United States Patent 55
`5,889,870
`[11] Patent Number:
`
` Norris [45] Date of Patent: Mar. 30, 1999
`
`
`US005889870A
`
`[54] ACOUSTIC HETERODYNE DEVICE AND
`METHOD
`
`5,357,578 10/1994 Tanishi.
`OTHER PUBLICATIONS
`
`[75]
`
`Inventor: Elwood G. Norris, Poway, Calif.
`.
`:
`.
`[73] Assignee: American Technology Corporation,
`Poway, Calif.
`
`[21] Appl. No.: 684,311
`:
`22]
`Filed:
`1. 17, 1996
`[22]
`File
`Ju
`,
`[SL] Unt, Cdees eesssseccssseccssseesssseeesnne HO04B 3/00
`[52] US. Ce ec eeesessenceseneenseeesseneenees 381/77; 381/79
`[58] Field of Search occ 381/79, 77, 82
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`2/1927 Sprague.
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`8/1968 Clark, III .
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`2/1972 Stover .
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`1/1973 Tischneretal. .
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`3/1973 Damon .
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`6/1973 Kay etal. .
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`9/1974 Geren et al.
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`3/1983 Clark .
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`6/1986 Nakamura .
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`4/1989 Tanakaet al. .
`4,991,148
`2/1991 Gilchrist.
`5,317,543
`5/1994 Grosch .
`
`.
`
`.
`
`Ultrasonic Ranging System—Polaroid.
`Helmholtz (Excerpts from On Combinaton Tones)—Edi-
`tor’s Comments on Paper 16.
`Aoki, K., et al., “Parametric Loudspeaker—Charateristics of
`Acoustic Field and Suitable Modulation of Carrier Ultra-
`sound,” Electronics and Communications in Japan, Part 3,
`1. 74, No. 9,
`pp. 76-82 (1991).
`vou
`i
`> PP
`Makarov, S.N., et al., “Parametric Acoustic Nondirectional
`Radiator,” Acustica, vol. 77, pp. 240-242 (1992).
`Westervelt, P.J., “Parametric Acoustic Array,” The Journal
`of the Acoustic Society of America, vol. 35, No. 4, pp.
`535-537 (1963).
`
`Primary Examiner—Forester W.Isen
`Attorney, Agent, or Firm—Thorpe, North & Western, LLP
`
`[57]
`
`ABSTRACT
`
`invention is the emission of new sonic or
`The present
`subsonic compression waves from a region resonant cavity
`or similar of interference of at least two ultrasonic wave
`trains.
`In one embodiment,
`two ultrasonic emitters are
`oriented toward the cavity so as to cause interference
`between emitted ultrasonic wave trains. Whenthe difference
`
`in frequency between the two ultrasonic wavetrains is in the
`sonic or subsonic frequency range, a new sonic or subsonic
`wave train of that frequency is emitted from within the
`cavity or region of interference in accordance with the
`principles of acoustical heterodyning. The preferred
`embodiment is a system comprised of a single ultrasonic
`radiating element oriented toward the cavity emitting mul-
`tiple waves.
`
`6 Claims, 7 Drawing Sheets
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`1
`ACOUSTIC HETERODYNE DEVICE AND
`METHOD
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`This invention pertains to compression wave generation.
`Specifically, the present invention relates to a device and
`method for indirectly generating a new sonic or subsonic
`compression wave without the use of a direct radiating
`elementat the source of the new compression wave genera-
`tion.
`2. State of the Art
`
`Sound waves in general are wave-like movements ofair
`or water molecules. Because these media are elastic and
`
`generally homogeneous, naturally occurring sound travels in
`all directions radially from the source of generation. A voice,
`instrument or
`impact,
`for example, will
`radiate omni-
`directionally in a unitary, integrated form, carrying multiple
`frequencies, overtones, and a full range of dynamics that
`collectively contribute to an instantaneous sound perception
`at the ear. This perception of naturally occurring sound at a
`healthy ear is deemed to be “pure” when it corresponds to
`the same acoustic content that existed at the point oforigin.
