On Angel’s Wings, Part 1 - A Speaker Design for the Bohlender Graebener RD-75 Planar Magnetic Driver
The Bohlender Graebener Corporation And David Graebener (1948-2015)
(updated from the original published version - J. Martins)
Bohlender Graebener (BG) was a company located in Carson City, Nevada, that produced high-quality planar magnetic audio transducers. In the late 70’s, David Graebener and four other speaker enthusiasts started a company called SpeakerLab in the Seattle area. At that time, David Graebener began developing the transducer and, ten years after, he set out on his own, completed the development of this technology, and marketed these speakers under the company name of Applied Technology Engineering. According to Ed Dell, the founder of audioXpress, David’s earlier involvement with SpeakerLab - one of the first employee-owned, mail-order speaker kit companies - was also the inspiration for Ed to start Speaker Builder. We therefore must thank David for more than just producing his line of planar magnetic transducers.
In 1994 David, Tom Bohlender, and Warren Kocmond concluded that this type of speaker technology had some serious potential in today’s marketplace. They gathered a group of sophisticated private investors and formed the Bohlender Graebener corporation for the purpose of further developing and marketing this unique technology. During those years, the technology was developed to the level where it could be manufactured in a repeatable and consistent manner, in various forms and sizes, using a variety of materials suited for specific performance requirements. The company manufactured drivers under OEM contracts for several prestigious loudspeaker firms. In addition, they produced two product lines using their planar magnetic drivers, a line of floor-standing speakers and another of in-wall speakers. David Graebener - 1948–2015David Graebener continued to be involved with the design and development of transducers and systems for the rest of his career and is rightfully considered an authority on planar-magnetic design. From the late 1970s through the 80s, Graebener was vice president, research & development of Speakerlab. He was owner of Advanced Transducer Engineering and co-founder of Solution Matrix, where clients included Boeing Commercial Airplane Company, Pacific Car and Foundry, NCT, Onkyo, and Carver/Toshiba. Graebener also worked with American Technology Corporation, Eastern Asia Technology Company, consulted for GCInnovations, and was owner of David Graebener Consulting and partner in Tyee Group. David was always sought after as a consultant for transducer development, was involved in many patents related to planar-magnetic transducer design and is associated with the development of more than 130 electro-acoustic designs. He also won many awards from both CEDIA and CES for his designs.
In 2006, David Graebener, joined Tom Bohlender and Ed Jaeger in their new company, Wisdom Audio, continuing to work on planar magnetic transducer development. As David explained then, “In my short time at Wisdom working with Tom [Bohlender] and the engineering team, I have seen and been part of some dramatic technological advancements in transducers for high quality audio reproduction systems and I quickly became excited to jump in with both feet. Wisdom’s new initiatives are unlike any others in the industry, and uniquely address packaging and performance challenges facing a number of audio applications today. Upon fruition in 2007 we will be launching some very unique and exciting systems that will satisfy even the most technologically savvy and discerning audiophiles."
Bohlender-Graebener (BG Corp.), continued with Warren Kocmond at the helm, as the company’s CEO. In 2008, the company was rebranded as BG Radia. Radia was BG’s line of freestanding, on-wall, and in-wall speakers. Warren Kocmond, the company’s CEO, stated: “in our fourteen-year history, our company has gone by many names, from Bohlender-Graebener to Radia to simply BG. Though many will still refer to us as BG, and that would be perfectly fine, we felt that formally adding our premium line of Radia to the brand name would help tie it all together.”
In January 2015, David Graebener passed away. That same year, Christie acquired all the assets of BG Radia including the intellectual property associated with the BG Radia Neo planar ribbon drivers and a patent for a curved coaxial loudspeaker design. Christie continues to offer opportunities to companies interested in licensing the BG loudspeaker designs and the patented Neo planar ribbon drivers.
