This paper reviews sound art (being defined here as music, composition, recording, etc.) in the context of art-science with particular focus on some of the properties, characteristics, applications and issues relating to ultrasound. By tracing methodologies art-science to musique concrète, acoustic ecology and bio-acoustics, a pretext for the emerging media of ultrasound can be established, which provide insights into the development of future applications. Examples of art-sciences practices that use ultrasound are cited. Further evidence will suggest that ultrasound is a medium to be distinguished from audible sound.
My intent in studying the invisible sonic spectrum is to understand the importance that these frequencies bare on the complex communicative fabric of the natural and technological aspects of our lives. If the medium is a language, eventually, we will exhaust the vernacular into cliche, or be made to feel insufficient in our abilities to communicate the novelty of our experiences. Either out of experimentation or necessity, the artist seeks to augment the language and extend the palette of expression. Technology allows us to expand our perspective, and artist, who are always interested in perspective, are taking more interest in participating in its development.
Perspective is essential. The only thing which keeps our intellect from floating off into the ether, is the tributary of sensory data which culminates into our notion of perspective. Here lies the affirmation of our existence and the realization of poetic beauty as can be found, in a groove of conifers at dusk, or how a babbling brook ringed with ice, reflects the moon. Inversely, it is the sense of beauty that compels our imagination and curiosity to discover how things work. Knowledge has never defeated beauty, it cultivates appreciation and begins to show us how little we really do know. This curiosity has been an elemental bond of the arts and sciences since antiquity, or rather, from a time preceding the distinction of the two. This paper does not intend to arrive at a solution, but strives to generate questions.
One possible context for sound art
"But since the dawn of the twentieth century, a recurring aesthetic dream has been the expansion beyond a fixed set of homogeneous materials to a much larger superset of heterogeneous musical material."
-Curtis Roads (1)
With the development of recording technologies have come innumerable titles for those who work in the medium of sound, I use the term sound art as a means of covering the multifarious ambiguities which extended from the performance-based composer/musician dialectic. Art is generally accepted as a pretense for odd behavior, while sound limits this behavior to a particular medium. In any case, musique concrète would not have been able to so profoundly impact the notion of music, and modern art, without recording technology. It challenged some of the traditional notions of what the sound artist was expected to produce. The creative gesture could be the skillful application of technology. It is not to say that the musician was, or will ever be, supplanted by the audio engineer, not that that was ever an issue. It simply meant there was a media through which an artist could record and manipulate sound, be it found or well-intended. Perhaps what was more provocative then the use of technology, was the way in which musique concrète was applied. Schaeffer's system for classifying sound as objects, compounds, and mixtures, was not an arbitrary injection of scientific nomenclature, but a formal scientific methodology being applied as a compositional technique.(2)
In the simplest of terms, it is a matter of taking a sound, equivalent to a musical gesture, out of its original context and juxtaposing it in a new set events, e.g. collage. This re-contextualization has a psychological effect on the listener. The new context allows the listener to hear the sound from a different perspective. They are also allowed to focus their attention on the medium itself, primarily, because there is no other associative sensory input or temporality. This is achieved namely from editing. A more dramatic perspective can be achieved by the recording technique. There are many sounds that humans just cannot perceive, simply because you can place a microphone in odd places, that you cannot, or may wish not to, place a human head. It is this novel approach to sound recording that truly changes our perception of a sound. Thus, we can say that one aspect of technology, is that it functions as an interface for events which lie outside the domain of human perception.
