Abstract:
An audible warning system which reduces the number of land vehicle traffic accidents where there are already traffic control devices installed such as at stop signs, intersection control lights and on vehicles in conjunction with side lights, tail lights and brake lights. The accident reduction results from the monitoring of approaching traffic with a radar-type range sensing system. Sound generators, based on parametric-array technology, permit the focusing of sound into waves of a few degrees width and aiming them at a particular vehicle or pedestrian located at a particular range from the sound projector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Provisional Application No. 60/223,213, entitled “Audible Warning System” filed on Aug. 4, 2000, and which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to communicating audibly with unequipped listeners, and, more particularly, to communicating audibly inside a compartment located at a distance from an audible source by modulating ultrasonic carrier waves with audible communication. 
     In recent years in the U.S., there are typically 16,000 fatal multiple-vehicle collisions See Eberhard, C. D. et al; Automotive Engineering March 1997, pp. 86-90; Miller, T.; Accident Analysis and Prevention Vol. 23 No. 3 ppl. 343-352 1997 and The U.S. Statistical Abstracts. The following five categories of multiple-vehicle collisions had direct annual monetary costs of: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Cross-Path Collisions 
                 $14.5 
                 billion 
               
               
                   
                 Rear-End Collisions 
                 $13.9 
                 billion 
               
               
                   
                 Opposite Direction 
                 $9.5 
                 billion 
               
               
                   
                 Sideswipe 
                 $2.8 
                 billion 
               
               
                   
                 Backing 
                 $0.8 
                 billion 
               
               
                   
                   
               
             
          
         
       
     
     Approximately 70% of the cross-path collisions took place where signs and signal were installed. The Federal Highway Administration recognized the problem in 1999 when they issued new standards mandating enhanced sign and signal visibility. Frustrated municipalities are changing their laws and investing in automatic cameras which photograph and then ticket drivers who “blow through” red lights. 
     In spite of arrays of flashing lights, rumble strips and other devices, turnpike toll booths need to be surrounded with concrete fortifications and are regularly the site of deadly rear-end collisions. Turnpikes are also the sites of deadly collisions with vehicles that have entered traveling in the wrong direction. 
     The U.S. Statistical Abstracts reports that there are 700 deaths and 30,000 injuries in highway construction zones. Drivers often fail to respond to numerous vehicle-mounted and roadside warning devices and crash into highway work crews at full speed. This is such a problem that many jurisdictions require highway work crews and their vehicles to be followed with crash-barrier trucks designed to absorb the impact of a crash from behind to save highway workers lives. 
     The reduced driving ability of the rapidly aging U.S. population is expected to make all this worse. In anticipation of the aging problem, the Federal Highway Administration (FHWA) this year issued guidelines for signs, signals and highway layout to deal with the older driver problem. Particular attention is being paid to standards for the how much light signs reflect when illuminated at night—the “retroreflectivity” of the signs. 
     In attempting to solve these problems, there is the danger of visual and audial pollution. This can be witnessed at an accident site where the wreckers, ambulances, fire and law enforcement vehicles are all outfitted with strobe lights. This can have a stupefying effect on passing drivers. Some jurisdictions have had to reduce the intensity of their LED traffic lights due to complaints from the public. Installing intense lighting and attention-getting coloration on every vehicle as well as on every sign and signal is unlikely to be accepted by the public. Sound emitted from rumble strips in roadways and well as audible blind-pedestrian crossing aids have been opposed by the public living nearby and their use has had to be curtailed or eliminated. 
     So there is the seemingly paradoxical need for a more intense means of warning, if not, a means of jarring, dangerously approaching drivers, while avoiding disturbing the majority of drivers who are approaching safely. 
     Research into vehicle-borne collision avoidance devices Smith, D.; Effective Collision Avoidance Systems for Light Vehicles, A Progress Report; Proc. ITS 2000, Intelligent Transportation Society of America, Boston May 2000, reports that automobile drivers react better to audible notifications than to visual ones. The literature on television advertising points to the superiority of sound over images. Trout, J.; The New Positioning; McGraw Hill NY, 1996; reports research showing the mind is able to understand a spoken work in 140 ms while 180 ms is required to understand a written word. The understanding of the written word fades in one second while the understanding of a spoken word lasts for 4 or 5 seconds. U.S. Army research, reported on by Trout, has shown that the intonation of speech can significantly affect the rate of information absorption. 
     Sound is used in traffic warning systems. For example, every vehicle is outfitted with a horn. Railroad crossing gates and toll-taking machines sound bells under certain conditions. Pedestrian crossings lights outfitted for blind pedestrians transmit sound to aid the blind pedestrian. Emergency vehicles are equipped with sirens and other sound emitting devices. Construction machines emit sounds when they are backing up. The effectiveness of these devices is limited by their inability to aim sound in a particular direction and their inability to focus it on a particular vehicle or pedestrian. This limitation is simply due to the need for a sound-projecting device, such as a horn, to be gigantic to focus its output into a narrow wave. An aperture with dimensions on the order of 50 wavelengths is needed to form a wave of a few degrees width. Since speech has frequency components as low as 300 Hz which implies sound with a 1-meter wavelength. To form 3° wide waves of 1-meter wavelength sound would require a horn with dimensions on the order of 50 meters! 
     Tanaka et al (U.S. Pat. No. 4,823,908) discloses directional speakers able to focus sound in a particular region of a large hall. These solve the directivity problem by using ultrasound whose wavelength in air is on the order of 5 to 10 millimeters. This implies a sound emitting aperture, to achieve 3° wide waves, of 8-to 16-centimeter dimensions. The audible message modulates the amplitude of the ultrasonic carrier wave in a way that is similar to what takes place with AM radio. Nonlinear properties of air in the presence of intense sound waves are used by Tanaka et al to demodulate the ultrasonic carrier and produce audible sound from the highly focused ultrasonic carrier waves. The sound emitter disclosed by Tanaka et al uses a complex baffling system which is unsuitable for mounting on a traffic control sign or signal or on a vehicle. Unfortunately, their technique yielded unacceptably high levels of harmonic distortion. 
     It is therefore an object of this invention to significantly improve the effectiveness of traffic control systems by giving them a means of communicating audible messages into the sealed passenger compartment of conventional approaching vehicles. 
     It is further an object of this invention to exploit the superiority of sound communication over visual communication. 
     It is further an object of this invention that the devices should be electrically compatible with and mount easily onto existing traffic control systems such as signs, signals and vehicles. 
     It is further an object of this invention to enable vehicles to communicate with other vehicles such as those that are approaching dangerously for the conditions at hand. 
     It is further an object of this invention for vehicles to communicate with pedestrians or the drivers of vehicles potentially in the path of movement of the vehicle issuing the warnings. 
     It is further the object of this invention to automatically control the acoustic projector&#39;s direction of transmission and the range of the focal point of the sound wave by coupling it with radar devices which measure direction, range and other characteristics of targets by analyzing skin reflections received from the targets. 
     It is a further object of this invention to communicate audible messages into a localized region without disturbing the whole area around the localized region. 
     It is a further object of this invention to delineate channels of movement such that unequipped people either walking or riding on a vehicle who depart from a channel will receive audible communication directing them back into the channel. 
     It is further the object of this invention to monitor ambient atmospheric conditions and modify the parameters of sound transmission as condition change. 
     It is a further object of this invention to provide a means to focus sound onto the upper window of a building from a distance and communicate with people located inside. 
     SUMMARY OF THE INVENTION 
     The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow. 
     This invention improves the ability of traffic control systems such as signs, signals, and officials, near roadways as well as vehicle-mounted lights and other visibility enhancers to communicate their messages to approaching vehicles. If an approaching vehicle is unresponsive, the intensity and urgency of the warning can be increased to the point of jarring the unresponsive driver or pedestrian. This is accomplished with a range sensing device, such as a radar system, which monitors approaching traffic and provides feedback on how well an approaching vehicle is reacting to the communications that the signs and signaling devices are presenting. 
     Vehicle drivers, pedestrians and others often fail to react to visual communications because they are inattentive, distracted, intoxicated or physically impaired. Research has shown that audible communication is superior to visual communication. This invention will give an audible voice primarily to traffic control devices as well as the ability to focus that voice on a particular vehicle or pedestrian as opposed to everyone in the general area. 
     The following examples of use of this invention are for purposes of examples and not as a limitation on the invention&#39;s use. For example, the invention can be used for example with: “STOP ” and “Wrong Way” signs; traffic lights at intersection curve ahead signs; speed limit signs; flagman or police officers guarding highway work zones; tail, brake and side lights on the rear of land vehicles; pedestrian crossing signals; a directed warning siren or vehicle backing indicator; and vehicles trailing a vehicle traveling in snow or fog. 
     In addition, the present invention could be used with for example: navigational marks at sea; navigation lights on watercraft, and watercraft traveling in fog or other low visibility situations. 
     The invention can also be used by fire and other public safety personnel to communicate with people behind the closed windows of a building. 
     The invention can also be used in aviation applications for ground and air traffic control. Aircraft taxiing to and from the terminal, runway, and maintenance hanger can be contacted by the control tower or specially equipped aircraft with greater speed and accuracy than the current reliance on radio transmission and reception. In-flight near misses will be eliminated with aircraft equipped with the present invention. Audio communication in the cockpit will no longer rely on the radio being turned on or being tuned to the correct frequency. 
     In general, the invention utilizes ultrasonic carrier waves that are demodulated, after a period of time, when they encounter and then compress a window whose stress-to-strain relationship is nonlinear, in order to exploit the propagation of the audible sounds, resulting from the demodulation process inside the window glass, into the air-filled compartment the window encloses. The intersection of a large number of modulated ultrasonic carriers at one point in open air can drive the air into saturation which will also demodulate the carrier waves. This permits the establishment of boundaries for channels of movement for pedestrians or vehicles that will direct unequipped errant travelers back into the channel of movement. 
     For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 pictorially represents an intersection traffic control light equipped with a range sensor and the modulated-ultrasound projector of this invention with the sound projector communicating to the driver in the interior of the approaching vehicle; 
     FIG. 2 represents a block diagram that includes a range-sensing radar whose returns are analyzed by a digital computer and the results of the analysis are passed to a second digital computer, which controls the parameters of the ultrasound projector in accordance with this invention; 
     FIG. 3 pictorially represents a modulated-ultrasound focused on a vehicle&#39;s windshield in accordance with this invention; 
     FIG. 4 schematically and pictorially represents a test range on which a vehicle is targeted by a modulated ultrasound wave from a projector; 
     FIG. 5 is a block diagram representing the transmission path of sound from an ultrasound projector to the received signal at the filtered output of a microphone in accordance with this invention; 
     FIG. 6 sets forth a computational procedure for preprocessing a waveform for transmission over an ultrasonic wave in accordance with this invention; 
     FIG. 7 is a flow chart of a digital computer program in accordance with this invention that would analyze the range information from the radar range sensor and issue warning in response to the situation at hand in accordance with this invention; 
     FIG. 8 pictorially represents a flat electrostatic speaker or sheet piezoelectric projector mounted on the face of a traffic control sign in accordance with this invention; 
     FIG. 9 schematically represents an array of electrostatic speakers or piezoelectric projectors in accordance with this invention; 
     FIG. 10 is a pictorial representation of an array of steerable ultrasound projectors of this invention. 
     FIG. 11 is a pictorial representation of a radar equipped pedestrian crossing signal communicating with a blind pedestrian at a particular distance from the traffic signal by focusing the modulated ultrasound so that audible sound is generated over the pedestrians head in accordance with this invention; 
     FIG. 12 is a pictorially representation of an ultrasonic sound system having a 4×4 array of speakers; 
     FIG. 13 is a pictorial schematic illustration of multiple ultrasound waves demodulated a range of r 1 , where the multiple waves intersect; 
     FIG. 14 schematically represents an ultrasound projector system steering its multiple waves so that ultrasound demodulation takes place at a range r 2 ; 
     FIG. 15 is a schematic illustration of four ultrasound waves emitted by four phased arrays and crossing at one point located at some distance from the phased arrays in accordance with this invention; 
     FIG. 16 pictorially represents a traffic sign equipped with a modulated-ultrasound projector communicating audible messages to the driver of the unequipped vehicle in accordance with this invention; 
     FIG. 17 pictorially represents a leading vehicle using an ultrasound wave in accordance with this invention to communicate to a trailing vehicle; 
     FIG. 18 pictorially represents wave steering with either phased-array or mechanical steering means of this invention. Steering could be in either elevation or azimuth or both; 
     FIG. 19 illustrates pictorially a remote ultrasound projector triggered by the radar range sensor of this invention located at an intersection; 
     FIG. 20 pictorially represents a modulated-ultrasound projector focused on cross traffic that might enter an intersection and be in the path of a dangerously approaching vehicle with the dangerous vehicle detected by the radar sensor mounted on the traffic control device in accordance with this invention; 
     FIG. 21 is a pictorial representation of a modulated-ultrasound megaphone that an official is using to focus his voice on a particular vehicle among many in accordance with this invention; 
     FIG. 22 is a pictorial representation of a public official communicating with people behind a window using focused modulated-ultrasound in accordance with this invention; and 
     FIG. 23 pictorially illustrates a channel of movement whose boundaries are established by waves of modulated ultrasonic waves in order for pedestrians or vehicles entering the waves to receive instructions on how to move back into the channel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention is a traffic control system that continuously monitors and communicates with, when necessary, approaching traffic. The time history of the range of an approaching vehicle is analyzed by a conventional digital computer coupled to the range-sensing device. For the purposes of the present invention, a vehicle is any means to transport people or cargo, including land vehicles, watercraft, and aircraft. A decision is made whether or not the approaching vehicle will be able to stop in the remaining distance. A message is transmitted to warn the dangerously approaching vehicle to the point of jarring a driver without special equipment installed in the vehicle. 
     The preferred embodiment of the above invention illustrated in the accompanying drawings, as illustrated in FIG. 1 is a pictorial view of the audible communication system constructed in accordance with the invention, same being generally indicated by numerical designation  1 . The system  1  generally includes an ultrasonic sound projection system  2  in wired or wireless communication with a range sensing system  10 . 
     A schematic of the audible communication system  1  is illustrated in FIG.  2 . As shown in FIG. 2, the ultrasonic sound projection system  2  preferably includes a digital computer  128  with storage capacity  133  for preprocessed messages, a digital-to-analog converter  134 , an amplifier  136 , and an ultrasonic sound projector  138 . The range sensing system  10  preferably includes a radar transceiver  130  with a radar antenna  129 , an analog-to-digital converter  131 , and a digital computer  132 . The range sensing system  10  signals the ultrasonic sound projection system  2  with information, such as vehicle range, vehicle type, and a message, when a subject vehicle is approaching a predetermined point at a dangerous speed. The vehicle type information is used to cross reference characteristics, such as model, make, year, windshield angle, windshield thickness, and windshield material, for the transmission of the appropriate warning signal that will demodulate to an audible sound once the warning signal passes through the windshield. All vehicle window information will be stored in either or both of the digital computers  128 ,  132 , in cases where the ultrasonic sound project system  2  is positioned to transmit a signal directed towards the side of the vehicle. The ultrasonic sound projection system  2  transmits a warning signal to the subject vehicle(s). 
     Provided below is a detailed analysis of the concepts underlying the various embodiments of this invention. More specifically, the production of sound from nonlinearities of air. 
     Air is excitable by an intense ultrasonic wave, which has been modulated with audible communications. The ultrasonic wave modulated by the audible communication, e (t), is given by: 
     
       
           p   1 ( t )= P   1   e ( t )sin(ω c   t )  Equation 1  
       
     
     and the secondary wave generated by the nonlinearities of the air demodulating the ultrasound wave given by: 
     
       
           p   2 ( t )=[(β P   1   2   A )/(16πρ o   αC   o   4   z )]θ 2   /θt   2   e   2 ( t )  Equation 2  
       
     
     Where: 
     θ 2 /θt 2  is the partial second derivative with respect to time 
     P 1  is amplitude of the launched ultrasonic carrier wave 
     p 1 (t) is the primary, ultrasonic carrier, wave pressure as a function of time, t. 
     e(t) is the modulation envelope of the ultrasonic carrier wave. 
     ω c  is the angular frequency of the carrier 
     p 2  (t) is the pressure of the secondary, audible, wave demodulated by the nonlinearities 
     β is the coefficient of nonlinarity: 
     
       
         β=(γ+1)/2  
       
     
     where γ is the ratio of specific heat 
     ρ o  is the ambient density of the medium 
     c o  is the small signal wave propagation speed 
     A is the wave cross section area 
     z is the axial distance is the absorption coefficient of the medium at ω c    
     Equation 1 sets forth a square-law nonlinearity due to the saturation of air in which the intense ultrasonic waves are traveling. 
     The amplitude of the secondary (demodulated) wave is proportional to the second derivative of the square of the modulation envelope. It is preferred that the pressure intensity be large with respect to the physical constants of the air as set forth in Equation 2. 
     The generation of audible sound is achievable by reflecting ultrasonic waves off of a solid surface in the direction of the source of the modulated ultrasonic waves. The nonlinear stress/strain relationship of the solid surface is responsible for demodulation of the audible communication and the generation of the audible sound. This takes place in much the same way as the nonlinearities of air generating sound in open air as described by Equation 2. 
     The present invention expands this known technique to the generation of the demodulated sound on the other side of a window or other panels enclosing a compartment so that a person can be hear the message on the opposing side of the window or compartment relative to the incoming wave. 
     FIG. 3 illustrates the arrival of the millimeter-wavelength ultrasonic wave  84  focused on the windshield  86 . The approximately 6 to 1 ratio of the velocity of sound in windshield  86  to the velocity in air causes the ultrasonic wave  84  to be reflected  89 . If the angle of incidence is within a few degrees of normal to the surface of the windshield  86 , the ultrasonic wave  84  is also refracted, refracted ultrasonic wave  91 . With or without refraction the ultrasonic wave  84  striking the surface of the windshield  86  undergoes a nonlinear interaction with the windshield  86  much as intense sound undergoes in air driven into saturation as described in Equation 2. The ultrasonic wave  84  interacts with the non-linear stress-to-strain relationship of the windshield  86 . This nonlinear interaction with the windshield  86  demodulates the ultrasonic wave  84  resulting in the reproduction of 0.1-meter wavelength audible sound  90  near the surface, but on the opposing side, of the windshield  86 . Further demodulation may take place if the ultrasound wave  84  is refracted through the windshield  86 . In many cases, the windshield  86  will be made up of a laminate of glass and plastic. 
     A further feature of the present invention relies upon preprocessing the transmitted sounds for optimum generation of audible sound by demodulating the ultrasonic wave as it passes through the windshield. In the present invention, the calibration process described in FIG. 4 is used for recording the audible sound  104  generated in the passenger compartment  110  by the ultrasonic wave  106  interacting with the windshield  108  of the vehicle  100 . These recordings are used to develop messages that have been preprocessed for optimal generation of intelligible audible sounds  104  inside the vehicle  100  utilizing the techniques of FIGS. 5 and 6 that are discussed below. These preprocessed messages are stored in the ultrasonic sound projection system  2  for later use. 
     FIG. 4 represents a test range used to demonstrate the concepts of the present invention and to preprocess messages for use with the system  1 . In FIG. 4, an ultrasound wave projector  94  is mounted on a suitable movable mount  96  so that the range  98 , r, can be varied during the data gathering process. (In some cases, it is necessary to vary the height of the projector  94  during the data gathering process.) A cross section of a land vehicle  100  is shown. A microphone  102  is mounted where a typical driver&#39;s head would be. The microphone  102  records the audible sound  104  that is generated by the demodulation of the ultrasonic wave  106  as it interacts with the windshield  108  and enters the passenger compartment  110  of the vehicle  100 . The audible sound  104  recorded by the microphone  102  is filtered by a receive filter  112  and then amplified by amplifier  114 . The amplifier  114  and filter  112  remove vestiges of the ultrasonic wave  106  and image frequency generated by the demodulation process in the windshield  108 . The audio signal is also band limited by the filter  112  to prevent distortions that can result from aliasing of noise and other signals above the Nyquist frequency as they are digitized by the analog-to-digital converter (A/D)  116 . The digital samples of the audio waveform are passed to the digital computer  118  for processing and storage. The digital computer  118  is also used to generate a stream of digital samples of messages modulating an ultrasonic wave  106 , which are converted to analog signals by the digital-to-analog converter (D/A)  120 . The output of the D/A  120  are filtered by the transmit filter  122 . The transmit filter  122  smoothes the so called boxcar effects of the digital-to-analog conversion process and removes other undesirable higher frequency components from the signal before they are amplified by the power amplifier  124 . The amplified signal drives an array of electrostatic or piezoelectric speakers  126  used to project the ultrasonic wave  106  onto the windshield  108  of the test vehicle  100 . 
     A block diagram of the flow of signals through the calibration of FIG. 4 is presented in FIG.  5 . The objective is to compute a source waveform, e (t), needed to generate a prescribed voice waveform, v (t). For example, the desired waveform, v d  (t), might be the waveform of the audible utterance: “stop.” In FIG. 4, one might desire an utterance of the word: stop  104  to be received inside of the passenger compartment  110  of the vehicle  100 . The problem is to compute which digital waveform needs to be transferred to the D/A converter  120  by the digital computer  118  for “STOP” to be audible inside the vehicle. 
     The transmission of the signal, e (t), through the system to generate v (t) is a nonlinear process. The ultrasonic wave compresses and decompresses the solid as it propagates through the surface of the solid. The stress-to-strain relationship of the solid will in general be nonlinear. A technique for compensating for the distortions introduced by the nonlinear interactions is disclosed by Singhal et al in U.S. Pat. No. 4,603,408, incorporated herein by reference. The synthesis of the transmitted waveform works with linear prefiltering of the waveform. 
     A procedure for the linear prefiltering is given in FIG.  6 . One begins with the desired audio waveform: V d (t). Then to begin the preprocessing procedure the starting input waveform, e(t) is computed: 
     
       
           e ( t )=[∫∫ V   d ( t ) dtdt]   1/2   Equation 3  
       
     
     The resulting input waveform, e o (t), is used to drive the sound projector  126  in the test range depicted in FIG.  4 . The audible sound  104 , v o  (t), received by the microphone  102  is then used to compute an improved wave form, e 1 (t). The received wave form, v o (t), is transformed to the frequency domain using the Fast Fourier Transform (FFT) algorithm: 
     
       
           V   o (ω)= FFT[v   o ( t )]  Equation 4  
       
     
     Similarly the input wave form, e o (t), is transformed with the FFT. 
     
       
           E   o (ω)= FFT[e   o ( t )]  Equation 5  
       
     
     Then the complex transfer function, H o (ω), is computed: 
     
       
           H   o (ω)= V   o (ω)/ E   o (ω)  Equation 6  
       
     
     The complex transfer function H o (ω) is used to compute the frequency-domain representation of the improved waveform: 
     
       
           E   1 (ω)= V   o (ω)/ H   o (ω)  Equation 7  
       
     
     The frequency domain version is then transformed to the time domain e 1 (t) with the Inverse FFT (IFFT): 
     
       
           e   1 ( t )=[ IFFT*E   1 (ω)] 1/2   Equation 8  
       
     
     The square root is taken to obtain the improved transmitted wave from, e 1 (t). (The square root is needed because of the squaring that takes place from the nonlinearities as modeled by Equation 2.) The preprocessing procedure then continues by inputting e 1 (t) and measuring v 1 (t). 
     The received audio signal, v 1 (t), is then transformed to V 1 (ω), the frequency domain equivalent: 
     
       
           V   1 (ω)= FFT[v   1 ( t )]  Equation 9  
       
     
     V 1 (ω) and E 1 (ω) then serve as the inputs to a recursive averaging operation: 
     
       
           H   1 (ω)= H   o (ω)+(1 /N )[{ V   1 (ω)/ E   1 (ω)}− H   o (ω)]  Equation 10  
       
     
     The recursive averaging process of Equation 10 yields an improved transfer function H 1 (ω). Note that in Equation 10, H o (ω) is subtracted from H 1 (ω). (H 1 (ω) is computed from the ratio of V 1 (ω) and E 1 (ω) in Equation 10.) After a few iterations, the latest update to the transfer function will be very similar to the previous version and thus the recursive averaging procedure will cease to change the values. The procedure then uses the desired frequency-domain spectrum and the latest estimate of the transfer function to compute an improved frequency spectrum: 
     
       
           E   1 (ω)= V   o (ω)/ H   1 (ω)  Equation 11  
       
     
     Then the IFFT and square root operations are carried out: 
     
       
           e   2 ( t )=[ IFFT*E   1 (ω)] 1/2   Equation 12  
       
     
     in the same way as in Equation 8. The procedure then continues by inputting e 2 (t) and measuring v 2 (t). This procedure can be iterated until convergence is reached. 
     For example, suppose the utterance “STOP!” was needed. The test range depicted in FIG. 4 would be setup and “STOP” will be processed with the procedure of FIG. 6 to obtain a version that will yield an intelligible “STOP” inside of the vehicle. It may be necessary to carry out the preprocessing of FIG. 6 for different ranges, type of vehicle, angle of incidence, atmospheric conditions, and amplitudes of signals due to the nonlinearity of the system. The preprocessed messages are stored in the digital computer  128  of the ultrasonic sound projection system  2 . Examples of other preprocessed messages include reduce speed or change course. 
     Returning now to FIG. 2, the block diagram illustrates an embodiment of the invention suitable for mounting on a sign or signal. A microwave radar transceiver  130  monitors the range of approaching vehicles by processing the radar returns  135  with the digital computer  132 . Though the range sensing system  10  is preferably microwave, any conventional radar system, including radio, laser, and acoustic, is acceptable. An analysis of the time histories of the approaching vehicle&#39;s range as well as an measurement of the approaching vehicle&#39;s radar cross section are input to a computer program, as illustrated in FIG. 7, executable by the digital computer  132 . The flow chart in FIG. 7 sets forth the mode of operation in words. The chart describes the control of the launching of available lights and sounds at unresponsive drivers. If the unsafe driver fails to respond to the lights or sounds, cross traffic is warned of the danger and police ticketing cameras can be triggered. The digital computer  132  then passes information such as: the number of the desired message, the range of the vehicle and the type of vehicle to the digital computer  128 . Digital computer  128  then selects the requested message from its set of stored preprocessed digital waveforms that are appropriate for the range and type of vehicle and transfer them to the D/A converter  134 . The analog signal output of the D/A converter  134  is then amplified by the power amplifier  136  and used to drive at least one sound projector  138 . It should be noted that computers  128  and  132  could be incorporated into a single computer (not shown). 
     A sound projector  138  can be electrostatic or piezoelectric thin sheets mounted directly on the face of a sign  142 , as illustrated in FIG. 8, or, as illustrated in FIG. 9, a 5×7 array of electrostatic or piezoelectric sheets. Each of the thirty-five individual sheets of the 5×7 array is a functioning speaker  154 . The individual speakers  154  are used as a phased array  152 . The resulting ultrasound waves (not shown) emitted from the individual speakers  154  are steered by controlling the phase between speakers with phase-shifter  153 . FIG. 10 illustrates a multiple-wave sound projector  182  steering, by conventional means, its waves  184  through an angle θ  180  so the waves  184  converge on the windshield  178  of an approaching vehicle  186 . Now returning to FIG. 9, the phase shifters  153  are driven by a control signal  151  generated by second digital computer  128 . The output of the phase shifters  153  drives the speakers  154  using power amplifiers  155 . The amount of phase shift introduced by the phase shifters  153  is under the control of the digital computer  128  in FIG.  2 . The digital computer  128  receives automatically the subject vehicle coordinates from the range sensing system  10  via the computer  132 , and calculates the phase shift for each of the individual speakers  154  to focus the wave upon the windshield of the moving vehicle. The preferred embodiment utilizes one amplifier and one phase shifter per speaker. In this case, there would be thirty-five phase shifters and amplifiers. The array of speakers can be implemented in several configurations not just the rectangular configuration illustrated in FIG.  9 . Circularly-shaped arrays and polygon-shaped arrays are also effective for phased-arrays. 
     The generation of audible sound in open air as opposed to projecting the sound into the interior of a vehicle can facilitate, as illustrated in FIG. 11, a blind pedestrian  78  crossing a roadway  72 . The ultrasound source  82  is directing a warning to the pedestrian  78  whose location is determined by the radar sensor  80 . The objective is to generate the audible message  76  only in the vicinity of the pedestrian  78 . This can be accomplished with the embodiment of the present invention depicted in FIGS. 12,  13 ,  14 , and  15 . 
     As illustrated in FIG. 12, the ultrasound projector  159  is made up of a number of phased arrays  161 . Each phased array is steerable and its wave can be moved in azimuth and elevation, as discussed above. As illustrated in FIG. 13, the projector  156  shows the individual waves  160  aimed so that they converge at point  162  which is located at a range of r 1  from the sound projector  156 . The convergence of multiple waves  160  whose waveforms are in phase in one region increases the intensity of the pressure of the ultrasound in that region. The intense sound drives the air into its nonlinear mode of behavior as given by Equation 2. The nonlinear behavior demodulates the ultrasonic wave  160  and generates an audible secondary sound  164  emanating from the region located at a range of r 1 . 
     In FIG. 14, a sound projector  168  makes use of its individual phased arrays  170  to aim its waves  172  so that they converge at point  166  located at a range  176  of r 2  from the sound projector  168 . This technique of moving the region of wave convergence permits the secondary source of the audible sound to be moved back and forth from the sound projector to address pedestrians at different locations in the cross walk. The different regions could receive different messages. For example, the pedestrian is being asked to return to the curb from which she came. A pedestrian close to the sound projector might be told to quickly mount the curb, as traffic would soon restart. 
     FIG. 15 illustrates the convergence of multiple sound waves emanating from a projector  178  on which are mounted phased arrays  180 , 182 , 184  and  186 . The waves  189  and  190  converge on point  192 . The dimensions of the phased array can be quite small since the ultrasound will typically have wavelengths in the range of a few millimeters. 
     Now returning to FIG. 1, an ultrasound sound projector system  2  is installed on a traffic light signal  4 . A narrow ultrasonic wave  6  is focused on an approaching vehicle  8 . The traffic light  4  is also outfitted with a vehicle range sensing means  10  such as a radar sensor. The range of the approaching vehicle  8  is detected by the radar range sensor&#39;s wave  12 . The range measurement is used to set the parameters of the ultrasonic sound projector system  2  and steer its ultrasound wave  6 . An example of a decision process for a system with visual and audible warning means as well as a police-ticketing camera is diagramed in FIG.  7 . The ultrasonic wave  6  is demodulated by the nonlinearities of the windshield  14  generating an audible message  16  inside the approaching unequipped vehicle  8 . Though a single approaching vehicle is illustrated as being detected by the sensing means, it is within the contemplation of the invention that any vehicle within the line of sight of the sensing means, whether it is the first, second or third vehicle in line from the sensing means, is detectable for the purposes of determining safety at a preselected location. 
