Elwood NORRIS -- Parametric Sound Gun **![](0logo.gif) [rexresearch.com](../index.htm)** --- **Elwood NORRIS** **Parametric / HyperSonic Projector** --- ![](norris.jpg) ![](woodynorris.jpg) --- **ABCNEWS.com** July 16 **Sound and Fury ~ Sonic Bullets to Be Acoustic Weapon of the Future** **by** **Judy Muller** ***A new technology emits so-called "sonic bullets" along a narrow, intense beam up to 145 decibels, 50 times the human threshold of pain.*** Anyone who has seen Tom Cruise fire his state-of-the-art sound wave gun at his pursuers in Minority Report no doubt assumes it is a weapon from the arsenal of science fiction. But such a weapon, or at least a less-glamorous version, is scientific fact. **Woody (Elwood) Norris**, the CEO of **American Technology Corporation** and a pioneer in ultrasound technology, has developed a non-lethal acoustic weapon that stops people in their tracks. "[For] most people," said Norris, "even if they plug their ears, it will produce the equivalent of an instant migraine. Some people, it will knock them on their knees." The device emits so-called "sonic bullets" along a narrow, intense beam up to 145 decibels, 50 times the human threshold of pain. It usually doesn't take that much to stop someone, as we learned in a demonstration in the company parking lot. The acoustic "weapon," in the demonstration model, looks like a huge stereo speaker, except this one sports urban camouflage. The operator chooses one of many annoying sounds in the computer in this case, the high pitched wail of a baby, played backwards and aims it at us. At 110 decibels, we were forced to walk out of the beam's path, our ears ringing. Had we stayed longer, Norris said our skulls would literally start to vibrate. Police departments and the Pentagon are flocking to Norris' headquarters in San Diego to see this revolutionary technology for themselves. The problem with past attempts to make an acoustic weapon is that sound traveled in every direction, affecting the operator, as well. Norris' narrow ultrasound beam takes care of that problem, meaning police could use it to subdue suspects or quell riots, without hurting bystanders or the operator, because the sound is directional. "Tear gas lingers long after you've fired off the canisters," said Norris. "This, you switch it off and it's gone. And the damage is only temporary." **Army to Use as Sonic Cannons** The U.S. Army has already ordered its own prototype of the non-lethal acoustic weapon. It will be packaged in a camouflaged cylinder and either be handheld or mounted on an armored car. Two security experts who were at the company on behalf of the Defense Department said it would be terrific for repelling suicide bombers and for rousting terrorists from their hideouts. Because the sound ricochets in tight, enclosed areas, said retired Marine Col. Peter Dotto, it would make it very uncomfortable for al Qaeda terrorists to stay in Afghan caves. "They would have to come out," said Dotto, "and they probably would come out with their hands over their ears so they would be very easy to subdue at that point." **Practical Uses, Too** Not all the applications of this new technology are pain-inducing. Norris has invented a related acoustic device called the Hypersonic Sound System. Only when he turns the speaker in your direction, do you hear the message. For instance, liquid being poured over ice was the sound requested by a soda company to inspire people within earshot of a vending machine to quench their thirst. Norris tried out the acoustic beam at a mall near his office and passers-by all stopped to listen when the sound was aimed at them. "That is absolutely amazing," said one woman, "it sounds like the sound is inside your head." There are dozens of potential commercial uses, from shooing away pesky birds (geese off of golf courses, for example) to directing television sound so it doesn't disturb a sleeping spouse. Whether friend or "friendly fire," this new technology is likely to affect almost every aspect of our lives, in ways we can only begin to imagine. --- [**http://www.parametricsound.com/Technology.php**](http://www.parametricsound.com/Technology.php) **HyperSound Technology** ![](transmitsound.jpg) ![](technologyImg.jpg) **HyperSound** **3D and Binaural Sound** ***Discover the remarkable novel way to transmit sound*** Since the 1960's there have been many efforts to control and direct sound into