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Patent No. 5889870 Acoustic heterodyne device and method (Norris, Mar 30, 1999)
Assignee: American Technology Corporation, 15378 Avenue of Science, Suite 100. :: San Diego, CA 92128 USA
Abstract
The present invention is the emission of new sonic or 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. When the difference in frequency between the two ultrasonic wave trains 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 multiple waves.
Notes:
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 element at
the source of the new compression wave generation.
2. State of the Art
Sound waves in general are wave-like movements of air 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 of origin.
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 into a 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 reproduction has been applied to recover this stored information,
whether the response is generated by a mechanical mechanism 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 reproduction.
A primary goal of modern acoustic science is to reproduce pure sound, based
on conversion of 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 overtones 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 generating 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 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
when the 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 electrical current that is proportional to the strength and
frequency of the signal to be broadcast is sent through a coil attached to a
rigid membrane or cone. The coil moves inside a permanent magnet, and the magnetic
field exerts a force on the coil that is proportional to the electrical current.
The oscillating movement of the coil and the attached membrane sets up sound
waves in the surrounding air. In brief, reproduction of sound has heretofore
required mechanical movement 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 symphony hears this broad range of sound, he receives it in an integrated
manner as a "unified" combination of sound waves, having a myriad of frequencies
and amplitudes. This complex array is responsively promulgated through the 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
successful in effectively capturing the full range of sound experience within
the recorded signal. For example, it is unnecessary to provide separate microphones
for recording both low and high range frequencies. Instead, like the ear drum,
the microphone, with its 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 appropriate media.
It is therefore clear that the microphone is 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 microphone
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 configuration
of the speaker, (d) the directional differences between high and low frequencies,
(e) the phase variation of low versus high frequency wave trains, (f) the difficulty
of coupling speaker elements to surrounding air, and (g) the loss of harmonics
and secondary tones. Again, these aspects represent just a few of the problems
associated with reconstructing 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 problem. 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 wave train. The initial design challenge is to compensate
for resistance against movement in speaker response due to inertia within the
speaker mass itself. 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 numerous
other physical problems, speaker development has focused mainly on improving
materials and components as opposed to developing a different concept of sound
generation. Diaphragm improvements, cone construction 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 methodology. Unlike the electrodynamic
loudspeaker with its cone shaped diaphragm, the electrostatic loudspeaker uses
a thin electrically conducting membrane. Surrounding the plate are one or more
fixed grids. When a signal voltage is applied to the elements, the electrostatic
force produced causes the diaphragm to vibrate. This low-mass diaphragm is particularly
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 detection 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 height is limited in its ability
to simulate the delicate spatial image of a much smaller piccolo or violin.
Another issue 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 tweeter to a listener, versus (ii) the substantially longer
path for low frequency signals from the horn or woofer speaker to a listener's
ear, can be in the range of thousands of percent in phase differential.
The preceding discussion of speaker technology is 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 concept that mechanical
systems, such as speakers, are 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 difficulties 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 apparatus for
indirectly emitting new sonic and subsonic wave trains from a region of air
without using a direct radiating element to emit the wave trains.
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 wave train.
It is still another object to cause at least two ultrasonic wave trains to interact
in accordance with the principles of acoustical heterodyning to thereby extract
intelligence from the interfering wave trains.
It is yet another object to indirectly generate new sonic or subsonic wave trains
by combining them with an ultrasonic carrier wave using amplitude modulation,
emitting the combined signal from an ultrasonic transducer, causing interference
between the carrier wave and another ultrasonic frequency wave train, to thereby
create the new sonic or subsonic wave trains.
It is still another object to affect a physical state of a living being utilizing
an indirectly created compression wave.
It is still yet another object to generate a new compression wave which is perceptible
to human senses using at 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 wave
train without having to overcome the 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
wave train 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
wave trains into the resonant cavity.
Yet another object is to omni-directionally generate a high frequency wave train,
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 wave train in
a localized area without coupling to an associated environment or enclosure
which would otherwise cause undesirable broadcasting of the sonic or subsonic
wave train.
Yet another object is to generate a new sonic or subsonic wave train wherein
characteristics of the new sonic or subsonic wave train 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 interaction of the at least two ultrasonic frequency
wave trains.
