Sound waves carry energy, and that energy exerts real physical force on anything in its path. The effect is subtle at everyday volumes, which is why your voice does not shove coffee cups off the table. But scale up the intensity, focus the beam, or shrink the target, and sound becomes a surprisingly effective tool for pushing, pulling, trapping, and even levitating solid objects. Researchers now routinely use sound to sort individual cells, reposition kidney stones inside living patients, and suspend liquid droplets in mid-air with no container at all.
How Sound Exerts a Physical Push
Any wave that carries energy also transfers momentum to whatever absorbs or reflects it. Light does this (it is the principle behind solar sails), and sound does too. When a sound wave hits an object, part of its energy is absorbed and part bounces back. Both processes deliver a tiny net push in the direction the wave was traveling. Physicists call this push “acoustic radiation force.” It arises from several mechanisms: the wave losing energy as it passes through a material, the wave reflecting off a surface, differences in how fast sound travels through different parts of an object, and variations in energy density within a standing wave field.
At normal loudness, acoustic radiation force is negligible. The pressure fluctuations in conversational speech are on the order of a few thousandths of a pascal, far too weak to budge anything you can see. But ultrasonic transducers can concentrate sound energy into tight beams at frequencies well above human hearing, and the radiation force climbs with the square of the pressure amplitude. Double the sound pressure, and you quadruple the force. This is why most practical applications of sound-driven motion use ultrasound rather than audible frequencies: it is easier to focus, easier to crank up in intensity, and it does not deafen everyone in the room.
Levitating Objects in Mid-Air
The most visually dramatic demonstration of sound moving matter is acoustic levitation. When two opposing sound sources (or a source and a reflector) create a standing wave, fixed points of high and low pressure form in the air between them. Small objects can be trapped at specific locations in this pattern, held in place against gravity by the radiation force. The equilibrium positions sit slightly below the pressure nodes of the standing wave, a detail that took decades of theoretical work to explain correctly.
Early levitation setups were limited: they could suspend a particle but not steer it. That changed with the development of ultrasonic phased arrays, which are grids of tiny speakers whose timing can be individually controlled. By optimizing the phase of each element, researchers can shape the sound field into structures that act like tweezers, twisters, or acoustic “bottles” that cage an object from all sides. A single-sided emitter array can translate, rotate, and manipulate levitated particles along programmable paths, no opposing reflector needed.
This is not just a lab trick. Acoustic levitation has practical value whenever you need to handle a sample without touching it. Protein crystallographers, for instance, have used levitating droplets to collect X-ray diffraction data at room temperature. Crystals of lysozyme were suspended inside tiny liquid drops, and the rapid spinning induced by the acoustic field helped sample the crystal structure efficiently. The resulting structures matched those from conventional methods down to a resolution of 1.8 angstroms, and the crystals showed no damage from the levitation itself.
Sorting Living Cells by Sound
Shrink the scale from millimeters to micrometers, and acoustic radiation force becomes a precision sorting tool. In microfluidic chips (tiny channels etched into glass or silicon), an ultrasonic standing wave can push cells sideways depending on their size, density, and compressibility. Larger, denser cells feel a stronger push toward the center of the channel, while smaller or softer cells drift toward the walls. This happens continuously as fluid flows through, separating mixed populations into distinct streams without any labels, dyes, or physical filters.
The technique, called acoustophoresis, has been used to separate viable mammalian cells from dead ones. Cells that have undergone programmed cell death are physically smaller than healthy cells, and the strong size dependence of acoustic radiation force in a microchannel exploits this difference for efficient sorting. Researchers have also separated functionally distinct subsets of bone marrow stromal cells based purely on their physical properties, no antibody tags required. Because the sound field does not damage the cells, they remain alive and usable for downstream experiments or therapies.
The appeal here is gentleness. Conventional cell-sorting methods often involve fluorescent labeling, high-speed droplet formation, or centrifugation, all of which can stress or alter delicate cells. Acoustic sorting sidesteps those issues. You pour a mixed cell suspension into one end of a chip, apply ultrasound, and collect separated populations at the other end. The cells never touch a filter, never get tagged, and never experience the shear forces of a centrifuge.
