147th ASA Meeting, New York, NY

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A Shot in the City:
Locating a sound source in an urban environment
[Click here for printable PDF version]

Lanbo Liu1,2 e-mail: Lanbo.Liu@erdc.usace.army.mil
Donald G. Albert1 e-mail: Donald.G.Albert@erdc.usace.army.mil

   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
Presented Friday morning, May 28, 2004
147th ASA Meeting, New York, NY

Image of Soldier firing a Rifle in an urban area.

Gunshot noise measurements in an urban area.
(Photo J. Aguiar, USA ATC)

     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?)

      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.

Arial view of training village used in this work.
Aerial photo of the training village where measurements
were made. Most of the buildings are two stories high
and made of concrete blocks. The distance from the
shot to microphone No. 8 is 100 m.

      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

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.

Image of equipment used in recording the sounds of the shot.

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.


Forward: Sound from the shot

Image of computer simulation showing sound waves from shot bouncing off buildings.  Click on this image to view movie of the propagation of the sound waves.

Reverse: Locating the shot

Computer simulation showing sound waves from a
shot (the black "X") bouncing off buildings in the
village. Click on the image above to view movie
of the propagation of the sound waves.

The result of modeling the sounds using the
Time Reversal Method. Notice the concentration
of sound waves (red blob) indicating the source
location of the original shot sound. Click on the
image above
to view a movie of the
model run.

     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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