`Because sound is a transient, temporary state of motion
`within a media,it is not self-sustaining. Indeed,the first and
`second laws of thermodynamics require that
`the sound
`eventually dissipate its motion into heat or other forms of
`energy. Therefore, if storage or preservation of the sound is
`desired,it is necessary to transmute such motion intoa fixed
`form of recording. This fixed form can then be recovered
`later by conversion of the fixed form back into sound waves.
`In the earliest experiences of recording, mechanical
`devices were moved by impact of the sound waves to
`inscribe or etch a corresponding grove into a plate. By
`positioning a needle or other tracking device over a set of
`moving grooves, crude reproduction of the original sound
`waves was accomplished. More sophisticated technologies
`have developed which enable capture of sound waves in
`other fixed forms such as magnetic, electronic, and optical
`media. Nevertheless, the same principle of sound reproduc-
`tion has been applied to recover this stored information,
`whether the response is generated by a mechanical mecha-
`nism or by digitally controlled laser reading devices.
`Specifically, stored signal is converted back to sound waves
`by recreating movement of an object, which then sets the
`surrounding air into motion corresponding to sound repro-
`duction.
`
`Aprimary goal of modern acoustic science is to reproduce
`pure sound, based on conversionof the electronic, magnetic,
`mechanical or optical record into compression waves which
`can be detected at the ear. The ideal system would play all
`original sound back through a resonating device comparable
`to that which produced the sound in the beginning. In other
`words, the violin sounds would be played back through a
`violin, regenerating the overtones and a myriad of other
`dynamic influences that represent that instrument. Similarly,
`a piccolo would be played back through a device that
`generates the high frequencies, resonance aspects and over-
`tones associated with this type of instrument. In short, one
`cannot expect a viola to sound like a viola in “pure” form if
`sound reproduction is actuated by a mechanical wave gen-
`erating device that does not embody unique characteristics
`of that instrument or voice. Accordingly, it would seem that
`the only practical way to reproduce the original “pure”
`quality of sound would be to isolate each instrument or
`source, record its sound output, and then reproduce the
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`output into the same instrument or acoustic resonator. It is
`apparent that such a solution is totally impractical.
`In the real world, the challenge of reproducing sound has
`been allocated to the speaker. The operation of a loudspeaker
`is relatively simple to understand whenthe interaction of the
`components is explained. A speaker is a transducer which
`receives energy in one form (electrical signals representative
`of sound) and translates the energy to another
`form
`(mechanical vibration). In a dynamic loudspeaker, an elec-
`trical current that is proportional to the strength and fre-
`quency of the signal to be broadcast is sent through a coil
`attached to a rigid membraneor cone. The coil movesinside
`a permanent magnet, and the magnetic field exerts a force on
`the coil that is proportional to the electrical current. The
`oscillating movementof the coil and the attached membrane
`sets up sound wavesin the surrounding air. In brief, repro-
`duction of sound has heretofore required mechanical move-
`ment of a diaphragm or plate. To expect a single diaphragm
`or plate to accurately supply both the shrill sound of the
`piccolo and the deep resonance of the base drum would
`indeed be unreasonable.
`
`It is important to note, however, that when the listener at
`a live performance of a symphonyhears this broad range of
`sound, he receives it in an integrated manneras a “unified”
`combination of sound waves, having a myriad of frequen-
`cies and amplitudes. This complex array is responsively
`promulgated throughthe air from its originating source to an
`ear that is incredibly able to transfer the full experience to
`the brain. Indeed, the full range of audible signal (20 to
`20,000 Hz)
`is processed as a unified experience, and
`includes effects of subsonic bass vibrations, as well as other
`frequencies which impact the remaining senses.
`It is also important to note that this same “pure” sound
`that arrives at the ear, can be detected by a microphone and
`consequently recorded onto a fixed media such as magnetic
`tape or compact disc. Although the microphone diaphragm
`may not have the sensitivity of a human ear, modern
`technology has been quite successfulin effectively capturing
`the full range of sound experience within the recorded
`signal. For example, it is unnecessary to provide separate
`microphonesfor recording both low and high range frequen-
`cies. Instead,like the ear drum, the microphone, withits tiny
`sensing membrane, captures the full audio spectrum as a
`unified array of sound waves and registers them as a
`composite signal that can then be recorded onto an appro-
`priate media.