Pushing Kidney Stones Inside the Body
One of the most striking medical applications of acoustic force is ultrasonic propulsion of kidney stones. The idea is straightforward: aim a focused ultrasound beam at a stone lodged in the kidney and push it toward the ureter, where it can pass naturally. In preclinical work, researchers moved stones and beads through pig kidneys, with all stones successfully displaced. About two-thirds were pushed the full distance to the renal pelvis or ureter. Each push lasted less than a second on average, and a typical procedure took around 14 minutes per stone. Examination of the treated kidneys showed no tissue damage.
The technique has since been tested in people. In the first human clinical study, stones were repositioned in 14 subjects. In patients who had previously undergone lithotripsy (stone-breaking treatment), residual fragments were moved to new locations in every case, and four out of six passed more than 30 fragments within days of treatment. Stones as large as 8 millimeters were successfully moved. In several cases, what clinical imaging had identified as a single large stone turned out to be a cluster of smaller, passable stones once they were nudged apart with ultrasound. The procedure was painless and caused no injury.
This matters because small kidney stone fragments often linger after lithotripsy, causing ongoing discomfort or growing into new stones. Having a noninvasive way to sweep those fragments toward the exit could reduce the need for repeat procedures. Ultrasonic propulsion is still being refined, but it sits within a growing family of therapeutic ultrasound tools for stone disease that also includes burst wave lithotripsy, a method that uses focused sound to fragment stones without surgery.
Measuring Tissue Stiffness With a Sound-Wave Push
Doctors already use sound waves to image the body, but a newer class of techniques uses acoustic radiation force not just to look but to poke. In acoustic radiation force impulse (ARFI) imaging, a brief, focused ultrasound pulse pushes on a small region of tissue, displacing it by a few micrometers. The same transducer then tracks how the tissue rebounds. Stiffer tissue (like a tumor or a fibrotic liver) moves less and snaps back faster than softer tissue. By mapping these displacement responses, clinicians get an image of tissue stiffness layered on top of the standard ultrasound picture.
A related approach monitors the speed of shear waves that ripple outward from the push point. These shear waves travel faster through stiff tissue and slower through compliant tissue, giving a quantitative measurement of elasticity. Both methods can be performed during a routine ultrasound exam with a standard transducer, requiring no extra equipment. They have become particularly useful for assessing liver fibrosis, where the degree of stiffness correlates with disease severity, and for characterizing breast and thyroid lesions without a biopsy.
Ultrasonic Cleaning and Industrial Cavitation
If you have ever dropped your glasses into an ultrasonic jewelry cleaner, you have seen sound move matter at a practical scale. The mechanism here goes beyond simple radiation force. At high intensities, sound waves in a liquid create and collapse tiny bubbles in a process called cavitation. When these microbubbles implode, they generate extremely fast microjets of liquid that slam into nearby surfaces. The impact creates localized stress that exceeds the bonding forces holding contaminants in place, blasting away grime, biological films, and industrial deposits.
Recent simulation work has shown the detailed mechanics of how this works on stubborn deposits like crude oil sediments. As a cavitation microjet hits the sediment surface, it generates stress concentrations that cause first elastic, then permanent plastic deformation. The stress wave propagates through the sediment to the interface between the deposit and the underlying surface, where it peaks, effectively peeling the deposit away from below. For oil droplets specifically, detachment depends on whether the microjet force overcomes the interfacial resistance between the oil and the surface it is clinging to.
This is not gentle cleaning. Cavitation can erode metal surfaces over time, which is why ultrasonic cleaning baths have recommended exposure limits for delicate items. But for industrial-scale tasks like removing scale from heat exchangers or cleaning surgical instruments, the controlled violence of cavitation is exactly the point. Sound is doing heavy mechanical work here, just at a microscopic scale.
Manipulating Droplets Without Touching Them
Acoustic levitation is also finding a niche in chemistry and materials science through contactless droplet handling. When two small liquid droplets are levitated in an acoustic field and then brought together, researchers can study their coalescence dynamics without any container walls influencing the process. Experiments have shown that the sound pressure acting on the droplets measurably decreases just before they collide, a subtle effect that reveals how the acoustic field itself participates in the merging process.
The practical vision is what some researchers call a “lab on a drop,” where chemical reactions, crystallization, and analysis all happen inside levitated droplets that never touch a surface. This eliminates contamination from container walls, avoids unwanted nucleation sites, and allows observation from all angles simultaneously. It is still largely a research tool, but the ability to hold, merge, split, and analyze tiny liquid volumes using nothing but sound has obvious appeal for pharmaceutical development and materials characterization.
