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Deciphering Gunshot Recordings

Deciphering Gunshot Recordings. Robert C. Maher and Steven R. Shaw Electrical and Computer Engineering Montana State University - Bozeman. Outline. Introduction Gunshot acoustics near the firearm Mechanical action Muzzle blast Shock wave (supersonic projectiles)

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Deciphering Gunshot Recordings

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

  5. Gunshot EvidenceNear 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. Expanding Shock Wave Shock WaveGround Reflection Microphones Ground Surface Multipath: Ground Reflection

  8. 3.7 meters L R Microphones qM = 23° B Shock Wave Trajectory X 6.3 meters C B’ A Gun Barrel Shock Wave Timing Example 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. Muzzle Blast 0.2 Shock Wave Ch 1 R Ch 2 L 0.15 0.1 0.05 Amplitude [linear scale] 0 -0.05 Muzzle Reflection -0.1 Shock Wave Reflection -0.15 -0.2 0 5 10 15 20 25 30 35 40 45 Time [ms] Gunshot Recording: Path 1 L R

  10. 0.2 0.15 0.1 0.05 0 Amplitude [linear scale] -0.05 -0.1 -0.15 -0.2 0 5 10 15 20 25 30 Time [ms] Gunshot Recording: Path 2 L R

  11. 0.2 0.15 0.1 0.05 Amplitude [linear scale] 0 -0.05 -0.1 -0.15 -0.2 0 5 10 15 20 Time [ms] Gunshot Recording: Path 3 L R

  12. 0.2 0.15 0.1 0.05 0 Amplitude [linear scale] -0.05 -0.1 -0.15 -0.2 0 2 4 6 8 10 12 14 Time [ms] Gunshot Recording: Subsonic 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. 1.25 s 1.0 s 0.75 s 0.5 s 0.25 s Projectile Deceleration 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. Shock Wave Muzzle Blast Downrange Examples 91 Meters Downrange

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

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

  20. Conclusion • Closely-miked gunshot 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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