What is HSS? Allows you to direct sound into a narrow beam. Sound travels hundreds of yards without experiencing attenuation. Does this by manipulating inaudible ultrasound. Open light bulb flash light.
Sound Waves “Sinusoidal” wave. Caused by vibrations of objects. Humans can hear from 20 - 20,000Hz (hertz are vibrations per second). Time Pressure
Why use ultrasound? Non-linearity occurs when systems aren’t consistent; most physical objects are non- linear. HSS uses frequencies of more than 60,000Hz because this negates sound’s natural non- linearity. This allows HSS to travel long distances and remain focused without attenuating. When HSS hits a non-linear object (like your ear or a wall), it resounds.
Difference/Tartini Tones Caused by tones with slightly different frequencies. Perceived as fluctuations in volume. Only works in a non-linear system.
Piecing it Together With difference tones, the tone that you hear is the difference in Hz of the 2 tones being played. i.e: 400 + 401 = 1 Hz… 400 + 450 = 50Hz. Since humans can’t hear less than 20Hz difference, a difference tone less than 20Hz is perceived as volume fluctuation. But, a difference of more than 20Hz is heard as a 3rd tone being played simultaneous to the first 2 tones.
Ultrasound + Difference Tone = Sound Even though the 2 tones are ultrasonic, you can still produce difference tones. i.e. 60,000Hz and 60,300Hz gives you a 300Hz audible tone. Because difference tones only work in non- linear systems, you only hear the tone when it hits your ear, the wall, or some other object. Since the first two tones are inaudible, however, you only hear the difference tone of 300Hz.
Practical Uses Advertising displays Military NRAD technology Performance halls and theaters Car stereo systems Computer speakers Guerilla military strikes Classrooms
Extensions into Music Because acoustic instruments--and your ears--are non-linear, difference tones (in music Tartini tones) are present. Since no tones in music are ultrasonic, all 3 tones are evident… not just the two being played or the one Tartini tone. How does this affect our perception of intervals and harmony? How does this affect consonance and dissonance?
Annotated Resources 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. 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.
Online Resources Wikipedia article on HSS. University of New South Wales' physics page--an excellent resource.University of New South Wales' physics page--an excellent resource. Woody Norris demonstrating HSS. PDF document explaining attenuation in greater depth.PDF document explaining attenuation in greater depth. Woody Norris's TEDtalks presentation of HSS.Woody Norris's TEDtalks presentation of HSS.