Still another object is to emit a new sonic or subsonic wave train from a region
of air as a by-product of modulating 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 waves trains. In one embodiment, a new sonic or subsonic wave
train is emitted from a region of interference of at least two ultrasonic wave
trains emitted from at least two ultrasonic transducers. The principle of operation
is based on incorporating 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 another ultrasonic wave train. The
ultrasonic wave trains interfere within a region of non-linearity in accordance
with principles identified by the inventor as "acoustical heterodyning," and
thereby generate by-products which include the difference and the sum of the
two ultrasonic wave trains.
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. When the difference in
frequency between the two ultrasonic wave trains is in the sonic or subsonic
frequency range, the difference in frequency is generated as a new, audible
sonic or new subsonic wave train emanating outward from within the region of
heterodyning interference.
A different embodiment of the system provides the advantage of being comprised
of only one ultrasonic direct radiating 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 cannot itself generate sonic or subsonic frequencies,
plus the audible evidence that the sound is not emanating directly from the
emitter, one might be deceived.
The importance of the first embodiment is that it teaches the concept of generating
a new sonic or subsonic wave train as a result of the interference between two
ultrasonic wave trains in accordance with the principles of acoustical heterodyning.
In essence, it is easier to see that two ultrasonic wave trains 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 embodiment
functions. It becomes apparent that the same acoustical heterodyning principle
applies when it is understood which wave trains are interfering in space.
A key aspect of the invention is the discovery that by superimposing sonic or
subsonic intelligence onto an ultrasonic carrier wave, this intelligence can
be retrieved as a new sonic or subsonic wave train. Whether the ultrasonic wave
trains 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 problems inherent to mass and the associated
limitations of inertia of a conventional direct radiating element. The present
invention eliminates a direct radiating element as the source of a new compression
wave so that the desired sound is generated directly from a region of air and
without the several forms of distortion all associated with direct radiating
speakers.
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 known that the transmission medium of air provides an elastic
medium for the propagation of sound waves. Thus, prior art research has treated
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 understood by those skilled in the art that in extreme conditions, air molecules
are less and less able to follow the vibration of a compression wave, such as
that produced by a diaphragm. Therefore, the tendency of research has been to
avoid non-linear conditions.
In contrast, the present invention appears to favor the 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 unobservable or inconsequential. However,
when ultrasonic compression 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) superimposing intelligence on an ultrasonic carrier wave and retrieving the
intelligence as the indirectly generated 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 compression 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" phenomenon inherent in emission of high
frequency compression waves from a direct radiating element, 10) generating
a new sonic or subsonic compression wave which is independent of the characteristics
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.
BRIEF
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the components of a state of the art conventional
loudspeaker system.
FIG. 2 is a block diagram of the components of 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 compression wave generation
using the apparatus of FIG. 2, including the acoustical heterodyning interference
effect.
FIG. 4 is a block diagram of the components of an indirect compression wave
generation system.
FIG. 5A is a graph showing how air responds increasingly non-linearly as the
amplitude or 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 frequency transducers to
indirectly generate compression waves.
FIG. 8 is another alternative configuration of ultrasonic frequency transducers
to indirectly generate compression waves.
FIG. 9 is an illustration of a resonant cavity with two ultrasonic frequency
signals being emitted from two transducers.
FIG. 10 is an illustration of a resonant cavity with two ultrasonic frequency
signals being emitted from one transducer.
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 which teaches reflection of the ultrasonic frequency
signals to develop acoustical effects.
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made to 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 conventional compression
wave generating system is thus comprised 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 frequencies at which the speaker element 10 will vibrate to produce compression
waves 14.
To improve the quality of sound from a sound reproduction system such as in
FIG. 1, a person skilled in the art presently looks at ways to improve the physical
radiating element, such as the loudspeaker 10. The loudspeaker 10 functions
as a transducer, attempting to accurately reproduce sound recorded in an analog
or 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 Hz is 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 provides electrical signals representative of
compression waves to 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 compression waves. The term "compression
wave" will interchangeably refer to the terms "sound wave", "longitudinal wave"
and "wave train" which are sonic, subsonic and ultrasonic waves propagating
through a transmission medium such as air.
The present invention in a preferred embodiment teaches 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 becomes the direct radiating element, and becomes an indirect
source of the compression wave.
Of greatest interest to the present invention are both sonic and subsonic frequencies.
