keeps secrets safe and sound
Technology Research News
Encryption usually means disguising data
using a numerical formula. Researchers from the Naval Postgraduate School
have come up with a scheme for encrypting sound that protects the information
by taking advantage of the way sound waves propagate.
The scheme focuses a clear audio signal at only one point in space, making
it impossible to listen in from any other point. "It is possible to create
a signal that will focus in a unique location," said Kevin Smith, an associate
professor of physics at the Naval Postgraduate School. At locations other
than the focus, the various sound waves that make up an audio signal arrive
at different times, producing interference instead of a coordinated signal.
The technique can also be used to improve sound quality in any room, including
home theaters, according to Andres Larraza, a physics professor at the
Naval Postgraduate School.
The scheme is based on time-reversal acoustics signal processing, which
can be used to transmit audio clearly in environments -- like rooms with
cement walls or underwater -- where reflections of sound off different
surfaces at different times cause reverberation or echo.
Audio signals get garbled when sound waves diverge and overlap, causing
the listener to hear the same sound at several different times. "A simple
pulse distorts and spreads in time due to the variety of paths between
source and receiver," said Smith.
A good demonstration is to clap inside the type of tubular slide commonly
found in playgrounds, said Larraza. "The apparent long duration of the
clap is due to all the different propagation paths [from] multiple reflections
of sound inside the slide," he said.
The overlap makes it difficult to distinguish different sounds, said Smith.
"When a sequence of symbols is transmitted... this multipath propagation
may cause the various symbols to overlap, degrading the ability of the
receiver to distinguish the information," he said.
Time reversal acoustics fixes the problem by transmitting sound to a point
and noting exactly when the parts of the signal -- or different paths
-- arrive, then transmitting the same signal with the arrival times reversed,
said Larraza. "This allows the slowest path a head-start and the fastest
path brings up the rear." This way the different sound paths arrive back
at the destination simultaneously, bringing the sound back into focus.
Another way of looking at the scheme is as an encryption
method for all the points except for those where the sound is in focus.
The inherent multipath environment of water "provides for a natural encryption
whereby only a single location can receive the unscrambled message," said
The researchers realized that the reverberant environment of an enclosure
could be used in a similar way as a natural encryption method. The encryption
code is inherent in the structure of the enclosure, and each enclosure
provides a unique type of scramble. Time-reversal acoustics encryption
is "always unique to the environment and independent of [a] signaling
scheme," said Smith.
To demonstrate the method, the researchers used a speaker as a sound source
and two microphones to record at different positions in a concrete chamber
2.59 meters long, 2.41 meters wide and 2.83 meters high.
This type of chamber causes a lot of reverberation, said Larraza. If a
few notes of Beethoven's Fifth Symphony were piped in, for instance "the
first note... would still be playing into the last quarter of the fourth
note, overlapping all along with the other two notes in between, resulting
in cacophony," he said.
Applying time-reversal acoustics to the first note in Beethoven's Fifth
would allow it to play its intended duration at the focal location, while
anywhere else in the room it would reverberate longer, said Larraza.
The researchers tested their time-reversal based encryption algorithm
by sending sets of signals representing binary bits -- the ones and zeros
of digital communications. They transmitted each signal with enough time
between transmissions for all the multipath signals to arrive at each
receiver, then used the time records for each receiver to build symbols
out of the time-reversed signals, he said. "In a room with complex geometry,
the time record of each reception measured by each receiver is unique."
In the researchers' experiment, the source transmitted simultaneously
to each of the microphones its own unique message. "This would be equivalent
to applying from the same source Beethoven's Fifth and the Beatles "All
You Need Is Love" and being able to listen... to each one in their unique....
location; at the Beethoven spot, "All You Need Is Love" [would] not be
playing. Anywhere else in the room there [would be] noise," said Larraza.
The method could be combined with traditional encryption for added security,
said Smith. It may also prove useful for enhancing sound quality in home
theaters and concert halls, said Larraza.
The research is interesting, novel and potentially useful, according to
Manuel Torres, a researcher from the Superior Council of Scientific Research
in Spain, and Jose-Luis Aragon, a researcher at the National University
of Mexico. "The application of this technique to encrypt some messages
is a clever and original idea," said Torres.
The technique also looks promising as a method to enhance sound quality
in buildings, Torres said. "The ability to focus a full message in space
and time... and simultaneously send multiple messages from one source
to different locations in [an] enclosure makes the technique potentially
applicable in architectural acoustics."
In addition, because these messages are destroyed if they are intercepted
before they reach their destination points, the method could find application
in military underwater communications, said Torres.
In general, waves of any kind are a potentially powerful alternative to
numerical encryption techniques, Torres added. Waves are the result of
periodic disturbances in any medium or in space. Sound waves, for instance,
result from vibrations in elastic media, including air, water or the earth,
that can be sensed by the human ear.
The technique could be applied immediately, according to Larraza. "Using
time reversal acoustics as a diagnostic tool for enhancing sound quality
is technologically plausible at this time," he said.
Larraza's and Smith's research colleague was Michael G. Heinemann. They
published the research in the January 28, 2002 issue of Applied Physics
Letters. The research was funded by the Office of Naval Research (ONR).
TRN Categories: Applied Technology; Physics
Story Type: News
Related Elements: Technical paper, "Acoustic Communications
in an Enclosure Using Single-Channel Time-Reversal Acoustics," Applied
Physics Letters, January 28, 2002.
29/June 5, 2002
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