Sound from ultrasound
Encyclopedia
Sound from ultrasound is the name given here to situations when modulated ultrasound
can make its carried signal audible without needing a receiver
set. This happens when the modulated ultrasound passes through anything which behaves nonlinearly and thus acts intentionally or unintentionally as a demodulator.
waves produced by a parametric array
using heterodyning. Ultrasound has wavelengths much smaller than audible sound and thus can be aimed in a much tighter narrow beam than any traditional audible loudspeaker system.
The first modern device was created in 1998, and is now known by the trademark
name "Audio Spotlight", a term first coined in 1983 by the Japanese researchers who abandoned the technology as unfeasible in the mid 1980s.
A transducer can be made to project a narrow beam of modulated ultrasound
that is powerful enough (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. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small well-defined area . 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 criticisms of this approach. Anyone or anything that disrupts the path of the beam will interrupt the progression of the beam, like interrupting the illumination of a spotlight. For this reason, most systems are mounted overhead, like lighting.
" weapon that aims a highly-directed high-intensity sound wave, causing debilitating pain. However, these devices, such as LRAD, are really just high-powered bullhorns, and contrary to popular misconception, do not use ultrasound at all for sound generation, and instead use traditional loudspeaker elements (tweeter
s). This type of loudspeaker
is unrelated to this article. Wikileaks
has published leaked technical specifications for military use of anti-crowd , anti-pirate sound weapons. "Ref"
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.
to create an audible "beam" of sound.
, 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.
. In December 2002, Popular Science
named HyperSonic Sound the best invention of 2002. Norris received the 2005 Lemelson-MIT Prize
for his invention of a "hypersonic sound". 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.
apparently offers a sound from ultrasound product named the "MSP-50E" but commercial availability has not been confirmed.
Electronic once listed their "AudioBeam" product for about $4,500. There is no indication that the product has been used in any public applications. The product has since been discontinued.
Related topics were researched almost 40 years earlier in the context of underwater acoustics.
Both articles were supported by the U.S. Office of Naval Research, specifically for the use of the phenomenon for underwater sonar pulses. It should be noted that 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 and underwater. 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 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 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. 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.
Where
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 with computer simulations by using a precompensation scheme based on Berktay's expression. In 1999 Pompei's article 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.
. 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 -ω² 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 -ω² 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.
, pain, dizziness and fatigue, 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.
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).
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). This is several times the highest levels used by commercial Audio Spotlight systems, so there is a significant margin for safety. In a review of international acceptable exposure limits Howard et al. (2005) noted the general agreement amongst standards organizations, but expressed concern with the decision by United States of America’s 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).
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 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) reviewed international guidelines for airborne ultrasound in a report published by the United Kingdom’s 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”. It should be noted that the ACGIH document also mentioned the possible need for hearing protection.
6778672 filed on 17 August 2004 describes an HSS system for using ultrasound to:-
Modulated ultrasound
Ultrasound can be modulated to carry an audio signal . This is often used to carry messages underwater, in underwater diving communicators, and short-range communication with submarines; the received ultrasound signal is decoded into audible sound by a modulated-ultrasound receiver.-Range...
can make its carried signal audible without needing a receiver
Receiver (modulated ultrasound)
In modulated ultrasound terminology, a receiver is a device that receives a modulated ultrasound signal and decodes it for use as sound, navigational-position information, etc. Its function is somewhat like that of a radio receiver...
set. This happens when the modulated ultrasound passes through anything which behaves nonlinearly and thus acts intentionally or unintentionally as a demodulator.
Parametric array
Researchers since the early 1960s have been experimenting with creating directive low-frequency sound from nonlinear interaction of an aimed beam of ultrasoundUltrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
waves produced by a parametric array
Parametric array
The parametric array is a nonlinear transduction mechanism that generates narrow, nearly sidelobe free beams of low frequency sound, through the mixing and interaction of high frequency sound waves, effectively overcoming the diffraction limit associated with linear acoustics...
using heterodyning. Ultrasound has wavelengths much smaller than audible sound and thus can be aimed in a much tighter narrow beam than any traditional audible loudspeaker system.
The first modern device was created in 1998, and is now known by the trademark
Trademark
A trademark, trade mark, or trade-mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or...
name "Audio Spotlight", a term first coined in 1983 by the Japanese researchers who abandoned the technology as unfeasible in the mid 1980s.
A transducer can be made to project a narrow beam of modulated ultrasound
Modulated ultrasound
Ultrasound can be modulated to carry an audio signal . This is often used to carry messages underwater, in underwater diving communicators, and short-range communication with submarines; the received ultrasound signal is decoded into audible sound by a modulated-ultrasound receiver.-Range...
that is powerful enough (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. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small well-defined area . 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 criticisms of this approach. Anyone or anything that disrupts the path of the beam will interrupt the progression of the beam, like interrupting the illumination of a spotlight. For this reason, most systems are mounted overhead, like lighting.
Commercial advertising
To aim a sound signal at a particular passer-by without everybody in the area hearing it. In commercial applications, it can target sound to a single person without the peripheral sound and related noise that a loudspeaker emits.Military and commercial security applications
Military applications have been speculated, such as a "sonic bulletSonic bullet
The phrase sonic bullet occurs in more than one context :* For weapons that use sound as a basis to injure people, see Sonic weaponry;* For a music CD maker, See Sonic Bullet;* For the album by The Bambi Molesters, see Sonic Bullets: 13 From The Hip....
