WHY IS THE DDD TRULY REVOLUTIONARY?

To answer this question we first need to look at the principle types of loudspeaker driver currently available.

All loudspeaker drivers may be categorised by two groups of fundamental characteristics. The first group describes the means of electromechanical transduction; that is the method by which electrical energy representing the audio waveform is converted into mechanical energy and this in turn breaks down into three basic types:


               
Electromagnetic or electrodynamic: A simple reciprocating motor drives the cone, diaphragm or sound producing element. Well over 95% of loudspeakers sold today fall into this class, which includes the DDD driver; its one point in common with conventional designs.
               
 
Electrostatic: Air motion is produced by varying the electrical charge on a diaphragm located between two electrodes.
 
 
Piezoelectric: This employs materials which flex in one dimension when an electric potential is applied across them.
 

While other methods of transduction such as magnetostriction and corona discharge modulation have on occasion been employed, none has achieved commercial significance, so the three categories above cover almost every loudspeaker currently available.

Both the electromagnetic and the electrostatic types can offer high levels of linearity. The piezoelectric type has generally been confined to applications where high sound quality is not a requirement.

Electrostatic loudspeakers are held in high esteem by many audiophiles, and deservedly so on the basis of their sound quality at moderate output levels, but they suffer from a number of serious limitations:


               
Restricted maximum output level especially at low frequencies.
               
 
Medium to low electrical efficiency.
 
 

High directivity particularly at high frequencies.

 
 

Poor reliability especially in humid environments.

 
 
They can be difficult loads for some amplifiers
 

We have done extensive research on electrostatic loudspeakers, but have concluded that they are inherently impractical and unlikely to progress much beyond their current state of development.

In consequence of the above, the electromagnetic type is in our view the transducer of choice.

The second fundamental group of characteristics deals with how the mechanical energy is converted into acoustic energy and breaks down into the following types:


               
Mass loaded piston drivers.
               
 

Tympanic or membrane drivers.

 
 

Flat panel loudspeakers such as the NXT® and BMR® devices.

 
 

The Heil® air motion transformer.

 
 
Transmission line drivers, chiefly represented by the Jordan Module® and its developments; the Manger® driver; the Walsh® driver and the German Physiks DDD driver.
 

Mass Loaded Piston Drivers: This includes nearly all conventional cone and dome drivers. The name itself refers to the dominant effect of mass on the acoustic output of the driver, and to the theoretical model of pistonic motion to which such drivers conform to a greater or lesser degree. According to this model, the driver actuator and diaphragm (usually a cone or dome), should move back and forth as a single unit within a single dimension, just like the piston in a reciprocating internal combustion engine. Ideally, the diaphragm of the driver should remain entirely rigid and should exhibit no internal vibration whatsoever, though in practice this condition is never met, except at the lowest frequencies.

In such designs, mass reactance will be the main component of the complex acoustic impedance of the diaphragm throughout most of the useful frequency range of the driver, hence the term, mass loaded. The mass in turn is loaded by the compliance or springiness of the driver’s suspension, and the two together, mass and compliance, form a resonant system like a weight suspended from a spring. Such a system tends to oscillate around a single frequency when excited, and, predictably, a large part of conventional loudspeaker design is taken up with damping such oscillations.

Unfortunately, a resonant system, even a damped resonant system, is poorly suited to sound reproduction. Audible sound spans 10 octaves, while a strongly resonant system is mechanically efficient over only a small range of frequencies close to its resonant frequency. Such systems are necessarily bandwidth limited due to this frequency dependent efficiency, while transient response is inevitably degraded due to the inertia of the mass and because of the energy stored in the loudspeaker’s suspension and which is returned subsequently.

Such a driver, unless it is loaded into a horn or acoustic lens, will also suffer from a non-constant directivity pattern, with dispersion normally narrowing with increasing frequency. This is often called beaming. This characteristic, perhaps more than any other, will cause a loudspeaker to sound musically unnatural since acoustic musical instruments almost never radiate sound in this manner.

The normal method for dealing with the limitations of mass loaded pistons is to use two or three per speaker system and to drive them over the narrow frequency ranges around their respective resonant frequencies. Many fine speaker systems have been designed this way, but we believe this approach is fundamentally flawed, and nearly always results in location dependant transient response, ragged directivity patterns and an overall sense of individual drivers imperfectly integrated.