a narrow band of space so that audio messages could be transmitted to a targeted area, such as emergency exits. Following years of intense R&D, the founder of Parametric Sound, Elwood "Woody" Norris has innovated the complex technology called the HyperSound System (HSS). Instead of using a vibrating membrane like traditional speakers, HSS electronically converts audible tones into ultrasonic waves transmitted at frequencies beyond human hearing. These audible tones are projected along an air beam of silent ultrasound energy. This sound (actually created in the air) can be directed to just about any desired point in the listening environment. The HSS parametric sound beam is a major breakthrough in sound that is available exclusively through Parametric Sound. HSS is compatible with any media input, but unlike other forms of sound reproduction, it can focus sound where you want it and nowhere else. transmit sound Due to its incredible directionality and 3D effects, HSS technology has numerous applications in the following main categories: Commercial: informational and marketing uses Consumer: 3D sound for home video and audio And new applications are being discovered as the benefits of directed sound become more widely recognized. Put sound where you want it and nowhere else. A HyperSound System (HSS) is similar to a flashlight. If you project the HSS emitter device directly, you hear the sound formed in the column of ultrasonic energy just like light from a flashlight. However, when a listener stands to the side of an HSS emitter, you hear only the sound that is reflected from a boundary surface, just like the light of a flashlight when it is reflected off a wall. No sound is created on the surface of the emitter; the listener can only hear sound from being in the beam or that is reflected from the wall or other surface. Never in the history of sound have you been able to create this degree of controlled directionality to audible sound. --- [**http://www.youtube.com/watch?v=wBEyDp5v4HY**](http://www.youtube.com/watch?v=wBEyDp5v4HY) **Parametric Sound Exhibits the HSS-300- Hyper Sound ... - YouTube** --- [**http://en.wikipedia.org/wiki/Sound\_from\_ultrasound**](http://en.wikipedia.org/wiki/Sound_from_ultrasound) **Sound from ultrasound** **1 Parametric array****2 Applications****2.1 Military****2.2 Commercial advertising****2.3 Personal Audio****3 History****4 Products****4.1 Audio Spotlight****4.2 HyperSonic Sound****4.3 Mitsubishi Electric Engineering Corporation****4.4 AudioBeam****5 Literature survey****5.1 Experimental ultrasonic nonlinear acoustics****5.2 Theoretical ultrasonic nonlinear acoustics****6 Modulation scheme****7 Attenuation of ultrasound in air****8 Safe use of high-intensity ultrasound****9 See also****10 Further resources****11 References****12 External links** Sound from ultrasound is the name given here to the generation of audible sound from modulated ultrasound without using an active receiver. This happens when the modulated ultrasound passes through a nonlinear medium which acts, intentionally or unintentionally, as a demodulator. **Parametric array** Since the early 1960s, researchers have been experimenting with creating directive low-frequency sound from nonlinear interaction of an aimed beam of ultrasound waves produced by a parametric array using heterodyning. Ultrasound has much shorter wavelengths than audible sound, so that it propagates in a much narrower beam than any normal loudspeaker system using audio frequencies. The first modern device was created in 1998,[1] and is now known by the trademark name "Audio Spotlight", a term first coined in 1983 by the Japanese researchers[2] who abandoned the technology as infeasible in the mid 1980s. A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough, at 100 to 110 dBSPL, to substantially change the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly and extracts the modulation signal from the ultrasound, resulting in sound that can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes. This technology allows a beam of sound to be projected over a long distance to be heard only in a small well-defined area;[citation needed] a listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam. There are some limitations with this approach. Anything that interrupts the beam will prevent the ultrasound from propagating, like interrupting a spotlight's beam. For this reason, most systems are mounted overhead, like lighting. **Applications** **Military** There has been speculation about military sonic weapons that emit highly-directional high-intensity sound; however, these devices do not use ultrasound, although sometimes thought to do so. Wikileaks has published technical specifications of such sound weapons.