This is 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 invention 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 do not impact upon the human ear 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 embodiment as
illustrated in FIG. 2. The fundamental elements of the system include at least
two ultrasonic acoustical transducers 20, an ultrasonic signal source 22, a
means for combining signals 24, and an input 26 to the means for combining signals
which provides a signal to be superimposed upon a carrier signal. The ultrasonic
signal source 22 also functions as a means for controlling the frequency of
signals being emitted from the at least two ultrasonic acoustical transducers
20. The dotted line 28 indicates that in this first embodiment, the orientation
of the transducers 20 are coaxial.
The apparatus above is able to function as described because the compression
waves 30, 32 interfere in air according to the principles of acoustical heterodyning
(a phrase chosen by the inventor which describes the effect). Acoustical heterodyning
is somewhat of a mechanical counterpart to the electrical heterodyning effect
which takes place in a non-linear circuit. For example, amplitude modulation
in an electrical circuit is a heterodyning process. The heterodyne process itself
is simply the creation of two new waves. The new waves are the sum and the difference
of two fundamental waves.
In acoustical heterodyning, the new waves equalling the sum and difference of
the fundamental waves are observed to occur when at least two ultrasonic compression
waves interact or interfere in air. Presently, acoustical heterodyning has only
been observed when both fundamental waves are ultrasonic, thus generally above
20 KHz.
The preferred transmission medium of the present invention is air because it
is a highly compressible medium that responds non-linearly under different amplitudes.
This non-linearity of air is what enables the heterodyning process to take place
without using an electrical circuit. However, it should be remembered that any
compressible fluid can function as the transmission medium if desired.
FIG. 3 illustrates that the indirect generation of a new compression wave is
made possible by the unexpected discovery that two ultrasonic wave trains 30,
32 experience a form of the acoustical heterodyning effect in a non-linear acoustical
transmission medium such as air when they interfere. Air will respond more and
more non-linearly in a region 34 as amplitude and frequency increase. This region
34 will extend as far as the compression waves 30, 32 are interfering with each
other.
As related above, the acoustical heterodyning effect results in the creation
of two new compression waves, the sum and the difference of ultrasonic compression
waves 30, 32. The sum is an ultrasonic wave which is of little interest and
is therefore not shown. The difference, however, can be sonic or subsonic, and
is shown as a compression wave 36 which is emitted generally omni-directionally
from the region of interference 34. The shape of the new wave is generally dictated
by the shape of the region of interference 34. In this illustration, the region
34 will be generally cylindrical as would be seen if drawn in three dimensions.
The shape of the region 34 can, however, be modified to produce a desired effect.
Furthermore, the illustration of opposing and generally coaxial compression
waves 30, 32 should not be thought to depict the only orientation that the waves
can have.
It is worth noting before proceeding further that the acoustical heterodyning
effect has been proven empirically. The evidence lies in the fact that at least
one new wave is created. The new sonic or subsonic compression wave 36 is verifiable
by direct audible detection as well as by measuring the frequency with an audio
spectrum analyzer. However, unlike direct audible detection, the sum of both
frequencies can only be verified through measurement using an instrument such
as the audio spectrum analyzer. Both the sum and the difference have been measured
to verify the accuracy of these predicted results.
As can be surmised, the particular acoustical heterodyning effect which is of
interest to the present invention is the difference or frequency subtraction
of one ultrasonic wave train relative to another. Consider a specific example
which explicitly provides the result of acoustically heterodyning two different
ultrasonic compression waves 30, 32. Assume the existence of a first ultrasonic
frequency wave train (first fundamental wave) 30 of 100,000 Hz. Assume a second
ultrasonic wave train (second fundamental wave) 34 occurs at 100,900 Hz. An
audible tone of 900 Hz is heard as the result of the first and the second ultrasonic
wave trains interacting when one or both are of sufficient amplitude. The frequency
subtraction caused by the acoustical heterodyning effect results in a 900 Hz
frequency tone being generated and heard as a new compression wave from a region
of interference.
The generation of a single-frequency merely illustrates the core inventive principle.
A greater appreciation of potential for acoustic heterodyning is found in the
following applications. For example, if a single new uni-frequency compression
wave can be generated, it should be realized that even bass intense, multi-frequency
signals such as live music, a voice or a transmission received via radio or
television can be amplified and played using the present invention. A tiny ultrasonic
frequency transducer in a pocket can conceivably reproduce with perfect clarity
all the recorded frequencies of a live symphonic recording, perhaps even approaching
the experience of being there.