Sound in Living Bodies
Any technology that pushes things around inside the body raises safety questions. The evidence so far is encouraging. In a study using live pigs, researchers used a phased-array ultrasound system to levitate and steer 3-millimeter glass spheres along preprogrammed paths inside the urinary bladder. The spheres followed their intended trajectories with less than 10% average deviation, and examination of the bladder wall and surrounding tissue showed no injury.
At a smaller biological scale, zebrafish embryos have been acoustically levitated at sound pressure levels of 135 to 144 decibels (measured at the pressure node and antinode, respectively) for up to about 33 minutes. Researchers compared the development of levitated embryos to controls and found no statistically significant differences in the size or number of trunk neuromasts and otoliths, key developmental markers. Mortality was somewhat higher in the levitated groups, but the difference did not reach statistical significance either. The results suggest that brief acoustic levitation does not derail vertebrate embryonic development, though longer exposures need more study.
These findings do not mean acoustic manipulation is risk-free at any intensity or duration. Ultrasound at high enough power levels causes tissue heating and cavitation, both of which can damage cells. The therapeutic and laboratory applications described throughout this article work precisely because they stay within intensity windows that move targets without harming surrounding tissue. Dialing up the power past those windows turns a precision tool into a destructive one, which is, after all, how lithotripsy shatters stones.
Everyday Sound Versus Lab-Grade Sound
A reasonable question after all of this: if sound can levitate objects and push kidney stones, why does a loud concert not send your drink flying? The answer comes down to intensity, frequency, and focusing. Concert speakers produce broadband sound that spreads out in all directions. The radiation force it exerts on a solid object is real but absurdly small compared to gravity. You would need sound pressure levels well beyond anything survivable to move a macroscopic object sitting on a table using unfocused audible sound.
The applications that work all cheat in the same direction: they use ultrasonic frequencies (typically tens of kilohertz to low megahertz), they focus or shape the beam tightly, and they target objects that are small relative to the wavelength. A standing wave at 40 kHz in air has a wavelength of about 8.5 millimeters, so it can trap objects a few millimeters across with ease. Scale up to a bowling ball, and you would need impractical amounts of power and a wavelength-matched setup that does not exist outside of thought experiments.
Infrasound, at the other extreme, involves frequencies below 20 Hz with wavelengths measured in meters or tens of meters. These very low frequencies can rattle windows and vibrate structures, and they carry enough energy to cause physical discomfort at high intensities. But the force they exert on small loose objects is spread over such a large area that it rarely produces visible displacement. The physical effects people report near infrasound sources (vibration of chest walls, a sense of pressure) are real mechanical interactions between sound and the body, just not the dramatic telekinesis that movies depict.
Phonons and the Nanoscale Frontier
At the very smallest scales, the concept of “sound moving things” takes on a different character. Inside solid materials, vibrations propagate as quantized packets called phonons, the solid-state equivalent of sound waves. Phonons carry heat through materials, and controlling them is central to designing better electronics, thermoelectric devices, and quantum technologies. Researchers have recently achieved nanoscale spatial mapping of individual phonon modes inside structures as small as a single quantum dot, using advanced electron microscopy techniques that probe volumes trillions of times smaller than conventional methods.
At these scales, phonons do not move objects in the everyday sense, but they move energy, and manipulating that energy flow has enormous practical consequences. By engineering nanostructures and interfaces, scientists can suppress thermal conductivity (how easily heat flows) while preserving electrical conductivity, a combination that is critical for efficient thermoelectric materials that convert waste heat into electricity. Observations of non-equilibrium phonons near nanoscale interfaces, where vibrational energy behaves differently depending on the atomic structure of the boundary, are advancing the understanding of heat dissipation in the transistors and circuits that power modern electronics.
The tools for studying these phenomena are themselves acoustic in nature. Momentum-resolved vibrational spectroscopy in the electron microscope can now map both optical and acoustic phonon modes across an entire range of wavelengths, all within a nanoscale volume. This capability lets researchers directly correlate how vibrations behave with the atomic-level structure and chemistry of the material they are traveling through, closing a gap between theory and measurement that persisted for decades.