[3] **Commercial advertising** A sound signal can be aimed so that only a particular passer-by, or somebody very close, can hear it. In commercial applications, it can target sound to a single person without the peripheral sound and related noise of a loudspeaker. **Personal Audio** It can be used for personal Audio, either to have sounds only one person, or group wants to listen to. The navigation instructions for example are only interesting for the driver in a car, not for the passengers. Another possibility are future applications for true stereo sound, where one ear doesn't hear what the other is hearing. [4] **History** This technology was originally developed by the US Navy and Soviet Navy for underwater sonar in the mid-1960s, and was briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 (105th AES Conv, Preprint 4853, 1998) fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker. **Products** As of 2012 there were known to be four devices which have been marketed that use ultrasound to create an audible beam of sound. **Audio Spotlight** F. Joseph Pompei of MIT developed technology he calls the "Audio Spotlight",[5] and made it commercially available in 2000 by his company Holosonics, which according to their website claims to have sold "thousands" of their "Audio Spotlight" systems. Disney was amongst the first major corporations to adopt it for use at the Epcot Center, and many other application examples are shown on the Holosonics website.[6] **HyperSonic Sound** Elwood "Woody" Norris, founder and Chairman of American Technology Corporation (ATC), announced he had successfully created a device which achieved ultrasound transmission of sound in 1996.[7] ATC named and trademarked their device as "HyperSonic Sound" (HSS). In December 1997, HSS was one of the items in the Best of What's New issue of Popular Science.[8] In December 2002, Popular Science named HyperSonic Sound the best invention of 2002.[citation needed] Norris received the 2005 Lemelson-MIT Prize for his invention of a "hypersonic sound".[9] ATC (now named LRAD Corporation) spun off the technology to Parametric Sound Corporation in September 2010 to focus on their Long Range Acoustic Device products (LRAD), according to their quarterly reports, press releases and executive statements.[10][11] **Mitsubishi Electric Engineering Corporation** Mitsubishi apparently offers a sound from ultrasound product named the "MSP-50E"[12] but commercial availability has not been confirmed. **AudioBeam** German audio company Sennheiser Electronic once listed their "AudioBeam" product for about $4,500.[13] There is no indication that the product has been used in any public applications. The product has since been discontinued.[14] **Literature survey** The first experimental systems were built over 30 years ago, although these first versions only played simple tones. It was not until much later (see above) that the systems were built for practical listening use. **Experimental ultrasonic nonlinear acoustics** A chronological summary of the experimental approaches taken to examine Audio Spotlight systems in the past will be presented here. At the turn of the millennium working versions of an Audio Spotlight capable of reproducing speech and music could be bought from Holosonics, a company founded on Dr. Pompei's work in the MIT Media Lab.[15] Related topics were researched almost 40 years earlier in the context of underwater acoustics. The first article[16] consisted of a theoretical formulation of the half pressure angle of the demodulated signal. The second article[17] provided an experimental comparison to the theoretical predictions. Both articles were supported by the U.S. Office of Naval Research, specifically for the use of the phenomenon for underwater sonar pulses. The goal of these systems was not high directivity per se, but rather higher usable bandwidth of a typically band-limited transducer. The 1970s saw some activity in experimental airborne systems, both in air[18] and underwater.