Humans are very good at filtering out the diverse information we receive by being highly selective about what information we choose to focus on at any given time. During the course of a day, our thoughts may take us well away from the details of our surrounding environment, perhaps even on a roller coaster of memories that are visual, aural, emotional and olfactic. The introduction of soundscape ecology or acoustic ecology by composers such as Barry Truax, R. Murray Schafer, and Annea Lockwood, in the sixties and seventies, carried with it not only a scientific methodology, but a prerogative which sought to remind us that the sonic relationship between humans and the environment is of essential importance. This was a direct response to a visually dominant culture which usurps a finite amount of natural resources.(3) In the process of this radical environmental transformation, the acoustic ecology of humans is neglectfully altered, becoming not much more than an industrial by-product that our conscious work exhaustively to filter out from our awareness. Thus, it is implied our acoustic ecology impacts our psychological condition. Because these ill-favored conditions coerce sensory withdrawal, it is often necessary to find a means through which we are able to activate our sense of hearing. The concept of a Soundwalk, as developed by Hildegard Westerkamp, promotes active listening as a means opening our sense of hearing, which, in turn balances all of our sensory organs, physical and mental facilities.
To better understand our experience of acoustic ecology, it helps to remember our human constraints. Consider basic psychoacoustics. The perception of sound can vary tremendously from the reality of sound. The graph below illustrates the average responsiveness of the human auditory system. Take into account, as well, that everyone is different. Think of your ears as microphones. All microphones have a particular frequency response profile. This subtly 'colors' sound, so everyone has a different experience. Also, we do not need a fundamental pitch in order to perceive a fundamental pitch. That is, if frequencies resemble a pattern which is particular to the partials of a given fundamental, then we will perceive the presence of that fundamental.(4) So, is there really a 'Truth' to sound, or only a variety of filters through which we are able to make interpretations?
At some point in our evolution, a snail crawled onto our head and fell asleep. We restlessly woke, but the snail did not. We call this sensory organ the cochlea. An art-science installation by Jacob Kirkegaard beautifully pays homage to this organ, in an installation called Labyrinthitis. The premise of this installation is based on certain anomalistic qualities of the cochlea. Specifically, it induces the phenomenon of Tartini Tones. These tones are produced when two pure tones of the ratio 1:1.2 simultaneously stimulate the ear to create a third, measurable tone. This is not a psychoacoustic effect, because the sound can be recorded.(5) Kirkegaard's composition is a spiraling set of intervals which physically crawl along the cochlea, inducing the effect of tartini tones. The physical experience is an awareness of the organ itself. The iteration of the cochlea's logarithmic structure, not only appears as a compositional system, but the physical manifestation of the installation as well. It is the work of artists, such as Kirkegaard, that provide us with a better understanding of the human apparatus which helps locate our perspective in acoustic ecology.
Certainly, techniques for manipulating recordings have grown more intricate since the movement of acoustic ecology began to flourish. Today, it is easy enough to acquire and use any number of software-based tools to manipulate sound. So the challenge is really becoming, not how, but why. Generative algorithmic composition derived from natural processes, is a comprehensive artistic practice and field of study. This research is not undermining the diversity of naturally occurring sounds, but it helps us to better understand and appreciate them. There is no indication as to how much acoustic phenomena in the natural world is still unheard. In the finite of terra firma, there is a much larger domain of events which lie outside the realm of human perception. The limits of our perception define us, but they are also what inspire us. Considering how essential sound is as a medium for communication, it makes sense to find distinct spectra on the sonic continuum, occupied by distinct species. An analogy for this would be radio stations. That is, if all radio stations transmitted signals at the same frequency, it would impossible to hear anything but the most powerful broadcast. It then becomes reasonable to to assume that human intervention effectively destroys species' communication by force of sheer amplitude.(6) It is such conflicts of interest which can be observed by practitioners of bioacoustics.
Elemental forms of communication provide the phenomenological impetus for sound artists to meaningfully engage in a scientific methodology. For some sound artists, this leads to an epistemological approach to ecology, in which, ecology is not organizing the sounds, as such, but the sounds are organizing the ecology. These of some of the ideas of Bernie Krause's Niche Hypothesis (7), which underlie his concept of Biophony. Biophony suggests using acoustic data to measure the health of global ecosystems. This idea has been implemented by researchers on a micro-scale as an efficient means of measuring bird populations.(8) Composer, David Dunn and physicist, Jim Crutchfield are art-science collaborators who have made the connection between the bioacoustic mechanisms of micro-ecologies as being part of the feedback loop driving global warming.(9) This is a magnitude of events which could only be ascertained from a multi-disciplinary approach. What is most valuable from these examples, is not so much, the knowledge that is gained, but the process by which things are observed and evaluated. Also, the degree of disciplinary variance regarding the questions which are produced. These cross-disciplinary approaches stress the structural limitations that prevent a single discipline from viewing a larger, more inclusive picture of the phenomena being queried.(10) So, the question arises, can shifts of a sound-based perspective effect the related scientific perceptions?
