Before the DDD driver

Welcome to the March 2022 German Physiks newsletter.

This is a continuation of the newsletter sent in December last year. It explains how the German engineer Peter Dicks was inspired by Lincoln Walsh’s brilliant Walsh omnidirectional driver, to design the DDD driver that is used on all German Physiks loudspeakers. It also explains the important improvements the DDD driver offers on the Walsh design.

The Walsh driver

Firstly a recap on the Walsh driver, how it was constructed and how it worked.

The basic construction is like a conventional driver that has been stood on end with the cone facing downwards. There is a magnet at the top and the voice-coil drives an elongated cone that is made of three sections:

• Top section: 0.064 mm thick titanium foil.

• Middle section: 0.076 mmm thick aluminium foil.

• Bottom section: 0.64 mm thick felted paper.

The driver was operated full range, without the use of a crossover and in the Ohm F loudspeaker that it was used in, it covered the range from 37Hz to 17kHz.

At low frequencies, all three sections of the cone moved together in unison, so here the Walsh driver worked just like the conventional pistonic drivers used in the vast majority of loudspeakers manufactured today.

The cone was comparatively heavy at about 125 grams and this would have limited the driver’s high frequency response. Walsh’s genius was to design the driver so that bending waves could be generated in the very thin and flexible metal sections of the cone. When the voice coil pressed down with sufficient force, rather than for the voice-coil/cone assembly moving in unison, the cone material nearest the voice-coil flexed and a wave travelled down the cone wall towards the open end. Because velocity of this wave was much higher than the speed of sound in air, it caused a sound wave to be pushed outwards. When the driver was working in bending wave mode, the mass of material that had to be accelerated, i.e. the moving mass, was that of the voice coil and the cone material in the wave. This was considerably less than combined mass of the voice coil and entire cone and it is this reduced moving mass that allowed the Walsh driver to cover frequencies up to 17kHz.

The shape of the cone and its vertical orientation caused the radiation pattern to be omnidirectional.

The angle and height of the cone and the cone materials were chosen so that it behaved like a pulsating cylinder and was thus phase linear i.e. it accurately preserved the phase relationships of all the individual frequencies within the audio signal.

The problems

The Walsh driver was capable of producing audiophile Nirvana for many very happy users, but it was not without its problems.

A review in the Stereo Buyer’s guide of the Ohm F, which used the driver shown above, recommended an amplifier with at least 200W rms per channel. For the Ohm A, which used an earlier version of the Walsh driver, they recommended 350 W rms per channel. In the 1970s and 1980s, high power amplifiers were not a common or as cheap as they are today.

Because of its construction, the driver was quite fragile. Consequently, it was easy to damage it when playing at high levels, which was when it really came alive and the magnificent dynamics that the driver was capable of producing could be appreciated.

Part of the energy in the bending wave could be reflected at the junctions between the different sections of the cone and this could lead to resonances. This was suppressed to some extent by damping applied inside the cone, but it remained a problem.

The complexity of the driver meant that it was time consuming and expensive to produce. This is what ultimately lead to the Ohm Loudspeaker Company ceasing production of the Ohm F loudspeaker and the Walsh driver that it used in 1984. Nearly 30 years later, the Ohm F, which is regarded as the best of the designs that used the Walsh driver, still has many enthusiastic users around the world.

Enter Peter Dicks

Compared to conventional drivers, the physics of the Walsh driver were very complex, both in terms of the mathematics involved, as well as the fact that a large number of parameters had to be optimised. These included the physical properties of the cone material, the cone material thickness, the magnet strength, the cone shape –  angle, minimum and maximum diameters and the properties of the cone termination. Trying to optimise so many parameters by experiment will have been impossible, so although the Walsh design’s concept was brilliant, its potential was never realised: until Peter Dicks came along.

Peter Dicks was a German engineer and mathematician. Although he had no direct involvement in the audio industry at the time, he was very interested in the behaviour of loudspeaker drivers. In 1978 he had become fascinated by the Walsh driver and decided to make a computer model of the driver in order to better understand how it worked. This was a complex task and took several months. Once he had completed the computer model, he then started to investigate to see how the Walsh driver might be improved. Over a period of several years he made a series of prototype drivers which he used to refine his new computer model until the model accurately predicted the performance of the new driver.