[19] Again supported by the U.S. Office of Naval Research, the primary aim of the underwater experiments was to determine the range limitations of sonar pulse propagation due to nonlinear distortion. The airborne experiments were aimed at recording quantitative data about the directivity and propagation loss of both the ultrasonic carrier and demodulated waves, rather than developing the capability to reproduce an audio signal. In 1983 the idea was again revisited experimentally[2] but this time with the firm intent to analyze the use of the system in air to form a more complex base band signal in a highly directional manner. The signal processing used to achieve this was simple DSB-AM with no precompensation, and because of the lack of precompensation applied to the input signal, the THD Total harmonic distortion levels of this system would have probably been satisfactory for speech reproduction, but prohibitive for the reproduction of music. An interesting feature of the experimental set up used in[2] was the use of 547 ultrasonic transducers to produce a 40 kHz ultrasonic sound source of over 130db at 4m, which would demand significant safety considerations.[20][21] Even though this experiment clearly demonstrated the potential to reproduce audio signals using an ultrasonic system, it also showed that the system suffered from heavy distortion, especially when no precompensation was used. **Theoretical ultrasonic nonlinear acoustics** The equations that govern nonlinear acoustics are quite complicated[22][23] and unfortunately they do not have general analytical solutions. They usually require the use of a computer simulation.[24] However, as early as 1965, Berktay performed an analysis[25] under some simplifying assumptions that allowed the demodulated SPL to be written in terms of the amplitude modulated ultrasonic carrier wave pressure Pc and various physical parameters. Note that the demodulation process is extremely lossy, with a minimum loss in the order of 60dB from the ultrasonic SPL to the audible wave SPL. A precompensation scheme can be based from Berktay's expression, shown in Equation 1, by taking the square root of the base band signal envelope E and then integrating twice to invert the effect of the double partial time derivative. The analogue electronic circuit equivalents of a square root function is simply an op-amp with feedback, and an equalizer is analogous to an integration function. However these topic areas lie outside the scope of this project. p\_2(x,t) = K \cdot P\_c^2 \cdot \frac{\partial^2}{\partial t^2} E^2(x,t) Where p\_2(x,t) =\, Audible secondary pressure wave K = \, misc. physical parameters P\_c = \, SPL of the ultrasonic carrier wave E(x,t) = \, Envelope function (such as DSB-AM) This equation says that the audible demodulated ultrasonic pressure wave (output signal) is proportional to the twice differentiated, squared version of the envelope function (input signal). Precompensation refers to the trick of anticipating these transforms and applying the inverse transforms on the input, hoping that the output is then closer to the untransformed input. By the 1990s, it was well known that the Audio Spotlight could work but suffered from heavy distortion. It was also known that the precompensation schemes placed an added demand on the frequency response of the ultrasonic transducers. In effect the transducers needed to keep up with what the digital precompensation demanded of them, namely a broader frequency response. In 1998 the negative effects on THD of an insufficiently broad frequency response of the ultrasonic transducers was quantified[26] with computer simulations by using a precompensation scheme based on Berktay's expression. In 1999 Pompei's article[15] discussed how a new prototype transducer met the increased frequency response demands placed on the ultrasonic transducers by the precompensation scheme, which was once again based on Berktay's expression. In addition impressive reductions in the THD of the output when the precompensation scheme was employed were graphed against the case of using no precompensation. In summary, the technology that originated with underwater sonar 40 years ago has been made practical for reproduction of audible sound in air by Pompei's paper and device, which, according to his AES paper (1998), demonstrated that distortion had been reduced to levels comparable to traditional loudspeaker systems. **Modulation scheme** The nonlinear interaction mixes ultrasonic