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Deciphering Gunshot Recordings

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Presentation on theme: "Deciphering Gunshot Recordings"— Presentation transcript:

1 Deciphering Gunshot Recordings
Robert C. Maher and Steven R. Shaw Electrical and Computer Engineering Montana State University - Bozeman

2 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

3 Gunshot Analysis Applications
Real Time Tactical Information Gunshot Detection Sniper Localization Forensic Reconstruction Timeline Assessment Shooter Location and Orientation Firearm Classification

4 Sound Characteristics
Acoustic behavior depends upon: Firearm type Projectile parameters Explosive load Distance Meteorology Obstacles

5 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)

6 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

7 Multipath: Ground Reflection
Expanding Shock Wave Ground Surface Microphones Shock Wave Ground Reflection

8 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°

9 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

10 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

11 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

12 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

13 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

14 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

15 Projectile Deceleration (cont.)

16 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

17 Downrange Examples Shock Wave Muzzle Blast 91 Meters Downrange

18 Downrange Examples (cont.)
Shock Wave Muzzle Blast 352 Meters Downrange

19 Downrange Examples (cont.)
Shock Wave Muzzle Blast 732 Meters Downrange

20 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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