From Hi-Fi World - March 2008 issue




(PART 1)

Adam Smith charts some of the many milestones in the development of the humble loudspeaker since its invention...


New recruits to the school of high fidelity are often surprised to hear that loudspeakers are something of a technological throwback. Despite the applications of space-age materials to cones, computer aided finite element analysis to the design of suspensions, the application of swish materials to motor units and tweeter domes, all topped off with some exotic veneers to cabinets, none of these can alter the fact that the resemblance to the very first moving coil loudspeaker, designed over eighty years ago, is still more than superficial.




Where it all started - the original Rice/Kellog speaker design drawing.


However, just because the basic principles remain the same, there have been more than a few steps taken along the road of loudspeaker design, and along that way there have been innovations, frustrations and more than a few ideas that have fallen by the wayside...



Of course it was all much easier in the old days, when all that was required for sound reproduction was a needle in a groove, which set up vibrations in a sound box, sent these up the "tone arm" and into a nice big horn. Bigger horns meant more volume, a simple flap could be utilised to control output level and that was that. Then, following work from Thomas Edison, John Fleming and Lee DeForest, the amplifying valve took hold and things would never be quite the same again.


It was actually in 1874 that Ernest Siemens was granted a patent for "obtaining the mechanical movement of an electrical coil from electrical currents transmitted through it" but he did not use it for sound reproduction. However, a second patent in 1878, following Alexander Graham Bell's invention of the telephone, did and this formed the basis of a parchment-based diaphragm to be used at the heart of a moving-coil transducer, allied to a suitable horn.


Various improvements quickly followed, such as Oliver Lodge's improved loudspeaker of 1898, with non-magnetic spacers to keep the air gap between the inner and outer poles of the magnet system. The development of what we now know to be the main cone followed and then, in 1909, the suspension used for centering the voice coil arrived. Finally, however, the basis of the modern loudspeaker was finally laid down in 1925 by Chester Rice and Edward Kellog when their General Electric research paper established the basic principles of a directly radiating loudspeaker with a small coil-driven diaphragm in a baffle with a uniform response over a broad frequency range.


This was only the first step and development has continued ever since, even if the modern result is remarkably similar in essence. As the quality of source material and valve amplifiers improved it became apparent that the single drive unit was becoming a limiting factor and in 1928 Herman Fanger invented what came to be known as the coaxial drive unit, featuring a small high frequency horn with its own drive diaphragm that was located in the centre of a larger unit. A patent was granted in 1933 and, as we know, this has led to some interesting developments over the years.


As our very own contributor and guru of all things vintage, Haden Boardman, puts it "lots of crusty old Japanese drivers from the likes of Coral, Pioneer, Diatone and even Panasonic had a mediocre HF pressure unit poking through the centre of a bass unit". On the UK side of things, the likes of the Whiteley Brothers' 'Stentorian' loudspeaker was one of the first exponents of this type and a little company called Goodmans gradually refined and developed their Triaxiom designs. In the US, possibly the most famous of the coaxial drivers was the Altec 604 introduced in 1944 and still in production now, thanks to Oklahoma's Great Plains Audio buying up and utilising the original tooling from Altec Lansing's final owners, the Telex Corporation.




Altec Lansing's 604 coaxial drive unit - first unveiled in 1944 and still in production today.

On this side of the pond, however, one of the most notable developments in coaxial technology occurred in 1946, when Tannoy developed the Dual Concentric drive unit. Where this differed from other designs was that it located the tweeter motor unit behind the motor unit of the bass driver and fired it through a specially designed 'acoustic waveguide', named after the analogous behaviour found in microwave electronics. This meant that the bass unit could operate properly and without interference, as there was no horn or other protuberance in the middle of its area of operation, meaning an even 'point-source' response with minimal distortion or reflection problems.




Tannoy's Dual Concentric drive unit, a design that refined the coaxial loudspeaker and one that forms the basis of the company's output to this day.


The other famous and successful UK coaxial development did not occur until 1988 when KEF introduced the Uni-Q drive system. Where this was different again was that it made use of a, by then conventional, dome tweeter in the throat of the bass driver. Such a move had only recently become possible thanks to the advent of rare earth magnetic materials such as neodymium that offered the same amount of magnetic flux in a much smaller package than a comparable ferrite item. This did away with the need for unwieldy horns either in the centre of the drive unit or in the middle of the bass cone and meant that the drivers could be made smaller. Of course, it introduced a whole new set of problems such as the siting of a tweeter dome at the centre of not only a cone that acted as a horn, but one that was moving as well! Fortunately KEF's use of computer aided design since the 1970s helped them immensely in taming the problems that arose, and Uni-Q has continued to have a long and successful life.




A B&W bass driver featuring their Rohacell sandwich style cone, some forty years after Leak came up with the idea.


Of course, not everyone went down the point source route and Bell Labs' 1931 development of what they called the 'divided range' loudspeaker paved the way for multi drive unit applications and the electrical crossover.



