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Deciphering Gunshot Recordings
Robert C. Maher and Steven R. Shaw Electrical and Computer Engineering Montana State University - Bozeman
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Outline Introduction Gunshot acoustics near the firearm
Mechanical action Muzzle blast Shock wave (supersonic projectiles) Gunshot acoustics near the target Propagation and Attenuation Reflections and multipath issues Limitations Conclusions
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Gunshot Analysis Applications
Real Time Tactical Information Gunshot Detection Sniper Localization Forensic Reconstruction Timeline Assessment Shooter Location and Orientation Firearm Classification
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Sound Characteristics
Acoustic behavior depends upon: Firearm type Projectile parameters Explosive load Distance Meteorology Obstacles
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Gunshot Evidence Near the Shooter
Mechanical Action Muzzle Blast Supersonic Projectile (shock wave) Surface Vibration Reflections Microphone Type and Location Audio Recording Issues (e.g., codecs)
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Supersonic Projectile
Shock Wave Front Trajectory ( velocity c ) Shock Wave Front Trajectory ( velocity c ) Bullet Trajectory qM, slow qM, fast ( velocity V ) Shock Wave Cone: Mach 3.0 Shock Wave Cone: Mach 1.05
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Multipath: Ground Reflection
Expanding Shock Wave Ground Surface Microphones Shock Wave Ground Reflection
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Shock Wave Timing Example
B C Gun Barrel Shock Wave Trajectory 3.7 meters 6.3 meters qM = 23° B’ X Microphones L R Bullet speed at muzzle: 2728 ft/sec (831.5 m/sec) Speed of sound (c): 1075 ft/sec (328 m/sec) Mach Number (V/c): 2.54 Mach Angle (qM): 23.2°
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Gunshot Recording: Path 1
5 10 15 20 25 30 35 40 45 -0.2 -0.15 -0.1 -0.05 0.05 0.1 0.15 0.2 Time [ms] Amplitude [linear scale] Muzzle Blast Muzzle Reflection L R Ch 1 R Ch 2 L Shock Wave Shock Wave Reflection
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Gunshot Recording: Path 2
5 10 15 20 25 30 -0.2 -0.15 -0.1 -0.05 0.05 0.1 0.15 0.2 Time [ms] Amplitude [linear scale] L R
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Gunshot Recording: Path 3
5 10 15 20 -0.2 -0.15 -0.1 -0.05 0.05 0.1 0.15 0.2 Time [ms] Amplitude [linear scale] L R
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Gunshot Recording: Subsonic
2 4 6 8 10 12 14 -0.2 -0.15 -0.1 -0.05 0.05 0.1 0.15 0.2 Time [ms] Amplitude [linear scale] L R
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Gunshot Evidence: Downrange Near the Target
Propagation effects Attenuation due to acoustical spreading Temperature gradient Wind gradient Obstacles: reflection and diffraction Path elevation trajectory Projectile deceleration
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Projectile Deceleration
0.25 s 0.5 s 0.75 s 1.0 s 1.25 s Figure 11. Shock wave profile versus time as a function of distance downrange
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Projectile Deceleration (cont.)
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Effect of Temperature The speed of sound (c) in air increases with increasing temperature: (T in °C and c0 = 331 m/s) Hotter air near ground: curves upward Cooler air near ground: curves downward
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Downrange Examples Shock Wave Muzzle Blast 91 Meters Downrange
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Downrange Examples (cont.)
Shock Wave Muzzle Blast 352 Meters Downrange
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Downrange Examples (cont.)
Shock Wave Muzzle Blast 732 Meters Downrange
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Conclusion Closely-miked gunshot recordings: Distant recordings:
Geometric acoustics works well Distant recordings: Propagation effects lead to greater uncertainty due to altered sound path Reflections and reverberation dominate Verification is needed to assess the validity of acoustic analysis claims
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