Hypersonic Sound (HSS) is the term used to describe the process by which audible sound waves can be produced using ultrasonic sound waves that are free from non-linearity. The first attempts at hypersonic sound were made in the 1960’s using underwater sonar. In the 1970’s it was proven that mathematically HSS could be produced in air, but by the 1980’s the technology was abandoned because of problems with distortion. In the late 1990’s HSS was again researched because of advances in sound production technology and in 1998 the first working, commercial prototypes were made under the name “Audio Spotlight”.
The advantage of using ultrasonic sound is that sound transmissions can be focused into a narrow, far-reaching beam that resists diffusion and attenuation; therefore, the beam can be transmitted over greater distances with pinpoint accuracy. Additionally, this sound beam can be targeted to only a single object or person, leaving the surrounding environment free of noise pollution. Already, this technology is being put to use in the advertising and automobile industries, and the United States military.
First, it is important to understand what a sound wave is. A sound wave is a series of alternating high (condensation) and low (rarefaction) pressures created by some object disturbing the environment through which the sound wave is traveling. This pressure wave, then, is received by the eardrum which converts it through the inner ear into an electric signal which the brain can process. The key thing to recognize in the case of hypersonic sound is that each of these small pressure changes is a different micro-environment; the small portions which are low-pressure have different densities (atmospheric density is related to pressure) than those that are high-pressure. This is extremely important to note when dealing with the transmission of a sound wave across distances.
Next, it is important to understand the terms diffusion and attenuation, which describe the behavior of a sound wave over time. Diffusion is the process by which a sound wave expands outward, and attenuation is the process by which a sound’s intensity diminishes. These two characteristics of a sound wave are very interrelated; as a sound wave expands and increases its area occupied, its intensity (which is inversely proportional to area occupied) decreases. Additionally, a sound wave’s absorption into the surrounding environment as well as its reflection off of objects and particles in the environment decreases its intensity and thus contributes significantly to its attenuation.
Next, it is important to understand what it means to be non-linear and how or why a sound wave is non-linear. Non-linearity simply means that as the wave advances through the environment and time elapses, the conditions of the environment in which the wave exists do not remain constant. Explaining how or why a sound wave is non-linear is a little more complicated and requires the piecing-together of some facts which have already been noted. Because a wave’s frequency depends on the speed of sound, and the speed of sound depends on the density of the environment through which the wave is traveling, and the density of a fluid (fluids are gasses and liquids) environment depends on the pressure—which is fluctuating due to the nature of the wave—of the fluid, a wave’s frequency depends greatly, although transitively, on the pressure of the fluid. As a wave moves through various pressures, its frequency and speed change. Because the wave’s speed changes, the rate of diffusion changes as a result of its rate of expansion changing. Because of both the rate of diffusion changing, and because of the amount of particles to reflect off of (because of the compression and rarefaction, where lower densities have fewer particles and vice-versa) changing, the rate of attenuation changes. All of these factors are even further affected as the sound wave travels outwards because of the diminished intensity and conversely the diminished compression, rarefaction, attenuation, and diffusion. Thus, a sound wave is non-linear both in small segments (from one micro-environment to the next) and as an entire segment (as its intensity diminishes from the source at point A to the target at point B).
Because of sound’s non-linearity, it is extremely difficult to project a sound across long distances, and when a sound is projected across long distances, it becomes extremely distorted. So, logically, to counteract these effects, sound has to be given a linear quality. To do this, ultrasonic sound waves are used. Ultrasonic sound is sound that is above the human range of hearing (20,000Hz); in HSS, frequencies in the hundreds-of-thousands of hertz are used both to improve the linearity of the sound and to prevent harm to animals whose range of hearing exceeds that of humans. Because the frequency of ultrasonic waves is hundreds or even thousands of times faster than audible waves, and frequency is a measure of number of pressure fluctuations per second, the pressure fluctuates between rarefaction and compression hundreds or even thousands more times per second. Because the micro-environments are now hundreds or thousands of times smaller than with audible sound, the effects of the micro-environments on the propagation of the sound wave become negligible and thus the non-linear characteristics which were present in audible sound waves are not present in ultrasonic waves; additionally, because of the new nature of the pressure differences, the air through which it is traveling loses its non-linearity (because it is essentially “part of the wave”) and thus fails to make the sound “audible” (being “audible” would cause the sound to lose intensity and attenuate). This is what enables HSS to travel over incredibly long distances without losing intensity, becoming distorted, or propagating spherically outward rather than in a straight line. When the wave then hits an object that is non-linear, such as a wall or a human, the wave disturbs that object (because a wave is a disturbance of the surrounding environment and the object is its new environment through which to propagate) and uses that object to once again become “audible”.
In the case of these ultrasonic sound waves, though, “audible” does not really mean audible (ultrasonic is by definition inaudible); rather, what it means is that the wave once again becomes non-linear so that a theoretical human ear capable of hearing over 20kHz would be able to decipher it. To make the ultrasonic waves audible, a phenomenon known in music as the “Tartini tone” or in physics as the “difference tone” is employed.
