(Part 3)

In the final part of his look at loudspeaker design, Adam Smith ponders the weird, the wonderful and the ones that didn't quite make it...


Over the past two months, we have looked at the ways in which loudspeaker designers have come up with a variety of different ideas in order to maximise the potential of sound reproduction in the home. The methods considered so far have all been successes, to a greater or lesser extent, but it must be remembered that for every Ferrari or Porsche, there is always a DeLorean! Consequently, it seems only fair to take a trawl through those ideas that, for one reason or another, have not had the success that their designers were hoping for. The first two, in fact still use the conventional moving coil drive unit as their basis, but with a couple of additions...


To me, one of the most elementary ideas with regard to a moving coil loudspeaker is in devising some way of monitoring and consequently controlling, the movement of the cone. Sadly, someone thought of it before me. Way back in 1958 G.H. Brodie was granted a patent for a loudspeaker drive unit that featured an assembly on the chassis, plus a contact on the cone, which formed a parallel plate capacitor, the capacitance of which varies with the movement of the cone.


This design ethos is still around today, courtesy of a company called Servo Speaker but uses a capacitor actually built within the drive coil of the loudspeaker - one fixed conductor is sandwiched between two insulating layers at the top of the pole piece and the moving plate is fixed to the voice coil itself. Both of these utilise the feedback signal generated by the capacitor to modify the signal being sent to the loudspeaker, thus ensuring that its movement is controlled.


Servo Speaker's drive unit design - Motional Feedback lives on!

Of course, other methods have been utilised over the years to achieve the same ends - probably the most well known hail from the 1970s and were marketed by Philips in their 'Motional Feedback' series of loudspeakers. These were active units, which also featured motional feedback control of their bass drivers through electronic control. Here, rather than using a capacitor, they used a piezo-electric accelerometer to provide information about the movement speed and direction of the cone to the amplifier, in order for it to be controlled, thus reducing distortion.


Motional feedback never really caught on in a big way. The Philips loudspeakers were expensive at a time when more high quality budget equipment was appearing and they effectively priced themselves out of the market. Where the technology does continue to be found is in subwoofers where it can be put to good use in monitoring the movement of a sub-bass drive unit to prevent damage at high excursions and volume levels. In their Servo 15 subwoofer Paradigm use an accelerometer to provide feedback to the amplifier about the cone's movement, as do Velodyne with their Digital High Gain Servo technology..

I SEE...

Whilst Tannoy's Dual Concentric layout and KEF's Uni-Q drivers are well known in terms of point-source technology, another way of approaching the problem came and largely went in the 1980s and 1990s. Developed by Elei Boaz and licensed to Goodmans Loudspeakers, Inductive Coupling Technology (ICT) was an intriguing approach to the old point source problem.


Unlike the Tannoy and KEF approaches, ICT did not use two separate drive units but one single one with a few modifications. Boaz discovered that a simple shorted turn in the centre of the voice coil, in the form of a small aluminium dome mounted on a resilient foam pad, with a shaped phase plug in front of it, acted as a single turn transformer which was most efficient at high frequencies.


Development of the shape of the cone, the thickness and weight of the ICT collar and careful shaping of the phase plug in front of it led to the development of a wide range of drive units that found their way into a variety of applications, including Sony televisions and a large number of cars - my very own Ford Scorpio has a set and very good they sound, too! [Any excuse to show off about his motor! - Ed.]


GLL's ICT drive unit utilised a small coil in the throat of the loudspeaker, driven by

magnetic induction from the main voice coil.

On the hi-fi side of things, GLL developed the Imagio 100 series of loudspeakers in the 1990s which gave a range of designs, from a standmounter with a single 6.5in (165mm) ICT driver up to a large floorstander that used a similar drive unit in tandem with a pair of additional 6.5in bass drivers. These were superseded by the 200 series that improved cabinet design and added transmission lines to the additional bass drivers in the floorstanders. Finally, the 300 series reduced the range to one standmounter and one floorstander and added real wood veneers.


Sadly, GLL pulled out of the hi-fi loudspeaker market in 1998 and their other businesses gradually declined, before their doors closed for the last time in 2005. The loss of the hi-fi units meant that development of the ICT technology effectively stopped and, despite some very interesting research papers detailing further ideas to push the HF extension even higher, nothing came of this. Tannoy and Martin Audio still use ICT today, the former making use of its robustness in the P.A. arena (as there are no tweeters to overheat and blow up when the volume level gets too high) and the latter using its compactness in a handful of custom installation designs.



