The Development
of the DDD Driver
Truly Revolutionary
Award-winning Speakers
The German Physiks DDD driver is unique. It is an omnidirectional driver with an exceptionally wide frequency range and a uniform phase response. This text describes how the DDD driver works and compares it with the other most commonly used types of loudspeaker driver.
At first glance the DDD driver looks like the pistonic drivers that are used in the majority of the loudspeakers on the market. It has a voice coil/magnet assembly that serves as the actuator and it has a cone, though this is longer and narrower than usual. The shape is where the similarity with a pistonic driver ends.
The pistonic driver
With a pistonic driver, when the voice coil moves, the entire cone moves together with it – or that is what we want it to do. This is why the cone and voice coil structure is made as rigid as possible. Pistonic drivers are generally placed facing towards the listener, as excepting low frequencies, the sound waves generated are projected foreword, moving in the same direction as the cone - figure 1.
The DDD driver
Despite its apparently simple appearance, the DDD driver operates in a much more complex way. It has three operating modes and effectively works as a mechanical three-way system:
1. Pistonic: Used at low frequencies up to about 200Hz. Like conventional loudspeaker drivers, the voice coil and cone move back and forth together.
2. Bending wave: The cone is made from a very flexible 0.15 mm carbon fibre sheet. As frequency rises, the voice coil force causes the cone to flex, creating a bending wave that travels toward the open end. These waves are dispersive, meaning their speed increases with frequency (proportional to the square root of frequency). Around 200Hz the wave velocity matches the speed of sound, allowing sound to detach from the cone surface. The radiation angle increases with frequency, reaching about 85°. The shift from pistonic radiation to bending-wave radiation is gradual: pistonic output decreases while bending-wave output increases, becoming dominant around 800Hz without phase changes.
3. Modal radiation: At higher frequencies, bending-wave wavelength shortens until it matches the cone length, creating a standing wave (the dipole frequency, about 4,000Hz). The cone enters controlled break-up, extending frequency response rather than causing distortion. Concentric ripple-like patterns form on the cone, acting as many small radiators; as frequency increases, more radiators form with smaller moving mass.
By optimising cone thickness, elasticity, weight, and stiffness, the three modes are carefully balanced. Although the cone shape provides rigidity at rest, it allows controlled wave excitation. Sound radiates sideways through 360°, so the DDD driver is mounted vertically.
Optimising the DDD Driver
The magnet
The DDD driver utilises a powerful ferrite magnet of our own design together with an under hung voice coil. The magnetic strength in the gap is approximately 1.2 Tesla. This gives the DDD driver a sensitivity that is on a par with that of conventional pistonic drivers of similar dimensions. The very high magnetic strength in the gap also provides a useful increase in efficiency in the upper two operating regions.
The linearity of the DDD driver’s magnetic circuit – a parameter that is crucial to the performance of any dynamic driver – is also extremely high. This ensures that the force that the voice coil produces to drive the cone very accurately follows the input audio signal.
To limit the moving mass, the weight of the voice coil must be minimised, consequently the upper frequency limit and maximum power handling become issues. In early prototypes, the upper frequency limitations posed quite a challenge.
Today we achieve a power handling capacity comparable with that of conventional woofers by using a voice coil constructed with flat wire that is wound on its edge, allowing an extremely densely packed winding and also by completely enclosing the coil within the magnetic structure, which then serves as a more efficient heat sink for the voice coil.
Controlling the Cone – Part 1
Minimising ringing
In any driver of this type, you want the motion of the cone to precisely follow the electrical signal that drives it. You do not want it to ring. This is where the cone continues to move after the impulse that excited it has finished. Essentially perfect control of the cone and an absence of ringing are easily achieved when the wavelength of the frequency propagated down the cone is greater than the length of the cone itself. It is difficult when the wavelength is shorter, since the full wave is reflected from the boundary of the cone – this is where the end of the cone opposite the voice coil attached to the driver chassis. The reflection will in turn produce re-reflections, as the travelling wave slowly loses energy over the course of several wave cycles. Imagine small ripples in a pool and how they are reflected back from its edge in a recurring pattern. The behaviour of a rippling cone is precisely the same, as the motion tends to persist for a considerable time, which ultimately has the effect of obscuring the information being reproduced.
Our approach to this problem is two pronged.
Firstly, Peter Dicks, the designer of the original DDD driver, found that bending waves disperse, i.e., the DDD driver cone exhibits an increasing velocity of propagation of waves (more correctly termed phase velocity) with increasing frequency and he was able calculate the velocity of propagation in the cone as a function of frequency and therefore determine at what frequency the velocity of a wave in the cone reached that of sound in air. This is called the Coincidence Frequency. At this frequency the waves start to detach from the cone’s surface at an angle given by the ratio of cone-speed to air-speed. A few Hertz above the Coincidence Frequency the detachment angle is close to zero, but with higher frequencies the elevation approximates to 90°. The detachment of waves from the cone describes the functional principle of the bending wave radiator.