`It is therefore clear that the microphoneis not the primary
`limitation to effective storage and subsequent reproduction
`of “pure” sound. Rather, the challenge of accurate sound
`reproduction arises with the attempt to transform the micro-
`phone output to compression waves through a mechanical
`speaker. Accordingly,the focus of effort for achieving a high
`quality unified sound system has been to develop a complex
`speaker array which is able to respond to high, medium and
`low range frequencies, combining appropriate resonance
`chambers and sound coupling devices, to result in a closer
`simulation of the original sound experience.
`This quest for improved sound reproduction has included
`studies of problems dealing with (a) compensating for the
`mass of the speaker diaphragm, (b) the resistance of air
`within an enclosed speaker, (c) the resonant chamber con-
`figuration of the speaker, (d) the directional differences
`between high and low frequencies, (e) the phase variation of
`low versus high frequency wavetrains, (f) the difficulty of
`coupling speaker elements to surrounding air, and (g) the
`loss of harmonics and secondary tones. Again, these aspects
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`3
`represent just a few of the problems associated with recon-
`structing the sound wave by means of a direct radiating
`physical speaker.
`As an example of just one of these issues, overcoming the
`mass of a speaker driver has remained a challenging prob-
`lem. Obviously,
`the purpose of the speaker driver and
`diaphragm is to produce a series of compression waves by
`reciprocating back and forth to form a wavetrain. The initial
`design challenge is to compensate for resistance against
`movement in speaker response due to inertia within the
`speaker massitself. Once the speaker driver is set in motion,
`however, the mass will seek to stay in motion, causing the
`driver to overshoot,
`requiring further compensation for
`delayed response to reverse its direction of travel. This
`conflict of mass and inertia recurs thousands of times each
`
`second as the speaker endeavors to generate the complex
`array of waves of the original sound embodied in the
`electrical signal received.
`In order to meet the difficulty of compensating for mass,
`as well as numerousother physical problems, speaker devel-
`opment has focused mainly on improving materials and
`components as opposed to developing a different concept of
`sound generation. Diaphragm improvements, cone construc-
`tion materials, techniques and design, suspensions, motor
`units, magnets, enclosures and other factors have been
`modified and improved. Nevertheless, the basic use of a
`reciprocating mass remains unchanged,despite an efficiency
`of less than 5 percent of the electrical power being converted
`to acoustic output.
`Electrostatic loudspeakers represent a different method-
`ology. Unlike the electrodynamic loudspeaker with its cone
`shaped diaphragm,the electrostatic loudspeaker uses a thin
`electrically conducting membrane. Surroundingtheplate are
`one or morefixed grids. When a signal voltage is applied to
`the elements,
`the electrostatic force produced causes the
`diaphragm to vibrate. This low-mass diaphragm is particu-
`larly useful as a high-frequency radiating element, and its
`operation can be extended to relatively low frequencies by
`the use of a sufficiently large radiating area.
`Although electrostatic speakers offer some advantages,
`they are large, expensive, inefficient and suffer from the lack
`of point source radiated sound. For example, sound detec-
`tion is accomplished by a microphone at a localized or
`approximate point source. To convert the detected sound to
`a non-point source, such as a large electrostatic diaphragm,
`may create unnatural sound reproduction. Specifically, a
`radiating electrostatic speaker 5 feet in heightis limitedin its
`ability to simulate the delicate spatial image of a much
`smaller piccolo or violin.
`Anotherissue in loudspeaker design is that the optimum
`mass and dimensions for low frequency radiating elements
`differ radically from those for high frequency. This problem
`is typically addressed by providing both woofer and tweeter
`radiating elements for each channel of a loudspeaker system.
`The implications of this design are highly undesirable. The
`phase shift introduced because of the differences in time
`delay for high frequency signals traveling (i) the shorter
`distance of the cone of a tweeterto a listener, versus (ii) the
`substantially longer path for low frequency signals from the
`horn or woofer speakerto a listener’s ear, can be in the range
`of thousands of percent in phase differential.
