MCS1, SCS3 and SS2 Speaker Design

One setup tip we learned was that THIEL recommends leaving the large allen head mounting screws installed if the MCS1s are not mounted to their stands since the threaded holes go all the way through the cabinet. If the four holes are left open then, the back pressure from the woofers will travel through these holes which may result in whistling noises.

There are only a few companies that I'm aware of that go the extra mile when it comes to mounting the drivers, and THIEL has been inaugurated as one of them. Unlike the mainstream wood screws in MDF, all of THIEL's drivers were mounted with machine threaded screws into metal threaded inserts. Most folks who do not review or dissect speakers will likely not notice this feature. My opinion is that the benefits of using threaded inserts exceed the extra manufacturing costs as it ensures the best possible rigid mounting of the driver to the cabinet. Plus, as someone who loves to dissect speakers, I found that the allen head screws helped reduce slippage when tightening or loosening them. I tend to be very cautious when using a phillips head since I always feel that I'm going to slip and go right through the cone. Thus we don't recommend doing this at home. As an added bonus, the screws were all gold plated. But when removing the grills, I thought the gold screws created too many different colors on the baffle. However, the gold screws do reveal a behind the scenes peak at how THIEL Audio paid attention to every detail.

One surprise we noticed during our investigation was that there was no damping material on the side walls of the cabinet and no gasket was used under the drivers to seal them. The only other cabinet we've seen that didn't have these features included the Canton Karat Reference 2 DC speakers which we reviewed several months earlier.

I found it peculiar how THIEL Audio recessed the drivers. The mid/tweeter driver had a conical shape to the frame as opposed to a more common flat shape. It was recessed about 3/4 of an inch with the cabinet continuing the same slope as the driver. I thought that the purpose of this was to line up the voice coil to the woofers so that the time coherence was maintained. I quickly noticed that the woofer was also recessed about 3/4 of an inch which seemed to misalign the voice coil thereby seemingly negating the time coherence. We asked Jim Thiel the reasoning behind such a design and he responded with the following memo.

“You are perfectly correct that "the recessing of the tweeter is to line up the voice coils for the purpose of time alignment". You are also correct that if we did not recess the tweeter there would be a little bit of diffraction eliminated. And, further, you are also correct in noting that if we did not recess the woofers we would not need to recess the tweeter to achieve alignment. With most drivers this would not be true. It would usually be that if none of the drivers were recessed they would be far from time-aligned because the woofer's coil would normally be much farther behind the baffle than the tweeter's. Why it is true with the MCS1 is because both the woofer and tweeter are unusual in their geometry. The tweeter has a built-in recess from the geometry of its chassis that places the midrange cone rim behind the baffle surface and the woofer has an extremely shallow diaphragm geometry that is made possible by the use of a thick, cast polystyrene foam layer laminated behind the aluminum layer of the diaphragm to obtain the required strength and stiffness. So, why did we not utilize these features as sufficient to achieve correct time-alignment and mount the drivers without any recess? Good question! The answer is that the woofer would actually not have its minimum diffraction mounted flush with the baffle.

 

This is because the surround itself is usually a source of significant diffraction because it protrudes in front of both the diaphragm rim and the baffle. What we have done instead is to design the two-layer diaphragm with enough thickness at its rim so that, by mounting the surround from behind the diaphragm, the surround's roll creates a (relatively) smooth continuation of the diaphragm's shape, rather than an abrupt protrusion.

To complete the desirable situation of a smooth shape from diaphragm to surround roll and on to the baffle, we have recessed the driver chassis enough so that, with the use of the ring, there is no significant, abrupt change from the surround roll to the baffle. So all this was done to reduce diffraction from the woofers' surrounds, with considerable measured success. Now, it is true, as you say, that this woofer diaphragm / surround design and driver mounting requires that we recess the tweeter. But the extra recess is only about 1/4 inch and by being very careful with the shape of the recess there is very little added diffraction from the tweeter. You notice that the recess does not continue from the tweeter chassis's edge but rather has a different size and shape that was found to produce less diffraction.”  

“There is no time lag between the mid and tweeter since they are driven by the same voice coil.

There is phase shift but this is opposite for the two diaphragms and cancel out in the speaker's total output. This is also how a first order crossover works; there is phase shift but it is equal and opposite and less than 180 degrees and so can be made to perfectly cancel in the acoustic output.”

“How the mechanical crossover works is that the coupling suspension's stiffness is high enough so that at low frequencies the two diaphragms move together as one unit. So the tweeter diaphragm is contributing to the low frequency output but at a much reduced level. At high frequencies the coupling's compliance allows the midrange diaphragm to "decouple" from the voice coil because high frequency force cannot be conducted through this compliance to the relatively heavier midrange moving system. So at these high frequencies the tweeter diaphragm is the only one driven by the voice coil, since it is directly connected.”

This explanation is a much better answer for both the time lag and crossover questions. It seems Mr. Thiel is saying that at the lower frequencies (slower speeds) the dome has such a low output you can't hear it but at the higher frequencies (faster speeds) the coupling suspension (spring) does not allow these frequencies through so the cone would not lag nor play the higher frequencies hence there is a mechanical crossover.

The SS2 subwoofer didn't raise nearly as many questions since it had a more traditional truncated cast frame and an aluminum cone with rubber surround. I also found the crossover quite interesting since all of the components were wired by their leads. In other words there was no printed circuit board and no wires connecting component to component thereby minimizing connection paths. All internal wiring used 18 AWG solid twisted pair. THIEL's logic behind the Speaker Wire can be found in our FAQ with Jim Thiel which is included at on Page 8 of this article.