tones in air to produce sum and difference frequencies. A DSB-AM modulation scheme with an appropriately large baseband DC offset, to produce the demodulating tone superimposed on the modulated audio spectra, is one way to generate the signal that encodes the desired baseband audio spectra. This technique suffers from extremely heavy distortion as not only the demodulating tone interferes, but also all other frequencies present interfere with one another. The modulated spectra is convolved with itself, doubling its bandwidth by the length property of the convolution. The baseband distortion in the bandwidth of the original audio spectra is inversely proportional to the magnitude of the DC offset (demodulation tone) superimposed on the signal. A larger tone results in less distortion. Further distortion is introduced by the second order differentiation property of the demodulation process. The result is a multiplication of the desired signal by the function -?2 in frequency. This distortion may be equalized out with the use of preemphasis filtering. By the time convolution property of the fourier transform, multiplication in the time domain is a convolution in the frequency domain. Convolution between a baseband signal and a unity gain pure carrier frequency shifts the baseband spectra in frequency and halves its magnitude, though no energy is lost. One half-scale copy of the replica resides on each half of the frequency axis. This is consistent with Parseval's theorem. The modulation depth m is a convenient experimental parameter when assessing the total harmonic distortion in the demodulated signal. It is inversely proportional to the magnitude of the DC offset. THD increases proportionally with m12. These distorting effects may be better mitigated by using another modulation scheme that takes advantage of the differential squaring device nature of the nonlinear acoustic effect. Modulation of the second integral of the square root of the desired baseband audio signal, without adding a DC offset, results in convolution in frequency of the modulated square-root spectra, half the bandwidth of the original signal, with itself due to the nonlinear channel effects. This convolution in frequency is a multiplication in time of the signal by itself, or a squaring. This again doubles the bandwidth of the spectra, reproducing the second time integral of the input audio spectra. The double integration corrects for the -?2 filtering characteristic associated with the nonlinear acoustic effect. This recovers the scaled original spectra at baseband. The harmonic distortion process has to do with the high frequency replicas associated with each squaring demodulation, for either modulation scheme. These iteratively demodulate and self-modulate, adding a spectrally smeared out and time exponentiated copy of the original signal to baseband and twice the original center frequency each time, with one iteration corresponding to one traversal of the space between the emitter and target. Only sound with parallel collinear phase velocity vectors interfere to produce this nonlinear effect. Even-numbered iterations will produce their modulation products, baseband and high frequency, as reflected emissions from the target. Odd-numbered iterations will produce their modulation products as reflected emissions off the emitter. This effect still holds when the emitter and the reflector are not parallel, though due to diffraction effects the baseband products of each iteration will originate from a different location each time, with the originating location corresponding to the path of the reflected high frequency self-modulation products. These harmonic copies are largely attenuated by the natural losses at those higher frequencies when propagating through air. **Attenuation of ultrasound in air** The Figure provided in[27] provided an estimation of the attenuation that the ultrasound would suffer as it propagated through air. The figures from this graph correspond to completely linear propagation, and the exact effect of the nonlinear demodulation phenomena on the attenuation of the ultrasonic carrier waves in air was not considered. There is an interesting dependence on humidity. Nevertheless, a 50 kHz wave can be seen to suffer an attenuation level in the order of 1dB per meter at one atmosphere of pressure. **Safe use of high-intensity ultrasound** For the nonlinear effect to occur, relatively high intensity ultrasonics are required. The SPL involved was typically greater than 100dB of ultrasound at a nominal distance of 1m from the face of the ultrasonic transducer.