I had the opportunity to listen to the new Thiel CS2.7’s (Coherent Source) alongside the CS3.7’s at Hanson Audio in Dayton, OH. This product is rather complex in nature, and the technology is very interesting. True, the concept of ports and passive radiators isn’t new, but the overall concept of what is going on in a speaker design like this is astounding. The advent of CAD allows the complexity of the material choices for all the drivers and resonance characteristics to finally come together and make some very compelling products that do indeed take advantage of what ports can offer, and to get around a ton of pitfalls using ports / radiators.
If you look at the CS2.7, or the CS3.7 you see the passive radiator approach to “porting”. Some of you speaker experts can fix my fuzzy conceptual math on this, but here goes.
The “active” woofer in a situation where the driver is playing as low a tone as it can prior to doubling up the frequency is actually moving at it’s LEAST cone excursion. At this resonance point, the PASSIVE driver is moving the most. This is a good thing as it removes the long cone excursions from the active woofer which reducing intermodulation distortion (Doppler effect) when the upper woofer is trying to play lower midrange energy. Another benefit is that the reduced cone excursions of the active woofer allow better control of the “port”, or passive radiator. The larger mass passive radiator can try to force the active driver to wander from it’s intended course.
At the opposite extreme, where the ACTIVE woofer is playing as high as it can, the passive driver is moving the least and the active drive is moving the most. The active cone excursions are pretty shallow, as the frequency has increased, though. So the trade off is better all around clarity and reduced distortion if this is done right.
Somewhere in the mid bass frequency, where both cones are playing equal energy, determines the active driver’s size and how big the box has to be since that’s where the -3 dB roll-off to the passive driver is “tuned”.
Looking at the CS2.7, it has a much smaller cabinet than the CS3.7 by volume. The smaller CS2.7 woofer drives a smaller passive radiator to keep the tuning right with amazingly little low frequency energy lost. All things being the same, a smaller box is stiffer than a big one. But, to make the box a nice easy size to manage, the passive radiator in the CS2.7 is oval. This limits the width of the cabinet more than making the radiator round. The bigger brother uses two ten inch active / passive drivers of differing material so all things aren’t the same. And. Since the box has to be bigger anyway, the passive driver is round. There is some secret sauce that allows the larger CS3.7 ten-inch driver to play into the upper midrange as well as the smaller CS2.7 eight-inch driver does. The CS3.7 active woofer is not the same animal as the CS2.7 active woofer at all. The larger CS3.7 box has to use extensive and expensive bracing to keep the box from voicing, too. All manner of intensive bracing is used according the Thiel. That makes the CS3.7 a much more expensive speaker. The smaller box benefits from a more ridged structure and the smaller active bass drive to help keep the midrange open and nice. The midrange / tweeter coaxial drive is identical. These are of course, first order filter speakers so the drivers response work well into the driver above or below them as the case may be. Let alone the bass, but the rest of the product is intensive to design.
So getting all the material to sound the same and the resonance math right is frighteningly complex. Did they get it right? That depends. With a ported system you can’t have warm bass without the lower midrange being effected. And, too tight bass also interacts with the lower midrange. To my ear, the midrange is the single best point on the CS2.7. It is extremely layered and detailed. The listening room favored the smaller CS2.7’s as they were closer together where the bigger brothers were too far apart to image well. The CS3/7’s sounded good, but I never heard the incredible imaging that the smaller CS2.7’s threw out. I still feel it was the room, and speakers being almost too far apart. The highs may be too hot on some systems, and I did hear it get harsh on digital sources at times. Then again, I hear this all the time. Consider that for a moment. Then, we’d switch source material and all was well again.
OK that crazy to get right bass. The trade-off for a warm bass verses tight bass leans towards the tight side. And, I like it that way. Pure taste mind you, but too warm a bass hurts the midrange too much to my liking. The CS2.7’s went impressively deep. Deep enough that when the dual ten-inch SmartSub SS2.2 woofer was in or out, no one could substantially tell when it was turned it off! The sub was naturally spliced in at the roll off to the speaker’s bass response so admittedly, it was only contributing below the -3 dB roll-off point. True, two 150 watt Plinius class A amps wire into 400 watt mono sure didn’t hurt the bass, either.
All-in all, this is a very well integrated sounding speaker. By that I mean it sounded like a single driver. It is NOT a warm speaker, but a fast and detailed speaker with tremendous imaging. Midrange was truly astounding and some of the best I’ve heard. These guys kind of play stuff way loud, but my previous exposure to Theil speakers shows an amazing clarity at low volumes. That coaxial driver really works. The fact that they got the active driver / passive radiator right is further attested to in the midrange clarity.
If you like a fast detailed sound this is a product worth a listen. I don’t think the CS2.7 will play nice with poor solid state electronics, though, or 16-bit Red Book CD audio. They seem to be made to sound good when the source material is good, and they did. They are not going to be nice to poor digital sources, and they didn’t. If you take the time to match these up with good stuff, the downside is mitigated some but the upside is really amazing midrange imaging and layering. You get tight bass and open extended (some might say too extended) highs with that. Treble is one of those things that you like, or you don’t. I would not pick this speaker for playing poor source material loudly. The treble would, to my ear, be too much after awhile. For the price and complexity of this design of speaker it is a remarkable achievement to even try to do all the things this product does. Jim Thiel may have passed on (we all know there is music in heaven!) but the company he left behind pulled this one off. May the fruits of his life benefit us all.
I won’t name names, but only a few manufacturers work with first order filter speakers, let alone ported / passive radiator models. And, one thing you will notice is that they have all been around a LONG time (THIEL since 1978). Practice makes for near to perfect. The CS2.7 sure “sounds’ like it for it’s attractive price.
Disclaimer – I’m not an owner of this speaker (I use DynAudio C4 signatures) as my wife lets me own one pair at a time so I’m somewhat of a neutral listener.