Voice Coil magazine published an obituary on David Graebener on its March 2015 edition. Vance Dickason writes, “I was sort of a disciple of David’s back in the 1970s. In 1975, I opened a retail store, Scientific Fidelity SoundLabs, which was indeed a Portland, or SpeakerLab clone. David and I talked a lot over the years, but especially about the perils and pitfalls of measuring large planar transducers. Without a doubt, he was one of the kindest souls I ever met in the industry.” The BG RD-75 driver Photo ABG’s four basic planar magnetic drivers, were sold and continue to be available through Parts Express
For the DIY person, BG offers four basic models, the RD-75, RD-50, RD-40, and RD-28.1. These model designations indicate the approximate length of the driver. Figure A gives the detailed dimensions of the RD-75 used in this project, and it is pictured in Photo A. The basic construction of the standard models consists of a frame made from heavy-gauge painted steel enclosing a push-pull array of Ceramic 8 magnetic-alloy field pieces.
The diaphragm resides within the field created by these magnets. It is a thin polyester film with an aluminum voice coil deposited on its surface. This design results in a load to the amplifier that is essentially resistive, with a 6Ω value in the case of the RD-75, making it a very easy load to drive for almost any amplifier.
The rated sensitivity of the RD-75 is 88dB/1W/1m. This may seem low, but since the RD-75 is a very long line source, you are listening in the near field at any distance in a normal room, even one that is quite large. The decrease in SPL with distance is half that of a standard small-diameter dynamic driver. Therefore, at normal listening distances, the SPL from the RD-75 will be higher than an equivalently rated dynamic driver. That’s the good news. The bad news is that the decrease in SPL will not match that of a dynamic driver used in the mid- to low-bass region, requiring some adjustment to get proper balance at a prime listening position.
Another advantage of a long, narrow driver is that it produces a cylindrical wave launch. The sound dispersion is very wide in the horizontal direction because of the narrow width of the driver, but is very restricted in the vertical direction because of its long configuration.
Unlike small drivers that tend to produce a spherical wave launch, this cylindrical launch minimizes interfering reflections from the ceiling and floor, one of the main causes of room problems. Using the RD series in a dipole format also produces nulls at the sides, eliminating some of the other nasties you can get from early reflections.
The other models are essentially just shorter versions of the RD-75. The resulting performance characteristics center on power handling and impedance. For example, the RD-75 can handle 200W of program material, while the RD-28.1 can handle 65W of program material. This is understandable when you look at the radiating area of both. The RD-75 has 144 in2 of radiating area to the RD28.1’s 48 in2.
Looking at the RD-75, you realize that 144 in2 is not too shabby for a mid/high frequency driver. Your typical 7″ driver comes in around 28 in2, and although it may have a higher XMAX if it is a combination bass/midrange, I know of no mid/high driver that can match the RD-75 in terms of volume displacement.
Because of this large area, the RD-75 diaphragm moves a relatively short distance even at high volume levels, resulting in lower IM distortion. And the most endearing feature of these drivers is that they cover the mid/high frequency range in one driver, including the all-important human voice range.
The impedance of the RD-75 is 6Ω, while that of the RD-28.1 is 4Ω. However, since they share most of the same acoustic properties, matching the different versions for applications such as surround sound should be easy. For all the drivers, BG specifies a crossover frequency of 150H at 24dB/octave. For full power handling, they recommend a crossover point of 300Hz. For the technically inclined, the following are some of the materials you can use to fabricate custom drivers:
Conductors:
Deposited aluminum
Etched aluminum
Etched copper
Various surface applied wound metal wires
Frames:
Carbon steel
Stainless steel
Various molded plastics
Magnetic Materials:
Strontium-ferrite ceramic
Isotropic and non-isotropic alnico
Samarium-cobalt
Neodymium-ferrite-boron
That shopping list should keep you busy for a while. However, for general home applications, the standard versions as supplied to the DIY market work just great!
Diaphragms:
