From the Sweet Spot

Suspending Disbelief: The Potential of Acoustic Levitation

Issue 145

Acoustic manipulation is a technique that can be used in medical applications, such as controlling a miniature camera within a human body, or removing a kidney stone. Call it “acoustic levitation.” It works by using multiple ultrasonic transducers whose phase can be controlled, resulting in interference patterns that can trap and move small particles.

Could this technology have applications in audio? Could it be used to physically show where standing waves occur in a listening room? Do you wear lossy clothing when listening to music? Just what is acoustic levitation anyway and how does it benefit us already? To find out, Copper interviewed the insightful scientists Dr. Rick Weber and Dr. Chris J. Benmore of the Argonne National Laboratory in Lemont, Illinois.

Russ Welton: Could you tell us about your background in physics as it relates to acoustics?

Dr. Rick Weber: My background is in materials science, with an emphasis on non-equilibrium materials – supercooled liquids, glasses and amorphous compounds. I worked with a company called Intersonics that developed an acoustic levitator that was flown on the Space Shuttle, and started using the technology for research in the lab.

 

Dr. Rick Weber.

Dr. Rick Weber.

 

The company started selling an acoustic levitator instrument for materials research in 2006.  We collaborate closely with Chris on materials experiments at the Argonne National Laboratory Advanced Photon Source. We have a very good synergy of capabilities, interests and areas where there is a strong need for acoustic manipulation technology such as pharmaceutical research.

Dr. Chris Benmore: My background is in physics, I also have a strong interest in liquid, glassy and amorphous materials. My training is in neutron scattering (at the Rutherford Appleton Laboratory, Los Alamos National Laboratory and Argonne) and high-energy synchrotron x-ray diffraction (at the Advanced Photon Source in Argonne). We started working with Rick to help develop his acoustic levitator for x-ray experiments to be used as a “sample holder,” around the time the company MDI (Materials Development Inc.) started to sell them. X- rays and ultrasound are used to analyze the suspended materials’ properties for medical applications. The main interest since then has been in pharmaceuticals, as it mimics what happens to a droplet in a spray dryer – a device that produces dry powder from a liquid.

 

Dr. Chris Benmore.

Dr. Chris Benmore.

 

RW: When did you first discover and get involved in acoustic levitation?

DR. RW: Around 1988 at Intersonics.

CB: (In) 2006 when we started to incorporate them into our x-ray experiments.

RW: What have been some of the greatest challenges in building these acoustic levitation systems?

DR. RW: For best results we use a high-Q (narrow tuning bandwidth) transducer that is driven at a precise resonance to put out a high sound pressure level over a narrow frequency band. Since the resonant frequency changes slightly as the system warms up, it needs active tracking. Stray reflections disrupt the sound field and can cause instabilities. Reflections need to be controlled by using sound absorbers and by the geometry of the set up.

RW: How would you describe standing wave resonance, and please explain why this phenomenon is such a good thing in this application?

DR. RW: We use an interference technique that creates a standing wave along the direction of sound propagation, between two active transducers operating at the same frequency. This approach also enables us to shift the levitation node position by electronically controlling the relative phase of the two sound waves by introducing a small frequency difference between the sound sources. Digital signal processing really helps, since doing these things in analog is difficult or sometimes not possible at all.

 

RW: How is your research currently being utilized, and helping in the medical field?

DR. RW: A lot of emerging drug compounds have very low solubility, since they are normally very stable crystals. Levitation has enabled us to make high-solubility glassy forms in many cases. This is an active area of pharmaceutical research being supported by the National Institutes of Health, and industry, along with university research labs.

CB: Levitating pharmaceutical compounds in an x-ray beam allows us to understand where the atoms are during the compound’s transition from a droplet to a glass. If the drug compound crystallizes, it loses its effectiveness, so by being able to understand why and how it remains in a glassy state is important in designing effective pharmaceutical formulations with high solubility.

RW: Tell us about the advancement of similar x-ray technologies such the Argonne Advanced Photon Source project.

DR. RW: Argonne has one of the most powerful x-ray sources in the world. It enables us to measure atomic structure in a few seconds, instead of the few hours it would take in a normal lab system. It is about the only way to “see” how a process takes place as it is happening on an atomic scale – with in-situ measurements during processing.

CB: The Advanced Photon Source produces very high energy x-rays, ~100 keV, that are able to pass through a levitated droplet and provide information on its atomic structure. The lack of any container makes the signal very clean and also enables the levitated droplet to more easily form a glass there are no container walls for it to crystallize on.

RW: There is much in the way of room audio correction software in the market. Could acoustic levitation technology be used to identify poor room acoustics?

DR. RW: If you levitate an array of ~2 mm diameter Styrofoam balls and turn the acoustic output of the device down so that they are just floating, the balls are very sensitive to small external forces. This might be a way to map out standing waves in a more observable way than probing a room with a microphone.

 

Suspended polystyrene balls.

Suspended polystyrene balls.

 

RW: The existence of standing waves in a listening room may be perceived in a negative light by some audiophiles, because of their resultant nodes, say at 50 Hz for example, where there are nulls or area of sound cancellation at certain positions in a typical music listening room. Other than moving your seating position to avoid sitting in a null, what would you advise?