Peter showed his new driver, later to be dubbed the DDD driver, to a number or European loudspeaker makers, but none showed any interest. It was only when he showed it to Holger Mueller, the owner of what was to become DDD Manufactur, the company that makes German Physiks loudspeakers, did he find someone who shared his enthusiasm for the new driver. Mueller was himself an admirer of the Walsh design, having first heard it on a visit to New York many years earlier. Together they worked to further refine the design. The finished version appeared in 1992, on the first German Physiks loudspeaker, the Borderland Mk I. The first version of the DDD driver, shown above, used a diaphragm made from 0.025 mm thick titanium foil. Sonically this worked very well, but the material was difficult to handle in production and the field it was easily damaged when customers, or more usually their wives, tried to remove dust from its surface.

In 2008 the titanium foil diaphragm material was replaced with carbon fibre. This material was much easier to handle in production and in the field it proved immune to all but the most determined dusters.

The German Physiks DDD driver

Principles and advantages

How the German Physiks DDD driver works
Like the Walsh driver, the DDD driver uses both pistonic and bending wave radiation. Peter Dicks’ research enabled an additional mode of operation called modal radiation to be employed and this comes into play above the bending wave frequency range. To give a very simplified explanation of modal radiation, when the driver is radiating modally, patterns that look like the concentric rings of ripples you see when you drop a stone into water are established on the cone surface. Each one of these vibrating areas acts like an individual sound radiator. The small amount of cone material involved in each one means that the effective moving mass of each one is very small. As the frequency rises, the number of these individual radiators increases and thus their size and so their moving mass gets smaller. This low moving mass allows the DDD driver to have a much higher upper frequency limit than the Walsh. In the case of the carbon fibre DDD driver, the response extends out to about 30kHz, but we start to roll this off at 24kHz, in order to use the flattest part of the driver’s response. Ultimately, the stiffness of the cone material prevents these “radiators” getting any smaller and this limits the upper operating frequency.

Advantages of the carbon fibre DDD driver over the Walsh

Exceptional dynamic response: The moving mass of the carbon fibre DDD driver is less than 3 grams - less than 1/40th that of the Walsh, yet it can move as much air as a conventional 6.5-inch driver. This gives it an exceptional dynamic response, which is especially noticeable in the way that it reproduces the snap and attack of percussion.

Easily scaleable: The compact size of the DDD driver makes it easy to use a number together in order to produce higher maximum output levels. The four DDD drivers on our Loreley Mk III loudspeaker allow levels up to 120dB to be produced, which in turn allows a closer approach to live-music-like dynamics.

Freedom from resonances: Because the diaphragm is made from one material, joins between materials with different mechanical characteristics, which can give rise to reflections of the bending wave and hence resonances, are eliminated. The use of a computer model to design the DDD driver allowed the mechanical characteristics of the cone material and surround to be selected to minimise the residual energy at the cone-surround interface. This, together with some other proprietary techniques, control resonances very effectively.

Very resistant to abuse: The carbon fibre DDD driver is probably able to withstand more physical abuse than any other loudspeaker driver on the market. If you click here you can see a video demonstrating this. Unlike the Walsh driver, the DDD driver can also be driven very hard without fear of mechanical failure. This allows a long service life.

Easier to manufacture: Carbon fibre is much easier to handle in production than the thin aluminium and titanium foils used in the Walsh driver, and this makes the carbon fibre DDD driver much easier to manufacture.

Lower reaction forces: The much lower moving mass of the DDD driver means that the reaction forces generated by its movement are smaller and so it is putting less energy into the cabinet, which could otherwise generate resonances.

Improved phase coherence: The use of computer modelling allowed the selection of cone material and dimensions, such that the design ideal of a pulsating cylinder could much more closely be approached, thus giving improved phase coherence and therefore better timbal accuracy.

For a more detailed description of the development of the DDD driver please click here.

Conclusion

The Walsh driver was an exceptional achievement, especially considering that it was designed without the benefit of computer modelling. Peter Dicks took Walsh’s concept and with the aid of computer modelling and a not inconsiderable amount of work over many years, produced the DDD driver. This allowed the potential of the Walsh design to finally be realised, to the benefit of audiophiles around the world.

• An online review of the Emperor integrated amplifier.

• A comparison of our DDD driver with another omnidirectional driver.