generated by an acoustic concentrator levitates a ring of aerosol
Finding out what is in a closed container
can be a daunting task when you can't open iteither because its
contents may be toxic or because it is someone else's property.
"Why not just tap and listen?" Dipen Sinha once suggested to
a group of government officials gathered to assess ways to verify compliance
with the 1990 U.S./Soviet Union Chemical Weapons Treaty. Requiring only
a metal key, his simple strategy was nonetheless effective.
Since formalizing that idea by developing a sound-based tool for noninvasive
fluid identification, Sinha has assembled a team of talented scientists
and technicians, inventors who seem capable of devising endless uses for
sound. With backgrounds in theoretical physics, chemistry, engineering,
and hardware and software design, this versatile team has tackled such
questions as "is this food fit to eat" and "where are the
best oil deposits?" From answering "what's in the drum"
to "what's in your blood," the team's sonic sniffers promise
continued solutions for practical problems.
Sound as Pressure
Underpinning many of the team's inventions is the basic science of sound
as pressure waves (see the sidebar on wave phenomena). The vibrations
of a loudspeaker inform us that its speaker cone is intermittently pushing
(exerting a force on) the surrounding air. Such intermittent pressure
on air molecules sets them into wave motion. That motion subsequently
vibrates your eardrums, the first step in sound perception.
But high-frequency sound pressure can also be applied to microscopic
structurescells, viruses, and the molecules in a broad range of
liquids and gases. The team's specialty is devising ways of carefully
controlling sound pressure to use it either as a probe for identifying
the contents of closed containers or as a microscopic mover, capable of
concentrating airborne or liquid-borne particles to facilitate their analysis.
Many of the team's inventions rely on the positive reinforcement of
sound-pressure waves to generate larger-amplitude wavesthe phenomenon
of resonance or "standing waves." Church bells in a carillon
exemplify this acoustic phenomenon. Differing in size and often in thickness,
bells not only produce a characteristic frequency (pitch) when struck,
but they continue to resonate with one or more frequencies thereafter.
Each bell's unique characteristics as a sound conductor define the frequencies
that reinforce one another, thus setting up standing waves, which we hear
as a bell's lingering reverberation.
When the physical properties of a container's contents are unknown, the
technique of swept-frequency acoustic interferometry can reveal them and
be used to characterize the contained substance(s). By generating sound
waves of many different frequencies (sweeping the frequency) and introducing
them, one at a time, into the wall of the container, an investigator can
empirically discover the characteristics of the container's wall and its
Because liquids differ in properties such as the speed at which they conduct
sound and how much they absorb sound waves, a container's contents affect
sound-wave transmission, which consequently exhibits peaks at certain
frequencies. The contents thus "pick out" their own resonant
frequencies. The resulting spectrum of standing waves superimposes two
sound signaturesone for container's wall and one for its contents. This
"resonance spectrum" is monitored by a sensitive detector, and
mathematical relationships are used to extract properties such as liquid
sound speed, sound absorption (attenuation), and density. When compared
against a database of acoustic signatures, the properties derived from
the resonance spectrum can identify a container's contents.
In addition, by improving on existing sound-projection
technology using carrier waves, the team can introduce its resonance-probing
sound waves into a container from distances of up to 15 feet. When containers
may enclose highly toxic or inflammable substances, such standoff diagnosis
is clearly desirable. [figure: Acoustic Interferometry]
One of many acoustic techniques whose development was sponsored by the
Department of Defense, acoustic interferometry also has medical applications.
An example is diagnosing arthritis or osteoporosis by comparing the acoustic
characteristics of diseased joints and bone with those of their healthy
counterparts. Novel applications are likewise anticipated as sound projection
techniques continue to improve. For example, using sound pressure to launch
decontaminating vapors could help to sanitize buildings, a need illustrated
by the massive post-9/11 effort required to decontaminate the U.S. Senate
offices of anthrax.
Corralling Particles with Sound: Acoustic Concentrators
Sound pressure and resonance also combine in the functioning of acoustic
concentrators. Using sound to move particles, concentrators are basically
small hollow cylinders of piezoelectric material. When stimulated with
low-power alternating voltage, the material changes shape and intermittently
pushes on any air or liquid contained inside the cylinder. These pressure
surges create standing waves in that internal medium, which force the
enclosed molecules into a set of concentric rings. Air or liquid molecules
and any suspended contaminants are more concentrated within the rings,
less concentrated between them.
