October 1999

Thiel Audio Factory Tour
by Marc Mickelson

Drivers and design

I also spent part of my time with Jim Thiel in his office/lab talking about speaker and driver design. As any audiophile worth his tweaks knows, Jim Thiel is known for his rigorous method, which is to say that he designs by measurement and ear. A quick glance around his office will reveal amplifiers, test equipment, drivers, parts to drivers and two computers: a Macintosh and a PC. It’s on the Mac that Thiel does the bulk of his computer work, having developed a series of Excel spreadsheets that allow him to plug in values and then which spit out thousands of line of code that can be uploaded to the CNC machines for the creation of speakers. This programming skill would be enough for some people, but for Jim Thiel it’s only a sideline. His enthusiasm for speaker technology is obvious as he crawls under his desk to pull out a few prototype cones to show me. He’s like that kid in high school who was always in the lab going far beyond the experiments that we all were doing for grades, and he has a strong element of the science teacher in him too, explaining the intricacies of his work to a greenhorn like me with incredible patience.

In terms of engineering principles, Thiel values flat frequency response, low distortion, wide dispersion and preservation of phase and time information. To accomplish these and other design principles, he designs in a methodical fashion, developing the drivers for a new speaker first, then the cabinet, and then the crossover. Next comes a prototype with its crossover mounted outside the cabinet for tweaking purposes. Measurements are taken at all points along the way, over 30,000 of them on the CS6 during its development, for instance. Finally, the listening phase takes place. Thiel admits that it is possible to make a product that measures well but sounds bad, and vice versa, so listening is as important but not more important than taking measurements, which help you "solve the real problem."

In the days before Thiel (the company) began making its own drivers, Jim Thiel would have to wait a month before his suppliers could make a prototype driver for him to work with. Now the whole process can be done in three hours, and it was my sense that this, and not the desire to have the capability to make drivers in house, is what drove Thiel to create their own drivers. Once again, efficiency reigns supreme: not only can the company now make prototypes faster, they can also make drivers to their exact specifications and needs, which in Thiel’s case means drivers with short-coil, long-gap motor systems; aluminum diaphragms of various optimized shapes; and powerful magnets. Why aluminum when polypropylene and Kevlar are so popular with other speakers? Greater compressive strength and stiffness. Thiel has experimented with a vast number of materials and found only one that’s superior to aluminum: beryllium, which is impossible to form, so it has to be machined and thus would add a great amount of cost to each driver.

Thiel advocates the use of coaxial tweeter/midrange drivers, which are tougher to design than either a tweeter or midrange alone but bring benefits that offset the difficulty. The suspension of the CS7.2's tweeter, which sits at the center of the midrange, is particularly interesting. It's of a low-loss rubber that’s much wider and thinner than normal, allowing greater linear excursions and increasing dynamic range by 15dB. Because of the short-coil, long-gap motor system, the driver's magnetic field has to be greater, so Thiel uses four radially magnetized neodymium magnets that together produce a much stronger field than one large standard magnet.

In terms of diaphragms, there isn’t one best shape because every diaphragm is, in Jim Thiel’s words, "a unique problem with a unique solution." The diaphragm for the CS7.2's upper midrange driver is an aluminum/polystyrene/aluminum sandwich, which is much stiffer than either material alone. From under his desk, Thiel pulls a box of prototype diaphragms, and one of these for the tweeter/midrange of the CS2.3 is marked "#71," meaning that 70 others came before it. I can only guess how many after.

Thiel woofers incorporate the same design principles as the coaxial tweeter/midrange drivers but on a larger scale. The CS7.2 woofer is massive and heavy, and it incorporates a couple of interesting refinements. A copper sleeve around the pole and copper ring between the pole and magnets reduce inductance and thereby the distortion elements of the inductance. In a Mr. Wizard-type demonstration, Jim Thiel showed me the effects of the two massive magnets of the woofer on the copper ring. He dropped the ring over the pole with the magnets in place, and it floated slowly downward.

To find out more about Thiel Audio,
visit their website at www.thielaudio.com