THE DDD DRIVER

A Brief Description

Figure 1: DDD Driver


At first glance the German Physiks DDD driver looks like a conventional piston cone driver. 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 piston driver ends. With a piston 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. The sound wave that a piston driver produces moves in the same direction as the movement of the cone - figure 1. This is why piston drivers are generally placed facing towards the listener.


Figure 2: Piston Driver


The DDD driver, despite its apparently simple appearance is rather more complex. It has 4 modes of operation and in essence works as a mechanical 4-way system.


               
1. The lower frequency end of its operating range can be described with Small/Thiele resonant parameters.
               
 
2.  In the next frequency band up to the Coincidence Frequency, it works like a pistonic driver.
 
 
3. Next an overlapping band follows where pistonic movement is progressively replaced by bending waves until all the radiation is generated purely by bending movement in the cone. Due to dispersion and the cone’s special shape, the Coincidence Frequency is spread over an extended frequency range, rather than occurring at a single frequency like the Dipole Frequency.

From the upper edge of the Coincidence Frequency band, it works like a pure bending wave converter where the velocity of the travelling waves in the cone increases with frequency.
 
 
4. The last mode of operation commences above the bending wave band at the Dipole Frequency, when the first standing wave occurs and where modal break-up begins.
 

By optimising the key properties of the cone material, namely thickness, elasticity and specific weight, together with the cone’s bending stiffness, which is achieved by selecting the correct cone-angle, all four frequency bands may be very closely balanced.

The last two modes cover the majority of the DDD driver’s operating range and are what differentiate it from conventional drivers. In these two modes, when the voice coil moves, the whole cone does not move together with it, as the open end of the cone is terminated by a rubber suspension and semi-rigidly attached to the driver chassis. Instead the motion of the voice coil causes a wave to travel from the top of the cone down to the open end. This occurs because unlike the piston driver, the DDD driver cone is made from a very light and flexible foil - 0.025 mm thick titanium or 0.15 mm thick carbon fibre. While the shape of the cone gives it rigidity at rest, it is relatively easy to excite waves in the cone material. The clever part is controlling these waves. In very simple terms, the resulting motion can be compared to that of the bell of a jelly fish when it is swimming, with the sound wave radiated sideways from the driver as shown in figure 2. For this reason the DDD driver is always mounted vertically. The actual situation is rather more complex, as the angle of radiation with respect to the cone wall becomes progressively more acute with increasing frequency due to dispersion of the bending waves. Additionally the driver’s operating mode changes with frequency, however a detailed description of the mechanisms involved is beyond the scope of this article.


Figure 3: DDD Driver


This type of driver is sometimes referred to as a transmission line driver because the cone carries a wave and so acts as a transmission line. We prefer to call it a bending wave radiator. Bending waves describe the type of wave set up in the cone by the actuator motion. The use of this nomenclature also avoids confusion with the acoustic path transmission line loudspeakers that most audiophiles will be familiar with, such as in the classic designs from Paul Voigt, A. R. Bailey and TDL®.