A liquid acoustic concentrator uses resonant sound pressure to move particles
suspended in fluids that are contained within the cavity of a cylindrical
transducer (a cylinder of piezoelectric material that converts electrical
signals to sound pressure). The cavity's resonance frequency changes as
the particles are concentrated. An investigator can query the liquid inside
the concentrator about its particle content by observing how the cavity's
resonance changes as a function of time. For example, a friendly yogurt
bacterium like acidophilus differs in size, shape, and other physical
properties from a food-spoiler like salmonella or a lethal bioterrorist
agent like anthrax, and so its influence on the liquid and how it concentrates
under sound pressure will also differ. Friend can thus be distinguished
from foe within a few seconds of examining a container suspected of bacterial
This technique builds on previous success in which acoustic methods were
used to detect the presence of salmonella contamination in unbroken eggs.
Nor are acoustic concentrators limited to threat reduction. Applied slightly
differently, resonant sound pressure can become a concentrator of blood,
gently separating cells (the suspended particles) from plasma (the liquid).
The team has also devised an aerosol acoustic concentrator capable of
concentrating airborne contaminants fifty to a hundredfold. Inserting
this simple, inexpensive device into the inlet of portable air monitorssuch
as those that would be used to screen a workplace for anthrax contaminationboosts
contaminant-detection sensitivity by that same fifty to a hundredfold,
making it less likely that potentially lethal contaminants will escape
Raising a Flag
Recently, the team has expanded its repertoire of threat-detection tools
beyond strictly acoustic ones. Suppose you're in charge of airport security
and need to rapidly screen passengers to narrow the field of candidates
for more detailed searches. You might find use for a fifty-dollar dielectric
sensor developed by the team. Held close to beverage or food containers,
the sensor will, with the click of a button, unobtrusively establish whether
they contain a benign water-based liquid or a possibly explosive hydrocarbon
By sending an electromagnetic pulse into the liquid and measuring the
capacitance of a circuit that includes container and contents, the sensor
assesses the liquid's dielectric propertyits ability to store charge
and potentially conduct a current. As anyone knows who has been ordered
out of a swimming pool during a thunderstorm, water is a good electrical
conductor. Hydrocarbons, however, are not. If a passenger's response to
a polite inquiry about a container's contents ("it's baby food,"
for example) did not match the sensor's response, you might justifiably
pursue a more comprehensive search.
What is remarkable about Sinha's team is its ability to see a host of
problems that could lend themselves to variations on its technologies
and then to respond by devising an invention. For example, the team is
currently engaged in discovering solutions to such problems as imaging
breast cancer without exposing women to the high-energy radiation involved
in mammography, monitoring blood-sugar levels without the need for needle
sticks, noninvasively determining whether a shipping container has been
tampered with, and remotely detecting structural defects in natural-gas
pipelines without interrupting delivery to consumers.
The team's contribution to safety, health, and security is evident in
each of these envisioned sound solutions. And with prospects for combining
many of its inventions into suites of progressively more useful tools,
the sounds seem destined to grow only sweeter.
Drop a pebble into a pond, and the disturbance will produce ripples
on the surface. If you float a scrap of paper at one point on the
ripple (wave) pattern, the paper will bob up and down as "surges"
of water molecules intermittently push on the paper's underside. Each
surge represents a momentary increase in pressure. Sound waves use
air molecules as the "pushing medium" and analogous surges
and decreases in pressure occur.
Example of the standing-wave pattern that develops in a liquid after sound
stimulation with an acoustic concentrator. Blue particles are more concentrated
in the concentric blue rings, less concentrated in the lighter-colored spaces
Dielectric sensor gives a positive indication (red light) that this liquid
is water based.
Dipen Sinha received a Ph.D. in physics from Portland
State University, after undergraduate and graduate education in
India. He holds twelve U.S. patents with several more pending and
has received three R&D 100 Awards.
Greg Kaduchak received his Ph.D. and M.S. in physics
from Washington State University and a B.A from Saint Louis University.
He is a recent recipient of the FBI Director's Award and an R&D
Chris Kwiatkowski received his Ph.D. and M.S.
in physics from Washington State University and a B.S. from the
University of Toledo. In addition to acoustics, his research interests
extend to optics and digital signal processing.
Other team members include Dr. Kendall Springer, Dr. Alexander
Kogan, David Lizon, and Greg Goddard.