" weapon that aims a highly-directed high-intensity sound wave, causing debilitating pain. However, these devices, such as LRAD, are really just high-powered bullhorns, and contrary to popular misconception, do not use ultrasound at all for sound generation, and instead use traditional loudspeaker elements (tweeter
Tweeter
A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...
s). This type of loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
is unrelated to this article. Wikileaks
Wikileaks
WikiLeaks is an international self-described not-for-profit organisation that publishes submissions of private, secret, and classified media from anonymous news sources, news leaks, and whistleblowers. Its website, launched in 2006 under The Sunshine Press organisation, claimed a database of more...
has published leaked technical specifications for military use of anti-crowd , anti-pirate sound weapons. "Ref"
History
This technology was originally developed by the US Navy and Soviet NavySoviet Navy
The Soviet Navy was the naval arm of the Soviet Armed Forces. Often referred to as the Red Fleet, the Soviet Navy would have played an instrumental role in a Warsaw Pact war with NATO, where it would have attempted to prevent naval convoys from bringing reinforcements across the Atlantic Ocean...
for underwater sonar
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
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
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
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
There are currently four known devices which have been marketed that use ultrasoundUltrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
to create an audible "beam" of sound.
Audio Spotlight
F. Joseph Pompei of MIT developed technology he calls the "Audio Spotlight", and made it commercially available in 2000 by his company HolosonicsHolosonics
Holosonics is a USA firm which is developing and manufacturing directional loudspeaker technology called the Audio Spotlight. The technology is based upon the sound from ultrasound effect. The company's founder is Dr. F. Joseph Pompei, an M.I.T. PhD who at age 16 became the youngest acoustic...
, 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.
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. ATC named and trademarked their device as "HyperSonic Sound" (HSS). In February 1998, HSS was named the Best of What's New for 1997 by readers of Popular SciencePopular Science
Popular Science is an American monthly magazine founded in 1872 carrying articles for the general reader on science and technology subjects. Popular Science has won over 58 awards, including the ASME awards for its journalistic excellence in both 2003 and 2004...
. In December 2002, Popular Science
Popular Science
Popular Science is an American monthly magazine founded in 1872 carrying articles for the general reader on science and technology subjects. Popular Science has won over 58 awards, including the ASME awards for its journalistic excellence in both 2003 and 2004...
named HyperSonic Sound the best invention of 2002. Norris received the 2005 Lemelson-MIT Prize
Lemelson-MIT Prize
The Lemelson Foundation awards several prizes yearly to inventors in United States. The largest is the Lemelson-MIT Prize which was endowed in 1994 by Jerome H. Lemelson, and is administered through the Massachusetts Institute of Technology...
for his invention of a "hypersonic sound". 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.
Mitsubishi Electric Engineering Corporation
MitsubishiMitsubishi
The Mitsubishi Group , Mitsubishi Group of Companies, or Mitsubishi Companies is a Japanese multinational conglomerate company that consists of a range of autonomous businesses which share the Mitsubishi brand, trademark and legacy...
apparently offers a sound from ultrasound product named the "MSP-50E" but commercial availability has not been confirmed.
AudioBeam
The German audio company SennheiserSennheiser
Sennheiser electronic GmbH & Co. KG is a private German audio company specializing in the design and production of a wide range of both consumer and high fidelity products, including microphones, headphones, telephony accessories, and avionics headsets for consumer, professional, and business...
Electronic once listed their "AudioBeam" product for about $4,500. There is no indication that the product has been used in any public applications. The product has since been discontinued.
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.Related topics were researched almost 40 years earlier in the context of underwater acoustics.
- The first article consisted of a theoretical formulation of the half pressure angle of the demodulated signal.
- The second article 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. It should be noted that 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 and underwater. 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 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
Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
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 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. 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 and unfortunately they do not have general analytical solutions. They usually require the use of a computer simulation. However, as early as 1965, Berktay performed an analysis 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.Where
-
Audible secondary pressure wave -
misc. physical parameters -
SPL of the ultrasonic carrier wave -
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 with computer simulations by using a precompensation scheme based on Berktay's expression. In 1999 Pompei's article 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 convolutionConvolution
In mathematics and, in particular, functional analysis, convolution is a mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...
. 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 -ω² in frequency. This distortion may be equalized out with the use of preemphasis filtering.
By the time convolution
Convolution
In mathematics and, in particular, functional analysis, convolution is a mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...
property of the fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
, 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 -ω² 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 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 levels of 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. Exposure to more intense ultrasound over 140dB near the audible range (20–40 kHz) can lead to a syndrome involving manifestations of nausea, headache, tinnitusTinnitus
Tinnitus |ringing]]") is the perception of sound within the human ear in the absence of corresponding external sound.Tinnitus is not a disease, but a symptom that can result from a wide range of underlying causes: abnormally loud sounds in the ear canal for even the briefest period , ear...
, pain, dizziness and fatigue, 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.
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).
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). This is several times the highest levels used by commercial Audio Spotlight systems, so there is a significant margin for safety. In a review of international acceptable exposure limits Howard et al. (2005) noted the general agreement amongst standards organizations, but expressed concern with the decision by United States of America’s 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).
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 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) reviewed international guidelines for airborne ultrasound in a report published by the United Kingdom’s 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”. It should be noted that the ACGIH document also mentioned the possible need for hearing protection.
Use in politics
There are rumors that this technology has been used to send information to candidates during live debates. The sharp sound gradient can be used to send information to a receiver without disturbing the nearby microphone. This was first reported during the infamous Romney Whisper—which NBC later identified was simply an audience member picked up by an "open mike" in the broadcast mix, and was not audible to the candidates.Further resources
USS PatentPatent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
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.