Much of the sonic activity that is interesting to sound artists working in a field of bioacoustics is occurring at frequencies extending beyond the range of human hearing, and, perhaps beyond the range of the recording device as well. For decades, consumer audio hardware has been standardized to recreate sound between 20Hz and 20kHz. Advances in digital recording, memory access, and storage have prompted specifications with greater bandwidth. Despite the prevalence of lossy media, obtaining a recording with a bandwidth higher than 44kHz is almost unavoidable. Even though the bandwidth of digital recording equipment has improved considerably, affordable options for devices which can sample rates high enough for certain bioacoustic applications, are difficult to find. For producing sound, analog playback is still mired by hardware limitations. The hardware which can produce ultrasonic frequencies are highly specialized; therefore, expensive to acquire. This is one of the first obstacles for the sound artist who is interested in bioacoustics, the hardware limitation. Although, this type-of technological dilemma can be considered standard fanfare for a multimedia artist in any discipline. One consequence of working with unusual technologies which are not mass produced, is that the artist has to look for, or create innovative solutions. Thus, out of necessity, the artist collaborates with a technical specialist, or, as an autodidact, develops a scientific set of skills.
"The sounds that first held our attention at the dawn of human consciousness are still buried deep in our Pleistocene past and embedded in the genetic code of our physiology."
- Bernie Krause (11)
The main function of our ears is so we can communicate amongst ourselves. As the graph for frequency responsiveness suggests, we are most receptive to vocal patterns. We are social creatures, like bats, who depend upon communication with each other as a measure of our health. Perhaps one could say, the acoustic ecology of our ear has developed our social propensities.
When we compare our acoustic responsiveness to those of other animals, we see that our sense of hearing is rather narrow. Compared to sea mammals, our hearing is impoverished. However, in fairness to ourselves, water is an ideal medium for sound, where it travels roughly four-times faster than in air. Considering the hearing ranges of many species, we may reason that a good deal of inter-species communication, both terrestrial and aquatic, occurs at ultrasonic frequencies. Thus, greater amounts of information can be acquired in shorter periods of time. (12)(13) These are important considerations to make when evaluating the effects of ultrasound on our own physiology. Exposure to environments rich in ultrasonic content predate our humanity. During the course of our evolution, we have spent a majority of our years acquiring neurological mass in environments which have a complex, communicative fabric transmitted by living organisms capable of producing a range of frequencies. From the vast aquatic soundscape, where our core brain facilities developed, to the dense terrestrial forest, where our senses became refined, environment is evidence our evolutionary development. How we perceive, communicate and interact with our surroundings, has largely determined our success as a species. Our pre-human form strove to absorb and grow around as much of the environmental stimuli as it could manage. Our brains managed this expansion, as a builder adds an addition to a house; primarily, by constructing new rooms onto an existing structure. So, newer neurological structures simply grew around the old ones. Therefore, the modern human brain still contains the primitive hindbrain region, or protereptilin brain. (14) Based on the environments of our evolutionary past, is there some reptilian sensors that still works?
It is from our necessity to communicate that researchers look for solutions to hearing loss. One option that was researched, used the ultrasonic spectrum as a means of speech perception. In this technique, speech samples were processed (double side-band with carrier frequency suppression) into the ultrasonic range at 28kHz and 40kHz. The sound was then transmitted through transducers which were attached to the skull. The results suggested that words were perceived as speech.(15) In this situation, it could be that the body is acting as a demodulator of the ultrasonic vibrations. However, for individuals who suffered from substantial auditory loss, transmission via the cochlea seems unlikely. So what was mechanism?