Returning to a more detailed discussion of specific elements of FIG. 2, an important
and practical element of the invention is the single ultrasonic signal source
22 being used to supply the electrical signals representing the ultrasonic frequency
wave trains 30, 32. The advantage of this arrangement is that signal differences
that might otherwise occur due to variations in temperature or performance of
two separate signal generators would likely lead to drift between the frequency
values of the ultrasonic wave trains 30, 32. Furthermore, because it is the
difference in frequency between the two ultrasonic wave trains 30, 32 which
is ultimately the frequency of interest, it is important to minimize unwanted
frequency variations of the ultrasonic wave trains 30, 32.
To eliminate drift, a single ultrasonic output source 22 generates a base frequency
for both ultrasonic wave trains 30, 32 so that the wave trains 30, 32 will drift
together, if at all. This configuration thus makes it easier to precisely control
the difference in frequencies and ultimately the frequencies of the new compression
wave.
FIG. 2 also lists as a component of the system a means for combining signals
24. This device performs the function of modifying one or both of the ultrasonic
wave trains 30, 32 being generated by the ultrasonic signal source 22. This
modification consists of the means for combining signals 24 by combining a first
ultrasonic signal 38 with an electrical signal 40, representing the new compression
wave 42 to be generated. The combination is defined as the sum of the first
ultrasonic signal 38 and the desired compression wave 42 and is transmitted
as the second ultrasonic signal 42.
The method of combining signals 38 and 40 in the present invention is preferably
accomplished through amplitude modulation. Therefore the means for combining
signals 24 in the first embodiment is an amplitude modulator. FIG. 2A illustrates
that amplitude modulation creates a signal having a fundamental frequency 60,
an upper sideband 62, and a lower sideband 64. In this invention, the upper
sideband 62 is used because it represents a non-inverted signal which carries
the information that will become the new "difference" compression wave.
It might be apparent that if the electrical signal which will become the new
compression wave 62 is amplitude modulated onto a fundamental frequency 60,
that the ultrasonic compression wave 30 or 32 (whichever is being modulated)
needs no demodulation in order to be heard as the new compression wave 62. The
last elements of the system shown in FIG. 2 are the two ultrasonic acoustical
transducers 20. These acoustical transducers 20 are designed to emit compression
waves at ultrasonic frequencies. Examples of transducers 20 can be piezoelectric
or electrostatic devices, but may obviously include other radiating elements
for the appropriate frequency range.
While the first embodiment uses a single ultrasonic signal source 22, it should
be realized that it is possible to provide separately generated electrical signals
to the ultrasonic transducers 20. FIG. 4 illustrates using two separate ultrasonic
signal sources 44, 46. The risk of this configuration is that frequency drift
becomes a possibility. As a practical matter, this embodiment might also require
some type of synchronization between the two ultrasonic signal sources 44, 46.
For example, a synchronizing controller 48 might coordinate emission of the
two ultrasonic frequency signals 30, 32.
FIG. 5A is a graph provided to illustrate the principle of acoustical heterodyning
by showing the relationship between the amplitude of a signal and the non-linearity
of air in response to that signal. The restoring force is the force which a
molecule of air will exert to get back to equilibrium when it is displaced.
If air were linear, Newton's laws would teach us that air would respond to a
given force which displaces it with an equal and opposite force. However, the
graph illustrates that the restoring force does not respond linearly (which
would be represented by a straight line) as the amplitude of a signal increases.
Instead, the equation of the curve 52 is y=x+x.sup.2, where air responds with
a linear component x, as well as a non-linear component x.sup.2. The curve 52
thus represents that as amplitude of a signal becomes significant, the non-linear
response of air begins to increase more rapidly than the linear component in
accordance with the equation.
FIG. 5B is a graph provided to illustrate properties which the signal must exhibit
so that air will respond to it non-linearly. The x-axis represents frequency
of the signal on a logarithmic scale. The y-axis represents the degree of absorption
in air by dB per 1000 feet. As shown, the line 50 is nearly flat up to about
10 KHz. This is consistent with the experimental results confirming that sound
waves at lower amplitudes do not appear to develop significant acoustical heterodyning.
Air becomes substantially more non-linear as amplitude increases, thus enabling
interference in accordance with the principles of acoustical heterodyning.