DR. RW: The standing wave is caused by interference between sounds, either from two sources and/or reflected sound and a source. Moving the sound source speaker might help. It might not eliminate the standing wave but it could change its location or possibly change its frequency slightly. Electronic control of phase, might help, but this could introduce its own colorations to the sound. I have heard speakers that can adapt to a room by adjusting their characteristics to generate the “best” sound. But it gets you back to the old problem – do you want something that sounds good and musical and doesn’t wear you out after an hour of listening, or are you seeking some kind of “acoustic truth” in the sound? It can be very subjective sometimes, if the components in the system are all basically doing a good job.

RW: I have seen levitated objects at the point of a node being moved by changes in frequency. Could transducers be built to move nodes to where you want them to occur in a room? That is, so they would not be in your listening position or where a problematic null may occur.

DR. RW: Control of relative phase determines the position of the levitation node. If the transducers are driven at slightly different frequencies, then a traveling wave at the difference frequency moves the node. It seems likely that controlling standing waves in a room would require control of phase, as well as room geometry and use of sound absorbing material. Digital signal processing (DSP) opens up all sorts of possibilities, including sound cancellation, and actively controlling the phase of different frequency bands.

 

Acoustic levitator control panel.

Acoustic levitator control panel.

 

It really depends how far you are willing to move away from the studio mix. One issue with typical home listening rooms is that they are “dynamic,” in the sense that people move around, wear lossy or reflective clothes, and are subject to other variations that can affect the sound. The listener is part of the equation. I’ve had reflections off my glasses cause instabilities in a levitated sample on a few occasions. [At least one audio reviewer, Robert E. Greene, recommends removing your glasses when doing serious listening – Ed.]

RW: Could you see a possible future where this technology may be used to manage recorded music itself, to optimize its playback for given room dimensions?

DR. RW: Our transducers can output up to about 160 dB at 22 kHz, using piezo drivers and optimized horn designs. We have built phased arrays using 40 kHz transducers, but the precision of the sound field is not as good even though the wavelength is shorter. Applications in music playback could use phased arrays to create 3-D imaging. Interference patterns between the sound waves would set up regions of high and low intensity, so that sounds could be heard only at very localized points. To an extent this happens in stereo sound reproduction. A phased array could provide even more precise location of sounds. A practical issue would be bandwidth – we typically aim for high Q, so driving even 50 Hz of resonance leads to a large drop in amplitude. A broadband audio frequency phased array might have some interesting capabilities for new types of sound experiences. Phasing would be frequency dependent so it would need to adapt its properties in some way to accommodate program material. Definitely some interesting possibilities but they start to move further from the source material and into sound processing.

RW: What do you see for future applications of this acoustic levitation technology?

DR. RW: Various types of levitation are currently used for research. Acoustic levitation is good for “soft matter” – pharmaceutical materials, water-based solutions, and organic solvents and compounds like polymers and oils. It is a useful tool for investigating how processing affects materials, and for use in characterizing them. Long-term, research will continue to be an important application. We have talked to several people about using the technology for food and beverage service applications – which would be a novelty and something out of the ordinary. Maybe now that restaurants are opening again we can resume some of those conversations.

CB: In terms of research, there will always be a need to steadily levitate or move objects without them being in a container, i.e., when handling toxic or reactive samples. A really neat application I reviewed recently was a non-invasive application to remove kidney stones, using a multi-element array operating at 1.5 MHz. The array could trap small objects and move them inside pig bladders! Click on the following to read relevant papers from the Proceedings of the National Academy of Science:

A sound option for the removal of kidney stones

Noninvasive acoustic manipulation of objects in a living body

RW: How do you personally most enjoy listening to music?

DR. RW: A dangerous question! I listen to a lot of music. Mostly Red Book CDs, along with vinyl, and some streaming. Live music whenever possible. I bought my first good system from The Sound Organisation when they were by London Bridge and I was a grad student. I still have the Linn LP12/Ittok turntable/arm combination with a couple of upgrades. My amplification is a Naim “chrome-bumper” 32/5-Hi-Cap 250 updated by Chris West, and some very old Linn Sara speakers on Sound Organisation stands. I had a Linn CD12 CD player but it broke and Linn won’t fix them anymore, so now (I have) an Esoteric K-03Xs – very good but perhaps not quite as musical as the CD12 on its best days. My office system is a Linn Ikemi CDP disc player, Logitech streamer, Tektron 300b single ended triode amp and Harbeth LS3/5A loudspeakers – a different sound style that is exceptional for vocals and midrange. Alyssa Allgood sounds almost as good as she does on stage at Winter’s Jazz Club!

One comment on “Suspending Disbelief: The Potential of Acoustic Levitation”

  1. Correct me if I’m wrong but Intersonics made a subwoofer that used a servo motor, cam, and belt attached to a speaker cone. Tom Danley I believe was responsible for its design. He caused a lot of commotion playing sub audible frequency sweeps at a NAMM show. People were frightened the structure of McCormick Place (Chicago) was giving way.

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