Our bodies are sensitive to sonic vibration, something we can observe during any amplified concert or thunder storm. Audible sounds can transmit legible vibrations on to the body through air, allowing even a world class musician, who is profoundly deaf, to work.(16) If there is a mechanism(s) for ultrasonic reception, they remain elusive to science. What researchers have come to understand, is that the body must be exposed to the ultrasound. The cochlea is not receptive to ultrasonic frequencies.(17) Thus, there appears to be different senses for ultrasound and audible sound.
Research of sound containing sample rates of roughly 150kHz, has produced some interesting data. The image above is an illustration of alpha-EEG data measuring the Hypersonic Effect, which is defined as:
"inaudible high-frequency sounds with a non-stationary structure may cause non-negligible effects on the humanbrain when coexisting with audible low-frequency sounds."
-Oohashi et al. (18)
Below, is an fMRI image showing the same effect. It seems fairly safe to allow a few reasonable deductions about the conclusions of this study, given the detailed documentation. Firstly, the brain is responding to ultrasonic stimuli, possibly within the deep-lying, archaic structure. Second, the test subjects take pleasure in listening to a wider (full range) spectrum of sound. Thirdly, there are certain requirements for inducing such an effect. They are:
- the frequencies between 50kHz and 100kHz need to have motion.
- the ultrasonic frequencies are most effective when transmitted to the brain via audible carrier frequencies.
- the body (and non-auditory transmitters) need(s) to be exposed to the sound.
- there is roughly 200 seconds of persistent cerebral activity after the sound has subsided.
As the Hypersonic Effect suggests, there is evidence that we have an undefined sense for ultrasound, and our brain seems to be processing this information. We are not conscious of the experience, but the effect seems to make recorded music more enjoyable. These points suggest a few different things. First, the manufacturing of audio hardware capable of producing higher frequency responses. Second, the sense for ultrasound, perhaps different from audible sounds, seems to have way of syncing up with them. Thirdly, there is roughly 200 seconds of persistence. For the artist who works with sound, you tend to think of this research in the very physical; the way of acoustics. If you are like me, at this moment, you are thinking of sympathetic resonance, possibly the space and materials that would be required to make such a resonance. Fourth, the sound needs to have motion, a discernible pattern.
It is important to realize that the traditional approach in measuring the responsiveness of hearing, is by using stationary frequencies devoid of harmonic content. In contrast, the research conducted for the hypersonic effect, used musical excerpts recorded at high sample rates. The natural world does not produce many, if any stationary sounds with little harmonic content. If sounds do not previously exist as part of our acoustic ecology, or have a call number in the library of our evolution, chances are we will not recognize them. Just consider how much useful information a sound without rhythm or harmony could have. Music is like nature, and nature is like music with lateral and vertical motion; and, most importantly, patterns of distinction. So, it stands to reason that evaluating any sonic effect on our neurology, would include sounds which are experientially relevant. By recording music with a broader frequency spectrum, much of the criteria for creating the hypersonic effect are met.(19) As more research is conducted into this area, it is possible that the disciplines of acoustics and psychoacoustics may have a wider spectrum and different senses to consider. In the meantime, we can imagine the possibilities of accessing a high bandwidth ultrasonic 'port'.
Ultrasound in the Art-Sciences
Ideas for solving problems, as well as inspiration for creating new ones, can sometimes be divined from a scientific journal. Perusing through any given database of scientific periodicals, with the topic of ultrasound, will produce an abundance of research that spans across many scientific disciplines. Ultrasound is truly a multifarious medium which is being applied and researched for a variety of applications because of it's unique non-linear properties which differ from audible sound. Let us consider a few these properties of ultrasound in an art-science context.