Definitely a 'nearly' rather than an outright success, ICT has largely faded from the public consciousness now - a shame I feel, as it had some unique strengths.



One interesting idea that has come up in the pages of Hi-Fi World a few times before is the Fane Acoustics Ionofane ionic discharge loudspeaker but few people know how far back in can trace its heritage. In fact, the date of this goes back before the invention of the loudspeaker, the amplifier or even the telephone!


Back in the late 1800s, Carbon Arc street lighting was in widespread use in the UK as Thomas Edison had yet to invent the filament light bulb, and this generated light through the generation of a spark between two carbon electrodes. The result was light but also a constant-pitched humming noise from the electric arc.


In 1899, Physicist William Duddell was appointed to investigate the problem and, during the course of his research, discovered that by altering the voltage applied to the lamps he could vary and control the resulting note. A German scientist, Dr. Simon, had made the same discovery the year before and he actually demonstrated in public the loud-speaking device that resulted. He also found that the phenomenon gave a modulated light beam in addition to the sounds, with which the German Navy managed to make telephone calls between ships using a modulated arc searchlight and a photosensitive selenium sensor.


The mains-powered, ionic discharge Fane Acoustics Ionofane tweeter.

Back in the UK, Duddell found that he could attach a keyboard to one of the lamps and make a musical instrument. When he showed this to the London Institution of Electrical Engineers it was noticed that arc lamps on the same circuit in other buildings also played music from Duddell’s machine and generated speculation that music delivered over the lighting network could be created - if followed up, this could have been the very first example of networked music! However, Duddell didn’t even file a patent for his instrument and it never became much more than a novelty.


The phenomenon was not applied to a proper loudspeaker design until 1946 when a French scientist, Siegfried Klein, confined the arc to a small tube and horn-loaded it to increase output. Development work took place on the design and DuKane were granted the patent license in the USA (Plessey were the UK recipients). The name Ionovac was coined in 1956 and Electro-Voice began marketing the design in 1958.


The Ionofane came along in 1965  and was produced by Fane Acoustics Ltd. of Yorkshire. Retailing for £29/8s the model 601 tweeter offered a sensitivity of 95dB at eighteen inches and also formed the high frequency part of the company's model 603 loudspeaker, allied to a 15in bass driver and a 5in midrange.


B&W's P2 loudspeaker used the Ionofane tweeter.

Possibly the most well known application of the Ionofane however, was in the B&W P2 loudspeaker of the mid 1960s. This mated the Ionofane to a 13x8in elliptical EMI bass driver which, by all accounts, rather held the overall design back from true greatness! Nevertheless, the higher power handling P2H version was used by the BBC as a monitoring loudspeaker and B&W also made units under license to Sony in Japan, so the ionic 'word' did spread across the globe. In the end, though, the necessity of regular electrode and control valve replacements and the need for a mains supply to the loudspeaker effectively killed off the bulky Ionofane. It also didn't help that one of the side-effects of the ionisation of the air that produced the sound was quantities of ozone gas (O3) plus a small amount of Nitrous Oxide (NO2)! Both of these were in very small quantities and would have caused no ill effects in a well ventilated room, but Ozone is poisonous and so is not to be recommended. Mind you, as Nitrous Oxide is also used for anaesthetic purposes and is better known as laughing gas, at least any listener locked in a sealed listening room would have passed away happy... Nevertheless, Ionofanes still have a loyal collectors' following and can still be serviced/repaired.



The Ionofane was not the only unit to make use of gas discharge as another, rather unique, design appeared in the 1970s. Designed by plasma physicist Dr. Alan Hill and unveiled at the 1979 CES, the Hill Plasmatronic loudspeakers were like nothing seen before, or since come to that! At the bottom end, a 12in bass driver and 4in midrange driver covered the frequency range up to 700Hz - so far, so conventional. However, above this, five plasma jets took over, using their own tank of helium (neatly tucked into the back of each loudspeaker!) to generate high frequencies up to a claimed 100kHz or more and pulsed along with the music at the same time!


The Hill Plasmatronics loudspeaker - helium extra!

