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 ExampleB 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 15 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 25 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 35 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: Subsonic2 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