`The preceding discussion of speaker technologyis recited
`primarily to emphasize the historical difficulty of changing
`a stored form of sound to a compression wave capable of
`reproducing sound in its original form. Nevertheless, the
`prior art has been virtually dominated for sixty years by the
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`that mechanical systems, such as speakers, are
`concept
`required to reproduce audible sound. Clearly, it would be
`very desirable to provide a means of sound reproduction
`which adopts a different approach, avoiding the many dif-
`ficulties represented by the choice of moving a diaphragm or
`speaker in order to generate sound.
`OBJECTS AND SUMMARY OF THE
`INVENTION
`
`It is an object of the present invention to provide a method
`and apparatusfor indirectly emitting new sonic and subsonic
`wave trains from a region of air without using a direct
`radiating element to emit the wavetrains.
`It is another object to indirectly generate at least one new
`sonic or subsonic wave train by using a by-product of
`interference between at least two ultrasonic signals having
`different frequencies equal to the at least one new sonic or
`subsonic wavetrain.
`
`It is still another object to cause at least two ultrasonic
`wavetrains to interact in accordance with the principles of
`acoustical heterodyning to thereby extract intelligence from
`the interfering wavetrains.
`It is yet another object to indirectly generate new sonic or
`subsonic wavetrains by combining them with an ultrasonic
`carrier wave using amplitude modulation, emitting the com-
`bined signal from an ultrasonic transducer, causing interfer-
`ence between the carrier wave and another ultrasonic fre-
`
`quency wavetrain, to thereby create the new sonic or
`subsonic wavetrains.
`
`It is still another object to affect a physicalstate of a living
`being utilizing an indirectly created compression wave.
`It is still yet another object to generate a new compression
`wave whichis perceptible to human sensesusingat least two
`imperceptible compression waves, but without directly
`propagating the new compression wave.
`Yet another object of the invention is to generate a new
`sonic or subsonic wavetrain without having to overcomethe
`mass and associated inertial limitations of a conventional
`
`direct radiating element.
`Still another object of the invention is to generate a new
`sonic or subsonic wavetrain without introducing distortions
`or undesired harmonics otherwise inherent to a conventional
`
`direct radiating element.
`Another object is to indirectly generate and enhance a
`new sonic or subsonic wave train from within a resonant
`
`cavity by emitting at least two ultrasonic wavetrains into the
`resonant cavity.
`Yet another object is to omni-directionally generate a high
`frequency wavetrain, thereby avoiding the highly focused
`and directional nature of high frequency signal emissions
`typical of a conventional loudspeaker.
`Still yet another object is to generate a new sonic or
`subsonic wavetrain in a localized area without coupling to
`an associated environment or enclosure which would oth-
`erwise cause undesirable broadcasting of the sonic or sub-
`sonic wavetrain.
`
`Yet another object is to generate a new sonic or subsonic
`wave train wherein characteristics of the new sonic or
`
`subsonic wavetrain are not limited by the characteristics of
`a direct radiating element.
`Another object of the invention is to emulate a sound
`wave detection process typical of an approximate point-
`source detection device such as a microphone, but without
`providing a physical detection device at a detection location.
`Another object is to control the volume of a new sonic or
`subsonic wave train by manipulating the degree of interac-
`tion of the at least two ultrasonic frequency wavetrains.
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`5,889,870
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`Still another object is to emit a new sonic or subsonic
`wave train from a region of air as a by-product of modu-
`lating a single ultrasonic wave train emitted from a single
`ultrasonic transducer into the region in accordance with the
`principles of acoustical heterodyning.
`The present invention is embodied in a system which
`indirectly generates new sonic or subsonic wavestrains. In
`one embodiment, a new sonic or subsonic wave train is
`emitted from a region of interference of at least two ultra-
`sonic wave trains emitted from at
`least
`two ultrasonic
`transducers. The principle of operation is based on incorpo-
`rating retrievable intelligence onto an ultrasonic carrier
`wave. The intelligence is retrieved as the desirable
`by-product of interference of the ultrasonic carrier wave
`train and anotherultrasonic wavetrain. The ultrasonic wave
`
`trains interfere within a region of non-linearity in accor-
`dance with principles identified by the inventor as “acous-
`tical heterodyning,” and thereby generate by-products which
`include the difference and the sum of the two ultrasonic
`wavetrains.