FIG. 6A illustrates the preferred embodiment of the invention. In a comparison
with FIG. 2, a significant difference is the elimination of one ultrasonic transducer
20. Otherwise, the remaining ultrasonic transducer 20, the means for combining
signals 24 and the ultrasonic signal source 22 remain substantially the same.
It would seem counter-intuitive, however, to think that this arrangement is
still able to accomplish the objectives of the present invention. However, an
analysis of the ultrasonic compression wave being emitted quickly proves that
the acoustical heterodyning effect is still taking place.
First, the electrical signals involved are the first ultrasonic signal 66 which
is the fundamental wave, and the electrical signal 68 which represents the new
sonic or subsonic wave to be combined with the ultrasonic signal 66. The combination
of the signals 66, 68 creates a new electrical signal 70 composite as a new
upper sideband that is the sum of signals 66 and 68, along with signal 66, both
of which are emitted from the ultrasonic transducer 20 as a compression wave
76.
A listener will hear the new compression wave 76 from a region of interference
74 which generally can begin at a transmitting face of the ultrasonic transducer
20. Except for the audible evidence to the contrary, this might lead the listener
to incorrectly conclude that the ultrasonic transducer 20 is generating the
new compression wave 76. By definition, the ultrasonic transducer 20 cannot
directly generate audible frequencies. Therefore, what one hears is the interfering
ultrasonic compression waves interacting in accordance with the acoustical heterodyning
effect. It was discovered that the two ultrasonic compression waves are created
from 1) the new electrical signal 70, and 2) the first ultrasonic signal 66.
These respective compression waves corresponding to signals 66 and 70 are propagated
at the transducer 20, providing the required two ultrasonic wave trains for
acoustical heterodyning interference.
FIG. 6B is also provided to show an alternative arrangement of components which
more intuitively illustrates the two distinct ultrasonic compression waves 66
and 70 being transmitted to the ultrasonic transducer 20 for emission therefrom.
The only meaningful difference between the two embodiments is that separate
ultrasonic signals sources 22 are shown for each of the ultrasonic compression
waves.
The embodiments of FIG. 6A or 6B are preferred for many reasons. For example,
the systems have one less transducer 20, and will therefore be less expensive
to produce. The systems will also be lighter, smaller and, most importantly,
will have the greatest efficiency.
The aspect of efficiency requires further discussion to understand some of the
implications of the various embodiments. Whereas the first embodiment shown
in FIG. 2 requires orientation of the ultrasonic transducers 20, no orientation
is required in FIG. 6 because the single transducer 20 functions as the radiating
element for both interfering signals.
Orientation of ultrasonic transducers 20 is important because the system of
FIG. 1 can be altered so as not to generate any new compression wave. For example,
if the transducers 20 are oriented so that the ultrasonic compression waves
30, 32 never substantially cross, no new compression wave can be created. Therefore,
FIG. 7 showing a slightly convergent path and FIG. 8 showing a generally parallel
path both depict ultrasonic transducer 20 orientations which will generally
create sufficiently large regions of interference so that a new compression
will be generated. However, neither of these orientations appear to generate
as significant a region of interference as the orientations of FIGS. 1 or 6.
due to the greater degree of interference represented. This greater efficiency
translates into greater energy transfer to the new compression wave and consequently
to a stronger or louder new wave.
In contrast, the preferred embodiment will always generate a new compression
wave which has the greatest efficiency. That is because no orientation of two
ultrasonic transducers 20 will ever match or exceed the perfect coaxial relationship
obtained when using the same ultrasonic transducer 20 to emit both ultrasonic
compression waves. This coaxial propagation from a single transducer would therefore
yield the maximum interference pattern and most efficient compression wave generation.
Before moving to other aspects of the invention, it is important to realize
that unusual sound effects are possible with the highly directional ultrasonic
transducers 20. It has been observed that reflecting the at least two ultrasonic
wave trains at an object or surface causes the reflected waves to give an impression
of localized source. In other words, the reflected new compression wave appears
to be coming from the object or surface of reflection.
This is represented in FIG. 13 and can be used to simulate a variety of interesting
acoustical effects, including three dimensional sound. By simply directing the
orientation of the ultrasonic transducer 20 toward a ceiling or wall 96, one
can simulate the experience of sound emanating from that location. If the transducer
target is placed in motion, the moving reflective location creates an impression
of movement for the sound or object being represented. By controlling the orientation
of the transducer with computer drivers, sound reproduction can be localized
to individual faces on a movie screen or even off the screen in an overhead
position, moving vehicles or aircraft, or any myriad of other sound effects
which can now only be imagined.
