- Written by Chris Groppi
- Published on 19 January 2009
The CS3.7 follows many of the same design philosophies as THIEL's earlier speakers. The main premise of the design is that the multi-driver speaker must be both phase and time coherent. This means that the crossovers must introduce a minimum amount of phase shift between the driver signals, and also not introduce any time misalignment that would effect impulse response. Imagine a snare drum hit. With a time coherent speaker, the impulse of the snare drum happens at the same time in all of the drivers. This turns out NOT to be the case with most speakers.
Crossovers typically produce both a phase shift between drivers, and cause time misalignment as well. They might measure perfectly in frequency response, but will show all manner of problems in their impulse response (yet another reason not to trust measurements blindly). THIEL uses rather complex first order crossovers that are phase coherent. When combined with the sloped front baffle, the speakers are also time coherent. In addition, the crossovers are tuned to accurately correct all the response irregularities of the driver in the cabinet. The result is a design with exceptional frequency, phase and time response, at the cost of very complex crossover networks.
The heart of the CS3.7 is the THIEL-designed and custom made coaxial midrange and tweeter driver. Very few companies design and manufacture their own drivers. This driver alone is likely responsible for a large portion of the increased cost of the CS3.7 over previous models. It's also like nothing I've ever seen before. The idea of coaxially mounting a tweeter inside the midrange to make a point source wideband driver is nothing new. What THIEL has done is re-examine how to build the midrange part of this combination. Mounting a dome tweeter in the center of a cone shaped midrange horn-loads the tweeter. This can dramatically affect its performance.
THIEL has combined the dome tweeter with a flat, ring shaped midrange diaphragm. Unfortunately, this was the hard part. A cone shaped midrange gets its strength from the cone shape. Make it flat, and maintain the required light weight, and all the strength goes away. Now the driver will not move rigidly in a piston-like way, but instead start to bend (called driver break-up). THIEL has used finite element analysis (FEA) to design a wavy-shaped ring driver that is flat as far as the tweeter is concerned, but maintains enough rigidity to avoid any breakup. In fact, the driver can be driven all the way past 20 kHz without break-up, vastly exceeding the performance of typical midrange drivers (and even many tweeters). This diaphragm is driven with a long-gap short coil motor for extra low distortion. The penalty here is that a huge magnet is required, since only the most uniform part of the magnet's field is used. A very large rare-earth magnet is used for this duty. This midrange is absolutely a no-holds-barred assault on the state of the art for dynamic drivers, and must cost an absolute fortune to make.
The CS3.7 is equipped with one woofer and one identical looking passive radiator. Both are aluminum diaphragms like the midrange, and also are FEA-designed with a flower petal-like molded shape to maintain stiffness and avoid driver break-up. All these drivers working together result in a speaker with 32 Hz to 35 kHz frequency response +/- 3dB, 33 Hz to 26 kHz +/- 2dB. Sensitivity is a rather high 90 dB/W/m, but with a nominal 4 ohm impedance (really, lower than that as we'll see later).
The cabinet is also a departure for THIEL. While maintaining the typical THIEL sloped front baffle, the CS3.7 has a much more curvilinear form than previous designs. The front baffle is a CNC-machined aluminum plate rather than typical MDF. The curved side pieces are made from a 15 layer thick laminate of hardwood laid-up to produce the curved shape. The only MDF pieces are the base and the narrow rear panel with the single set of 5-way heavy-duty binding posts. A cast aluminum "head" provides both increased rigidity and reduction of internal standing waves. The result is a cabinet that is one of the most inert I've seen. It feels like it's a solid block of concrete. While heavy at 90 lbs each, they are not anywhere near as heavy as they could be. The inert behavior comes from good engineering, not the willy-nilly application of more mass.
Fit and finish is exactly what I expect from a pair of speakers costing as much as a new Toyota Yaris: impeccable. Quality of the wood veneers, the black chrome plated floor spikes, the 5/8" thick CNC machined and anodized aluminum Outriggers and even the boxes the speakers were delivered in show not a trace of penny pinching. It is all as good as it ever needs to be, without being excessive.