A difference tone is a frequency that is generated when two other frequencies interact (in music: form a chord). This phenomenon is a result of both physical interaction between the two frequencies and neurological processing of the two frequencies. This phenomenon is almost like interference beats, which are caused when two frequencies of the same pitch are sequenced out-of-phase and thus cause the amplitude to fluctuate. A difference tone, however, is caused when two pure tones (perfect sine waves) are played in-phase but at different frequencies. When done at differences in frequency of over about 100Hz and not including the pitch which is at twice the frequency of the lower tone (in music: the octave)—which would be inaudible when using ultrasonic tones anyway--, a new tone with the frequency of the difference between the two original tones is created. For example: a tone at 440Hz (in music: the note “A”) and 660Hz (in music: the note “E” a perfect fifth higher) would produce the tone of 660Hz – 440Hz, which is the tone 220Hz (in music: an “A” an octave lower than the original “A”). What would not work is playing the tone 440Hz and it’s doubling at 880Hz, because the resultant difference tone would have the same frequency as the original 440Hz tone. Due to the capabilities of the human ear and aural processing centers in the brain, frequency differences which are almost a doubling of the original tone and frequency differences that are so small that the two tones are almost the same tend not to work; the ideal differences in tones are from 5:4 to 3:2 (in music, from a major third to a perfect fifth). This is because when the two tones are as such, the brain simply processes the sounds as being very dissonant, rather than allowing the difference tone to become clear.
Because the frequencies of ultrasonic tones are so high, however, a ratio of 5:4, when the original tones are at 400,000Hz and 500,000Hz, would still produce a difference tone of 100,000Hz--5 times the highest tone that is recognizable to a human. However, due to the temporary non-linearity of ultrasonic sounds, the ratios can be shrunken so that the difference of the tones can be as little as about 200Hz, producing a tone well within the human hearing range. The only disadvantage of this method of sound transfer is that bass tones (lower than about 200Hz) are unable to be reproduced. In applications like music, this would affect the harmonies and could make a performance sound “top-heavy”. In speech, the absence of bass tones, while not detracting from the decipherability of what is said (which is primarily affected by the mid- and high-range tones), would alter the timbre of a voice, which would affect the audience’s recognition of familiar voices and their ability to judge which person is speaking when 2 or more speakers are present (such as in a stage performance).
Hypersonic Sound technology is already being used in advertising displays so that sound can be projected only at one person without disrupting people who are not in the beam’s path, by the US military both to convey messages over long distances and, in a more powerful form, as a sound stun gun (LRAD). Future uses for this technology could include installation into automobiles—each passenger could hear their own music; concert halls—true surround sound projected from a central location and reflected off of the walls--sound projections could even move around the room in real-time; laptop computers—listen to podcasts, videos, or music without disrupting anyone else; and even in megaphones—whisper a message to one person instead of yelling over an entire crowd. HSS, now only in its relative infancy, will soon become a technology that will be seen frequently in a myriad of applications as prices shrink and popularity grows.
Equations:
Frequency (f) (measured in Hz) equals wavelengths (λ) per second (s) through a certain point:
f = λ/s
Speed of sound (v) equals wavelength (λ) times frequency (f):
v = λf
Speed of sound through an ideal gas (v) equals the square root of {[(the ratio of specific heats at a constant pressure {γ}) times (Boltzmann’s constant {k}) times (temperature in Kelvin {T})] divided by mass of one molecule of the gas (m)}:
Intensity of spherically radiating sound (I) equals power (P) (measured in watts) divided by surface area of a sphere (4πr):
Bibliography + Annotations:
Feynman, Richard P., Robert B. Leighton, and Matthew Sands. The Feynman Lectures on Physics. Reading, Mass.: Addison-Wesley, 1963.
Chapter 47 of this book explores the topic of sound waves and eventually their relation to electromagnetic waves and atomic harmonics. Equations are given in calculus format.
Kock, Winston E. Sound Waves and Light Waves. Garden City, NY: Anchor Books, 1965.
This book provides the fundamentals of sound- and light-wave motion and delves into the topic of propagation and dissipation of waves.
This book provides the fundamentals of sound- and light-wave motion and delves into the topic of propagation and dissipation of waves.
Levitin, Daniel J. This Is Your Brain on Music : The Science of a Human Obsession. New York: Plume, 2007.
This is an excellent book about how the brain processes music and sound. In addition, a section of the book is devoted to explaining the basics of music notation and jargon.
"Sound Attenuation." Sound Attenuation. 4 May 2002. Silex Exhaust Systems. 6 Feb. 2009 . This PDF file discusses various issues related to sound, sound transfer, and sound dampening (forced attenuation). Equations and explanations are provided here.
Wolfe, Joe. "Interference Beats and Tartini Tones." Music Acoustics. University of New South Wales. 2 Feb. 2009. .
This website from the University of New South Wales's physics department provides an excellent discussion of interference beats and difference tones. Audio examples are also provided here.
Links:
A link to a video of Woody Norris, a pioneer in popularizing Hypersonic Sound, provided by TED. Woody talks about the development of HSS technology, demonstrates the technology, and talks about the future of the technology. 15 minutes.
http://www.ted.com/index.php/talks/woody_norris_invents_amazing_things.html
http://www.youtube.com/watch?v=5imaJwfJMZ8
The Wikipedia article on "Sound from Ultrasound."
http://en.wikipedia.org/wiki/Sound_from_ultrasound
http://www.phys.unsw.edu.au/jw/beats.html#Tartini
The official website of Audio Spotlight.
http://www.holosonics.com/
The official website of Woody Norris's Hyper Sonic Sound.
http://www.atcsd.com/site/content/view/34/47/
1 comments:
Thank you for explaining this as you did. I understand now how and why this technology works as it does!
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