`
`A system which easily demonstrates the principle of
`acoustical heterodyning comprises two ultrasonic frequency
`transducers which are oriented so as to cause interference
`between emitted ultrasonic wave trains. Whenthe difference
`
`in frequency between the two ultrasonic wavetrains is in the
`sonic or subsonic frequency range,
`the difference in fre-
`quencyis generated as a new,audible sonic or new subsonic
`wave train emanating outward from within the region of
`heterodyning interference.
`Adifferent embodimentof the system provides the advan-
`tage of being comprised of only one ultrasonic direct radi-
`ating element. The advantage is not only in the decreased
`amount of hardware, but the perfect alignment of the two
`interfering ultrasonic wave trains because they are emitted
`from the same radiating element. In effect, the new sonic or
`subsonic wave train appears to be generated directly from
`the ultrasonic emitter. If it were not for the inescapable
`conclusion that the ultrasonic emitter cannotitself generate
`sonic or subsonic frequencies, plus the audible evidence that
`the sound is not emanating directly from the emitter, one
`might be deceived.
`The importanceof the first embodimentis that it teaches
`the concept of generating a new sonic or subsonic wavetrain
`as a result of the interference between two ultrasonic wave
`
`trains in accordance with the principles of acoustical het-
`erodyning. In essence,it is easier to see that two ultrasonic
`wavetrains are coming from two ultrasonic emitters. But the
`principle of acoustical heterodyning taught by this first
`embodiment prepares the way for understanding how the
`second embodimentfunctions. It becomes apparentthat the
`same acoustical heterodyning principle applies when it is
`understood which wavetrains are interfering in space.
`A key aspect of the invention is the discovery that by
`superimposing sonic or subsonic intelligence onto an ultra-
`sonic carrier wave, this intelligence can be retrieved as a
`new sonic or subsonic wave train. Whether the ultrasonic
`
`wavetrains are generated from two emitters or from a single
`emitter, the effect is the same.
`Another aspect of the invention is the indirect generation
`of new compression waves without having to overcome the
`problemsinherent to mass and the associated limitations of
`inertia of a conventional direct radiating element. The
`present invention eliminatesa direct radiating element as the
`source of a new compression waveso that the desired sound
`is generated directly from a region of air and without the
`several formsof distortion all associated with direct radiat-
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`ing speakers.
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`6
`Another aspect which is helpful to utilize the present
`invention is to understand the nature of the transmission
`
`medium. More specifically, the region of air in which an
`acoustical heterodyning effect occurs is referred to as the
`transmission medium. It is well knownthat the transmission
`
`medium ofair provides an elastic medium for the propaga-
`tion of sound waves. Thus,prior art research hastreated air
`as a passive element of the sound reproduction process. Air
`simply waits to be moved by a compression wave.
`Consequently, little practical attention has been devoted
`to the nature of air when it behaves non-linearly. In the past,
`such non-linearity has perhaps been perceived as an obstacle
`to accurate sound reproduction. This is because it is under-
`stood by those skilled in the art that in extreme conditions,
`air moleculesare less and less able to follow the vibration of
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`a compression wave, such as that produced by a diaphragm.
`Therefore,
`the tendency of research has been to avoid
`non-linear conditions.
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`the present invention appears to favor the
`In contrast,
`existence of a non-linear transmission medium in order to
`
`bring about the required heterodyning effect. Although air is
`naturally non-linear when a compression wave moves
`through it, the degree of non-linearity is relatively unob-
`servable or inconsequential. However, when ultrasonic com-
`pression waves are emitted so as to interfere in air, the
`non-linearity causes a surprising and unexpected result
`which will be explained and referred to as the acoustical
`heterodyning effect or process.
