| United States Patent |
6,052,336 |
| Lowrey, III |
April 18, 2000 |
Apparatus and method of broadcasting audible sound using
ultrasonic sound as a carrier
Abstract
An ultrasonic sound source broadcasts an ultrasonic signal which is amplitude
and/or frequency modulated with an information input signal originating from an
information input source. If the signals are amplitude modulated, a square root
function of the information input signal is produced prior to modulation. The
modulated signal, which may be amplified, is then broadcast via a projector
unit, whereupon an individual or group of individuals located in the broadcast
region detect the audible sound.
| Inventors: |
Lowrey, III; Austin (c/o Austin Lowrey
Associates 7212 Reservation Dr., Springfield, VA 22153) |
| Appl. No.: |
070850 |
| Filed: |
May 1, 1998 |
| Current U.S. Class: |
367/139; 367/134; 367/137
|
| Intern'l Class: |
H01M 029/02; H04R 025/00 |
| Field of Search: |
367/137,139,134 381/77 455/46
607/56 |
References Cited
U.S. Patent Documents
| 5159703 |
Oct., 1992 |
Lowery |
455/42. |
| 5889870 |
Mar., 1999 |
Norris |
381/77. |
Other References
"In The Audio Spotlight" by David Schneider; Scientific
American, News and Analysis (Oct. 1998); pp. 40-41.
|
Primary Examiner: Pihulic; Daniel T.
Attorney, Agent or Firm: Staas & Halsey LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims
the benefit of U.S. provisional application Ser. No. 60/046,803, filed May 2,
1997, entitled A METHOD FOR TRANSMITTING AUDIBLE SOUNDS THROUGH THE AIR USING
ULTRASONIC SOUND AS A CARRIER by Austin Lowrey, III and incorporated by
reference herein.
Claims
What is claimed is:
1. An apparatus, comprising: a unit
amplitude modulating an ultrasonic signal with a square root of an information
signal to produce a modulated signal; and a projector coupled to said
unit and projecting the modulated signal to a listener.
2. The apparatus
according to claim 1, wherein said unit comprises: a circuit producing
the square root of the information signal; and a modulator amplitude
modulating the ultrasonic signal with the square root of the information signal.
3. The apparatus according to claim 2, further comprising: a
first sound source, coupled to an input of the modulator, outputting the
information signal; and a second sound source, coupled to the input of
the modulator, outputting the ultrasonic signal.
4. The apparatus
according to claim 1, wherein the information signal comprises a voice signal.
5. A method, comprising: modulating an ultrasonic signal with a
square root of an information signal to produce a modulated signal; and
projecting the modulated signal to a listener.
6. The method
according to claim 5, wherein the modulating further comprises producing a
square root signal from the information signal.
7. The method according
to claim 6, further comprising: amplifying the modulated signal; and
transmitting the amplified modulated signal.
8. The method
according to claim 5, wherein the modulating is an amplitude modulation.
9. An apparatus, comprising: a first modulator frequency
modulating a first ultrasonic signal with a first input signal to produce a
first modulated ultrasonic signal; a first ultrasonic signal source
providing a second ultrasonic signal; and a broadcasting system, coupled
to the first modulator and the ultrasonic signal source, broadcasting the first
modulated ultrasonic signal and the second ultrasonic signal to a listener.
10. The apparatus according to claim 9, wherein the broadcasting system
comprises: a first projector, coupled to the first modulator, projecting
the first modulated signal; and a second projector, coupled to the
ultrasonic signal source, projecting the second ultrasonic signal.
11.
The apparatus according to claim 9, further comprising: a first input
sound source, coupled to the first modulator, outputting the first input signal;
and a second ultrasonic signal source, coupled to the first modulator,
providing the first ultrasonic signal.
12. An apparatus, comprising:
a first modulator frequency modulating a first ultrasonic signal with a
first input signal to produce a first modulated ultrasonic signal; a
first input sound source, coupled to the first modulator, outputting the first
input signal; a first ultrasonic signal source providing a second
ultrasonic signal; a second ultrasonic signal source, coupled to the
first modulator, providing the first ultrasonic signal; a second
modulator amplitude modulating the second ultrasonic signal with a second input
signal to produce a second modulated signal; a second input sound
source, coupled to the second modulator, outputting the second input signal;
an amplifier, coupled to an output of the second modulator, amplifying
the amplitude modulated signal; and a broadcasting system, coupled to
the first modulator and the ultrasonic signal source, broadcasting the first
modulated ultrasonic signal and the second ultrasonic signal to a listener.
13. The apparatus according to claim 12, wherein the first and second
ultrasonic signals produce a difference signal for the listener in an audible
range of the listener.
14. The apparatus according to claim 12, wherein
the input signal comprises a square root of an information signal.
15.
The apparatus according to claim 14, wherein the information signal comprises a
voice.
16. A method, comprising: frequency modulating a first
ultrasonic signal with a first input signal to produce a first modulated signal;
providing a second ultrasonic signal; and broadcasting the first
modulated signal and the second ultrasonic signal to a listener.
17. A
method, comprising: frequency modulating a first ultrasonic signal with
a first input signal to produce a first modulated signal; providing a
second ultrasonic signal; amplitude modulating the second ultrasonic
signal with a second input signal to produce a second modulated signal; and
amplifying the amplitude modulated signal; and broadcasting the
first modulated signal and the second ultrasonic signal to a listener.
18. The method according to claim 17, further comprising:
emitting the first and second modulated signals in the audible range of
the listener.
19. An apparatus for broadcasting an audible sound using
an ultrasonic sound as a carrier such that at least one person exposed to the
ultrasonic sound can hear the audible sound, without detecting the source of the
sound, comprising: an information sound source outputting an information
signal; an ultrasonic sound source outputting the ultrasonic sound;
a square root circuit, coupled to the sound source, producing a square
root of the information signal output by the information sound source;
an ultrasonic modulator, coupled to the square root circuit and the
ultrasonic sound source, amplitude modulating the ultrasonic sound with the
square root of the information signal to produce a modulated signal; an
amplifier, coupled to the ultrasonic modulator, amplifying the modulated signal;
and a projector unit, coupled to the amplifier, projecting the modulated
signal such that the audible signal is detected by the at least one person.
20. An apparatus for broadcasting first and second audible sounds using
first and second ultrasonic sounds, respectively, as a carrier such that at
least one person exposed to the ultrasonic sounds can hear the audible sounds,
without detecting the source of the sounds, comprising: a first
information sound source outputting a first information signal; a second
information sound source outputting a second information signal; a first
ultrasonic sound source outputting a first ultrasonic signal; a second
ultrasonic sound source outputting a second ultrasonic signal; a first
modulator, coupled to the first ultrasonic sound source and the first
information sound source, frequency modulating the first ultrasonic signal with
the first information signal to produce a first modulated signal; a
second modulator, coupled to the second ultrasonic sound source and the square
root circuit, frequency modulating the second ultrasonic sound with the second
information signal to produce a second modulated signal; a first
projector unit, coupled to the first modulator, projecting the first modulated
signal to the at least one person; and a second projector unit, coupled
to the second modulator, projecting the second modulated signal to the at least
one person.
21. An apparatus, comprising: a unit modulating an
ultrasonic signal with the square root of an information signal to produce a
modulated signal in which the information signal is intelligible to a listener;
and a projector coupled to the unit and projecting the modulated signal
to the listener.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an apparatus and method of
broadcasting an audible sound, and in particular, to an apparatus and method of
broadcasting an audible sound using an ultrasonic sound as a carrier modulated
by the audible sound as an input signal.
2. Description of the Related
Art
Over the past few years, several situations have arisen in military
and civil areas where crowds, with or without leaders, have posed a serious
problem to Government forces.
For example, in Somalia, leader General
Aideed would almost never remain outside unless surrounded by a crowd of
sympathizers. Troops attempting to seize or capture the leader would have to
engage the crowd, probably killing or injuring some, in order to get close
enough to capture him. Hence, forces were not likely to attempt to capture the
leader.
Another example is the U.S. invasion of Haiti, where a ship with
troops was sent to perform various actions that would have been helpful to the
population living there. The landing of these troops was, however, opposed by a
crowd on the dock. Hence, in order to land, the crowd on the dock must first be
disposed of. Again, crowd members would likely be hurt, resulting in the troops
deciding not to act.
Still another example is any situation where an
angry crowd gathers. In this situation, the crowd frequently turns to looting
and destruction of property. It is a constant challenge for, for example, police
to disperse such a crowd without causing casualties, perhaps fatal ones.
All of these examples have a common theme, namely a crowd or leader that
one would like to influence such that they leave or stop their hostile
activities.
SUMMARY OF THE INVENTION
It is an object of the
present invention to provide a nonlethal individual or crowd control device
which uses an audible sound broadcasted using an ultrasonic sound as a carrier.
It is another object of the present invention to provide a device that
will allow the hearing impaired to hear speech.
It is still another
object of the present invention to provide a device that will emit audible sound
to listeners located in a defined area.
It is yet another object of the
present invention to provide a low frequency sound, either audible or
sub-audible frequency, in the heads of listeners.
In one embodiment of
the present invention, there is provided an apparatus including a unit amplitude
modulating an ultrasonic signal with a square root of an information signal to
produce a modulated signal, and a projector coupled to the unit and projecting
the modulated signal to a listener.
In one aspect of the embodiment, the
apparatus further includes a circuit producing the square root of the
information signal, a modulator amplitude modulating the ultrasonic signal with
the square root of the information signal, a first sound source outputting the
information signal, and a second sound source outputting the ultrasonic signal.
In another aspect of the embodiment, the information signal is a voice
signal from, for example, a microphone.
In another embodiment of the
invention, there is provided a method of modulating an ultrasonic signal with a
square root of an information signal to produce a modulated signal, and
projecting the modulated signal to a listener.
In one aspect of the
embodiment, the method further includes producing a square root signal from the
information signal, modulating the ultrasonic signal with the square root of the
information signal to produce the modulated signal, amplifying the modulated
signal, and transmitting the amplified modulated signal.
In another
aspect of the embodiment, the modulating is an amplitude modulation.
In
yet another embodiment of the present invention, there is provided an apparatus
including a first modulator frequency modulating a first ultrasonic signal with
a first input signal to produce a first modulated signal, an ultrasonic signal
source providing a second ultrasonic signal, and a broadcasting system, coupled
to the first modulator and the ultrasonic signal source, broadcasting the first
modulated signal and the second ultrasonic signal to a listener.
In one
aspect of the embodiment, the apparatus further includes a first projector
projecting the modulated signal, a second projector projecting the second
ultrasonic signal, a first input sound source outputting the first input signal,
a second ultrasonic signal source providing the first ultrasonic signal, a
second modulator amplitude modulating the second ultrasonic signal with a second
input signal to produce a second modulated signal, a second input sound source
outputting the second input signal, and an amplifier amplifying the amplitude
modulated signal.
In another aspect of the embodiment, the first and
second ultrasonic signals produce a difference signal for the listener in an
audible range of the listener.
In yet another aspect of the embodiment,
the input signal is a square root of an information signal.
In still
another aspect of the embodiment, the information signal is a voice from, for
example, a microphone.
In still another embodiment of the present
invention, there is provided a method of frequency modulating a first ultrasonic
signal with a first input signal to produce a first modulated signal, providing
a second ultrasonic signal, and broadcasting the first modulated signal and the
second ultrasonic signal to a listener.
In one aspect of the embodiment,
the method includes amplitude modulating the second ultrasonic signal with a
second input signal to produce a second modulated signal, amplifying the
amplitude modulated signal, and projecting the first and second modulated
signals in the audible range of the listener.
In yet another embodiment
of the present invention, there is provided an apparatus including a unit
modulating an ultrasonic signal with an information signal to produce a
modulated signal in which the information signal is completely intelligible to a
listener, and a projector coupled to the unit and projecting the modulated
signal to the listener.
These together with other objects and advantages
which will be subsequently apparent, reside in the details of construction and
operation as more fully hereinafter described and claimed, reference being had
to the accompanying drawings forming a part hereof, wherein like numerals refer
to like parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG.
1 illustrates an exemplary embodiment of a two projector system for broadcasting
an audible sound using an ultrasonic sound as a carrier.
FIG. 2
illustrates an exemplary embodiment of a one projector system for broadcasting
an audible sound using an ultrasonic sound as a carrier.
FIG. 3
illustrates an exemplary embodiment of a projector system using computer based
signal processing.
FIG. 4 illustrates exemplary embodiments of one and
two projector systems employed in a stationary and mobile environment.
DESCRIPTION OF PREFERRED EMBODIMENTS
An apparatus and method of
using ultrasonic sound to create audible sounds in the heads of individuals, or
of a crowd of individuals, will be described. The sounds could be voices, music
or ringing sounds to cause discomfort, disorientation, or low frequency
vibrations that have been shown to induce discomfort.
The physical
principles involved in such devices will be described together with the way that
they would be used to disrupt or affect the actions of an individual or crowd.
Additionally, examples will be used to indicate ways in which to handle the
situations described above.
QUALITATIVE DESCRIPTION
PRELIMINARY
CONSIDERATIONS
The use of sound controlling groups or crowds in both
civil and military situations was considered at least as far back as World War
II. Psychological studies on sounds that produce an aversive effect, or a
pleasant effect, have been reported since the early decades of this century.
Work in the nineteenth century by Helmholtz and Lord Rayleigh (detailed below)
show an understanding of the effect that combination tones, or beats, with low
frequencies (less than about one hundred Hertz) have on the pleasant or
unpleasant quality of a sound.
Crowds can largely be divided into two
kinds; those with leaders, and those without. A crowd with a leader can be
affected either by limiting the communication between the leader and the crowd,
or by directly affecting the crowd. A crowd without a leader (such as a looting
mob) can only be affected by something that influences everyone in the crowd.
Thus, it is necessary to develop physical methods to either impair
communication, or to produce a physical or psychological effect in all persons
exposed to the system.
Some of the methods suggested for affecting an
entire crowd involve very high intensity sound (120 dB or more above the
standard level of 2.times.10.sup.-5 N/m.sup.2). Sirens or very low frequency
vibrations (less than 100 Hertz) are frequently spoken of.
Studies
indicate several classes of sound to be of interest in crowd control. One class
of sounds includes those which are aversive in themselves. Examples of these
are: (1) scraping noises, such as that of chalk on a blackboard, (2) the crying
of a baby, and (3) screams of pain. These sounds almost involuntarily cause a
person to avert his/her attention from what he/she is doing, at least
momentarily. Repeated exposure to this class of sounds, if the occurrence is not
predictable, tends to produce jumpiness and sometimes leads to irrational
behavior.
Secondly, there are a class of sounds which will cause a
person to be startled and divert his/her attention from the task that he/she is
doing because they indicate a possible imminent danger to him/her. Examples are:
(1) gunfire, and (2) automobile crash noises.
Both of these classes of
sound will likely produce a "startle" reaction in a crowd the first time that
they are used. If the crowd, however, identifies the source as, for example, a
loudspeaker, they will likely adapt to and ignore the noise. If the crowd mills
around for a longer time, the sounds could cause headaches and other symptoms of
stress. The crowd may disperse, but this is not at all certain.
If, on
the other hand, one is trying to stop a fleeing person, a sudden noise, such as
the screech of brakes, would undoubtedly cause the person to be startled and try
to see whether the noise indicated impeding danger to him/her. He/She will
clearly be disoriented for a few seconds, although the average time will vary
from person to person. The sound of a gunshot may, for example, cause the same
effect, or it may simply cause the person to run faster or run in a weaving
manner.
A third class of sound includes low frequency vibrations either
slightly above or below 20 Hertz; the lower audible limit. Vibrations in this
frequency range produce several effects on a person's body.
Resonances
of several internal organs lie in this frequency range. It has been shown that
exposure to vibrations at an organ resonance cause nausea and a general feeling
of malaise. Vibrational amplitudes that are too high will cause physical damage
to the organs, whereas vibrations at a constant frequency or starting very low
an rising through the range appear to cause a feeling of unease and tend to
increase the suggestibility of a crowd.
The effects on a person's mood
appear to be caused by hitting frequencies close to the alpha frequency of the
brain waves. A phenomenon called "entrainment" occurs when the brain is
stimulated at frequencies close to 10 Hz. This means that the brain's natural
frequency is pulled close to, and sometime equal to, the stimulating frequency.
A normal brain displays a prominent "alpha" pattern (8 to 12 Hz) at a time of
relaxed alertness. Tense alertness, such as caused by freeway driving, leads to
a "beta" pattern with a frequency of 13 Hz or higher. A relaxed, dreamlike state
causes a "theta" pattern of frequencies from 4 to 8 Hz.
Other
experiments, such as the ones cited by Norbert Wiener in "Nonlinear Problems in
Random Theory", found that "a decidedly unpleasant sensation" was produced by
stimulating the brain at "about 10 Hz." In fact, Helmholtz argues that beats of
frequency less than 40 Hz are not perceivable as tones, but rather create a
jarring feeling and are responsible for the unpleasant sensation of dissonant
combinations of notes in music. As the low notes of the organ are in the range
of 30 Hz, it would seem that tones ranging in frequency from about 12 to 40 Hz
will produce an unpleasant feeling, or suggestibility. These tones are probably
useful in crowd control used either by themselves, or to induce a mood that
could then be triggered by another sound.
In producing low frequency
vibrations with a conventional loudspeaker, several problems arise. First, at
frequencies as low as this, loudspeakers are not very efficient in producing
sound. The speaker will have to be quite large. Second, there is very little
directivity possible with frequencies this low. Directive arrays would be huge,
making it almost impossible to define an area where the effect occurs or to draw
a line in the sand where individuals start feeling the effect when they cross
it. Finally, the signal would be strongest at the speakers, requiring protective
gear for at least the operators, and probably for all of the crowd control
personnel.
Methods to impede communication between a speaker and a crowd
have also been examined. One of the most interesting is techniques includes
playing back to a speaker his/her own voice with a slight delay (less than a
second). The speaker stutters and trips on his/her words unless he/she slows
down his/her rate of delivery a great deal.
If two moderately loud
audible tones of different frequency are received by the ear simultaneously,
then, in addition to the two original tones, somewhat weaker tones with
frequencies given by the sum and the difference of the original frequencies can
be heard. This is called the Beat Frequency phenomenon when the two frequencies
are close together, and the Combination Tone phenomenon when they are not. The
combination tones are caused by a non-linear response by the ear to somewhat
loud sounds. The details of the production of these tones are discussed in more
detail herein below.
It is important, and in fact one of the critical
physical principles in this invention, that an audible combination tone can be
heard even when the two original tones are ultrasonic so that their frequencies
lie above 20,000 Hz, the upper limit of audibility. In this case, the
combination tone corresponds to the difference of the two original frequencies
and is audible if it lies in the 20-20,000 Hz range of audibility.
The
present invention shows ways in which, by altering the frequency and amplitude
of one of the ultrasonic tones, the difference tone can be made to be a single
tone (possibly of very low frequency), a scream or shot, or a voice.
More detailed discussions will be deferred until the "Technical
Description" hereinbelow. We will discuss a way of causing a targeted person (or
group) to hear whatever pattern of sound, be it speech or anything else, that we
wish.
A combination tone is produced in the heads of all individuals
exposed to both ultrasonic tones. If you are exposed to only one, you hear
nothing, Additionally, these individuals are unable to detect the source of the
sound.
The following exemplary systems are described. One includes two
directional projectors, each capable of generating a powerful ultrasonic tone
and directing the beam to a desired point or area. At least one of the
projectors is capable of modulating the sound either in frequency, amplitude or
both. The projectors would probably be separated by some distance, but this
depends on the desired effect, which governs the design.
Another
includes a single projector with the capability of modulating the amplitude of
the projected signal.
More specifically, FIG. 1 illustrates an exemplary
embodiment of a two projector system for broadcasting an audible sound using an
ultrasonic sound as a carrier. Two projector system 10 includes, for example,
first projection unit 12 and second projection unit 14. First projection unit 12
includes, for example, ultrasonic sound source 15 (such as a conventional
ultrasonic signal generator) which generates an ultrasonic signal, modulation
sound source 20 (such as a microphone) which generates an information signal
(such as a tone), modulator 25 (such as a conventional frequency modulator)
which frequency modulates the ultrasonic signal with the information signal to
produce a modulated signal, amplifier 27 (such as a conventional amplifier)
amplifying the modulated signal, and projector 30 (such as an ultrasonic
speaker--a tweeter type speaker) which emits the amplified signal. Second
projection unit 14 includes, for example, ultrasonic sound source 35 (such as a
conventional ultrasonic signal generator) which generates another ultrasonic
signal, modulation sound source 40 (such as a microphone) which generates
another information signal (such as a voice), modulator 45 (such as a
conventional amplitude modulator) which amplitude modulates the ultrasonic
signal with the information signal to produce a modulated signal, amplifier 37
(such as a conventional amplifier) which amplifies the modulated signal, and
projector 50 (such as an ultrasonic speaker--a tweeter type speaker) which emits
the amplified signal. Sound overlap region 55 is the region where the sound
waves of projectors 30 and 50 overlap. Two projector system 10 is not, however,
limited to the embodiments described above. For example, two projector system 10
may include first projector unit 12 and second projector unit 14, wherein
projector unit 12 includes, for example, ultrasonic sound source 15, modulation
sound source 20, modulator 25, and projector 30, and projector unit 14 includes,
for example, ultrasonic sound source 35 and projector 50. That is, projector
unit 14 need not have sound source 40. Similarly, sound overlap region 55 is the
region where the sound waves of projectors 30 and 50 overlap.
FIG. 2
illustrates an exemplary embodiment of a one projector system for broadcasting
an audible sound using ultrasonic sound as a carrier. This system could be for
crowd control, an improved hearing aid for the hearing impaired, or to emit
audible sound to listeners located in a defined area. One projector system 60
includes, for example, ultrasonic sound source 65 (such as a convention
ultrasonic signal generator) which generates an ultrasonic signal, modulation
sound source 70 (such as a microphone) which generates an information signal
(such as a voice), modulator 75 (such as a conventional amplitude modulator)
which modulates the ultrasonic signal with the information signal to produce a
modulated signal, amplifier 80 (such as a conventional amplifier) which
amplifies the modulated signal, and projector 85 (such as a conventional
ultrasonic speaker a tweeter type speaker) which emits the amplified signal.
FIG. 3 illustrates an exemplary embodiment of a projector system using
computer based signal processing. For example, in a one projector system, such
as illustrated in FIG. 2, computer 87 operates as ultrasonic sound source 65,
modulation sound source 70 and modulator 75. Computer 87 generates an ultrasonic
sound signal and generates or inputs an audible sound signal, and then modulates
the two signals. Computer 87 can modulate the two signals using, for example,
conventional frequency or amplitude modulation techniques or the techniques
described hereinbelow. The modulates signal produced by computer 87 is then
transmitted to digital-to-analog (D/A) converter 89, whereupon the digital
signal is converted to an analog signal. The analog signal produced by D/A
converter 89 is then amplified by amplifier 91, and transmitted to projector 93.
Projector 93 then emits the amplified signal to a listener. The same principles
can be applied to the two projector system illustrated in FIG. 1.
FIG. 4
illustrates exemplary embodiments of one and two projector systems employed in a
stationary and mobile environment. For example, reference numeral 100
illustrates two projector system 10 (illustrated in FIG. 1) in a stationary
environment. In this example, projectors 30 and 50 are mounted on the top of a
building, and directed towards sound overlap region 55. A person or crowd
located in sound overlap region 55, located, for example, 50M from projectors 30
and 50, detects the broadcasted sound(s). Reference numeral 105, on the other
hand, illustrates two projector system 10 in a mobile environment. In this
example, projectors 30 and 50 are mounted in the back of a vehicle. The vehicle
may then be directed to move with the individual or crowd, as the individual or
crowd moves, such that the individual or crowd remains in sound overlap region
55.
When employing two projector system 10, system parameters may
include, for example, the following: (1) sound source=loudspeaker/crystal, (2)
frequency=.about.30 kHz, (3) sound intensity=100 db (max) at 50 meters, (4)
total source power (sound)=0.14 Watts, and (5) minimum focal spot size=1.3
meters, as illustrated by reference numeral 115 in FIG. 4.
Reference
numeral 110 illustrates one projector system 60 (illustrated in FIG. 2) in a
man-portable environment. In this example, an individual, such as a police
officer, may direct projector 85 of one projector system 60 toward, for example,
a fleeing individual. An individual located within the broadcasting area of one
projector system 60 will detect a modulated signal projected by projector 85.
The modulated signal will include, for example, an ultrasonic sound, such as a
whistle, amplitude modulated with an information signal, such as a voice. System
parameters may include, for example, the following: (1) sound source=fluidic
oscillator (whistle), (2) frequency=.about.100 kHz, (3) sound intensity=100 db
(max) at 10 meters, (4) total source power (sound)=0.2 Watts, and (5) minimum
focal spot size=53 cm.
Listed below are some of the useful features that
a system employing audible tones carried by ultrasonic frequencies would have.
(1) Power: As long as the size of a sound generator is smaller than the
wavelength, the power output is proportional to the fourth power of the
frequency. That is, the power output of a given sized generator is much higher
at high frequencies than it is at low frequencies. Hence, this property makes it
simpler to produce high output at high frequencies with smaller generators. This
would imply that a 30,000 Hz generator could produce the same sound intensity as
a 30 Hz generator 10.sup.12 times its size. This property makes it fairly simple
to produce high power outputs with fairly small generators.
(2)
Directivity: The diffraction angle of a reflector or lens in a projector is
proportional to the wavelength of the sound divided by the diameter of the
reflector lens. Since a 30,000 Hz sound wave has a wavelength of 1 cm.,
parabolic reflectors with diameters of about 1 mtr. will provide excellent
directivity. In addition, the short wavelength will make it possible to quickly
design "beams" that will possess features, such as fairly sharp shadow regions,
so that persons will have a definite perception of the desired effect in the
"illuminated" region, but little in the "shadow". Invisible barriers are thus
possible. In addition, the relatively small arrays can conceivably be mounted on
helicopters, remotely powered aircraft, or balloons.
(3) Stealthiness:
Combination tones are produced in the heads of those exposed to both beams.
Since the sounds from the individual projectors are inaudible, it will not be
easy to identify them as the source of the sound. This will make it difficult
for the crowd to respond by attacking the system. In addition, the appearance of
sounds in their heads from no apparent source will create alarm or fear in the
exposed group. This effect by itself will probably cause a crowd to disperse,
particularly if the crowd were composed of unsophisticated or superstitious
people.
Effects of the System
The primary psychological
difference between this system and other proposed systems using sound for crowd
control is the property creating the sound within the head of the target
individual. The effect on a person who suddenly starts to hear sounds with no
apparent source is not known.
Since most cultures attribute inner voices
either as signs of madness, or as messages from spirits or demons, both of which
will invoke powerful emotional reactions, it is expected that the use of a voice
will have an immediate intense effect.
Another effect is the low (less
than 100 Hz) frequency sound. There are several reasons for this. First, these
low frequency sounds will have a higher amplitude, in general, than the voice
frequency sounds. Second, sounds at these low frequencies have been shown to
increase the suggestiveness or apprehensiveness of exposed persons.
A
system using a barrier array so that a person would feel more and more
apprehensive as he/she moved in a given direction, and less if he/she turned
around and went out. This may require a "trigger", such as a soft voice
suggesting that it is dangerous and one should go back might work, in addition
to the low frequency sound.
In addition, interference with the brain's
alpha rhythm of a targeted individual or group may be achieved. This may cause
temporary incapacitation, intense feelings of discomfort which would cause
immediate dispersal of the crowd, or departure of the targeted leader.
Other sound patterns are possible, either alone or in combination.
Sounds such as random shots, or screams may be very effective when combined with
low frequency sounds producing apprehensiveness.
A leader could be
singled out by using highly focused beams projected from one projector system
60, that target only the head region of a single person. The sound patterns
described above could be used, or one could use the speaker's own voice, with an
appropriate delay. The pattern selected would depend on whether it is desired to
disrupt the speaker or his speaking ability.
Return to the Situations
Described in the Background Section
Whether to use two projector system
10 or one projector system 60 depends on the applicable situation. For example,
in the "Somalia" situation, the best effect could probably be achieved by using
projector system 10, wherein one projector focused on the individual and another
broad beam device targeting the crowd. A frequency near the alpha frequency
would be directed at the individual to disorient him/her and perhaps make
him/her collapse.
The crowd could be handled in a different way, for
example, with sounds that induce apprehensiveness, without disabling. Ideally,
the crowd would disperse, leaving the leader to be apprehended. In fact, certain
characteristic sounds may be known to a particular culture that indicate that a
person has a dreaded disease, such as the plague. This, together with sounds
causing general apprehensiveness, might work.
The crowd on a dock
described in the Haiti example, would be handled in roughly the same way. Sounds
causing general discomfort would be mixed with other, for example, culturally
specific sounds that would incite fear and discomfort. The intensity of the
sounds could be increased for a while, then followed by a scream, or some
related noise. Since the source of the sounds is not readily obvious, there will
probably be general panic and fleeing.
An ultrasonic device may also be
used to control looting crowds, instead of the more harmful tear gas after hard
to control crowds. Additionally, the difficult task of removing residual tear
gas is eliminated. An ultrasonic device would be used to control the crowd by
exposing them to disorienting sounds, and sounds inducing fear.
Technical Description of the Method
The operation of the
embodiments illustrated in FIGS. 1-3 will now be described. The system depends
largely on the operation of the response of the ear to "moderately" loud sounds,
where "moderately" implies sounds loud enough to drive the ear into a non-linear
response mode. The non-linear response of the ear to high amplitude sounds is
discussed by, for example, Helmholtz.
The Response of the Ear
Let S(t) represent the total pressure incident on an eardrum, and the
net vibrational response of the mechanism involved in hearing by:
R(t)=F(S(t))
which simply states that the response is a function
of stimulus.
A power series expansion of the function F, results in:
R(t)=A*S(t)+B*(S(T)).sup.2 +C*(S(t)).sup.3 + . . .
The higher
powers having been dropped. A possible constant term is also dropped since it is
clear that there is no response when there is no stimulus.
The
expression when the stimulus includes two tones with frequency f.sub.1 and
f.sub.2, respectively are:
S(t)=a*cos(f.sub.1 t)+b*cos(f.sub.2 t)
The amplitude of the two sounds are a and b.
From the
expressions above:
R(t)=A*a*cos(f.sub.1 t)+A*b*cos(f.sub.2 t)+B*{a.sup.2
*cos.sup.2 (f.sub.1 t)+b.sup.2 *cos.sup.2 (f.sub.2 t)+2*a*b*cos(f.sub.1
t)*cos(f.sub.2 t)}+(terms of the third power)
Using standard
trigonometric identities, the terms in the second line of the equation become:
B*{a.sup.2 *(1+cos(2f.sub.1 t))/2+b.sup.2 *(1+cos(2f.sub.2
t))/2+a*b*(cos[(f.sub.1 +f.sub.2)t]+cos[f.sub.1 -f.sub.2)t]}
If all of
the constants in the expansion of F except A are zero, the response would be
perfectly linear. That is, any number of tones would produce a response which
contains all of the frequencies in the incident pressure wave and no others. The
amplitude of any tone in the response would be proportional to its amplitude in
the incident wave.
If B is not zero, the bracketed terms in the last
expression will be present in the response. Assuming that a and b are "small"
(less than one) and about the same size, then a.sup.2, b.sup.2, and ab will be
smaller than a or b. Even if B were equal to A, the quadratic terms in the
response would be smaller than the linear terms. However, as a and b get larger,
the relative size of a.sup.2, b.sup.2, and ab to a and b grows. Mathematically
this occurs where a and b are greater than one. Thus, the relative amplitudes of
the quadratic terms in the response:
B*a.sup.2, B*.sup.b2, and B*a*b
increase relative to A*a and A*b.
The behavior discussed above
describes the behavior of the ear. When sound amplitudes are small, the ear
hears the incident tones and no others. When the amplitudes are larger,
combination tones corresponding to frequencies (f.sub.1 +f.sub.2) and (f.sub.1
-f.sub.2) are heard. Recent studies at 350 Hz have measured that when the
primary tones have an amplitude of about 55 db, the second harmonic has an
intensity about 40 to 45 db below the fundamental. At primary tone levels of 80
db the harmonic tone is only a few db below the fundamental. A similar behavior
is expected when the primary tones are ultrasonic, although the relative sizes
of the linear and quadratic terms may be frequency dependent.
All of
this leads to the conclusion that B is not zero, but that it is smaller than A.
Experiments suggest that C is also not zero, but is probably smaller than B
since frequencies corresponding to the third harmonic (although seen) are weaker
than the second order terms.
Single Tone Effect
The quadratic
terms in the response will now be discussed.
The cosine squared terms
lead to the terms:
a.sup.2 /2 (1)
and
a.sup.2
/2*cos(2f.sub.1 t) (2)
with similar terms involving b and f.sub.2.
Equation (1) is independent of the frequencies of the original tones and
represents a constant pressure if the amplitude a is constant. The pressure
represented by this term is present even if there is only one ultrasonic
projector, and results in inducing audible sound with a single projector if the
amplitude is not constant.
Equation (2) is twice the frequency, which
will be inaudible if the original frequency is ultrasonic.
If the
amplitude of the ultrasonic tone is modulated at a frequency much less than that
of the ultrasonic tone (such as an audible frequency), the pressure in the ear
would also be modulated. A voice, or any other complex tone, should be rendered
audible by this mechanism.
Additionally, the "constant" term that
results from the square of the primary tone is the square of the amplitude of
the primary. If desired, signal processing can be used to induce voices since
the amplitude of the original tone needs to be the square root of the voice
signal. A bias can also be applied to prevent the signal going to the square
root circuit from ever being negative. The square root technique can be
accomplished using, for example, conventional analog circuits with, for example,
a square root output, or a computer using, for example, a digital square root
function.
The theory for the single tone effect will now be described.
First, assume that the voice that one wishes to transmit is Fourier analyzed.
f(t)=a*cos(f.sub.1 t)+b*cos(f.sub.2 t)+ . . .
where only two of
the components are retained to illustrate the principle. If F is the ultrasonic
(carrier) frequency, the transmitted signal is:
C*[A+f(t).sup.1/2
*cos(Ft)
where C is large enough to invoke the non-linear square
response:
C.sup.2 *[A+a*cos(f.sub.1 t)+b*cos(f.sub.2
t)]*[1+cos(2*Ft)]/2(3)
Equation (3) can be broken inot the following
terms:
C.sup.2 *A/2+C.sup.2 *(a*cos(f.sub.1 t)+b*cos(f.sub.2
t))/2+C.sup.2 *(A+a*cos(f.sub.1 t)+b*cos(f.sub. t))*cos(2*Ft)/2 (4)
The
second term in equation (3) (C.sup.2 *(a*cos (f.sub.1 t)+b*cos(f.sub.2 t))/2) is
directly proportional to the corresponding term in f(t). Equation (4) includes
the terms with the frequencies (2*F+/-f.sub.1). With F as an ultrasonic
frequency, these tones will be inaudible. The same will be true by extension for
the entire voice f(t). Thus, an audible voice signal together with inaudible
ultrasonic tones will be induced by this mechanism.
Combination Tones
The terms with frequencies given by the sum and the difference of the
frequencies of the original tones are called combination tones. If the two tones
are ultrasonic, the sum frequency will also be ultrasonic, and hence inaudible.
The difference frequency, however, will be audible if it lies in the audible
range for the ear. The production of an audible difference from two inaudible
ultrasonic tones was reported by Lord Raleigh. This shows that the
non-linearity, experimentally verified for audible sounds, is not appreciably
different for ultrasonic sounds.
In summary, a non-linearity of the ear
exists giving rise to a quadratic term in the ear's response. This effect occurs
both when the original sounds are audible, or ultrasonic.
If the two
ultrasonic tones of different frequencies were beamed at an individual, or a
crowd, the difference frequency would be heard, assuming that it lies in the
audible range. The frequency could be changed in any desired pattern, or left at
a constant frequency, such as a low frequency to increase apprehensivess.
If one wishes to induce a wave with a complex frequency pattern such as
a voice, the wave could be used to amplitude modulate one or both of the
ultrasonic waves. The frequency of the two waves would likely be the same, or
else there would be a background note consisting of the difference tone.
Although, it might be preferable to deliberately induce a low frequency to
increase apprehensiveness.
The quadratic terms also imply that the
"constant" term exists even with only one tone. Thus, a single high amplitude
ultrasonic source, amplitude modulated with a voice (a square root of the
voice), would induce the voice in the heads of those exposed to the signal. A
one projector system, such as system 60, would be the system of choice for, for
example, man-portable devices.
Beat Tones
In addition to the
combination tones described above, which are due to the quadratic response of
the ear, a phenomenon called the "Beat Frequency" effect occurs if the two
frequencies are very close together. Beat tones are of importance in the use of
very low frequencies, since they are caused by a linear response term, which is
generally larger in amplitude than the combination tone.
Observing two
waves with slightly different frequencies f and f+.delta.f (with .delta.f
small), the linear response will be:
R(t)=A*{a*cos(ft)+b*cos[f+.delta.f)t]}
using trigonometric
identities:
cos{(f+.delta.f)t]=cos(.delta.ft)*cos(ft)-sin(.delta.ft)*sin(ft),
and
R(t)=A*{[a+b*cos(.delta.ft)]*cos(ft)-b*sin(.delta.ft)sin(ft)}
Both of these terms include waves with frequency f whose amplitude is
modulated at frequency .delta.f. When f is an audible frequency, the pulsing
changes in amplitude (beat) are clearly audible.
