Fighting crime with acoustic detection systems

It was good news for me a few days back when I spotted a Seattle Times headline reading: Seattle mayor pushes for gunshot-detection system. Hoping that the City of Seattle was purchasing acoustic cameras that might be used to prove the real estate mobbing in my real estate speculators’ paradise of a neighborhood in northeast Seattle, I wasted no time in reading the article.

Seattle Mayor Ed Murray’s plan is to deploy an acoustic detection system in Rainier Valley and the Central District. The system combines the use of microphones, sensors and overheard cameras to detect and alert police to the gunfire. The purely acoustic facet of the system is the microphones that listen for the sound of gunfire.  Once gunshots are heard, the system triangulates the sound of the gunfire (not unlike the real estate mobbers say they’ve “triangulated” me) to locate its source (“Seattle mayor pushes for gunshot-detection system,” Seattle Times,

Both the acoustic gunfire detection system and the acoustic camera find the source of sound. The difference is that probably that the gunfire detection system sends a message that provides the source location, for example, the GPS coordinates of the source and system sensors may trigger the overhead cameras to pan from the source of the gunfire to another location, perhaps the trajectory of the gunfire or perhaps after the system is alerted to gunfire the camera zooms out and a wide-angle view of the street shows any movement that occurs in the aftermath of the shots. According to Mayor Murray, police will be alerted even before there’s time for residents to make 911 calls about the gunshots. The hope is that it will become easier to identify perpetrators and to arrest them.

The gunshot-detection system, also called an acoustic detection system, is not an acoustic camera, and I suspect that its listening and recording capabilities are specifically limited to guard against the invasion of privacy rights.  In fact, I wonder if it can tell a gunshot from another sound, for example, the backfire of a car, and listen only for sound with the characteristics of gunfire. Just the same, a gunfire-detection system, like the acoustic camera, is another interesting application of sound (Polytec: Acoustic Camera Applications,

An acoustic camera also shows the source of the sound; these cameras can be used to find stubborn sounds that cannot otherwise be tracked in automobiles, for example. The acoustic camera, however, identifies a “sound emission” by visualizing the sound. The imagery appears something like a holograph or Kirlian photography (“Kirlian photography” in Wikipedia, Applications for acoustic cameras include analysis of sounds for noise reduction in urban areas, to locate troublesome engine noises, and to analyze noise in passenger compartments of trains and planes.

This is an image of an ACOEM acoustic camera with an array of microphones. (01DB Noise Inspector Acoustic Camera Solutions,



Here is an array of acoustic cameras.


These product pictures for Polytec acoustic cameras show the visualization of sound in “3D acoustic fields.” Polytec promises the resolution of “sound sources on small devices all the way up to large and awkward structures.”


These holographic images use color-coding to help to identify and locate the sound. With imaged sound, the analyst can mouse over the image and “play” the sound, at least for certain sound camera applications.


With this kind of flexibility, the acoustic camera is a promising device to show the process of mobbing a house from locations of proximity by directional speaker and ventilation systems. The image I envision frames the victim house, cropping out the suspected mobbing houses on either side. Tighter framing should still be able to show the direction of the sound and the likely location of its origin, hopefully even from directional sound forces like parametric and parabolic speakers.

Parametric speakers use ultrasonic sound that is emitted from the speaker as two separate beams, one of which is a reference tone. When they strike an object, the beams collide and the ultrasonic sound falls back into the human-audible range. This adds a bit of a variable to its detection and it is possible that the best shot at detecting ultrasonic sound is to step into its trajectory. For directional sound that is transmitted onto the surfaces—most likely window panes—of the victim house, however, I would guess that the acoustic camera might render the victim home as though it is “haloed” by sound.

One of the problems in proving mobbing is its careful pairing with intense monitoring of the street for police, cars with radios, or those who might come close enough to the houses to detect the mobbing. This makes it difficult to get forensic evidence to back up the claims of those who are mobbed. And this is why the criminal real estate speculators or nasty neighborhood watch organizations who dare to commit a slew of felony crimes to turn over houses for development believe they are protected from exposure.  Obviously setting up a large sound camera would not be easy, at least not in front of a house being mobbed. Perhaps one could be used from a distance depending on the landscape of the neighborhood. But handheld cameras also exist for the field. With such a camera, it might be possible to secrete it in a mesh or other permeable sack that will allow it to sample the acoustic field discretely. It could probably also be affixed to a tinted back window of a car. Handheld acoustic cameras like the  SeeSV-S205 Sound Camera shown below would work for this approach to capturing mobbing (SeeSV-S205 Sound Camera,


In a photograph of a house undergoing mobbing, there should be space between the frame edges and the structure of the house so that the direction of the acoustic emission can be shown even if privacy laws make it difficult to frame in the mobbing house for the source. Where this is possible, and it would not be possible in cities like San Francisco or New York where so many homes abut neighboring structures, the photographer should be able to avoid challenging privacy laws in a manner that could shut down investigation of the sound source. In the case of the gunfire-detection system, it is difficult to limit the sampling of sound and picture in a manner that completely guards privacy rights, but I would be surprised if any victim of real estate mobbing would not welcome some investigator from police or city to frame her home in the lens of the acoustic camera and capture the sound being projected into the environment from nearby locations.

I do not know enough about real estate mobbing in Europe and Spain to know if the acoustic camera might be helpful there. If European mobbing capitalizes on the methods of private investigators or the military by using parabolic speakers or puts to work products gaining strength in consumer markets like directional speakers or contact-based speakers that transmit sound through vibration, I would suspect it could be a useful tool to clamp down on criminal and covert forced eviction.



One response

  1. Pingback: Sound is a pressure, and a tip on getting the most out of acoustic board (part 2) « On being Mobbed

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