A startling consequence of the present invention is the generation of a new
omni-directional compression wave. Specifically, the new compression wave will
generally radiate outward omni-directionally from a region of interference,
generally in accordance with the shape of the region. However, the remarkable
control which the present invention provides over the shape of the region of
interference enables a perception of the described directionality to be manipulated
in unexpected ways.
For example, one or two ultrasonic transducers might be aimed at a wall or other
object. The increased amount of interference between two ultrasonic compression
waves which will occur because of the reflection will cause most of the sound
to be generated omni-directionally from near the object being reflected from.
Likewise, bringing the two ultrasonic frequency transducers 20 of FIG. 2 closer
together limits the length of interference and consequently more closely approximates
a near point-source of sound.
Another significant advantage of the present invention is that sound is reproduced
from a relatively massless radiating element. In the region of interference,
and consequently at the location of new compression wave generation, there is
no direct radiating element. This feature of sound generation by acoustical
heterodyning can substantially eliminate distortion effects, most of which are
caused by the radiating element or conventional speakers. For example, harmonics
and standing waves on a loudspeaker cone, cone overshoot and cone undershoot
caused by inertia, and the imperfect surface of the cone itself are all factors
which contribute to signal distortion attributable to a direct radiating element.
A direct physical radiating element has other undesirable characteristics as
well. Despite certain manufacturer claims to the contrary, the frequency response
of a conventional loudspeaker is not truly flat. Instead, it is a function of
the type of frequency (bass, intermediate, or high) which it is inherently best
suited for emitting. Whereas speaker shape, geometry, and composition directly
affect the inherent speaker character, acoustical heterodyne wave generation
utilizes the natural response of air to avoid geometry and composition issues
and to achieve a truly flat frequency response for sound generation.
In general, it should be noted that this aspect of the present invention means
that the final step in achieving truly indirect sound generation has been achieved.
While the state of the art has advanced the ability to convert an analog signal
to a digital recording, and to even process the signal digitally, the quality
of sound reproduction remains limited by the characteristics of the analog transducer
which has always been required as a speaker element. This is no longer the case
because the present invention achieves distortion free sound which is not hindered
by a direct radiating element, with its attendant mass and inertial limitations.
Distortion free sound implies that the present invention maintains phase coherency
relative to the originally recorded sound. Conventional speaker systems do not
have this capacity because the frequency spectrum is broken apart by a cross-over
network for propagation by the most suitable speaker element (woofer, midrange
or tweeter). By eliminating the direct radiating element, the present invention
makes obsolete the conventional cross-over network. This enables realization
of a virtual or near point-source of sound.
Another application of the present invention involves unobtrusively generating
crowd-controlling subsonic sound waves. Very low frequencies, such as those
around 12 Hz, have been shown to nauseate or disorient human beings and other
animals. Prior efforts in using low frequency disorientation has been hampered
by a limited ability for localized application. The present invention has demonstrated
its adaptation for reflected amplification, and thereby allows a more focused
field of influence. For example, acoustic heterodyne generation of low frequency
sound could be directed to a building, window or other reflective surface near
a group of disorderly persons. The primary affect of this disorienting sound
would be in the immediate area of reflection, avoiding undesirable application
to innocent bystanders.
Other advantages arise directly from the unique nature of the ultrasonic transducers
20. Because of their small size and low mass, such transducers are generally
not subject to the many limitations and drawbacks of conventional radiating
elements used in loudspeakers. Furthermore, the use of ultrasonic transducers
20 at extremely high frequencies avoids the distortion, harmonics and other
undesirable features of a direct radiating element which must reproduce sound
directly in the low, mid and high frequency ranges. Consequently, the many favorable
acoustic properties of a relatively distortion free ultrasonic transducer system
can now be transferred indirectly into sonic and subsonic by-products.
FIG. 9 illustrates an additional aspect of the present invention relating to
an ability to generate and enhance sound within a broadly resonant cavity 80.
A resonant cavity 80 is any cavity 80 which enables interacting ultrasonic compression
waves 30, 32 to interfere in accordance with the principles of acoustical heterodyning.
