The dispute stems from student Joseph Pompei's accusation that Sennheiser Electronics GmbH (Berlin), founded by Jorg Sennheiser, a professor at the University of Hannover, used technology that Pompei had demonstrated a year before. The company denies Pompei's charge.

Despite the controversy, commercialization of the technology seems to be progressing on both sides of the Atlantic, as exemplified by the recent unveiling of a Chrysler concept car with a sound system based on the MIT technology. Without headphones, the technology enables each passenger to hear a different audio signal.

Ears on the prize

Discord began early this year, when Sennheiser Electronics won a German innovation prize for its AudioBeam. Pompei had demonstrated his own "audio spotlight" at the Audio Engineering Society in 1998 and claims to have been approached afterward by a Sennheiser employee. According to Pompei, "Just after my presentation, Dr. Wolfgang Niehoff, director of research at Sennheiser GmbH, approached me and complimented my work, saying that he was very interested in this field. He handed me his business card, and invited me to an 'all-expense paid trip to Sennheiser,' as it was having tremendous difficulty making a working prototype and wanted my help. I was flattered, but politely declined, and instead handed him a copy of my just-presented paper."

"A year later," said Pompei, "a colleague of mine attended AES99 and told me that Sennheiser GmbH was showing off a device at its trade show booth — without a paper — which was very similar to my own." According to Pompei, the colleague "was told by the engineers at the booth — after they saw his Media Lab nametag — that they essentially followed what I had published. This didn't bother me at all, as they didn't seem to be misrepresenting the work or making egregious claims." However, when he heard about the innovation prize, he said, "I was rather surprised. They certainly were well aware of my work, and I was unable to find any publication from Sennheiser. I was greatly more offended when I saw that the descriptions they were using to describe the technology as well as their demonstration material were very similar to those on my Web site and what I had demonstrated at AES98." According to Pompei, he had been credited nowhere.

Rolf Meyer, president of Sennheiser, sees things differently. In response to a letter of complaint from Pompei, he claimed that the company had done more than two years of research and development before the meeting between Niehoff and Pompei. This included a master's project that, said Meyer, just happened to be very similar to Pompei's research. "The duplication of work is something that is happening in all areas of research and development," he said. "We regret that Dr. Niehoff did not give . . . a complete overview of our position regarding this technology during his visit in 1998. Due to competitive and intellectual-property rights research he was not allowed to discuss this matter by our legal counsel."

The technology under dispute is an extension of work first done decades ago. Ultrasound has the advantage of being highly directional (compared to audible sound) because of its relatively short wavelength. Researchers realized that they could use this property to create directional sound beams by using intermodulation of ultrasound beams in air. This is possible because, at high-enough intensity (above 125 dB — as opposed to the normal volume of 30 to 90 dB) the air stops behaving linearly. When this happens, the propagation of one beam modulates the propagation of the other. This is similar to the phenomenon of frequency-doubling in optics: Both phenomena require a nonlinearity and both result in the production of controllable outputs. The two original beams, plus those with frequencies that are their sum and difference, and their harmonics can be generated in a controlled way. And, by choosing the ultrasound frequencies to be relatively close to each other, the difference beam can be audible.

By creating a complex ultrasound waveform (using a parametric array of ultrasound sources), many different sources of sound can be created. With their phases carefully controlled, they interfere destructively and constructively in the forward direction, resulting in a collimated sound beam or "audio spotlight."

This phenomenon was first demonstrated in the '70s, but early implementations had two major problems. The first was that they used relatively low-frequency ultrasound, near the audible range. Because of the high intensities needed to get the nonlinear response, such beams could be considered dangerous. The other problem is the distortion caused by using very basic demodulation strategies to produce the audio signal. Using the simplest envelope function created an unwanted signal that varied with the square of the audio modulation depth. Turn it up and it distorts, turn it down and it becomes inaudible and highly inefficient.

As a Media Lab PhD student, Pompei, who started his career as the youngest engineer at Bose Corp. (Framingham, Mass.), developed the technology by using computational techniques to preprocess the audio signal to minimize the distortion. To do so, he investigated the basic physics of the phenomena and found that the problem could be eliminated mathematically.

Earlier attempts had used an AM modulation technique to create the ultrasound signal. However, after doing some detailed mathematical analysis of the non-linear transformations that produce the audible signal, Pompei realized that the generation of realistic audible sound would require more sophisticated ultrasound generators. The mathematical model could be reversed: Starting with the desired output, Pompei worked backward to find the characteristics of the generating ultrasound beam. That led to a preprocessing control algorithm for the ultrasound transducers.

He soon realized that to implement the solution perfectly would require an infinite series of harmonics, most of which could not be reproduced on a real array. On the other hand, the correct solution could be approximated using a wideband ultrasound source. Pompei compared the options, testing the sound produced by the array in an anechoic chamber using a high-frequency linear microphone. He submitted a paper for the 1998 Audio Engineering Society conference and publicly demonstrated his system at the meeting.

Sennheiser president Meyer said that the German industry innovation prize was "not given for original principles, but for the development of an innovative product and bringing that product to market." Meyer also claimed that Pompei was only making a fuss because his graduation would depend on his thesis being original, adding that the work at Sennheiser suggested it wasn't.

Meyer offered two explanations for this. First is the master's project at the University of Hannover, supervised by Prof. Jorg Sennheiser. Despite the thesis' importance, Pompei's requests for a copy were neglected until the document was requested by EE Times.

After reading the translated document, Pompei said, "This thesis, which was sponsored and supervised by Prof. Sennheiser and conducted in Sennheiser's facility, describes a parametric array device, whose performance fell well short of even those devices described in publications more than 15 years ago . . . Had Mr. Gelhard the student and Prof. Sennheiser read these papers . . . they might have avoided making the very same mistakes which caused the widespread abandonment of the technology in the mid '80s, not revived until my published work of 1998."

Claim of ownership

Pompei rejected the claim that the thesis formed the basis for the present Sennheiser AudioBeam. Pompei asserted that, "An objective interpretation of their development program leads to the clear conclusion that the AudioBeam device is based not on Mr. Gelhard's thesis, but on my published paper and their director of research witnessing my public demonstration." The absence of any peer-reviewed publications by Sennheiser makes it impossible to prove what the company did or didn't understand about the technology, and when.

Meyer's contention that Pompei's work is unoriginal is based on a paper on ultrasound preprocessing published two months before the original AES demonstration in 1998. The paper discusses the same basic mathematical model as used by Pompei. But given the long time-scale of academic publishing and the refereeing process, that could be an indication of simultaneously duplicated work, rather than copying.

Meyer's most significant claim is that — regardless of the originality of the distortion preprocessing — the Sennheiser device simply doesn't use that technology. In the absence of peer-reviewed technical papers or access to the device, the claim is impossible to verify. Keeping trade secrets may make business sense, but in doing so a company likely forfeits the opportunity to be acknowledged for the invention. Publishing carries with it the risk of theft but makes the intellectual credit clear.

Pompei would like to see Sennheiser GmbH give back the innovation prize and apologize for failing to credit him. However, he said, this is a matter of principle, not intellectual property. Regardless of the ethical difference, he said, patents have been applied for and — if necessary — can be tested in the courts.

But both he and his rivals in Germany agree on one thing: The potential for the technology is enormous. Hardware is not an issue — the transducers required to produce these beams are half an inch thick and lightweight — and the system required to drive it has similar power requirements to those of conventional amplifier technology.