From the beginning, our aims for what became the CDD range were intentionally bold. We came to the conclusion that to stand out from the crowd, we would need to produce a product that was strong in all aspects of technology, performance, industrial design and affordability.
We’d noticed that philosophically, coaxial drivers were revered by many as the ‘right’ way to design a point source system, due to both the low and high frequency drivers radiating from (almost) the same point in space. Whilst we agreed with this thinking, we also considered that the coincidence of the sources made it impossible to adequately control directivity with the technology existing at the time.
From there, the determination to overcome these limitations led to the idea of what is now a core, patented driver technology of ours. Not only did we seek to control the directivity of a coaxial driver to an extent that had never been achieved over such a wide bandwidth before, but we also decided it was appropriate for the dispersion to be asymmetrical.
This asymmetrical pattern is something we term Differential Dispersion. Its aim is to more consistently cover the listening plane than a classic X by Y or conical pattern. Before CDD, we achieved this using individual HF horn designs, coupled to direct-radiating cone drivers. The biggest challenge we faced with CDD was how to take the kind of horn profiles we’d created for our two-way products (DD6 and DD12) and somehow recreate that on the low frequency cone, without detriment to its own performance.
Ultimately, the answer was simple: Take the same material as the cone was constructed from and continue the profile of the central, static, high-frequency waveguide via moving waveguides made out of—yes, you’ve guessed it—cellulose fiber (paper) pulp. Attaching these waveguides on to the low frequency cone actually braces it and improves the LF performance, which was a welcome bonus.
The differential dispersion pattern was achieved by shaping both the static and moving waveguides to create the necessary wavefront shape. Typically CDD drivers have an unusually wide dispersion pattern close up (100 degrees or more), which then narrows to around 60 degrees as throw distance increases. In the typical deployment application of wall-mounting, above the listener’s head, this creates a rectangular coverage pattern. In contrast, X by Y and conical dispersion systems tend to create elliptical dispersion patterns. Given the shape of most rooms, it is evident CDD’s rectangular pattern will cover the listening plane more consistently… and with less speakers.
Horn design in itself is quite a challenge if constant directivity—or at least a preferentially defined directivity— is sought. Differential dispersion horns typically exhibit complex and varying surface geometries, which are almost impossible to model in clay or plasticine (how we used to do it in the X by Y days). The answer was to turn to our boundary element methodology (BEM), which was used extensively in the realization of MLA technology, and apply this to the simulation of the surface geometries we derived for CDD drive units.
BEM allowed us to iterate to the right geometry very quickly, which was important for CDD, as we needed to tool the moving waveguides before we could fully understand how they would work when mounted to the low frequency cone. Doing that by trial and error would have been prohibitively expensive in both time and cost.
As well as moving transducer technology forward, we also wanted to take a radical approach to the cabinet construction, which would then allow us to create CDD’s distinctive appearance. What we wanted was a material that could be molded like a plastic cabinet, yet also possess the stiffness of a high-quality plywood cabinet.
The answer was UPM’s Formi material. It is a biocomposite: part polypropylene and part cellulose fiber. In fact, the cellulose fiber is a by-product of UPM’s main business, which is making paper. So unwittingly, the same material that made the driver technology work, was also allowing us to achieve our aims for the cabinet construction too.
After the success of CDD in the installation market, we were convinced that the technology also had application within the portable market. Again, our aims were bold in that we wanted to create the ultimate point source, powered speaker, which also incorporated computer control and digital audio networking. Just like CDD, we knew that these aims had to be achieved at an affordable price point.
We considered that CDD Live! could also be thought of as an “end point” in a digital signal chain, as in the point at which an abstract stream of 1s and 0s was converted into a signal we could hear. The network control, audio transport, the DSP, the amplifier and ultimately the transducer would all come together in the one product. So, in this way, we saw CDD Live! as more than just a powered loudspeaker.
Something that we were adamant about was that there would be no compromise on the output capability due to under-powered amplifier stages, which seem to be a common feature of many powered loudspeakers. This meant that we had to figure out how to package the electronics to maintain a compact footprint, whilst delivering enough cooling capability to give full output at up to 40C/104F.
Ultimately, market acceptance is the real measure of the worth of our objectives and I’m pleased to say that CDD has been a runaway success for us since its launch in 2015. CDD Live! has started shipping more recently, so its early days, but the factory has been rather busy of late. As for the future, don’t be surprised if you see further developments of CDD technology in other areas of the pro audio market.
Jason Baird is the R&D director of Martin Audio.