Parametric Loudspeakers: A New/Old Way To Deliver Sound
By Mel Lambert
ATC develops another unconventional technology for tightly focused audio.
ATC’s DSP box that is crucial to the ultrasonic concept.
Most of us are familiar with a number of different transducer designs, ranging from traditional cone drivers through planar/flat-panel devices to exotic ribbon affairs. But not many of us, I would wager, are familiar with the concept of using ultrasonic sound waves to produce both direct and virtual sound images.
It’s a reasonably simple concept to explain, but devilishly difficult to produce commercially, as a new San Diego-based firm, American Technology Corporation (ATC), can attest. In essence, the idea is to use powerful DSP functions to prepare a complex ultra high-frequency carrier wave that you then deliver to a suitable transducer or emitter. This carrier wave can be reflected off any normal surface or aimed directly at the audience, while at the ear, the high-frequency components are removed, and we hear the original sound.
EASY WHEN YOU KNOW HOW
ATC’s new products are based on the nonlinearity of air. Because sound comprises a small pressure wave that travels through the air, as the pressure increases and decreases the nonlinear nature of air causes the sound wave to be changed slightly, and new frequencies are formed within the wave.
Since we know how the air affects sound waves, we can predict exactly what new frequencies will be added into the sound wave by this nonlinear behavior of air. An ultrasonic wave can be sent with sufficient volume to cause the air to create such additional frequencies. But, because the ultrasonic sound is inaudible, all we hear are new sounds that are formed by the air’s nonlinear action.
And here is the magic: our music or vocal source is converted into a complex ultrasonic signal by a DSP system that takes into account how the air will react to it and produce the frequencies that comprised our input signal. And since the ultrasonic energy is highly directional, it forms a virtual column of sound directly in front of the transducer. Along that column, air is recreating the sound originally converted into an ultrasonic wave; because its source is within the column of ultrasonic energy, it doesn’t spread in all directions (unlike a conventional loudspeaker) but remains locked tightly inside the column. So, to hear the sound, the listeners’ ears must be in-line with the column of ultrasound; sound can also be heard after it reflects off a hard surface. If we point the ultrasonic emitter towards, for instance, a wall, we will only hear the audible sound after it has reflected off that surface.
PRACTICAL ADVANTAGES ARE OBVIOUS
Since the ultrasonic beam is highly directional, it forms a virtual column of tightly focused sound directly in front of the emitter, much like the light from a flashlight. Sound can be heard from within the beam, or after it reflects off any hard surface. This latter “virtual mode,” as ATC calls it, is similar to shining a flashlight at a wall in a dark room - you do not see the light from the flashlight, only the spot of light on the wall. And, systems can be built with thin, flat ultrasonic devices that do not require a mounting cabinet; since such systems do not follow the traditional inverse square law, focused or directed sound travels much farther in a straight line than conventional loudspeakers while maintaining intelligibility; and you can control dispersion in a way that would be very hard, if impossible, with conventional designs.
APPLICATIONS
Count the ways: museums, where narrations about a specific display could be directed only to the people standing directly in front of the material; rear channels of a surround-sound sales presentation system could be reflected from the rear wall of a conference room, by focusing the transducers mounted on the large-screen display or video monitor; and, for crowd control, you could focus sound into a large gathering of people at a football game, for example, and directly address selected members without disturbing others in the throng.
RESEARCH
If some of this sounds a wee bit familiar, you have a good memory. ATC’s patented HyperSonic Sound/HSS technology and speaker arrays are based upon some landmark research into what the firm refers to as “parametric loudspeakers,” which concept is based on two ultrasonic sources driven independently at different frequencies that define the signal of interest. Why “parametric loudspeakers?” Simply, it is because of the dual-frequency - sum and difference - nature of the propagation technique.
Helmholtz was aware of how combination tones (sum and difference) are generated by a non-linear restoring force on displaced air molecules.
A highly directional, virtual column of sound.
And because his theory and formulas predicted results that initially seemed to match what Helmholtz measured, it appeared to be a valid theory. Several researchers developed mechanisms for such “propagation distortion,” while in late-1962, Peter Westervelt, a professor at Brown University, refined the idea, as did H. O. Berktay with his “Berktay’s Far-Field Solution,” which defines a mathematical expression for the output of parametric acoustic arrays.
Fast forward through more academic probing to the mid-Nineties, when the ATC team focused its attention on the problem of converting theory into practical hardware, in addition to licensing its proprietary HSS technology. Currently, the company offers two units that were developed specifically for museum displays, retail environments, trade shows, point-of-purchase, kiosk sound, and other applications where highly directional sound is required. Models R120A and R220A are fully self-contained with audio input processing, power amps and output device(s). The 220 Series features twice the surface area of the 120 and provides higher SPL levels with a slightly tighter audio beam pattern. Other companies are also in the process of offering OEM devices based on HSS principles.
An obvious question: How loud can you run an HSS system? At the current stage of development, the company reports, HSS can produce in excess of 99 dB of audible sound at 20 feet from an 11-by-11-by-0.4 inch emitter array. And it can maintain a similar SPL performance over greater distances. In terms of durability, HSS is said to be equal to conventional loudspeakers.
All in all, something tells me that such devices are destined to dramatically shake up the transducer paradigm. While current arrays cannot match the power handling of conventional cone-based systems, it’s just a matter of time before market demands will up the ante with these remarkable designs.
Mel Lambert founded Media&Marketing more than a dozen years ago to provide communications and consulting services for pro-audio firms and facilities. Contact Mel at www.mel-lambert.com |
