Eric Acuña
// Sound in suspension. Light as conduit. Perception as process.
01192024-050125
Satellite dish, RTL-SDR receiver, Low Noise Amplifier, 150mm diameter parabolic mirror, LED light, Laser diodes, solar panel, Analog to Digital Converter, Aluminum steel, various cabling
Signal Dissonance, is part of an experimental research series, which investigates converting digital signals to analog formats via satellite radio, asking, "Can vibrations from radio signals carry data through light?" Using a DIY laser transmitter and solar panel receiver, it combines practical demonstration of viewing acoustic patterns by exploiting the principle of refraction. The presentation features diagrams, guides, videos, and audio samples. Visitors can view the setup, listen to audio, and read technical notes about astrophysics and atmospheric science, and how these tools are being used to see where the radio waves create an acoustic shockwave or a heat signature, which the schlieren system can see. The focus is on developing clear audio transmission through light, exploring component choices and light-sound principles, and highlighting future applications in communication and art.
Intro
The ability to visualize and characterize pressure waves has remained an important research challenge for many years. Although the principles of acoustics were established relatively early in the development of modern physics, comparable advances in optical science emerged later, particularly following the formulation of Maxwell’s equations and, subsequently, the development of quantum theory and laser technology.
The use of optical techniques for acoustic measurements relies on the interaction between light and sound within a medium. This interaction is commonly described through the acousto-optic effect, in which variations in acoustic pressure modify the molecular density of a transparent material and, consequently, its refractive index. Any change in the refractive index influence the propagation of light, allowing acoustic disturbances to be detected through modifications in the optical wave. Depending on the measurement configuration, i.e, line-of-slight, parabola focal length, or atmospheric control, these changes can appear as variations in the direction of light propagation or, importantly, as changes in the phase of the optical electric field.
Because interferometric techniques are inherently sensitive to optical phase variations, they provide a particularly effective approach for detecting and characterizing acoustic phenomena. Optical interferometry therefore offers a powerful means of translating pressure-induced changes in a medium into measurable optical signals.
I wanted to demonstrate the fragility, contingent, and material aspects of signal processes and systems. I discovered a range of research on 2D and 3D visualization of acoustic waves by optical feedback interferometry. Here, I learned how over the years many methods have been developed to visualize sound through a visual representation. In the field of acoustics, scientists have attempted to develop a visual representation of sound waves using transducers detecting two fundamental components of sound: sound pressure and particle velocity. In other words, the measurement of kinetic and potential energy, whose quantities provide information on the physical phenomenon of acoustic propagation.
The title Schlieren originates from the German word "Schliere" (plural: Schlieren), which translates literally to "streak", "smear", or "stria". Long before it was a scientific method, German glassmakers used the word Schlieren to describe the unwanted, wavy streaks or internal textures found in low-quality glass. These streaks occurred when the molten glass didn't mix perfectly, leaving behind tiny regions with a slightly different density and chemical makeup. When light passed through these spots, it bent irregularly, making the glass look blurry or smeared.
Schlieren techniques for making visible density gradients in transparent media seem to be older than had been supposed. English physicist who discovered the law of elasticity, Robert Hooke, developed a method of this kind as early as 1672 known as Hooks Law. Unlike dutch mathematician, Christian Huygens, who described a similar technique in 1685. This concept has been used by Huygens in his La Dioptrique 12 published in 1653, but his schlieren method was evidently invented much later-- obviously aroused no interest.
Schlieren
August Toepler, one of the first to visualize shock waves
In the 18th and 19th centuries, early astronomers and meteorologists adopted the word to describe the blurry, shimmering "heat waves" seen in the atmosphere on hot days. They realized that these atmospheric streaks were caused by the exact same optical physics as the imperfections in the glassmakers' ovens: pockets of air with differing densities bending the paths of light rays.
