Polymer films tapped as potential subs for speakers
By Chappell Brown
EE Times
July 21, 2003 (11:19 a.m. ET)
Hancock, N.H. - An audio technology called hypersonic sound-so highly directional that its output can target listeners in a specific area-will go into plasma screen digital signs made by Sony Corp.'s European arm. But while Sony's embrace raises expectations for hypersonic sound, proponents insist that its potential goes far beyond talking signs to offer a totally new approach to all audio applications.
American Technology Corp. (San Diego) builds the hypersonic-sound units. It was founded by Elwood (Woody) Norris, the technology's inventor, and holds multiple patents on the technology. Hypersonic sound began as a way to replace bulky, expensive speakers with a simple ultrasound emitter. Norris reasoned that two high-frequency signals close together could generate an audible signal via nonlinear mixing in the air. The air, in effect, would become the mechanical source of sound, eliminating speaker cabinets, voice coils and speaker cones.
The theory behind the effect is simple. If two signals are fed into a nonlinear mixer, the output contains two additional signals: the sum of the two inputs and their difference. Radio receivers use the effect to convert a high-frequency signal from the antenna to an easier-to-process intermediate frequency (IF). The antenna signal is mixed with a local oscillator and the difference signal becomes the IF.
"I had no idea that anyone else had worked on this concept," Norris said. "But when I did a patent search, I was amazed to see how much work had already been done in this area." Previous research demonstrated that air could act as a nonlinear mixer and audible tones could be generated from ultrasound, similar to IF generation in radios. But the effect was too weak and distorted to be useful in practical audio applications.
Norris became convinced that the key to good-quality sound output lay in the design of the ultrasound emitter. "The first TV remote control units used ultrasound transducers, which were cheap and available, so I bought hundreds of them, trying to build arrays with a high-amplitude output," said Norris. Amplitude was critical in producing a strong difference signal. "Textbook theory says that audio dynamics are linear, but that is only a first approximation," he said. "If you get the amplitude high enough, nonlinear effects begin to appear." But the arrays produced too much distortion, Norris found, due to interference effects generated by slight variations between them. So he set out to find a better ultrasound emitter.
All ultrasound transducers are based on the piezoelectric effect. Piezoelectric materials change their physical dimensions in response to an electrostatic field. Conversely, mechanically changing the shape of a piezoelectric material will generate an electrostatic field. A recent discovery that polymer films can also be piezoelectric proved critical to Norris' quest. Thin sheets of polyvinylidine fluoride are formed in the presence of a strong polarizing electric field, producing a unique molecular arrangement of the long-chain molecules in the plastic. Instead of being randomly oriented inside the film, the polymer chains are stretched out in linear arrays, creating the piezoelectric effect in the plastic.
"The film itself will shrink and expand in response to a varying field, but that won't produce sound," he said. "You have to find some way to buckle it so that shape change will cause the film to pop in and out."
Norris fashioned 28-micron-thick polyvinylidine fluoride films with an array of dimples. Behind them were tiny vacuum wells. Each dimple became an ultrasound transducer and the array could output distortion-free high-amplitude waves.
Experimentation continued and more-efficient configurations were discovered. A corrugated film organized with an array or linear folds bumped up the output by several orders of magnitude. Norris also found a way to eliminate the vacuum and other design simplifications followed. "We own the emitter," he said. "We spent years trying everything the human mind could come up with on transducer designs." Norris is now submitting another series of patents based on a highly simplified approach that will, he believes, let the technology compete with existing audio.
Sony's first steps will be modest ones. Sony Business Europe initially plans to use hypersonic sound only for commercial signs with low audio quality. A highly directional commercial message associated with a sign will be heard only by a customer standing directly in front of the sign.
Since Sony has built its name on high-quality audio products, the low-audio-quality signs it's going with for now indicates where the technology stands today. Norris said he expects his new round of patents to produce the needed high-quality audio within a year. If so, that would be a revolution in the audio and multimedia arena. |
