This article originally appeared in the July 2018 issue of Pro Sound News. Innovations is a monthly column in which different pro audio manufacturers are invited to discuss the thought process behind creating their products of note.
Product development is a process of continual refinement. At Celestion, our engineers have long made use of finite element analysis techniques to advance the mechanical, acoustic and electromagnetic performance of each new product beyond that of previous designs, even those that have become industry standards.
Celestion’s new CDX14-3040 compression driver is a perfect example; it stands on the shoulders of industry giants. It’s based on a traditional design approach to deliver a high-quality HF response with a familiar sonic character. The device offers several performance advantages over its predecessors, however, including lower overall distortion and reduced size and weight.
In 2014, we introduced the CDX14-3030, a 1.4-inch exit/3-inch voice coil compression driver with an all-in-one titanium diaphragm and surround. During its development, we took the opportunity to take what is essentially a tried-and-tested approach to large-format compression driver design and put a little bit more science into it. We rigorously applied finite element analysis (FEA), which essentially breaks down a complex puzzle into many small equations that a computer can solve within minutes, enabling us to better optimize and improve the design.
This device was developed during a period when neodymium prices were skyrocketing, which meant that it featured a heavier, bulkier ferrite magnet. Fast-forward a few years and neodymium prices are more stable, so we decided to develop an updated version of the 3030 on a neodymium platform. The resultant CDX14-3040 also has a 1.4-inch exit, a single-piece titanium diaphragm, 3-inch edge-wound copper-clad aluminum voice coil, and an annular three-slot phase plug, but instead of ferrite, it features an equivalent neodymium magnet. Like the 3030, it delivers 75W RMS (AES standard) power handling with a sensitivity of 106.5 dB from 500 Hz to 20 kHz.
Again, we applied FEA to advance the performance of the compression driver design. We investigated the shape of the surround to improve the linearity of the excursion. We also did more work on the phase plug to improve the modal suppression. Titanium can be subject to fatigue; as the diaphragm flexes under certain conditions, it can become brittle and eventually crack. By applying FEA to the geometry and the surround, we were able to reduce stresses and eliminate the possibility of fatigue.
The CDX14-3040’s diaphragm surround has a periodic geometry, which minimizes the modes around its circumference. The shape of the surround’s geometry has effectively eliminated the circumferential modes, yet still allows a linear excursion. Further, the geometry of the high-temperature plastic clamp ring is optimized for stiffness to ensure that the dome is always held rigid, preventing the diaphragm from bending. The diaphragm assembly is located into the magnet with a precision-cut aluminum locating ring, ensuring that the coil is always held concentric to the magnet gap. Obviously any offset of the voice coil can allow it to hit the magnet assembly. The locating ring is probably one of the most important parts of the driver, but it’s so often overlooked.
Between the cover and the clamp ring, we have a semi-rigid layer of plastic—a polymer similar to rubber—that spreads the clamping pressure more evenly across the diaphragm assembly. This means that the diaphragm assembly is more likely to stay flat, which holds the dome without allowing it to deform.
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The aluminum cover is die-cast, and inside, we’ve included some acoustic treatment to damp the resonances in the cavity to absorb the radiation to the rear of the dome. It also fools the sound into thinking the cavity is slightly bigger than it is. Having a large cavity behind the driver allows the device to work down to a lower frequency, because the air load behind the diaphragm can be the limiting factor to its inward excursion. That helps eliminate another potential source of distortion: that it may work in one direction better than the other.
We recommend a crossover frequency of 1 kHz, but it really depends on the power being applied and the size of the horn. Our recommendation is made on the understanding that customers will test it thoroughly to their satisfaction before they release their product.
Now that line arrays are commonplace, issues of size and weight are very important. By keeping the diameter of the drivers as small as possible, the exit holes in the front of those drivers can be positioned as close together as possible. The distance between the exit holes will determine how well the drivers couple and up to what frequencies.
If you can get the drivers really close together, they’ll couple to a higher frequency, and you’re less likely to get lobes in the response. By using a neodymium magnet, we have reduced the size and weight of the CDX14-3040 compared to its predecessors, facilitating closer coupling. Neodymium is a more magnetically powerful material, which means that for the same gap flux, you can use a whole lot less of it. The diameter of the CDX14-3030 is 180 mm, or 7 inches. Compare that to the diameter of the CDX14-3040, which is only 131 mm, or just over 5 inches. We have also put a bevel on the back cover, allowing it to fit into narrower or more trapezoidal-shaped speaker cabinets. That also helps reduce the weight. We made significant use of FEA on the magnet assembly, too, to minimize the amount of steel required.
Celestion’s new CDX14-3040 is one of the most compact compression drivers and lightest in its class: it’s around 500 grams (1 pound) lighter than other, similarly performing products out there. Individually, that might not sound like a great deal, but when you’re touring with a serious line array, performing in large venues night after night, that can add up to a significant saving.
Paul Cork is head of engineering at Celestion.
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