Secondly, he devised an effective means of balancing damping of the cone and the characteristics of the cone termination in order to minimise the residual ringing in the cone.
The calculations required to determine this were extremely involved, which is why to the best of our knowledge, no one prior to Peter Dicks had been able to devise and solve the necessary equations.
Controlling the Cone Part – 2
In addition to dispersion, the velocity of the bending wave in the cone is also affected its varying stiffness, which decreases as you move from the top, to the open end. Consequently, the velocity of a wave in the cone is proportional to frequency and inversely proportional to the distance from the cone’s peak.
The carbon fibre material we now use for the cone material and the titanium foil we used in the past, both have a high velocity of propagation in their bulk form. The velocity of propagation is especially high when they are formed into a very steep, thin-walled cone. Due to dispersion, the velocity of propagation of the wave in the cone is highest at the top, consequently, the upper part of the cone reaches the Coincidence Frequency much earlier than the lower part. Additionally, in the lower part, the wave lengths are shorter and all the waves are denser and therefore more efficiently radiated into the surrounding air. This higher efficiency of radiation means that there is less energy left in the cone to be reflected at the termination (i.e., the surround) and by maximising the amount of energy being radiated, a simpler form of termination for the cone may be used. This is important as the reflections would otherwise cause ringing in the cone. The theoretical way to eliminate this would be to ensure that all the remaining energy in the wave was absorbed by the termination, but this can only be done by having a termination that is the complex conjugate of the cone’s impedance and this cannot be physically realised.
With its current dimensions, the carbon fibre DDD driver reaches the Dipole Frequency below 4kHz. At this point the first standing wave starts to build up and above this frequency, the wavelengths become progressively shorter than the cone. Once beyond the Dipole Frequency, dispersion and radiation work as damping mechanisms to control the motion of the cone, however, additional damping may be employed to smooth the pulse and frequency response even more. These control ringing very effectively and are important factors in the carbon fibre DDD driver’s exceptionally extended high frequency response.
Minimising Doppler distortion
Another advantage of the DDD driver is that because it produces bending waves in a wide frequency range above the bass region, movement of the cone caused by bass signals produce almost no Doppler distortion, the bane of conventional loudspeakers.
The Free Lunch – with two courses
Omnidirectional radiation pattern
The DDD driver mounted vertically possesses an added advantage that is arguably just as great as the outstanding linearity of the design itself. That is, it is a nearly ideal point source with an omnidirectional radiation pattern.
The DDD driver propagates sound in a uniform hemispherical pattern. The frequency and phase responses are uniform from all listening angles, which is never the case with multi-way cone and dome loudspeakers, nor with dipole electrostatics or ribbons.
An omnidirectional radiator has several important audible advantages:
1. The window in which good stereo imaging can be perceived is considerably widened.
Unlike with a conventional loudspeaker, where you have to sit in a small area of the room to enjoy a good stereo image, with an omnidirectional loudspeaker, you can enjoy a good stereo image with the correct tonal balance in a very wide area within the room: just as you would in a concert hall.
2. The loudspeaker’s behaviour tends to be much more predictable from room to room, because the reflected sound is timbrally matched to that of the direct sound. Having the reflected and direct sound timbrally matched is also important for good stereo imaging.
3. The sound of an omnidirectional loudspeaker has decay characteristics more closely resembling large room reverberation than is the case with the narrowly focused output of typical monopole direct radiators. The sound has a naturalness about it that powerfully suggests the sense of space experienced at a live musical performance.
4. Because of the DDD driver’s physically compact size and exceptionally wide frequency range, it avoids the smearing of the stereo image that can occur when different parts of a musical instrument’s frequency range are reproduced by different types of driver, which are in different physical locations on the loudspeaker’s cabinet. This is because the distances between these drivers and the listener’s ears are not the same, which results in their contributions arriving at the listener’s ears at slightly different times.
Linear phase response
Of equal importance is the DDD driver’s uniform phase response. This is more often referred to as a linear phase response. A device that is linear phase, preserves the phase relationships of all the frequencies that pass through it. Music is a complex signal that can be broken down into a wide range of sine wave signals with specific frequencies and specific amplitudes, and very importantly, specific phase relationships between these frequencies. This mix is continuously varying. If an audio component does not accurately preserve these phase relationships, then its output will not be a faithful copy of its input.
Because the DDD driver is linear phase over its very wide frequency range, its tonal accuracy and transient response are both exceptionally good. Having a good transient response is especially important as music is a series of transients.
Conclusion
To come back to the original question, why is the DDD driver so revolutionary?
At a stroke most of the limitations of conventional drivers have been eliminated. The combination of high displacement, low mass, and high acceleration allows the DDD driver to operate linearly over a very wide frequency range, nearly the whole of the audible spectrum and to achieve excellent impulse response, low distortion and a flat phase response into the bargain.
The German Physiks DDD driver is able to offer an improvement in sound reproduction that we feel quite justified in describing as revolutionary.
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