The first image that usually comes to mind when we hear the word ultrasound, is baby. If you have ever had ultrasonic sonogram of your body, you would remember the cold liquid being applied to your skin. This liquid facilitates the conduction of sound waves in and out of your body. The frequencies used for medical imaging, do not travel well through the air. However, air-coupled ultrasonic transducers are being developed, for a high-resolution, acoustic-microscopic systems. This development is dealing with the impedance issues that ultrasound contends with when traveling through air.(20) In imaging fetuses, medical technicians are mindful of the mechanical and thermal indices. A high thermal index will cause tissue temperatures to rise, while a high mechanical index will trigger non-linear oscillations of tiny air bubbles. Due to the impedance mismatch between the liquid and the gas, acceleration of this oscillation can lead to an instantaneous collapse of the bubble in an event called cavitation.(21) In this situation, cavitation is not desirable, as it is capable of causing complications. But, there are art-science applications for this phenomenon.
Cavitation or sonoluminescence is a phenomena around which artists at Optifonica (22) center their installation Camera Lucida: Sonochemical Observatory. In this installation, the artists' intention is to create a mirror stage, where the threshold between the outer and inner space is removed.(23) It is not only the intent of the artists to develop a synaestetic experience for the viewer, but to make "a work of art/nature and a scientific research tool," which "seeks to blur the discrepancy between such definitions of intent."(24)
A second interesting property of ultrasound is acoustic radiation pressure which is the mechanical energy produced by sound that moves in the direction normal to sound waves.(25) An application using this property, is currently being developed, as an airborne ultrasonic, tactile display. It is designed to provide tactile feedback in 3D free space.(26) Optifonica have used acoustic radiation pressure in a performance called Sonolevitation. This performance is intended to draw attention to concepts of space travel and the possibilities of what artist will have to contend with, in near-vacuous environments.(27) Optofonica have been collaborating successfully with a scientist for some years now, in fabricating and in developing ideas which are beneficial to both disciplines. This type of working relationship not only expresses their approach to customizing technology, but it also illustrates the relevance and validity of collaborations between the arts and sciences.
A third property is acoustic heterodyning. When Helmholtz was conducting his studies in sound, he observed:
|"When two sound sources are positioned relatively close together and are of a sufficiently high amplitude, two new tones appear: one lower and one higher then either of the original tones." (28)
For example, tones produced at frequencies of 90kHz and 91kHz, respectively, will produce tones at 181kHz and 1kHz. The audible effect is an undertone. Acoustic heterodyning has seen some commercial applications, however the designs are proprietary. The underlying principle which makes this device work, is the impedance of ultrasound and air. (29) My own project, entitled Species, seeks to create this effect in water with an aesthetic presentation to match the delicate, chaos of the sounds being produced.
This is, by no means a comprehensive evaluation of applications being developed with ultrasonic properties. But, it does provide some insight into some behaviors of ultrasound which differ from audible sounds. It is also an indication of how some artists direct their imaginations, and where their inspiration is being derived from. I think the artisit Evelina Dominitch best summarized her relationship to science when she said in an informal interview:
|"If you look at the sciences and arts right now, the exciting breakthrough's are happening in science."
There are several consequences for cross-disciplinary convergences. One being, the function of art is only becoming more ambiguous. Not that art has ever been straight and narrow, but the confluence of art and science is being defined more by a methodological approach, which, in practical terms, resembles more of a science due to the novel technological applications that are required of emerging medias.
Space in which to dwell
Our senses are really interested in things that are not static. That is, if our senses are consistently streaming data, it is necessary to have thresholds of distinction which trigger our awareness. It would then seem, that in order to understand environmental stimuli, our neurological processes are required to make an analog model. For simplicity's sake, this would explain our ability to adapt to different cultural, sociological and environmental situations. This would also explain the magical quality by which musique concrète changes our perception of sound. Perhaps we are not so much a product of our evolutionary success, in as much as, we are a product of the success of our environment. We have succeeded so well, in fact, that we have been greeted by the knowledge, via technology, to alter our own course of evolution, as well as, those of other species. Technology gives us the ability to expand our perspective on the universe. But, by neglecting to consider which variables are or are not alterable, what we are really doing is undermining the value of the forces which have shaped and sustained our existence. I believe this is the awareness acoustic ecology and bioacoustics attempts to raise.