     An alternative application, FIG. 16, illustrates a system  1  mounted on a traffic warning sign  18 . The modulated-ultrasound projector  20  is mounted on the face of the sign  18 . An ultrasound wave  22  is focused on the windshield  24  of the approaching vehicle  26 . An audible message  28  is generated inside of the vehicle  26  by demodulation of the ultrasonic wave  22  as it interacts with nonlinearities of the windshield  24 . The approach of the vehicle  26  is sensed by a magnetic loop detector  30  or the like implanted in the roadway  32 . A radar system, television camera or other range sensing means could also be used. 
     Another alternative application, FIG. 17, illustrates the mounting of an ultrasound projector  35  on the rear of a land vehicle  27  in order to communicate with a trailing vehicle  33  via an ultrasonic wave  29 . Communications can be automatically issued by a radar system  30  that detects the distance to and approach speed of the trailing vehicle  33  with, preferably, microwaves  32  or other waves. An audible message  32  is generated inside of the vehicle  33  by demodulation of the ultrasonic wave  29  as it interacts with nonlinearities of the windshield  31 . 
     Additionally, the ultrasound projector illustrated in FIG. 17 can be place in the front of a vehicle (not shown), such as a police car, to transmit an audible message to a leading vehicle or an approaching vehicle. 
     Additionally, the ultrasound projector illustrated in FIG. 17 can also be place in the front and rear of a vehicle (not shown), such as a delivery truck, to transmit an audible message at a preselected range to warn pedestrians and other vehicles of the approaching vehicle, where the view of the vehicle is obstructed by buildings, trees, shrubs, or other vehicles. The range of the demodulated audible message is a function of the speed of the vehicle and the safe stopping distance of the vehicle at the speed of the vehicle plus an additional distance as a safety margin. 
     Yet another application, FIG. 18, illustrates the ultrasound wave  34  being steered using either conventional mechanical means or electronic phased-array techniques. The steering commands come from the computer (not shown) analyzing the radar returns  35  or data from another range sensing system. 
     A further application, FIG. 19, illustrates the ultrasonic sound projection system  36  at a location remote from the intersection traffic signal  38 . The approaching vehicle  42  is monitored by a radar wave  40 . A message is communicated to the remote ultrasonic sound projection system  36  that transmits an ultrasonic wave  44  against the vehicle  42  side, rear or front glass. An audible message is generated inside of the vehicle  42  by the nonlinear interaction of the ultrasound wave  44  with the window of vehicle  42 . 
     FIG. 20 illustrates a dangerously approaching vehicle  46  whose progress is monitored by a radar-type range sensor&#39;s wave  48  emanating from the traffic light  49 . An analysis of the time history of the approaching vehicle&#39;s range is carried out by a conventional digital computer running the programming flow chart of FIG.  7 . This analysis shows that it is unlikely that vehicle  46  will stop before the light  50  at the intersection changes to red. A warning carried by an ultrasound wave  54  is then issued to a vehicle  52  that might enter the intersection and be in the path of the dangerously approaching vehicle  46 . 
     FIG. 21 shows a sound generator  58  in use by a public safety official  56  to communicate with one vehicle  68  among many. An example would be a multilane toll plaza or a large parking lot. In this case the megaphone-like sound generator  58  consists of the sound generating array  60 , a range and direction sensor (which maybe a microwave radar and/or TV camera)  62  and a microphone  64  for the user  56  to speak into. The measurements of the range sensor  62  are used to set the preprocessing parameters of the ultrasonic wave  69  so that audible sound  66  is generated inside the vehicle  68  by the ultrasonic wave  69  being demodulated by the windshield  70 . 
     FIG. 22 shows a public safety application of the system  1 . An official uses a megaphone device  57  to transmit an ultrasound wave  61  onto a window  63  of the building  59 . An audible warning message is generated inside the building by the interaction of the ultrasound wave  61  and the window  63 . 
     Returning to FIG. 11, the system  1  in use at a pedestrian crossing  72  whose crossing control light  73  is outfitted to assist, in particular, blind pedestrians  78 . In this case the ultrasonic wave  74  interacts with the air and generates the audible sounds  76  near the blind pedestrian  78  using information from a distance and angle sensor  80  to set the parameters of the sound transmitter  82 . An interesting characteristic of this embodiment is that the sound  76  is generated in the vicinity of the pedestrian  78  and not closer to the sound projector  82 . This is accomplished by focusing multiple waves of sound on the targeted region as illustrated in FIGS. 14 and 15. 
     There are other embodiments, such as controlling a crowd, when sound generation in the air can be used. 
     Yet another application is illustrated in FIG. 23, where waves  89  of modulated ultrasound are projected by sensors  75  and  77  such that a pathway  79 , or channel, is defined between the waves. The pathway  79  could include, but is not limited to, use by pedestrians, watercraft or land vehicles. Exiting the pathway  79  and entering one of the waves  89  results in a message  85 ,  87  directing the pedestrian  81  or vehicle  83  back in to the pathway  79  will be transmitted to the intruding object  81 ,  83 . The messages  85  and  87  could be transmitted after a radar-type scanner has detected the intrusion of a wave  89 , or the wave  89  can continuously transmit instruction for returning to the pathway  79  which will be heard whenever the very narrow waves  89  have been entered. 
     Now returning to FIG. 10, an additional feature to the present invention is a microphone  188  that monitors the transmissions and relays characteristics of the transmitted wave  184  back to the transmitting system  182 . Should rain, snow, blowing sound, fog or other substances change the nonlinear properties of the air as described by Equation 2 or otherwise scatter the ultasonic waves  184 , the transmitting system  182  would use the detected changes to modify the parameters of the transmission such as transmitter power, carrier frequency, degree of modulation and preprocessing filtering to compensate for the effects of the substances that have entered the path of the wave  184 . 
     Yet another application of the present invention is aviation ground and air traffic control. Aircraft taxiing to and from the terminal, runway, and maintenance hanger can be contacted by the control tower (not shown) or specially equipped aircraft (not shown) with greater speed and accuracy than the current reliance on radio transmission and reception. In-flight near misses will be eliminated with aircraft equipped (not shown) with the present invention. Audio communication in the cockpit will no longer rely on the radio being turned on or being tuned to the correct frequency. 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.