'Steven Beck, principal scientist at BAE Systems, along with FBI agents Hirotaka Nakasone and Kenneth Marr, performed gunshot accoustic studies in a controlled environment by placing microphones at a range of angles and distances from each blast in order to capture the sound pattern of a single round from multiple points of view. Gunshots produce two different sounds -- a "bang," caused by rapid expansion of gasses that push the bullet through the barrel, and a "crack," caused by shock waves in the air made by supersonic bullets.....Beck said analysts need to understand the underlying accoustics of impulse sounds, and then know what variations can occur and how the recording conditions can affect the signals.
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According to this research, a shot fired into the air produces a wildly different waveform from one fired in the direction of a recorder, even when both shots came from the same gun. Likewise, a heavy rifle blast fired at a great distance from the microphone can produce a sound signature that is almost indistinguishable from one produced by a smaller, lighter gun fired at c;lose range.
There was another surprise -- a bonus "bang" caught by recorders positioned at 90-degree angles to the barrel. The Beck team traced thios sound to gasses leaking and expanding out the side of certain types of firearms. This means investigators could potentially exploit this extra "bang" to identify the direction of the weapon from the recording device....more recent work has shown that sound level and waveform details between on-[axis and off-axis recordings of the same firearm are often significantly greater than the difference between two firearm types at the same azimuth.
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Maher said if the mobile device is located near the shooting position, the high level sound of the gunshot will typically overload the microphone, input amplifier, and related electronics, resulting in a severely clipped and distorted waveform. Then this distorted gunshot signal is presented to the device's audio coding system, which is usually a speech coder designed to represent intelligible speech, not gunshots and highly distorted waveforms. "Thus, interpretation of gunshot accoustic evidence that has been passed though a cell phone's audio system is an area in need of further research," he said.
"In situations where a relatively pristine recording is obtained there may be opportunities to identify useful forensic features from gunshot recording, but in most real-world cases the quality of the recorded waveform will be poor," he said.
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"If more than one recording is available, such as multiple devices in different locations capturing the same event, it may be feasible to reconstruct the most plausible location of the firearm and its orientation," he said.'
The apparatus would be sequentially placed along hallways and stairwells, which either-or plausibility would activate alarms and automatic partitions.