Will it be the function of artists, who are willing to take cross-disciplinary approaches, to contextualize this technological and ethical progress? Artists are certainly being challenged to develop novel approaches for the use of technologies, for the sake of self-expression, metaphorically, and as a means of mediating complex relationships and phenomena that are best understood as an experience.
Ultrasound is just a small cog in the clock of emerging media. However, it is a medium which embodies much potential. By mentioning a few unique ultrasonic characteristics that are perceived by an unknown sense, I am not merely suggesting that it is a medium different to sound. Perhaps, it is too earlier to make such a distinction. But, at the bottom of this process lies the notion, that we, in the course of our evolution as a species, have some hidden potentials. In our eagerness to alter the realty-at-large, we have a tendency to develop at the cost of other species, our own health and strength. If instead, the tiny niche of space and time that our conscious occupies, has the potential to be expanded, even slightly, through the practice of art-science, then perhaps we could begin to learn how to be a little more considerate with our notions of progress.
A cross-disciplinary approach has been taken on by many artists through art-science collaborations which have been motivated by a variety of factors, whether it is Schaeffer's fascination with recording, Optifonica's aesthetics of phenomena, or Dunn's and Crutchfield's curiosity for bark beetles and global devastation. Collaborations do not need any particular guideline, nor any particular outcome. What is reassuring, is the diversity of approaches towards an art-science practice. The poetic ambiguity of art is not endangered. However, it can be observed that there is a characteristic which has taken form. It is the artist who is mindfully engaged in developing technologies, a methodology, and a set of outcomes which can be articulated to, and beneficial for, other disciplines.
(1) Roads, Curtis. Microsound. Cambridge, Massachusetts: The MIT Press, 2001. ("Time Scales of Music" pp. 18, pp. 33-34)
(2) Frank, Robert J., "Analysis of Form and Cadence in Musique Concrète via Temporal Elements" Electroacoacoustic Music Studies Network International Conference, 3-7 June 2008 (Paris) - INA-GRM and University Paris-Sorbonne (MINT-OMF) http://www.ems-network.org
(3) Wrightson, Kendall. "An Introduction to Acoustic Ecology." Journal of Electroacoustic Music, Volume 12 (March 1999): 11-15. http://homepage.mac.com/kendallwrightson/ae/aecology.html
(4) Christopher J. Plack, Andrew J. Oxenham, Richard R. Fay, and Arthur N. Popper, "Pitch: neural coding and perception (Springer Handbook of Auditory Research)." Springer Science and Business media.(2005): 13.
(5) Kirkegaard, Jacob, "Labyrinthitis." summer 2007. http://www.fonik.dk/works/labyrinthitis.html
(6) Reynolds, Joel R., "Submarines, Sonar, and the Death of Whales: Enforcing the Delicate Balance of Envirnmental Compliance and National Security in Military Training." William & Mary Environmental Law & Policy Review 32, (October 2008): 759-802.
(7) 4 Krause, Bernie L., "Bioacoustics, Habitat Ambience in Ecological Balance." Whole Earth Review, no. (Winter 1987). http://www.wildsanctuary.com/niche.pdf
(8) 5 Deanna K. Dawson, and Murray G. Efford, "Bird population density estimated from acoustic signals." Journal of Applied Ecology 46, no. 6 (November 2009): 1201-1209. http://www3.interscience.wiley.com/cgi-bin/fulltext/123192974/HTMLSTART
(9) 7 David D. Dunn And James P. Crutchfield, "Climate Change: Insect Bioacoustics and Future Forest Ecology." Leonardo 42, No. 3 (June 2009): 239-244.
