Lessons Learned At DOD Indoor Firing Ranges Chuck Jokel Noise Control Engineer, Army Hearing Program...

Lessons Learned At DOD Indoor Firing Ranges

Chuck JokelNoise Control Engineer, Army Hearing Program Office

UNCLASSIFED

National Hearing Conservation Association Annual Conference25 February 2011

2

PURPOSE: To share information about the challenges of assessing the noise hazard of indoor firing ranges and optimizing solutions for noise control.

UNCLASSIFIEDChuck Jokel MCHB-IP-MHC [email protected]

• Indoor ranges and their noise hazard in general

• Special Operations Command (SOCOM) ranges in particular

• Special challenges dealing with impulsive noise and complex noise

exposures

• Project Approach; comparison and modeling

• Modeling details

• Noise criteria

• Resulting weapon restrictions for untreated spaces

• Potential benefit of different kinds of treatments

Items Covered

3UNCLASSIFIEDChuck Jokel MCHB-IP-MHC [email protected]

First Example of Multi-lane Indoor Firing Range


Second Example of Multi-lane Indoor Firing Range


Narrower Multi-lane Indoor Firing Range


• Multiple locations are in various stages of construction

• All exposed surfaces are acoustically “hard;” either steel or concrete

(except behind the most distant firing position, where there is

acoustical treatment in the ceiling)

• Are multi-shooter tactical ranges: weapons may be fired from any

point up to about 50 m distance across an approximate 30 m width

• For these ranges, the control room has a view of the range and the

bulletproof glass window faces the range

• Other occupied spaces in the buildings housing the range are

physically isolated

SOCOM Indoor Ranges


• Reflections

Special Challenges with Indoor Weapon Noise


The Reflected Ceiling Level is Incorrect


Primary blast and wall reflection

Ceiling reflections

• Reflections

• Clipping



Special Challenges; a case in point


• Reflections

• Clipping

• Standard design tools used to control continuous noise do not apply

to impulsive noise:

– Cannot predict sound field using standard equations

– Cannot predict hazard reduction resulting from using different

acoustical materials, making it hard to

• Determine which material is best to use

• How much material is needed

• Where the material should be applied



• Historically, noise studies have shown the firing of any small arms

weapon indoors (or outdoors) is hazardous

• These findings have led to generalized recommendations:

– Wear hearing protection

– Line wall and ceiling surfaces with acoustically absorbent

materials

– Double-glaze windows for control rooms

– Watch out for flanking paths that could conduct noise to other

noise-sensitive areas

• No distinction in treatment of tactical range vs fixed point range

Indoor Ranges and Their Noise Hazard


• Measure levels in physically similarly sized ranges, but one treated,

one not treated, to gain some data on treatment benefit

• Model the situation to enable treatment optimization, and verify the

model. Base it on the following assumptions:

– Each impulse contributes a quantifiable “dose” of noise that

depends on the assigned hazard

– Actual exposures consist of individually identifiable impulses

– Each impulse either comes directly from the expanding pressure

wave originating at the muzzle or from a reflection of that wave,

both of which can be calculated from a few close-in measurements

Project Approach


Impulse Noise Hazard Depends on: Peak Level and B-duration


• The dose concept follows from the MIL-STD 1474D method of accounting for what is a just safe noise exposure

• For pure impulse noise, the “just safe” noise exposure depends on peak level and B-duration

Noise “Dose”


Typical Exposure Does, In Fact, Consist of Individually Identifiable Impulses


Putting it All Together


Table 5. Multiple shooter Doses and Daily ANR for the Untreated Range (All Lanes Occupied), Based on Impulse Noise Criterion (and Worst Case Measured Levels).

NOISE WEAPONSniper Rifle

M4 Rifle

M4 burst 45 cal 9 mm M240B M249

Directly From Shooter's Own Weapon 0.069 0.233 1.259 0.070 0.035 0.955 0.191

Directly From All Other Weapons 0.069 0.023 0.130 0.074 0.037 1.428 0.281

From Nearest Reflections 0.013 0.004 0.024 0.001 0.000 0.088 0.075

Total dose/round 0.151 0.260 1.413 0.145 0.072 2.471 0.547

Resultant ANR for single hearing protection 662 385 71 690 1389 40 183

Resultant ANR for double hearing protection 13245 7692 1415 13790 27772 809 3657

• Tables similar to the one shown on the previous slide were developed for:

– