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133rd Meeting Lay Language Papers


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HyperSonicTM Sound

Elwood G. Norris - woody@atcsd.com
American Technology Corporation
12725 Stowe Dr.
Poway, CA 92064

Popular version of paper 2pEA
Presented Tuesday Afternoon, June 17, 1997
133rd ASA/NOISE-CON 97 Meeting, State College, Pennsylvania
Embargoed until June 17, 1997


HyperSonicTM Sound (HSSTM) from American Technology Corporation employs ultrasonics to create audible sound in the air. It works by using harmless ultrasonic tones that we can't hear. These tones use the property of air to create new tones that are within the range of human hearing. The result is audible sound. The acoustical sound wave is created directly in the air molecules by down-converting ultrasonic energy to the frequency spectrum we can hear.

HyperSonic Sound is produced without the excess baggage of conventional speakers--there are no voice coils, cones, crossover networks, or enclosures. The result is sound with a potential purity and fidelity never before attained.

Sound quality is no longer tied to speaker size. The HyperSonic Sound system holds the promise of replacing conventional speakers wherever they are used: in the home, in movie theaters, in automobiles--everywhere.

A Brief Look at Loudspeakers

About a half-dozen commonly used speaker types are in general use today. They range from piezoelectric tweeters that recreate the high end of the audio spectrum, to various kinds of mid-range speakers and woofers that produce the lower frequencies.

Even the most sophisticated hi-fi speakers have a difficult time in reproducing clean bass, and generally rely on a large woofer/enclosure combination to assist in the task. Whether they be dynamic, electrostatic, or some other transducer-based design, all loudspeakers today have one thing in common: they are direct radiating-- that is, they are fundamentally a piston-like device designed to directly pump air molecules into motion to create the audible sound waves we hear. HSSTM technology produces sound in the air indirectly as a by-product of some other process. Acoustical engineers and loudspeaker designers have struggled for nearly a century to produce a speaker design with the 20 Hz to 20,000 Hz capability of human hearing. In their attempts to do so they have been less than successful.

As electronics have advanced and speaker technology has been pushed to its limits, a whole array of terms have come to define the various forms of distortion associated with the conventional loudspeaker: amplitude distortion, harmonic distortion, intermodulation distortion, phase distortion, crossover distortion, cone resonance, and so forth.

Every form of distortion contributed by a loudspeaker is traceable to some aspect of its mechanical nature: mass, magnetic structure, enclosure design, cone construction, etc. All form an important part of the final product's capability to perform its function in as perfect a manner as possible.

Speaker cone motion is subject to the laws of physics. This all-important element, more than any other in a speaker system, affects the overall purity of sound and can be a source of various forms of distortion. Ideally, when reproducing sound, the speaker cone should follow precisely the delicate nuances of any electrical waveform presented to it. The cone or radiating surface of a perfect loudspeaker would have virtually no mass nor resonances over the entire range of hearing, and would offer perfect linearity while at the same time being able to couple enough energy into the air to produce any sound level desired.

HyperSonic Sound technology does precisely that--it provides linear frequency response with virtually none of the forms of distortion associated with conventional speakers. Physical size no longer defines fidelity. The faithful reproduction of sound is freed from bulky enclosures. There are no, woofers, tweeters, crossovers, or bulky enclosures.

HSS works by emitting a beam of high frequency ultrasonic energy which is converted to an audible acoustic wave in mid-air. An important by-product of the technique is that sound may be projected to just about any desired point in the listening environment. This provides outstanding flexibility, while allowing an unprecedented manipulation of the sound's source point.

It helps to visualize the traditional loudspeaker as a light bulb, and HSS technology as a spotlight. As with the light bulb, a traditional loudspeaker radiates sound fairly uniformly in all directions. A listener can stand anywhere in an acoustical environment and point to the speaker as the source of the sound. HSS technology is much more analogous to the beam of light from a flashlight. If you stand to the side or behind the light, you can only "see" the light when it strikes a surface. HSS technology is similar in that you can direct the ultrasonic emitter toward a hard surface, a wall for instance, and the listener perceives the sound as coming from the spot on the wall. The listener does not perceive the sound as emanating from the face of the transducer, only from the reflection off the wall.

However, look directly into the lens of a spotlight and you will see the highest intensity of light and it will appear to emanate from the face of the light itself. If you direct an HSS ultrasonic emitter directly towards a listener, the listener will perceive the sound as emanating directly from the face of the emitter. A by-product of this ability to "shine" sound is to tightly control the dispersion and project the sound to much further distances than conventional loudspeakers. Also, it is now possible to dramatically minimize room effects in a listening environment.

Dispersion of the audio wavefront can be tightly controlled by contouring the face of the HSS ultrasonic emitter. For example, a very narrow wavefront might be developed for use on the two sides of a computer screen while a home theater system might require a more broad wavefront to envelop multiple listeners.

Technology Overview

Range of Hearing

The human ear is sensitive to frequencies from 20 Hz to 20,000 Hz (the "audio" range), and can detect the vibration amplitudes that are comparable in size to a hydrogen atom.

If the range of human hearing is expressed as a percentage of shift from the lowest audible frequency to the highest, it spans a range of 100,000%. No single loudspeaker element can operate efficiently or uniformly over this range of frequencies. In order to deal with this speaker manufacturers carve the audio spectrum into smaller sections. This requires multiple transducers and crossovers to create a 'higher fidelity' system with current technology.

Using a technique of multiplying audible frequencies upwards and superimposing them on a "carrier" of say, 200,000 cycles the required frequency shift for a transducer would be only 10%. Building a transducer that only needs to produce waves uniformly over only a 10% frequency range. For example, if a loudspeaker only needed to operate from 1000 to 1100 Hz (10%), an almost perfect transducer could be designed.

If the audio spectrum could be superimposed on this high frequency carrier and emitted into the air as an ultrasonic acoustical wavefront, the only thing remaining would be to "down convert" the ultrasonic energy to sonic energy we could hear.

Non-Linearity of Air

When two sound sources are positioned relatively closely together and are of a sufficiently high intensity, two new tones appear: a tone lower than either of the two original ones and a tone which is higher than the original two.

There are now four tones where before there were only two. It can be demonstrated mathematically that the two new tones correspond to the sum and the difference of the two original ones, which we refer to as combination tones.

For example, if you were to emit 200,000 Hz and 201,000 Hz into the air, with sufficient energy to produce a sum and difference tone, you would produce the sum - 401,000 Hz - and the difference - 1,000 Hz, which is in the range of human hearing.

The HSS concept originates from this theory of combination tones, a phenomenon known in music for the past 200 years as "Tartini tones." It was long believed that Tartini Tones were a form of beats because their frequency equals the calculated beat frequency. However, it was Hermann von Helmholtz (1821-1894) who completely re-ordered the thinking on these tones. By reporting that he could also hear summation tones (whose frequency was the sum rather than the difference of the two fundamental tones) Helmholtz demonstrated that the phenomenon had to result from a non-linearity. Could a method be found today to utilize this non-linearity of air molecules in a manner similar to the non-linearity of an electronic mixer circuit?

In theory, the principle appears quite simple. Yet, until now, no one has succeeded in making it work. Nobody has been successful in producing useful levels of sound output in this difference frequency range.

Basic Benefits

Small Size

Not only has the conventional speaker's crossover network and enclosure been eliminated, but HSS' ultra-small radiating ultrasonic emitter is so small and light-weight that the inertial considerations ordinarily associated with traditional direct-radiation speakers are virtually non-existent. (And so is just about everything else associated with the conventional speaker: the voice coil and support structure normally used to attach the moving cone in place.)

Point Source

The ability to produce the entire audible spectrum of frequencies from a single point source has been the goal of transducer engineers for the past 50 years. The improvement in phase response, time alignment, and frequency response becomes obvious.

Performance

Preliminary testing of the ATC proof-of-concept prototype shows the HSS technology should have the potential for the following performance specifications:

What about our animal friends?

In reality, sound waves contain a relatively small energy content. For instance, if every man, woman, and child in New York City spoke loudly at the same time, the total acoustical energy produced would barely brew a single cup of coffee.

The most familiar applications for ultrasonics today are in the medical field, and do not generally involve radiating into the air. More commonly, ultrasound is used to image the brain and other organs. The most familiar application of ultrasonic waves is the sonogram, an imaging device used regularly in the prenatal treatment of pregnant women to monitor fetal development. More recently, ultrasound is also being used to speed the healing process of bone fractures and other injuries. Abundant data in medical literature validates the fact that ultrasound at these frequencies is harmless. There is no need to worry about pets, either. Dogs and cats can hear sounds up to perhaps 40,000 Hz, and HSS operates well above this range.

Applications for HSS

HSS technology applications are limited only by the imagination. High volume applications are numerous and include:

Besides consumer electronics, the entertainment industry is expected to be fundamentally influenced by this development. In a movie theater, sound can be made to emanate directly from an actor's mouth on the screen. Special effects will no longer be limited to the capability of loudspeakers positioned around the auditorium.

You might want to project concert sound throughout an audience instead of using huge speaker stacks in front. A small table radio might project sound around an entire room. Why not equip your back yard with tightly focused HSS emitters to project sound all around your yard for that next pool party.

Until now, it has been difficult for a hearing aid--regardless of price--to reproduce the entire audio spectrum. This no longer need be the case. With HSS, hearing aids may also shrink further in size.

Virtual reality, in large-scale applications, has been brought another step closer.

No longer is the quality of the sound related to the size or type of a speaker's enclosure. Everywhere and anywhere a speaker is in use today--ships, aircraft, hospitals, automobiles--the HSS technology can replace the bulkier, inefficient speakers, and provide far better results than we have ever heard.

Truly, this is a quantum leap, a paradigm shift.


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