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Loudspeaker design – the certainty of change

New materials and improved DSP are helping to create innovation in speaker design. Simon Duff looks at some of the latest developments.

Alternative materials and the availability of new resources are a major concern for speaker designers who are increasingly looking to other industries to provide new avenues for ideas and inspiration.

For the future of transducers the two phrases that come to mind are ‘material science’ and ‘digital electronics integration’. These are the areas that have accelerated over the past decade and are determining how loudspeakers or sound systems that employ them will be different in the future. It is in the area of material science that loudspeakers have made the greatest advances since their commercialisation in the 1920s and ’30s, when the compression driver was invented.

Since then, the materials available to make diaphragms and create the magnetic field have changed and advanced, leading to huge improvements in loudspeakers. Whether that’s shifting from aluminium to titanium or beryllium or some composites for the domes on tweeters and compression drivers, or going from plain paper to carbon fibre, Teflon, fibreglass,

Kevlar or mixtures of all those materials with paper to make speaker cones. There has also been progress from original field coil magnet assemblies, which are essentially electromagnets, to alnico magnet material, to the lower cost ferrite magnet material and the more modern samarium cobalt, (a rare earth magnet developed in the early 1970s) and neodymium magnet materials. Technology advances in materials allow speakers to operate more efficiently, go deeper in bass, or, of particular importance, to have lower distortion than the generation before them.

So what is driving change in the current PA market place? Giacomo Previ, sales and marketing manager at Italian transducer manufacturer 18 Sound, believes that minimising the size of PA systems without decreasing the SPL output and sound quality is key. He says: “One of our development directions at the moment is to release loudspeaker components able to reach this goal. The overall benefit will be in reducing the cost for a PA system needed for the same audience size.”

Talking about materials, Previ continues: “Raw material commodity prices such as iron and neodymium are a great uncertainty today, that’s why we are focusing on developing loudspeakers able to use the magnetic motor energy in the most efficient way. This big challenge is an invaluable occasion to focus our creativity on finding a practical solution to the problem. One good example of this work is in our brand new 18” neodymium subwoofer 4” voice coil model.”

German studio monitor manufacturer, and developer of the ART ribbon tweeter, ADAM Audio, last year overhauled and relaunched its flagship S Series monitor line. Part of the speaker includes the X-ART tweeter, with a design derived from the use of new materials. Klaus Heinz, co director and designer, comments: “The big change is we now have a big block of neodymium behind the diaphragm. This not only improves the magnetic properties of the product, but it can also improve the frequency response because the diaphragm geometry has been changed to make use of this. X means it’s an extended frequency response. We have a real -3dB point at 50kHz, a tweeter that does 95-96dB SPL at 1m, and we have the efficiency better by 4dB across the whole range, which is an awful lot. That might sound like just technical data but it is really outstanding, no one else can offer that without a horn in front of the tweeter.”

18 Sound believes that when it comes to tweeter design, the best solution for obtaining high SPL levels in the high-frequency audio spectrum is still represented by the HF compression driver design. Previ explains: “We are currently investigating new composite materials for extending the transducer linear working area beyond its actual limits, in order to widen the frequency response and minimise the signal distortion. There are some new interesting transducer technologies around that seem to be very attractive, but it will take a long time before they will be attractive from a market point of view.”

Today’s power amplifier energy capabilities are able to manage a tremendous output current, so when it comes to subwoofers 18 Sound has developed a new generation of subs – the 9000 family – capable of managing very high power ratings and consequently able to withstand increased electrical and mechanical stress. Previ adds: “A very important part of efficiency in the loudspeaker is due to the power compression factor, that measures the loss in SPL output when the voice coil is heated up due to the current delivered by the amplifier’s output. When comparing our 9000 family speakers with traditional loudspeakers, power compression figures show an average reduction of almost 2dB. The final result is an increased capability to generate impressive SPL when driven at extremely high power for long periods of time.”

The latest theory in advanced research and development for many loudspeaker technologies includes powerful DSP that manages functions such as crossover, EQ, soft limiting, compression and dynamic boost bass. These elements can be optimised to match and complement each other resulting in superior performance. Alessandro Manini, head of RCF’s R&D department, comments on the recently introduced ART 4 Series speakers, which are medium power (400W) portable plastic moulded speakers equipped with digital amplifiers: “All the models of the ART 4 Series feature two special equalisation presets: Extended Frequency Response, which slightly emphasises the bass response for playback music and low volume applications; and Linear Frequency Response, for live music applications. Inside/outside voice coil wiring technology is used on all the transducers mounted on the 4 Series allowing the speaker to stay cooler and therefore play louder for longer. The use of neodymium magnets drastically reduces the weight of the final cabinet.”