`The present invention draws on a variety of technologies
`and aspects which have sometimes perceived as unrelated
`topics. These aspects of the invention include 1) indirectly
`generating a new sonic, subsonic or ultrasonic compression
`wave, 2) superimposingintelligence on an ultrasonic carrier
`wave and retrieving the intelligence as the indirectly gen-
`erated compression wave, 3) causing at least two ultrasonic
`compression waves to interact
`in air and using the
`by-product of the interference, 4) using the principle of
`acoustical heterodyning to indirectly generate the new com-
`pression wave, 5) generating the new compression wave
`from a relatively massless radiating element to avoid the
`distortion and undesirable harmonics of conventional direct
`radiating elements, 6) affecting a physical state of a living
`being by generating subsonic frequencies in close proximity
`thereto, 7) generating an approximate point-source of sound
`that is phase coherent over the entire audio spectrum, 8)
`eliminating distortion in playback or broadcasting of sound,
`9) eliminating the “beaming” phenomenoninherent in emis-
`sion of high frequency compression waves from a direct
`radiating element, 10) generating a new sonic or subsonic
`compression wave which is independent of the characteris-
`tics of the direct radiating element, and 11) the detection of
`sound without using a direct detection device at a detection
`location.
`
`It should be remembered that all of these aspects of the
`present invention are possible without using a speaker or
`other form of direct radiating structure. Furthermore, these
`sonic or subsonic frequencies are generated absolutely free
`of distortion and in a generally omni-directional orientation.
`The surprising result is the ability to recreate “pure” sound
`in the same form as when it was originally captured at a
`microphone or other recording system.
`These and other objects, features, advantages and alter-
`native aspects of the present invention will become apparent
`to those skilled in the art from a consideration of the
`following detailed description, taken in combination with
`the accompanying drawings.
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`5,889,870
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`7
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of the componentsofa state of
`the art conventional loudspeaker system.
`FIG. 2 is a block diagram of the componentsof an indirect
`compression wave generation system which is built
`in
`accordance with the principles of one embodiment of the
`present invention.
`FIG. 3 is an illustration of the indirect and new compres-
`sion wave generation using the apparatus of FIG. 2, includ-
`ing the acoustical heterodyning interference effect.
`FIG. 4 is a block diagram of the componentsof an indirect
`compression wave generation system.
`FIG. 5A is a graph showing how air respondsincreasingly
`non-linearly as the amplitudeor intensity of sound increases.
`FIG. 5B is a graph showing when air responds non-
`linearly to a specific signal of a defined frequency and
`amplitude.
`FIG. 6A is a block diagram of the components of an
`indirect compression wave generation system.
`FIG. 6B is an alternative embodiment of FIG. 6A.
`
`FIG. 7 is an alternative configuration of ultrasonic fre-
`quency transducers to indirectly generate compression
`waves.
`
`FIG. 8 is another alternative configuration of ultrasonic
`frequency transducers to indirectly generate compression
`waves.
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`FIG. 9 is an illustration of a resonant cavity with two
`ultrasonic frequency signals being emitted from two trans-
`ducers.
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`FIG. 10 is an illustration of a resonant cavity with two
`ultrasonic frequency signals being emitted from onetrans-
`ducer.
`
`FIG. 11 is a diagram of a hearing aid and headphones
`where the human ear canal is the resonant cavity.
`FIG. 12 is a block diagram illustrating using the present
`invention to detect sound.
`FIG. 13 is an embodiment whichteachesreflection of the
`ultrasonic frequency signals to develop acoustical effects.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Reference will now be madeto the drawings in which the
`various elements of the present invention will be given
`numerical designations and in which the invention will be
`discussed so as to enable one skilled in the art to make and
`use the invention.
`
`The present invention is a dramatic departure from the
`teachings of the present state of the art. The creation of
`compression waves is generally perceived to be a direct
`process. A direct process is defined as causing a radiating
`element 10 to vibrate at a desired frequency as shown in
`FIG. 1. The system of FIG. 1 is typically used to directly
`generate audible and inaudible compression waves, both
`above and below the range of human hearing. A conven-
`tional compression wave generating system is thus com-
`prised of a speaker element 10 which can be any dynamic,
`electrostatic or other direct radiating element, and a signal
`source such as a signal generator or amplifier 12. The signal
`source 12 supplies an electrical signal representative of a
`compression wave having a specific frequency or frequen-
`cies at which the speaker element 10 will vibrate to produce
`compression waves 14.