As the beats increase,
the beat frequency becomes harder to distinguish and is gradually perceived as a
weaker, independent tone. The beat phenomenon can thus be said to shade the
combination tone phenomenon.
Two ultrasonic tones whose combination tone
is a very low frequency produce a beat phenomenon, where the beat frequency
would equal the combination tone frequency. In this case, the ear would perceive
the beat. The amplitude of the beat will be higher than the combination tone
since it arises from the linear, rather than the quadratic, response of the ear.
Hence, its likely that low frequency sounds, can be induced with
particularly high amplitudes.
Producing Undistorted Sounds Using Two
Ultrasonic Sound Sources
Real time computer based signal processing can
be used to produce an understandable, non-distorted signal from a pair of
ultrasonic projectors in the following way.
Assume a sinusoidal signal
of frequency f.sub.1 is fed into one of projectors 30 and 50, and the signal for
broadcasting is Fourier analyzed in real time by a computer and can be written:
f(t)=.SIGMA.a.sub.i *cos(f.sub.i t).
The computer takes each of
the frequencies f.sub.i and adds f.sub.1 to it, and then constructs the signal:
g(t)=.SIGMA.a.sub.i *cos[(f.sub.1 +f.sub.i)t].
If g(t) is
amplified and then fed to the second projector, the signal in the regions where
the two beams cross (sound overlap region 55 is:
Acos(f.sub.1 t)+Bg(t).
The square of this signal is:
A.sup.2 cos.sup.2 (f.sub.1
t)+B.sup.2 g.sup.2 (t)+2ABcos (f.sub.1)g(t).
We will ignore the first
two terms as being both ultrasonic. The third term is:
2AB{.SIGMA.a.sub.i [cos(f.sub.1 t)cos(f.sub.1 +f.sub.i t)]}
Using standard trigonometric identities, this is:
AB{.SIGMA.a.sub.i [cos[(f.sub.1 +f.sub.1 +f.sub.i)t]+cos[(f.sub.1
-f.sub.1 -f.sub.i)t]}
The first set of these sums will again be
ultrasonic, thus not audible. The second set, however, is
AB.SIGMA.a.sub.i cos(f.sub.i t)
(noting that: cos(-ft)=cos(ft))
Hence, an amplified form of the signal that we wish to transmit.
Similarly, real time computer based signal processing can be used to
produce an understandable, non-distorted signal from a single ultrasonic
projector, wherein the square root of an input signal is produced by the
computer.
Propagation and Focussing of Ultrasonic Sound
One of
the great advantages of using an ultrasonic sound as a carrier for audible
sounds is the ease of focussing due to the short wavelength involved. Sounds are
a wave phenomenon, just as light, and can be treated mathematically by the same
equations that describe light, with appropriate changes in the interpretation of
the quantities involved.
Assume a point source of sound placed at or
near the focus of sound mirror. Since sound is reflected by a sudden difference
in the density of the material of propagation, most materials, such as metals or
plastics, will serve as mirrors. As in the case of light, the position of the
source at, in front of, or behind the focal point will determine the character
of the wave reflected from the mirror. When using the "geometrical optics"
approximation, the focal point in front of the mirror is more important. We will
be most interested in the case where the sound would be brought to a point focus
at some distance in front of the mirror, if we were to use the "geometrical
optics" approximation. The sound wave should then be represented by a spherical
wave centered on the geometric focal point of the mirror. The wave would not be
a complete sphere, however, since the mirror has a finite size. Sound emitted by
the source that passed beyond the mirror boundary will not be reflected and
focussed at the focal point. The finite size of the mirror causes the wave to
exhibit diffraction and not to focus to a geometrical point.
The most
pertinent part of the analysis lies in the fact that there is a diffraction
circle surrounding the focal point. The radius of the circle is 00/.610*(1/a)*f.
In this formula, 1 is the wavelength (the speed of sound (3.30-10.sup.4
cm./sec.) divided by the frequency), a is the radius of the mirror, and f is the
distance to the focal point of the mirror. About 80% of the total energy
striking the mirror from the source passes through the diffraction circle
described above. This is the basis for the calculation of the source power
required to produce a given power flux at the focal point.
By moving the
source away from the close focal point of the mirror, the energy will be spread
over larger areas in the vicinity of the far focal point. This will be the
technique used when a crowd, rather than an individual, is to be exposed.
Another important feature of ultrasonic sound is that it is absorbed by
the air to a much greater extent than audible sound. At 1 mhz, an attenuation
coefficient for air is 15(1/mtr.), varying as the square of the frequency. This
coefficient is for the pressure, so double the calculated value must be used to
obtain the attenuation of the intensity, which depends on the square of the
pressure.
Absorption is moderate for frequencies around 30 kHz, but
becomes severe for 100 kHz waves. This will lead to tradeoffs between the better
focussing properties of shorter waves and the lower absorption of longer waves.
The many features and advantages of the invention are apparent from the
detailed specification and, thus, it is intended by the appended claims to cover
all such features and advantages of the invention which fall within the true
spirit and scope of the invention. Further, since numerous modifications and
changes will readily occur to those skilled in the art, it is not desired to
limit the invention to the exact construction and operation illustrated and
described, and accordingly all suitable modifications and equivalents may be
resorted to, falling within the scope of the invention.
* * * * *