[citation needed] Exposure to more intense ultrasound over 140dB[citation needed] near the audible range (2040 kHz) can lead to a syndrome involving manifestations of nausea, headache, tinnitus, pain, dizziness and fatigue,[21] but this is around 100 times the 100dB level cited above, and is generally not a concern. Dr Joseph Pompei of Audio Spotlight has published data showing that their product generates ultrasonic sound pressure levels around 130 dB (at 60 kHz) measured at 3 meters.[28] The UK's independent Advisory Group on Non-ionising Radiation (AGNIR) produced a 180 page report on the health effects of human exposure to ultrasound and infrasound in 2010. The UK Health Protection Agency (HPA) published their report, which recommended an exposure limit for the general public to airborne ultrasound sound pressure levels (SPL) of 100 dB (at 25 kHz and above).[29] OSHA specifies a safe ceiling value of ultrasound as 145dB SPL exposure at the frequency range used by commercial systems in air, as long as there is no possibility of contact with the transducer surface or coupling medium (i.e. submerged).[30] This is several times the highest levels used by commercial Audio Spotlight systems, so there is a significant margin for safety[citation needed]. In a review of international acceptable exposure limits Howard et al. (2005)[31] noted the general agreement amongst standards organizations, but expressed concern with the decision by United States of Americas Occupational Safety and Health Administration (OSHA) to increase the exposure limit by an additional 30 dB under some conditions (equivalent to a factor of 1000 in intensity[32]). For frequencies of ultrasound from 25 to 50 kHz, a guideline of 110dB has been recommended by Canada, Japan, the USSR, and the International Radiation Protection Agency, and 115dB by Sweden[33] in the late 1970s to early 1980s, but these were primarily based on subjective effects. The more recent OSHA guidelines above are based on ACGIH (American Conference of Governmental Industrial Hygienists) research from 1987. Lawton(2001)[34] reviewed international guidelines for airborne ultrasound in a report published by the United Kingdoms Health and Safety Executive, this included a discussion of the guidelines issued by the American Conference of Governmental Industrial Hygienists (ACGIH), 1988. Lawton states This reviewer believes that the ACGIH has pushed its acceptable exposure limits to the very edge of potentially injurious exposure. The ACGIH document also mentioned the possible need for hearing protection. **Further resources** USS Patent 6778672 filed on 17 August 2004 describes an HSS system for using ultrasound to:- Direct distinct 'in-car entertainment' directly to passengers in different positions. Shape the airwaves in the vehicle to deaden unwanted noises. **References** ^ 105th AES Conv, Preprint 4853, 1998 ^ a b c Yoneyama, Masahide; Jun Ichiroh, Fujimoto (1983). "The audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design". Journal of the Acoustical Society of America 73 (5): 15321536. Bibcode:1983ASAJ...73.1532Y. doi:10.1121/1.389414. ^ "LRAD technical specifications for anti-crowd, anti-pirate sound weapons, 2009". WikiLeaks. September 27, 2009. ^ Norris, Woody (February 2004). "Hypersonic sound and other inventions". TED. Retrieved 07.11.2012. ^ AudioSpotlight web site ^ ABC news 21 August 2006 ^ http://www.parametricsound.com/AboutUs/HistoryandBackground.aspx ^ Corporation, Bonnier (1997-12). Popular Science. ^ "Inventor Wins $500,000 Lemelson-MIT Prize for Revolutionizing Acoustics" (Press release). Massachusetts Institute of Technology. 2004-04-18. Retrieved 2007-11-14. ^ http://www.lradx.com/site/content/view/369/55/ ^ Executive quotes from ATC. ^ (Press release). 