(10) 8 Barry Blesser and Linda-Ruth Salter, "Sound Artists and Scientists as Complementary Partners in Inquiry." Soundscape, The Journal of Acoustic Ecology 7, No. 1 (Fall/Winter 2007): 36-37.
(11) 25Krause, Bernie, "The Meaning of Wild Soundscapes." the green museum, 2004. http://greenmuseum.org/generic_content.php?ct_id=181
(12) 22 Gill, Victoria, "Bats 'recognise others' voices." BBC, June 5, 2009. http://news.bbc.co.uk/2/hi/science/nature/8085477.stm
(13) 23 Pollak, George, "Speaking of Bats." University of Texas at Austin, October 9, 2008. http://www.utexas.edu/features/archive/2004/bats.html
(14) Ohare, Libby, "Discovering the Things that Make Us Human: Evolution of the Brain." SERENDIP, January 1, 2008. http://serendip.brynmawr.edu/exchange/node/1750
(15) 27Martin L. Lenhardt, Ruth Skellett, Peter Wang, Alex M. Clarke, "Ultrasonic Speech Perception." Science, New Series , No. 5015 (July 1991): 82-85. http://www.jstor.org/stable/2876207
(16) 29 Evelyn Glennie, "The Hearing Essay." http://www.evelyn.co.uk/Evelyn_old/live/hearing_essay.htm
(17) 28 Tsutomu Oohashi, Norie Kawai, Emi Nishina, Manabu Honda, Reiko Yagi, Satoshi Nakamura, Masako Morimoto, Tadao Maekawa, Yoshiharu Yonekura, and Hiroshi Shibasaki, "The role of biological system other than auditory air-conduction in the emergence of the hypersonic effect." Brain Research 1073 - 1074 (2006): 3 3 9 – 3 4 7. http://www.sciencedirect.com/
(18) Tsutomu Oohashi, Emi Nishina, Manabu Honda, Yoshiharu Yonekura, Yoshitaka Fuwamoto, Norie Kawai, Tadao Maekawa, Satoshi Nakamura, Hidenao Fukuyama, and Hiroshi Shibasaki, "Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect." Journal of Neurophysiology 83, (2000): 3548-3558.
(19) 31 Reiko Yagi, Emi Nishina, Manabu Honda, and Tsutomu Oohashi, "Effect of inaudible high-frequency sounds on human acoustic perception." Neuroscience Letters 351 (2003): 191–195.
(20) Sadayuki Takahashi and Hiroji Ohigashi, "Ultrasonic imaging using air-coupled P(VDF/TrFE) transducers at 2MHz." Ultrasonics 49, Issues 4-5, (May 2009): 495-498. http://www.sciencedirect.com/science/journal/0041624X
(21) Ultrasound – Technology Information Portal. "Mechanical Index." US TIP http://www.us-tip.com/serv1.php?type=db1&dbs=Mechanical%20Index
(22) Optofonica http://www.optofonica.com/
(23) Portable Palace. "Camera Lucida: Sonochemical Observatory."
(24) Evelina Domnitch and and Dmitry Gelfand. "LUCIDA: Sonochemical Observatory." Leonardo, No. 5, (October 2004): 391–396. http://www.portablepalace.com/lucida/index.html
(25) Beyer, Robert., "Radiation pressure--the history of a mislabeled tensor." Journal of Acoustical Society of America63, Issue 4, (April 1978): 1025-1030. http://scitation.aip.org/journals/doc/ASALIB-home/
(26) Takayuki Iwamoto, Mari Tatezono, and Hiroyuki Shinoda, "Contact Method for Producing Tactile Sensation Using Airborne Ultrasound." EuroHaptics, 5024, (2008): 504–513. http://www.alab.t.u-tokyo.ac.jp/~siggraph/08/Tactile/SIGGRAPH08-Tactile.html
(27) Portable Palace. "Sonolevitation." http://www.portablepalace.com/levitation.htm
(28) Roads, Curtis. Ibid.
(29) Audio Spotlight by holosonics, http://www.holosonics.com/