In the second part of his trilogy, Adam Smith continues his exploration of loudspeaker design history with a look at electrostatic, ribbon and planar designs...
Even the most ardent students of loudspeaker design are often surprised to discover that the idea of operating a loudspeaker through a means other than a dynamic setup - using a cone, suspension, voice coil and magnet - goes back further in history than might be expected.
The Quad ESL57 - surely the most famous electrostatic loudspeaker of all...
In fact, it was way back in 1929 that Edward Kellog (yes, he of the Rice-Kellog moving coil loudspeaker fame!) first described the design of an electrostatic speaker that was made up of many small sections and had the ability to directly radiate sound without the use of a dynamic type system involving magnets or cones. The first practical design came about in 1953 when Arthur Janszen was granted a patent for an electrostatic high frequency loudspeaker.
The electrostatic loudspeaker design principle.
For those unfamiliar with the principle, the electrostatic loudspeaker generates sound by utilising a thin, flat diaphragm covered in an electrically conducting film, sandwiched between two electrically charged grids. Two grids are required to control the diaphragm in both directions, thus minimising distortion. For operation, the diaphragm is held at a potential of several thousand volts and the audio signal is stepped up and fed to the grids, each one out of phase with the other. An electrostatic field proportional to the signal results and the centre diaphragm moves within it, creating sound.
Of course, the most famous electrostatic design is the Quad ESL57, designed by Peter Walker and unveiled in the mid 1950s. This caused something of a stir at the time and rightly so, given that these were the days when the vinyl record was only just starting to take a hold, soft-sounding valve amplifiers were common and the most popular way to retrieve information from a record groove was a ceramic cartridge!
The ESL 57 wowed listeners with its spaciousness and impressively even frequency response. The only area it fell slightly short was in the bass, where the size of the panel became a limitation. A bigger panel would have meant more bass but this would have required even higher voltages inside the loudspeaker and would have been more difficult to control. Peter Walker struck a superb balance between size and performance and its enduring appeal pays testament to his design.
Relatively few other manufacturers explored the electrostatic route. In the UK, B&W dabbled briefly with the electrostatic tweeter panel allied to a conventional woofer in the DM70s. Overseas, KLH introduced the model 9s towards the end of the 1960s and these were large full range designs. Other notables were models from Acoustat and the Infinity Servo-Statik, amusingly described as "appallingly expensive" by Stereophile in 1975, owing to the fact that the two treble panels, conventional bass unit and combined electronic crossover/bass amplifier would set you back $2,000 at the time!
Today, Quad are still flying the flag for electrostatics, and the latest 2805 and 2905 models show just what this technology can achieve. They are joined by the likes of Martin Logan in the US and Kingsound in the Far East to give an often stunning alternative to a box loudspeaker.
THE RIBBON METHOD
One of the most enduring problems facing the loudspeaker designer for many years has been at the treble end of things, where a good response really requires a diaphragm that weighs as close as possible to nothing. This allows for pin-sharp transients and stunning levels of detail, but is not something that can be easily achieved with a dome or cone. For this reason, the idea of a very fine ribbon, suspended in a magnetic field, was first proposed after the First World War. Unfortunately, whilst this technology proved suitable for use in microphones, as developed by Erwin Gerlach and Walter Schottky of Siemens in 1924, the magnetic materials available at the time were not suitable for it to be turned into reverse and used as a loudspeaker. It was not until the late 1930s that high flux permanent magnets became available to make them a reality.
In a ribbon loudspeaker, the ribbon itself is usually a light metallic material such as aluminium, and it effectively acts as both the diaphragm and voice coil. It is supported in a very strong magnetic field and thus is very well controlled, allowing it to respond instantly to the signal input. One deficiency of early models, however, was their sensitivity, which was very low. To raise it, they were generally used within a suitably designed horn.
One of the first well known ribbon loudspeakers also hailed from the same company as the ESL57, being Quad's 1940s Corner Ribbon loudspeaker, utilising a horn-loaded ribbon tweeter allied to a conventional moving coil bass unit. A bigger step forward was taken a few years later when Stanley Kelly developed his own ribbon tweeter, which was later to be taken over by Decca to become the classic Decca Ribbon.
Mordaunt Short MS737 - ribbons and cone drivers in harmony...
This was a big step forward and achieved great popularity with both DIY enthusiasts as well as established loudspeaker manufacturers, who bought in these units to use in their own designs. One example of this was the Mordaunt Short MS737, mating a pair of KEF B200 bass drivers to a Decca ribbon with impressive effect.
Ribbons are gaining in popularity today, as modern materials and technologies have made them much more sensitive and user friendly. The likes of Monitor Audio, JAS Audio, Rountree Acoustics and Ruark have all made use of such tweeters in recent times, to great effect.
Of course, the small size and weight of a true ribbon has generally limited its use to treble units. However, various manufacturers have adapted the technology in order to extend the frequency range of their loudspeakers so that the full frequency spectrum is covered. Diehard enthusiasts cry that these are not true ribbon loudspeakers, but they stay true to the basic principle of operation and often achieve stunning results, so we'll leave them to their debates on various forums!
One of the most enduring manufacturers of this type of design has been Magnepan in the USA, with their Magneplanar range of loudspeakers. To their credit, Magnepan actually describe their drivers as 'Quasi-ribbon' and they work by having a series of bar magnets lined up side by side, with a gap between each, and a very thin Mylar sheet stretched out above them. Strips of conductive ribbon are then laid out along this sheet so that they sit above the gaps between the magnets.
The advantage of this is that there is no need for the transformer that is usually required with a small ribbon in order to raise the very low impedance to a level that can be driven by an amplifier. Also, the size of the transducer allows for proper full range operation and means those ribbon benefits stretch right down to the bass region.
Quasi ribbons - Magnepan loudspeaker construction.
In fact these designs hark back somewhat to the original Blatthaler loudspeaker design of the 1930s. Information about these is a little sketchy but they appear to have been an electromagnetic approximation of an electrostatic design, with a large flat magnet behind a diaphragm with a conventional winding inside it. As mentioned with electrostatics, a grid is needed both in front of and behind the diaphragm in order to control the movement properly, and thus minimise distortion. This would not have been possible with the Blatthaler as the front control would have been another magnet that blocked the sound! Despite its apparently prodigious power handling capabilities and achievable SPLs, the Blatthaler quietly faded away and gave way to the more well known electrostatic and ribbon designs.
One very interesting adaptation of ribbon technology came about in 1973, when Dr. Oskar Heil invented the air-motion transformer, which was subsequently licensed exclusively to ESS and became a staple feature of their loudspeakers from that time.
The Heil Air Motion transformer squeezes air out from a concertina ribbon.
The air-motion transformer took the ribbon idea in another direction. It was, again, a high frequency unit and consisted of a mylar diaphragm, which was bonded with conductive aluminum strips. The surface area of this construction was actually equivalent to that of an eight inch conventional cone driver, but it was then folded, in an accordion-like manner, down to a compact size of around one inch, to give better approximation of a point source. This diaphragm assembly was suspended in a magnetic structure that concentrates a very strong magnetic field around it.
When a signal passes through the aluminum strips, the whole assembly acts just like the aforementioned accordion, squeezing itself in and out with the input signal and moving air out of the front to create sound. It can move much faster than a conventional dome driver and gives superb clarity and high frequency transient response.
Next month: the oddities, rarities and ones that didn't quite succeed!