Tympanic or Membrane Drivers: This includes film and leaf transducers, electrostatic and ribbon type dynamic loudspeakers.

All of these use a diaphragm that has a low mass and a large surface area and which is stretched over a frame; in other words the diaphragm and suspension are one and the same. In such designs the resistance of air dominates the acoustical impedance of the diaphragm except at the lowest frequencies. Consequently transient response and frequency range can be excellent. These designs normally lack high power handling capabilities and are incapable of large excursions, so they are best confined to the higher frequency ranges and indeed are thoroughly impractical in bass applications. Moreover, they typically dictate a line source configuration which makes them difficult to use in domestic listening environments. They also tend to be difficult to integrate with mass loaded pistons due to their very different acoustical characteristics. As a result hybrid systems incorporating them very rarely provide satisfying results.

Flat panel loudspeakers: The NXT® driver can be described as a special membrane driver where its technology is based on the modal excitement of a stiff panel diaphragm where a number of areas are stimulated, but using a single exciter. The number, location and size of these areas vary with frequency. While optimising the location of the exciter point for a given panel may flatten the frequency response of the loudspeaker, it always lacks the phase coherence and wide dispersion of energy which are mandatory for high fidelity loudspeakers.

Balanced Modal Radiator: This technology, a spin off from NXT®, is based on pistonic movement, where the area of radiation becomes progressively smaller with increasing frequency. It uses a single exciter, but in contrast to the NXT® panel, only one vibrating area is employed and the size of this area varies inversely with the frequency of the stimulus. The result; radiation with coherent phase and without directivity. Although it is still early in its development, the BMR® could be the basis for an ideal full-range driver.

The Heil Air Motion Transformer: This occupies a class of its own and is worthy an essay in itself. It is an extremely ingenious design with high output, high efficiency, wide bandwidth, good impulse response, and low distortion. Its sole real drawback is its low frequency limit - not much under 1kHz - and the consequent necessity of mating it with a conventional woofer. Sadly, most attempts at doing so have resulted in audible mismatches.

The Transmission Line Driver: This is the class that our DDD driver falls into, which when optimised offers what is currently the best overall performance in sound reproduction. Whilst similar in appearance to a mass loaded piston driver because of its conical diaphragm and use of a conventional voice coil and magnet type actuator, it differs in one major aspect. The cone is securely anchored at its mouth and is flexed by the motions of the voice coil, and only pushed back and forth up to its Coincidence Frequency. From there on sound propagation is at angles up to 90 degrees to the wall of the cone, rather than parallel to the path of the voice coil, as is the case in the lower pistonic frequency range.

The cone itself ideally has an extremely high stiffness to mass ratio, but because it is very thin and its moving mass is extraordinarily low, so also is the bending resistance. Consequently, the cone will be excited into bending modes quite easily, particularly when the velocity of the waves on the diaphragm is higher than the velocity of sound in the surrounding air and consequently the wave energy will detach from the cone surface.

Without going into the physics of traverse wave propagation across a plate structure - which essentially is what the cone is in this design - we can say that when the cone is bent by an actuator, the actuator itself - in this case the voice coil - sees only a very small increment of mass from the cone. Rather than being mass loaded, it is loaded instead by the differential stiffness per specific weight of the cone-material and secondarily by the radiation resistance of the air load on the cone.

In simple terms, the voice coil is exciting shock waves across the surface of the cone which in turn excites motion in the air. As distinct from a conventional cone, there is almost no mechanical inertia to overcome, thus there is a very direct translation of the electron motion of the audio signal into the motion of air molecules in the listening space.

In a real sense, the acoustic behaviour of the system is much closer to that of an electrostatic membrane speaker than to a mass loaded cone, to which the transmission line driver bears a misleading external resemblance. The moving mass of the German Physiks DDD driver is under three grams, less than that of most tweeters, and yet its ability to displace air is roughly equivalent to that of a 6 1/2 inch woofer. So while it shares the cone shaped diaphragm of the latter, its behaviour could not be more different.


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.