Andrew Richardson, general manager at Adam Hall, distributors of Eminence speakers, a leading design of guitar speaker, agrees and says what he believes is driving progress: “Lighter speakers with greater output to keep up with today’s amplifier technology and computer analysis. Stiffer, lighter cones built for hot, humid, ambient conditions are becoming more in demand as well. All of the Eminence cast frame neodymium models are optimised using Finite Element Analysis, thus affording the lightest, most efficient motor possible.”

In terms of improvement to voice coil design Manini reports that RCF has enhanced its production process to reach higher temperature resistance and consistency: “We have developed an extensive use of our unique Impedance Control Coil technology for our Mytho Series Studio Monitors (pictured). The ICC technology is a secondary coil wound on the speaker yoke and driven in opposite phase to the primary coil, which has the function of cancelling the primary coil inductance, increasing the speaker sensitivity and reducing the speaker distortion. A primary effect of this technology is the development of the temporal behaviour of the speaker, improving the crossover transition from the midrange to the compression drivers.” Richardson comments that Eminence voice coil development has involved “improved power handling through high temperature bobbin materials and thermally optimised magnet structures”.

Loudspeaker designers are continually working to find new ways to minimise loudspeaker distortion, whether it be mechanical, acoustical or electromagnetic. At UK speaker manufacturer Celestion, recent innovation includes advances in voice coil design that is able to withstand higher temperatures. Double-sided coil-winding facilities enable quicker heat dissipation. Improved LF piston linearity has also been implemented.

John Paice, marketing manager at Celestion, comments: “Our ability to model, measure and analyse more accurately enables us to pinpoint specific characteristics we wish to concentrate on. For example, we can look closely at the symmetry of cone movement (whether the cone moves in the same way when it moves ‘forwards’ as when it moves ‘backwards’). In general, the more symmetrical the cone movement, the more linear the performance of the speaker (and that means less distortion). Celestion has developed a way of ensuring a more even clamping force holding down the compression driver diaphragm.

“This is yet another innovation that has enabled us to further reduce distortion (caused by minute deformations in the diaphragm material when you get uneven clamping).” The company has patented a new phase plug design method and this is being used with all its high-end compression drivers, such as the new CDX14-2420 device, featuring a deep drawn titanium diaphragm that increases stiffness and hence linearity, thereby reducing distortion. In terms of other products, 2010 will see the release of two 1” exit, ferrite compression drivers: CDX1-1445 with a PETP diaphragm and CDX1-1440 with a titanium diaphragm. These devices offer an extremely compact, lightweight and cost-effective HF solution. Their latest LF driver is a 21” diameter neodymium subwoofer, the NTR21-5010JD. It features a paper cone that is reinforced with glass and carbon fibre.The R&D team at Celestion uses a modelling and measurement technique called Finite Element Analysis (FEA) in initial speaker phase. This uses a set of computationally intensive mathematical techniques to simulate a particular design, enabling the creation of a virtual loudspeaker and is able to predict its performance in a given environment. Paice continues: “Only when a virtual model comes close enough to the desired target performance do we need to make a physical prototype. This results in a huge saving in terms of cost and development time.

To make best use of these techniques we need an in-depth understanding of the materials we are using (eg, magnets, cones/ diaphragms or the chassis metal work). The resultant information makes for a better computer model. We can provide significant weight savings by employing lightweight materials, such as neodymium magnets. We can also use our FEA modelling expertise to optimise the amount of material used in a
particular design.”

The possibilities of material science, plus some smart thinking, is leading to other ways of making speakers. For instance: Fraunhofer’s carbon nanotubes (see next issue); FeONIC’s F-Drive’s which turn resonant surfaces into loudspeakers; the University of Warwick’s FFL foils which, as reported last in last month’s PSNE, received more VC funding; even Yamaha’s TFL innovation.

But in spite of all the breakthroughs and advancements in technology, it seems traditional speakers requiring high SPL will, in essence, remain constructed around speaker cones and voice coils, and it looks set to remain this way for some time.

Why in 2010 are we still stuck with cones? The answer is simple. Because they work really well and they’re still very cost effective.

The bottom line in acoustics is that a loudspeaker must move air and vibrate air molecules that are butted up against a diaphragm. The harsh reality is, the air we breathe and the air that we listen to sound in isn’t particularly linear, and doesn’t accurately reproduce the vibrations that it gets. So over distance, distortion is created just from the fact that the air, which is non linear, is the propagation medium.

And as sound pressure levels increase, so high levels of distortion are generated, just because of the air. Can we change the laws of physics? Maybe not, but keep watching to see if we can at least bend them a little…