A Shot in the City:
Locating a sound source in an urban environment
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| Lanbo Liu1,2 e-mail: Lanbo.Liu@erdc.usace.army.mil Donald G. Albert1 e-mail: Donald.G.Albert@erdc.usace.army.mil 1USA ERDC Cold Regions Research and Engineering Laboratory 72 Lyme Road, Hanover, NH 03755 2Department of Geology and Geophysics University of Connecticut 354 Mansfield Road, Storrs, CT 06269 Popular version of paper 5aPAb5 |
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Sound propagation in an
urban environment is complicated by the presence of buildings. The
sound waves reflect from building walls and diffract (or bend) around
corners. Because of the many diffractions and echoes, it is difficult
for a listener to locate the source of the sound waves, especially
when the source is not directly visible. (Where is that ambulance siren
coming from?) |
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At the last Acoustical Society of America Meeting, we discussed measurements of these effects made in an artificial urban setting (a full scale model village used for training) and a method of mathematically simulating these sounds on a PC. In this paper, we apply a technique called time reversal processing to locate sound sources in urban areas. This method has been previously used in medical imaging and treatment, underwater communication, and other applications, and a short course on this method was taught at the last ASA Meeting by ASA Fellows Mathias Fink and William Kuperman. We are applying it to urban sound propagation for the first time. |
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How does it work? In the time
reversal method, a network of simple, low-resolution sensors in
the urban area detects sound waves. These signals are complicated,
since they include all the many echoes and |
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other arrivals
from the wave "bouncing" around amongst the buildings. We
then turn completely to a computer. It receives the sound signals from
the sensors, and we use it to generate a backwards version of the detected
sound waves. Then, using a model of sound propagation in the urban
area, we rebroadcast the sound waves in a virtual urban environment.
The waves eventually return and focus at the original source point,
enabling us to pinpoint the location of a sound, such as an explosion
or gunshot! By having a few microphones somewhere in the village, we
can use the computer model and the time reversal technique to locate
the sound source. |
 Equipment used in recording the sounds of the shot. |
By measuring the sound at a few locations in the village and using the computer model to propagate them back from the receiver locations, a focus of sound energy occurs at the original source location. As you watch the model run, notice that the sound waves emanate from the various speakers, bounce off of the buildings, and concentrate (red blob, image below, right) at the approximate source of the original shot, ("X" in the image below, left). This model run shows that the correct source location is found from just a few sensors, all of them being shielded from the view of the source location. With further development this method has the potential of improving sound source (sniper) detection and other sensor systems.
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Forward: Sound from the shot
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Reverse: Locating the shot
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Computer simulation showing
sound waves from a |
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In the ASA short course, Drs. Fink and Kuperman showed that time-reversal procedure also works if you physically rebroadcast the sounds into the actual environment, such as an underwater minefield, or a kidney-stone patient's body. They gave many examples of applications in the medical, communication, and signal processing fields. [We thank the many colleagues who have helped with this project, and the U.S. Army PM-CCS and the U.S. Army Corps of Engineers for funding this research.] |
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