Although the broadly resonant cavity 80 is not necessary to create the effect
of interference, it seems to enhance or amplify the effect by increasing interference,
as well as reinforcing the audio byproduct or "difference" frequencies. This
means that two ultrasonic frequency signals 30, 32 can be transmitted into the
cavity 80 from almost any perspective. For example, FIG. 9 shows two ultrasonic
frequency transducers 20 emitting ultrasonic frequency signals 30, 32 into cavity
80. The signals 30, 32 are reflected off the walls of cavity 80 a multiple numbers
of times to increase interference.
FIG. 10 shows an improved configuration of the broadly resonant cavity of FIG.
9 which only requires a single ultrasonic transducer 20 to generate a new compression
wave. The system is improved because of the perfect coaxial relationship between
the two ultrasonic compression waves 30, 32.
One implication of the broadly resonant cavity 80 of FIGS. 9 and 10 is that
the human ear canal is also a broadly resonant cavity, and can thus be used
to enhance the new compression wave. This result offers a particular advantage
for the headphone and hearing aid industry. For example, a hearing aid 90 as
shown in FIG. 11 which embodies the present invention can be used to reproduce
the entire audio spectrum of sounds for a listener, enabling a high fidelity
reproduction, rather than the characteristic "tinny" sound of a conventional
hearing aid. Likewise, any headphone or headset 92 can be modified to take advantage
of the present invention, and generally with less weight and size than conventional
systems with a dramatic extension to frequency response.
Another interesting aspect of the invention facilitates privacy of communication
as part of a wireless system. This arises because of the "beaming" effect inherent
with the use of an ultrasonic transducer 20. By nature, ultrasonic compression
waves propagate in a narrow beam, which can easily be targeted on specific objects
or locations. It is therefore possible to aim a transducer 20 across a noisy
or crowded room and direct audible messages only into an ear of an intended
listener. Those around the listener would be unaware of the audible communication
because of the non-reflective character of the ear and the narrow beam width
of the ultrasonic waves. Private instructions could therefore be given on radio
and television production areas, performance stages for cueing, and other applications
where one-way prompting would be helpful.
Surprisingly, the present invention can also eliminate unwanted environmental
noise pollution. Our society has coined the phrase "boombox" to refer to portable
stereo systems which have relatively large bass speakers. The boombox derives
its name from the annoying side affect of a booming and repeated "thump" of
the bass speakers driving large volumes of air. However, the term is also sometimes
used to refer to a car or other vehicle with even larger bass speakers. Because
the speakers are integrally attached to the vehicle, the frame or any loudspeaker
enclosure in general, the enclosure itself becomes a radiating element. Consequently,
persons outside the vehicle will be hit with wave upon wave of dull thumping
sounds, a nuisance at best.
The present invention can thus advantageously eliminate the coupling of the
enclosure to the direct radiating element by generating the new compression
wave in midair. The listener inside can still enjoy the experience of loud bass
frequencies within the confines of the vehicle. However, the lower frequencies
will not be directly coupled to the vehicle frame because the radiating element
is now a point in air. Consequently, undesirable bass broadcasting into the
environment beyond the immediate vicinity of the listener is significantly reduced.
An interesting twist of the invention is a reverse application of the technology
for sound detection. In other words, instead of reproducing sound, the invention
might be used to detect sound as shown in FIG. 12. More specifically, the invention
can function as a substitute for a point-source sound detection device such
as a microphone. Typically, a microphone must be physically positioned at a
desired location of sound detection in order to operate. The present invention
enables compression waves to be converted into an electrical signal by a transducer
20 without providing a physical microphone element at a detection location.
Essentially, a single transducer 20 might be used to focus ultrasonic compression
waves 30 at the desired detection location 102. Acoustical vibrations, such
as a voice or music, will interact with the ultrasonic compression wave 30.
By monitoring a decrease in output level of the ultrasonic compression wave
30, it should be possible to determine the frequencies of the audible compression
wave which is impacting on the ultrasonic compression wave 30. This might be
done by using a waveform analyzer 104 to determine the decrease in output level
caused by coupling of the ultrasonic compression wave 30 with the audible sound
waves 102.It is to be understood that the above-described embodiments are only
illustrative of the application of the principles of the present invention.
Numerous modifications and alternative arrangements may be devised by those
skilled in the art without departing from the spirit and scope of the present
invention. The appended claims are intended to cover such modifications and
arrangements.
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It
is to be understood that the preceding description is given to illustrate various
embodiments of the present inventive concepts. The specific examples are not
to be considered as limiting, except in accordance with the following claims.