Reverb keeps secrets safe and sound

By Kimberly Patch, 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 Smith.

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).

Timeline:   Now
Funding:   Government
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.


May 29/June 5, 2002

Page One

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Reverb keeps secrets safe and sound


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