In 1864, a German physicist named August Toepler invented the optical system we use today. He wanted a way to see and study these invisible density streaks in a controlled laboratory setting. Because he designed his system specifically to detect and photograph these optical Schlieren, the apparatus became universally known as Toepler's Schlieren Method.
Because sound waves move at roughly 343 meters per second, they were entirely impossible to capture with standard 19th-century equipment. To defeat this speed barrier, Toepler invented a brilliant dual-spark synchronization system using a high-voltage electrostatic generator to trigger a large electrical spark. This spark violently expanded the local air, creating a sharp, miniature acoustic shockwave (sound wave).
He wired a second spark gap into the exact same circuit, but added a built-in delay (using primitive capacitors called Leyden jars). This second spark fired a fraction of a microsecond after the first one, acting as an ultra-fast, blindingly bright photographic flash. (Shock wave).
The light from the second spark flashed through his newly invented knife-edge mirror system right as the sound wave from the first spark was expanding. By freezing the motion with light lasting only a few nanoseconds, Toepler successfully projected the shadow of a circular sound wave onto a photographic plate.
Point light source → collimating lens → test area → concave mirror → knife edge at focal point → camera or screen.
Light in Motion : Is light a wave or a particle?
The double-slit experiment is one of the most famous experiments in physics because it shows that light does not behave the way we normally expect. When light shines through two tiny slits, it creates a pattern of bright and dark bands called an interference pattern, which is what waves produce when they overlap.
This discovery, first made by Thomas Young in 1801, helped prove that light behaves like a wave rather than only as tiny particles. Scientists later repeated the experiment using single particles of light, called photons, sending them through the slits one at a time. Surprisingly, even though each photon hits the screen as a single point, many photons together still build up the same wave-like interference pattern. This suggests that each photon somehow behaves like a wave while traveling, even though it appears as a particle when it is detected.
The experiment becomes even stranger when scientists place a detector at the slits to see which path each photon takes. Once the path is measured, the interference pattern disappears, and the photons behave like ordinary particles instead of waves. Physicists still do not fully understand why this happens, making the double-slit experiment one of the greatest mysteries of quantum mechanics. Today, scientists describe light as having both wave-like and particle-like properties, depending on how it is observed.
The double-slit experiment demonstrates that light behaves as a wave, meaning it has a phase (where the wave is in its cycle) that can combine with other waves. When two waves meet, they can reinforce each other (bright bands) or cancel each other out (dark bands). This interference pattern is only possible because light is a wave.
Schlieren optics, on the other hand, takes advantage of the fact that light bends when it passes through materials with different refractive indices. Hot air, cold air, pressure waves, and gases all have slightly different densities, which change the speed of light by tiny amounts. Those tiny speed changes cause the wavefronts of light to bend. A schlieren system converts these otherwise invisible bends into visible light and dark regions.
|||||))))))|||||
Moiré- Fringe Method of flow visualization
-
A central gradient of blues and yellows, often indicating index variations in the medium.
-
Fine banding or wave-like patterns, consistent with air currents, thermal gradients, or gas flow.
These effects result from light rays being deflected slightly due to density differences in air (such as heat rising or gas movement). A lens-grid light block, partially blocks the deflected rays, turning invisible changes in air into visible contrasts of light and dark—or in this case, colorful gradients with chromatic characteristics, arranged and sized to coincide in the image producing field of view.
Ripples of Waves: Radio and Signal
A radio telescope measures changes in a radio wave caused by variations in plasma, gas, magnetic fields, or motion.
When you point a radio telescope toward the Milky Way, aka. the Hydrogen Line, you're receiving radio waves that have traveled through enormous amounts of neutral hydrogen, ionized plasma, molecular clouds, magnetic fields, dust. Each region changes the radio wave slightly. Instead of seeing bends with your eye, the telescope measures phase shifts, delays, polarization changes, Doppler shifts, and light intensity-- These are the radio equivalent of the wavefront distortions that schlieren optics reveals.
|||||))))))|||||
The Hydrogen Line (1420 MHz) is especially interesting because of the "spin-flip" transition of the electron. When astronomers map the hydrogen line, they don't simply detect its brightness, they also measure, frequency shifts, line width, phase, intensity, which reveal cloud motion, rotation of the Milky Way, turbulence, and gas density. One can think of this as a spectral schlieren image, where the distortions appear in frequency rather than visible brightness.
Radio Schlieren : Signal and Image Processing
Researchers have proposed and used techniques sometimes called radio schlieren or phase-gradient imaging.
Instead of using a knife edge, computers calculate, phase gradients, wavefront curvature, or spatial derivatives. Since the Hydrogen Line is an observable increase in RF power at 1420.4058 MHz created by Hydrogen atoms. It is most easily detected by pointing a directional antenna towards the Milky Way as there are many more hydrogen atoms in our own galaxy. This effect can be used to measure the shape and other properties of our own galaxy. For example:
Incoming radio wave ~~~~~~~~~~~~~~~~~~~~~~ Travels through plasma ~~~~~~~))))))))~~~~~~~ Wavefront distorted Radio array measures phase arrival time amplitude ↓ Computer reconstructs electron density map.
Modern radio arrays like the Very Large Array or Square Kilometre Array resemble this process as they compare the radio wave's phase as it arrives at many antennas. This comparison lets astronomers reconstruct the shape of the incoming wavefront with remarkable precision. In that sense, the array acts like a giant wavefront sensor, conceptually similar to what a schlieren system does for visible light.
Radio interferometry is a technique used in radio astronomy that combines signals from two or more radio telescopes, effectively simulating a much larger telescope. This allows astronomers to achieve a higher resolution than what a single dish can offer.
Just for fun, I created a "collapsing noise" audio-reactive network inside TouchDesigner to visualize coordinates from a sky scan with an RTL-SDR. In this network, audio transients drive the contraction, compression, or chaotic particle emission of a field. This simulates the response of a combined radio signal I recorded with a radio telescope consisting of a hacked W2 Wineguard dish, RTL-SDR and a few filters and LNAs.
As expected, because the Winegard's internal dish reflector is only 18 inches wide, it's barely two wavelengths wide. A dish that small cannot properly focus a 21 cm wave. It was a learning curve, and instead of reflecting the signal to a focal point, the radio waves simply feel over the edges, rendering the dish practically useless due to the size of the dish and tuning range of the LNA.
I later upgraded to a Nooelec mesh satellite, which is roughly 39 inches wide—about 4.7 wavelengths wide. This set-up gives me enough surface area to focus the weak cosmic energy into a concentrated beam with 20dBi of gain.
A collapsing noise signal-reactive network inside TouchDesigner, visualizing coordinates from a 1420 MHz sky scan with an RTL-SDR.
When the antenna scans empty space, a 1.0 multiplier applies chaotic 3D noise at full strength, causing the GPU to render the coordinate points as randomly swirling cosmic dust.
As the antenna sweeps across the Milky Way, the radio amplitude spikes, forcing the multiplier down to 0.0. With the displacing noise zeroed out, the points instantly collapse back into their exact, original celestial coordinates—drawing the physical structure of the galaxy out of pure chaos.
Visualizing the invisible: A physical cymatics network driving iron filings with raw cosmic static.
By piping un-demodulated radio noise frequencies from an RTL-SDR sky scan straight into an acoustic transducer plate, the chaotic static creates a complex web of physical standing waves.
The jagged, high-energy radio spikes violently reject the iron filings, forcing them to settle along the quiet nodal lines of the plate.
The result is a physical manifestation of electromagnetic chaos, where cosmic static patterns are drawn in real-time by the sheer absence of vibration.
These dominant frequencies will force the iron fillings into primary geometric structures, which then are 'frozen' in space using a spray solvent adhesive on Mulberry paper.
