The firing of a gun or an explosive device creates a loud, impulsive sound that can, under optimum environmental conditions, be detected above urban background noise up to two miles away from the firing incident location. The operation of ShotSpotter is understandably subject to the laws of physics and acoustic propagation.
The source of a pulse (a sound that goes bang, boom, or pop) is located using a mathematical process called trilateration. Trilateration requires a minimum of three sensors that surround the source of the sound to accurately measure the time when the sound was detected by each sensor. When three or more sensors participate in the detection, an automatic calculation will find a location solution which minimizes the error to the greatest extent possible.
ShotSpotter detects and properly geo-locates (provides latitude and longitude) over 90% of incidents within the coverage area, accurate to within a circle whose radius is 25 meters.
SST, Inc. does not guarantee 100% detection because real world, urban environments may contain intervening buildings, topography, foliage, periods of increased traffic or construction noise, and other urban acoustic noises that may mask, muffle, or entirely prevent the sound of a weapon discharge from reaching the sensor(s) thereby preventing a sufficient number of sensors from detecting the sound of the discharge to generate an alert.
These same obstructions may also alter or modify the audio characteristics of the discharge significantly enough so that it no longer fits within the sensor’s programmed detection parameters causing it to be disregarded by the sensor(s) as not impulsive.
Other factors, such as obstructed or attenuated muzzle blast, weapon discharge in an enclosed space, or if the weapon discharged is of .25 or smaller caliber, may also prevent the sensor(s) from not detecting all, or some shots fired.
Acoustical data analysis of a gunfire incident is complex and not comprehensive. The data and conclusions of an Incident Report should be corroborated with other evidentiary sources such as recovered shell casings, and witness statements.