Another area that has undergone continuous development since the early days is in the shape and material composition of the cone itself. In the early days, parchment or paper were the obvious choices, as a result of their light weight and the relatively weak motor units in the early days of the technology. Paper is still popular today, but more exotic materials have gradually become more prevalent. One of the major steps in this was in 1961 when Dr. Don Barlow of Leak announced the debut of their 'Sandwich' cone consisting of a layer of expanded polystyrene between two thin layers of aluminium. This gave the benefit of rigidity, whilst keeping the overall structure relatively light in weight.



JVC's wooden coned loudspeaker drivers were made possible by soaking the wood in sake to prevent it splitting when formed.



The idea has remained popular to this day and one of the most well-known exponents of the art currently is B&W, whose cones on their larger bass drivers, such as that used on the 801D, consist of a layer of 'Rohacell' material, which is a hard foam designed mainly for the aerospace and automotive industries, between two carbon fibre skins - new materials, certainly, but a tried and trusted formula!


Considering single layer cones, virtually all materials you care to think of have been tried at one time or another. From Kevlar to aluminium, via carbon fibre, some most unlikely results have been obtained. Possibly the strangest was around 2000 when, after twenty years of research, JVC's Toshikatsu Kuwahata discovered that soaking sheets of wood in sake made it pliable enough to form into loudspeaker diaphragms! Apparently Suntory whisky did not work, and this was suspected to be as a result of the extra distillation after fermentation of the whisky (sake is just fermented). Either way, it must have been one heck of a development session...



The Acoustic Research AR1 loudspeaker introduced the concept of the small, sealed box loudspeaker.



Possibly the most enduring materials however, have been plastics and the most notable exponents of this were Spendor, with their development of the Bextrene cone in the 1960s. Legend has it that the BBC's development work during this time resulted in the need for drive units that exceeded the capabilities of the paper-coned drivers then commonly used. Spendor's Spencer Hughes put an iron bedstead, a compressor working in reverse and an electric fire to uses that their manufacturers could never have envisaged, and finally succeeded in producing the first eight inch cone through the use of this very basic vacuum former. This found its way into the BC1 and the rest is history. Spendor never stopped refining the driver design or experimenting with different materials, however, leading to the improved SP1 in 1983, featuring a homopolymer polypropylene cone.



Going back to the early days, one of the aspects that was limiting both the abilities of the designers of the times, as well as the visual acceptability of their designs, was that early loudspeakers tended to be either open baffle, infinite baffle with a very large closed space behind the driver, or horn-loaded, using a design like the famous 'Tractrix' developed by Paul Voigt in 1926. Although horns remained in use for many years and still have a strong following today, even Voigt's Folded Corner Horn of 1943 failed to reduce the horn to a domestically acceptable size.

One alternative was discovered by Albert Thuras whilst working at Bell Labs when he filed the patent for the bass reflex enclosure in 1931. Here, the driver is located in an enclosed box, with one or more port tubes to the outside world. This does not just act as a vent, but actually sets up a Helmholtz resonance within itself and acts as an independent radiating source, albeit one with a phase shift.




Cross section of Radford and Bailey's original transmission line enclosure design, first featured in Wireless World in 1965.


A design that was outwardly simpler, but technically not necessarily so, came along in 1954 when Acoustic Research's Edgar Villchur invented the 'acoustic suspension' enclosure. As he explains it, the idea was to replace most of the mechanical restoring forces of the drive unit, namely the surround and suspension, with that of a more linear force - the air pressure inside the enclosed box. This method actually lent itself perfectly to shrinking the size of the enclosure, in order to give the air strength required inside the cabinet, so was viewed as a win-win situation.


The third main cabinet configuration arrived in 1965, when Arthur Radford and Dr. A Bailey described a design where the rear wave of the loudspeaker was to be completely absorbed without damping the loudspeaker’s motion or modulating it from internal reflections and resonance. Their reasoning was that this rear wave had to be channelled down a tube or channel long enough, and filled with absorbent stuffing, to resist resonance below the desired frequency. In this way, the rear wave is absorbed and, unlike the bass reflex enclosure, not used for reinforcement. In practical terms this meant the line had to be very long and, for domestic applications, the inherent resonance (typically at a quarter wavelength) is exploited to enhance the bass response in this type of enclosure, albeit with a less absorbent filling.




Thiele Small parameters for the SEAS H1215 drive unit as used in World Designs' WD18BR

loudspeaker. These parameters still form the basis of loudspeaker design.


Development of all of these designs tended to be rather hit and miss until the work of Neville Thiele and Richard Small in the early 1970s. They came up with a method of predicting the response of a loudspeaker drive unit, and its behaviour in an enclosure, through its physical properties. Nowadays, Thiele-Small parameters form the basis of any loudspeaker design, whether closed box, ported, transmission line, or some of the more recent variations such as the bandpass enclosure. They changed the way people designed loudspeakers for ever, and life wouldn't be the same again


Next month - ribbons, electrostatics and planar designs.  (PART 2)



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