`To improve the quality of sound from a sound reproduc-
`tion system such as in FIG. 1, a person skilled in the art
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`presently looks at ways to improve the physical radiating
`element, such as the loudspeaker 10. The loudspeaker 10
`functionsas a transducer, attempting to accurately reproduce
`sound recorded in an analogor preferably a digital format by
`converting an electrical signal into compression waves 14.
`Therefore, generating compression waves has previously
`been a direct process as defined above. The reproduced
`sound is generated directly by a physical radiating element
`which vibrates at the frequency or frequencies which drive
`it. This vibration typically drives a loudspeaker cone or
`diaphragm, which creates compression waves the human ear
`can hear when within the range of 20 to 20,000 cycles per
`second. For example,
`if the diaphragm vibrates at 1500
`cycles per second, an audible tone of 1500 Hzis generated.
`Before proceeding further,
`it will be helpful to define
`several terms to be used hereinafter. A “signal source” will
`interchangeably refer to a “signal generator” or “amplifier”
`which provideselectrical signals representative of compres-
`sion wavesto be emitted from a speaker. The term “speaker”
`will
`interchangeably refer to the terms “transducer”,
`“emitter”, “loudspeaker”, “diaphragm”, “physical radiating
`element” or “direct radiating element” which converts the
`electrical signals to a mechanical vibration causing com-
`pression waves. The term “compression wave” will inter-
`changeably refer to the terms “sound wave”, “longitudinal
`wave” and “wave train” which are sonic, subsonic and
`ultrasonic waves propagating through a transmission
`medium suchas air.
`
`The present invention in a preferred embodimentteaches
`a method and apparatus for indirectly generating a new
`compression wave. Indirect generation refers to the absence
`of a direct radiating element at
`the source of the new
`compression wave generation. Surprisingly,
`there is no
`physical radiating element vibrating at the frequency of the
`newly generated compression wave. Instead, air molecules
`are caused to vibrate at
`the desired sonic, subsonic or
`ultrasonic frequency to thereby function as the radiating
`element and generate the new compression wave. The air
`itself becomesthe direct radiating element, and becomes an
`indirect source of the compression wave.
`Ofgreatest interest to the present invention are both sonic
`and subsonic frequencies. Thisis largely due to the difficulty
`of directly generating these frequencies without distortion.
`In contrast, it is the nature of ultrasonic frequencies to be
`capable of generation with much greater precision and with
`less distortion. This occurs because the radiating element is
`typically more efficient, smaller in size, and is less massive.
`Accordingly, the ultrasonic radiating element is not subject
`to the same causes of distortion or to the same degree as are
`conventional speakers. Although it should be remembered
`that the invention can generate new compression waves at
`ultrasonic, sonic or subsonic frequencies indirectly,
`the
`present focus looks at more significant applications with
`respect to reproduction of music, voice and all other forms
`of sound.
`
`To generate a new compression wave, the present inven-
`tion 1) makes use of at least two ultrasonic signals, 2)
`superimposes a desired sonic or subsonic signal onto one or
`both of the ultrasonic signals, 3) emits the ultrasonic signals
`from at least one ultrasonic emitter 4) causes the ultrasonic
`signals to interfere according to the principles of acoustical
`heterodyning, and 5) generates a new compression wave
`from a region of heterodyning interference of the ultrasonic
`compression waves.
`The advantages of this arrangement are immediately
`observable. For example, the ultrasonic component waves
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`do not impact upon the humanear in a perceptible form and
`are therefore non-distracting. Consequently, only the desired
`new compression wave is perceived by a listener and in a
`form capable of recreating the original dynamics of more
`ideal sound reproduction.
`Introduction of the present invention is best understood by
`reference to FIG. 2. Other preferred embodiments will be
`explained hereafter, based on the principles of this initial
`discussion.
`
`Indirect compression wave generation is accomplished in
`a first embodimentasillustrated in FIG. 2. The fundamental
`
`two ul