NIOSH Research Activities Related to Impulse Noise

Auditory Hazards of Bullet N-Wave Exposure Gregory A. Flamme, SASRAC William J. Murphy, NIOSH The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.. Products mentioned in this presentation are not endorsed by CDC/NIOSH. National Hearing Conservation Association 2018 Conference, Orlando FL

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Stephenson and Stephenson Research and Consulting

What Causes an N-Wave? • Displacement of air in the path of a supersonic projectile • Temperature changes alter the speed of sound, permitting the build-up of acoustic energy

Settles, 2006; U.S. Army HEL TM 11-65; sadefensejournal.com

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Who Might Be Exposed to N-Waves? Law Enforcement and Civilians Military Personnel • Occupational and recreational tactical shooting teams • Gun violence

www.youtube.com, Polenar Tactical

• Warfighters advancing on an objective • Engagement in standard formation

U.S. Army Field Manual 3-21.8

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Acoustic Characteristics of N-Waves Rapid rise, gradual fall, rapid return to baseline Details depend on

8000

6000

• Projectile dimensions

• Projectile velocity

• Pressure increases with velocity

• Miss distance

• Pressure decreases with distance

Pressure, Pascals

• Pressure increases with diameter • Duration increases with length

4000

2000

0

-2000

-4000

-6000 -0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

Time Relative to Wave Onset, milliseconds

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Research Questions • Can a .50 Beowulf® rifle generate N-waves likely to produce auditory hazard? • If so, how close does it have to be to the ear? • How can the miss distance be measured acoustically? • Do we have the nerve to shoot toward a microphone?

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Methods • Environment

• Unoccupied outdoor firing range used by law enforcement personnel

• Sound source

• Semi-automatic rifle chambered for .50 caliber Beowulf® ammunition • Firing stand • Ammunition • • • • • •

200 g, 2000 fps,ARX 335 g, 1880 fps,FMJ 335 g, 1900 fps,HP 350 g, 1661 fps,Brass Spitzer 350 g, 1570 fps,Brass HP 400 g, 1820 fps,FP

• Instrumentation

• Data Acquisition system • National Instruments PXIe-4499 • 200 kHz, 16-bit

• Transducers, preamplifiers, power supplies • G.R.A.S. Type 12 AA power supplies • G.R.A.S. Type 26 AC or Type 26 TC preamplifiers • G.R.A.S. Type 40 DP 1/8” pressure-calibrated microphones

• Analyses

• MATLAB SASRAC

Microphone Locations 15 cm from left 35 cm

30, 45, 90, or 135 cm

Locations formed a plane from three microphones, plus a rear microphone to determine bullet speed.

Rear Microphone Front Microphone Target image from Wikipedia.com

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N-Wave Time Differences Arrival order:

1. Front left 2. Lower right, 0.050 ms after front left 3. Upper right, 1.985 ms after front left

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Source Localization Principles r1 Right ear

constant ∆t=

r2 Left ear

(𝑟𝑟1−𝑟𝑟2) 𝑣𝑣

r1

r2

• General:

• Three microphones produce three time differences ∆t21 = (t2-t1), ∆t31 = (t3-t1), ∆t32 = (t3-t2) • Delays for any microphone pair specify a hyperbolic surface of potential source locations having the same difference in path length. • Like the “Cone of Confusion” in spatial hearing SASRAC

Example Localization Process

∆t31 = 0.1985 ms

∆t32 = 0.1945 ms

∆t21 = 0.050 ms

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Calculation of Time Differences • Time differences represent differences in path length to each microphone pair, divided by the speed of sound. • Hyperbolas were identified by finding the source locations satisfying the observed time differences. • The source is at the intersection of the three hyperbolas. SASRAC

Estimated Plane Intersections • Plane intersections approximated intended trajectory. • Effects of rifle repositioning were evident.

15 cm from left 35 cm

30, 45, 90, or 135 cm

0.2

60 cm plane boundary

Estimated vertical distance, m

0.25



0.3


0.35

0.15

0.1

0.05

0 0

0.15

0.3

0.45

0.6

1.35

Estimated horizontal distance, m

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Challenges of a Moving, Off-Center Source • If the trajectory is not equidistant from the microphones, signals come from different locations. • Without correction of source location, trajectory locations are approximate.

Left microphone

Animation by Dan Russell, Ph.D, Penn State University, used with permission

Right microphone

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Peak SPL by distance 190

190

180

180

170

170

170

160

Whitham

Single Shot Limit re: Hodge & Garinther 150

160

Peak SPL, dB

180

Peak SPL, dB

Whitham


1


Unprotected Limit re: MIL-STD 1474E

130

130

130 0.5

Whitham

140

140

0

160


Unprotected Limit re: MIL-STD 1474E 140

1.5

0

2

0.5

1

1.5

0

2

180

180

170

170

170

Peak SPL, dB

180


160

Whitham


1



130

130

130 0.5

Whitham

140

140

0

160


Unprotected Limit re: MIL-STD 1474E 140

1.5

Estimated distance from microphone, m

2

2

Threshold for Eardrum Rupture



Whitham

1.5

190

190

160

1

Far-Field Expectations

400 Gr. Hawk Flat Point

350 Gr. Hornady XTP 190

0.5




Peak SPL, dB

Peak SPL, dB




Peak SPL, dB

350 Gr. Millenium Brass Spitzer

335 Gr. Rainier FMJ

200 Gr. PolyCase Inceptor ARX 190

0

0.5

1

1.5


Hodge & Garinther, 1970; Whitham, 1952

2

0

0.5

1

1.5

2


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Hazard estimates

90

20

88

18

86

16

84

14

Maximum Permissible Exposures re: 85-dB limit

A-weighted 8-hour equivalent continuous level, dB

• Analyses of 3-ms intervals including only the Nwave • Rapid changes in level and permissible exposures at close distances

82

80

78

76

74

72

70

12

10

8

6

4

2

0 0

0.5

1


1.5

2

0

0.5

1

1.5

2


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Summary and Next Steps Summary

• N-waves produced by the .50 Beowulf® rifle exceed the unprotected single-shot limit. • Very few exposures are permissible at miss distances within 0.5 m. • Source localization can be approximated by analyses of time differences • We didn’t have the nerve to fire toward a microphone.

Next steps

• Utilize all four microphone planes • Refine bullet velocity calculations • Allow for multiple source locations • Repeat study with multiple • Rifles and ammunition • Trajectories through the plane

• We did have the nerve to ask an expert to do it.

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Acknowledgements • First Sergeant, Matt Burkhardt Indiana State Police, Versailles District • Terry Heath, Range owner.

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Questions:

Greg Flamme: [email protected] CAPT William J. Murphy: [email protected]

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Improved Measurement Strategy

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