedoms for designers of loudspeakers, and for designers of real installations, be it residential or commercial.

It is now eight years since the first flat panel loudspeakers came onto the market. Compared to the 100 or so years that the conventional speaker has been available, we are standing at the beginning of long line of innovations yet to come. In our ever more design-conscious world, full of aspirational individuals and families, we are likely to see a lot more of this technology in the future.

Richard Newlove is the Managing Director of Amina Technologies Ltd, manufacturer of plaster-in-wall and customised flat panel loudspeakers.

www.amina.co.uk