A control unit offered flow monitoring meters and adjustments for each of the individual jets and, apparently, the Hills were largely easy to use and require little in the way of regular maintenance. Sadly, most of us are unlikely to find out as only sixty pairs were ever made and they retailed for between $8,000 and $16,000 during the 1980s. Once again, Ozone is a by-product of their operation and so they require a good supply of fresh air in order not to injure the listener.


Personally, I cannot help but smile at the idea of a pair of loudspeakers where listening has to be suspended every now and then whilst a new tank of helium is obtained - I have propane canisters delivered regularly for home heating duties as our house does not have mains gas, so I suppose I could just add them to the order...


Once again, the plasma loudspeaker continues to live on, thanks to a band of dedicated enthusiasts. A German enthusiast by the name of Ulrich Haumann even has a design for a DIY version on his website - have a look at


Of course, we cannot recommend that any of our readers have a go at this, due to the hazards involved, and even Ulrich himself points out that the design uses high voltage and very high RF power in the region of 27MHz. He rightly points out that he is, "not responsible for (people) doing something stupid after reading this page". Wise words!



One of the biggest steps forward in loudspeaker technology over the last few years has been in NXT, or the "Distributed Mode Loudspeaker". This uses one or more small activators to generate sound in a suitable panel, which gives great gains in terms of efficiency and size of resulting product. As a result, NXT has found a home in many smaller MP3 player speaker systems and has also made inroads into the hi-fi market thanks to the likes of  Mission M-Cubes and the Cyrus Icon X4.


It is not just in video that projectors are making themselves known, as 1 Ltd. are a new company that have developed the sound projector. This consists of a whole series of very small drive units that are digitally 'steered' by the controller to bounce of the walls, floor and ceiling of the listening area to arrive at the listener's ears and generate an impression of surround sound. Most notably used by Yamaha on their YSP series sound projectors, this technology has been shown to be very effective and is highly likely to move forward to be more commonplace in the future, generating highly feasible surround sound without the necessary array of loudspeaker boxes.


A very recent development in sound reproduction comes courtesy of airSOUND and their Orbitsound products. These generate a similar result to the sound projectors mentioned above, but use no digital signal processing to achieve their ends - simply a single forward-facing mono loudspeaker plus additional side-firing drive units on either side that generate spatial information for each channel, leaving the brain to process the resultant information and recreate the stereo image. Initial demonstrations show this to be quite effective, and as the technology is cheaper and simpler than the digital sound projector, this could well be an important step in the evolution of the loudspeaker enclosure


Another amazing idea is the SENNHEISER AUDIOBEAM. According to Sennheiser, "AudioBeam is a new type of directional loudspeaker that is able to focus sound like the light beam of a torch, creating an audio spotlight for museums, exhibitions, theme parks and many other applications.



AudioBeam works with ultra-sound, modulating the audible sound onto an ultrasonic carrier frequency, much like a radio station does, and then emitting this signal via 150 special piezoelectric pressure transducers.  Audible sound is only generated at a distance from the AudioBeam, when the signal is demodulated because of the non-linearity of air.  This can be imagined like the creation of many small, virtual loudspeakers in the ultrasonic zone.


No sound is audible beside or behind an AudioBeam — you will only hear the audio information if you are directly within the sound beam, or if the sound is reflected by a smooth surface.  This makes AudioBeam an ideal tool for information terminals, exhibitions, or even conferences where various AudioBeams emit several languages to a defined part of the auditorium. "


Of course, the above two items are based on moving coil loudspeakers but operated in a certain way. Possibly the only really new design I have come across recently has to be the ultimate ionic discharge loudspeaker. The premise is quite simple and actually harks back to the Carbon Arc lamp principle - take one high voltage Tesla Coil and modulate the signal to it to make music! Such a device was demonstrated at the DucKon 2007 exhibition by Steve Ward, an Electrical Engineering student, using a solid-state Tesla coil, modulated from the control unit in order to generate tones. He explains that, "it is the actual high voltage sparks that are making the noise. Every cycle of the music is a burst of sparks at 41kHz, triggered by digital circuitry at the end of a 'long' piece of fiber optics". Obviously the danger level is high (850 Amps anyone?) and the ozone generated must be vast, but you can't help but be impressed! Steve's site is here - - and you can see the singing Tesla Coil in action here - Needs a little more work to really capture the emotional essence of a string quartet, I feel...



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