gunshot recordings from a criminal incident: who …...muzzle reflection shock wave shock wave...
TRANSCRIPT
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Gunshot recordings from a criminal incident: Who shot first?
Robert C. Maher Montana State University – Bozeman
171st Meeting of the Acoustical Society of America – Salt Lake City – May 2016Paper 2aAAa7
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Outline• Introduction
– Audio forensic analysis– Gunshot acoustics– Forensic acoustical interpretation
• Example 1: who shot first?• Example 2: who shot first?• Conclusion
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Introduction
• Audio Forensics is the field of forensic science relating to the acquisition, analysis, and evaluation of sound recordings that may ultimately be presented in court or some official venue.
• Primary forensic concerns:i. authenticityii. enhancementiii. interpretation and documentation
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Gunshot Acoustics
• Confined combustion of gunpowder propels the bullet out of the gun barrel.
• The explosive gases expand rapidly behind the bullet causing the muzzle blast.
• The muzzle blast can exceed 150dB in the vicinity of the firearm.
• The muzzle blast is directional.
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Gunshot Acoustic Evidence Near the Shooter
• Mechanical Action• Muzzle Blast• Supersonic Projectile (shock wave)• Surface Vibration• ReflectionsNote:
Limitations of microphone and audio recording chain (e.g., codecs)
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Example Gunshot Recording
0 5 10 15 20 25 30 35 40 45-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
Time [ms]
Am
plitu
de [l
inea
r sca
le]
Ch 1 RCh 2 L
L R
Muzzle Blast
Muzzle Reflection
Shock Wave
Shock Wave Reflection
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9mm Handgun (anechoic)
0 2 4 6 8 10 12-800
-600
-400
-200
0
200
400
600
800
Time [msec]
Am
plitu
de [P
a]
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Forensic Example 1
• Two armed perpetrators on a city street– Witnesses and evidence that both individuals
shot handguns, but not clear who fired first.– One of the individuals died due to gunshot
wound, the other individual survived and claims self defense.
– A residential surveillance system recorded the sound of gunfire, but not video of the scene.
– Who shot first?
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First two shots…
Echo: 0.54 s
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First two shots (cont.)
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Echo 0.54 seconds
• Air temperature 16 celsius: c = 341 m/s• 0.54 seconds == 184 meters
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Street Geometry
Distance: ~90 meters
Additional path: ~180 meters
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Echo 0.54 seconds
• Echo explanation fits geometry, but why not a distinct echo for both gunshots?
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Witnesses agree…
• Individual #1 (deceased) was pointing firearm southward
• Individual #2 (claiming self defense) was pointing firearm northward
• Conclusion: northward-pointing gun facing reflecting surface caused more distinct echo, so Individual #2 fired first.
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Forensic Example 2
• Law enforcement officers approach a knife-wielding individual.– One officer has a Taser, the others have
handguns pointed at the suspect.– Shots are fired and the Taser is deployed.– Audio recordings made by Taser and by
nearby dashboard cameras in police cars– Which was first, gunshot or Taser?
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Taser and Shots
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Taser Sequence
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Dashcam Recordings
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Analysis• Initial sound of Taser is obscured.
– Dashboard recorders pick up gunshot sounds, but not Taser.
– Plan: work backward to align timing based on last gunshot.
• Conclusion:– First gunshot at 15.4 seconds– Taser deployment at 15.8 seconds– Gunshot precedes Taser by 0.4 seconds.
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Conclusion• Care is needed to assess the validity of acoustic
analysis claims involving gunshot evidence.• Forensic gunshot recordings generally contain
background noise, distortion, clipping, multi-path reflections and reverberation.
• In some cases, timing analysis of the gunshot sounds and acoustic reflections is appropriate.
• Situations involving multiple concurrent recordings of the same event are particularly useful.