2007-07-26. Retrieved 2008-11-23. ^ AudioBeam ^ Audiobeam discontinued ^ a b Pompei, F. Joseph (September 1999). "The use of airborne ultrasonics for generating audible sound beams". Journal of the Audio Engineering Society 47 (9): 726731. ^ Westervelt, P. J. (1963). "Parametric acoustic array". Journal of the Acoustical Society of America 35 (4): 535537. Bibcode:1963ASAJ...35..535W. doi:10.1121/1.1918525. ^ Bellin, J. L. S.; Beyer, R. T. (1962). "Experimental investigation of an end-fire array". Journal of the Acoustical Society of America 34 (8): 10511054. Bibcode:1962ASAJ...34.1051B. doi:10.1121/1.1918243. ^ Mary Beth, Bennett; Blackstock, David T. (1974). "Parametric array in air". Journal of the Acoustical Society of America 57 (3): 562568. Bibcode:1975ASAJ...57..562B. doi:10.1121/1.380484. ^ Muir, T. G.; Willette, J. G. (1972). "Parametric acoustic transmitting arrays". Journal of the Acoustical Society of America 52 (5): 14811486. Bibcode:1972ASAJ...52.1481M. doi:10.1121/1.1913264. ^ http://www.coolmath.com/decibels1.htm. Everyday Sound Pressure Levels. ^ a b http://www.hc-sc.gc.ca/ewh-semt/pubs/radiation/safety-code\_24-securite/index\_e.html Guidelines for the safe use of ultrasound: Part II - Industrial and Commercial applications. Non-Ionizing Radiation Section Bureau of Radiation and Medical Devices Department of National Health and Welfare ^ Jacqueline Naze, Tjotta; Tjotta, Sigve (1980). "Nonlinear interaction of two collinear, spherically spreading sound beams". Journal of the Acoustical Society of America 67 (2): 484490. Bibcode:1980ASAJ...67..484T. doi:10.1121/1.383912. ^ Jacqueline Naze, Tjotta; Tjotta, Sigve (1981). "Nonlinear equations of acoustics, with application to parametric acoustic arrays". Journal of the Acoustical Society of America 69 (6): 16441652. Bibcode:1981ASAJ...69.1644T. doi:10.1121/1.385942. ^ Kurganov, Alexander; Noelle, Sebastian; Petrova, Guergana (2001). "Semidiscrete central-upwind schemes for hyperbolic conservation laws and hamilton-jacobi equations". Society for Industrial and Applied Mathematics Journal on Scientific Computing 23 (3): 707740. doi:10.1137/S1064827500373413. ^ Berktay, H. O. (1965). "Possible exploitation of nonlinear acoustics in underwater transmitting applications". Journal of Sound and Vibration 2 (4): 435461. Bibcode:1965JSV.....2..435B. doi:10.1016/0022-460X(65)90122-7. ^ Kite, Thomas D.; Post, John T.; Hamilton, Mark F. (1998). "Parametric array in air: Distortion reduction by preprocessing". Journal of the Acoustical Society of America 2 (5): 10911092. Bibcode:1998ASAJ..103.2871K. doi:10.1121/1.421645. ^ Bass, H. E.; Sutherland, L. C.; Zuckerwar, A. J.; Blackstock, D. T.; Hester, D. M. (1995). "Atmospheric absorption of sound: Further developments". Journal of the Acoustical Society of America 97 (1): 680683. Bibcode:1995ASAJ...97..680B. doi:10.1121/1.412989. ^ Pompei, F Joseph (Sept 1999). "The Use of Airborne Ultrasonics for Generating Audible Sound Beams". Journal of the Audio Engineering Society 47 (9): pp 728. Fig. 3. Retrieved 19 November 2011. ^ AGNIR (2010). Health Effects of Exposure to Ultrasound and Infrasound. Health Protection Agency, UK. pp. 167170. ^ Noise and Hearing Conservation Technical Manual Chapter: Noise and Health Effects (App I:D) ^ Howard et al. (2005). "A Review of Current Ultrasound Exposure Limits". The J. Occupational Health and Safety of Australia and New Zealand 21 (3): 253257. ^ Leighton, Tim (2007). "What is Ultrasound?". Progress in Biophysics and Molecular Biology 93 (1-3): pp 69. doi:10.1016/j.pbiomolbio.2006.07.026. Retrieved 16 November 2011. ^ Safety Code 24. Guidelines for the Safe Use of Ultrasound: Part II Industrial and Commercial Applications - Guidelines for Safe Use ^ Lawton (2001). Damage to human hearing by airborne sound of very high frequency or ultrasonic frequency. Health & Safety Executive, UK. pp. 910. ISBN ISBN 0 7176 2019 0. --- [**http://www.youtube.com/watch?v=wBEyDp5v4HY**](http://www.youtube.com/watch?v=wBEyDp5v4HY) **Parametric Sound Exhibits the HSS-300- Hyper Sound ... - YouTube** --- **** **Woody Norris' TED Talk** --- **Patents** **WO2006086743****IN-BAND PARAMETRIC SOUND GENERATION SYSTEM** **[ [PDF](WO2006086743A2.pdf) ]**Parametric sound reproduction in high-intensity audio signaling, for example in hailing and warning at relatively large distances, is disclosed in one example by producing a primary audio signal in the audio frequency range, and producing a secondary audio signal in the audio frequency range by modulation of the primary audio signal, wherein the primary signal is chosen to enable an improved effect, for example one of directional reproduction, exploiting greater sensitivity of human hearing, exploiting an efficient or maximum intensity frequency range of a transducer used to reproduce the audio signals, and another parameter effecting distance, intelligibility, or intensity of an audio signal. **US2006233404 [ [PDF](US2006233404A1.pdf) ]****Horn array emitter** A system and method is disclosed for a parametric emitter array with enhanced emitter-to-air acoustic coupling. The system comprises a plate support member having opposing first and second faces separated by an intermediate plate body. The plate body can have a plurality of conduits configured as an array of acoustic horns. Each horn can have a small throat opening at the first face and an intermediate horn section which diverges to a broad mouth opening at the second face. An emitter membrane can be positioned in direct contact with the first face and extending across the small throat openings. The emitter membrane can be biased by (i) applying tension to the emitter membrane extending across the throat openings, (ii) displacing the emitter membrane into a non-planar configuration, and (iii) capturing the emitter membrane at the first face using an adhesive substance. A variable electrical signal can be applied to the emitter membrane for propagation through the intermediate horn section and out the broad mouth opening at the second face. **US8199931 [ [PDF](US8199931B1.pdf) ]****Parametric loudspeaker with improved phase characteristics**A method is disclosed for increasing a parametric output of a parametric loudspeaker system. The method can include the operation of providing multiple ultrasonic frequency emission zones that output signals in a frequency band. The phase relationships of the ultrasonic frequency emission zones can be correlated and controlled to increase phase coherence between each ultrasonic frequency emission zone to maximize parametric output. Correlating and controlling the phase relationships can include offsetting a frequency of a carrier signal applied to each emission zone from a resonant frequency of each emission zone in view of a rate of change of phase of each emission zone in a vicinity of each resonant frequency. Ultrasonic energy from the ultrasonic frequency emission zones can be generated, using the correlated phase relationship to increase the parametric output. **WO2008036321 [ [PDF](WO2008036321A2.pdf) ]****HIGH INTENSITY VEHICLE PROXIMITY ACOUSTICS** An acoustic human and animal behavior modification system (10) that is capable of creating a zone of exclusion immediately adjacent a surface vehicle (12) comprises an array of acoustic transducers (16) disposed on the vehicle in a location not readily seen nor accessible by humans adjacent the vehicle, and is configured to project an acoustic output radially outward in a radial sector at sound pressure levels above the ordinary human pain threshold to motivate animals and humans to move away from a vehicle or change their behavior. **US2005226438 [ [PDF](US2005226438A1.pdf) ]****Parametric ring emitter** A sound emitting device ( 10 ) for providing at least one new sonic or subsonic frequency as a by-product of emitting a waveform of at least two ultrasonic frequencies whose difference in value corresponds to the desired new sonic or subsonic frequency. The device includes a parametric emitting perimeter or plurality of emitter segments ( 13 ) positioned around a central open section ( 15 ). This open section ( 15 ) is structured with a diagonal width greater than a cross-sectional diagonal of the parametric emitting perimeter. An ultrasonic frequency source ( 60 ) and sonic/subsonic frequency generator ( 62 ) arc coupled together to a modulating circuit ( 61 ) for mixing an ultrasonic frequency signal with an electrical signal corresponding to the at least one new sonic or subsonic frequency. The modulator output is coupled to the emitting perimeter ( 64 ) which comprises ultrasonic frequency emitting material for propagating the mixed waveform into air for demodulating the waveform to generate the at least one new sonic or subsonic frequency. **Parametric loudspeaker with electro-acoustical diaphragm transducer****US2005244016****Parametric virtual speaker and surround-sound system****US2004247140****SYSTEM AND METHOD FOR DELIVERING AUDIO-VISUAL CONTENT ALONG A CUSTOMER WAITING LINE****WO2005002199** **Parametric virtual speaker and surround-sound system****US2003215103 (A1)****PLASMA FILTER ANTENNNA SYSTEM****WO03073555****SONIC EMITTER WITH FOAM STATOR****WO03034787****IMPROVED PARAMETRIC SIGNAL PROCESSING AND EMITTER SYSTEMS AND RELATED METHODS****CA2802862** ---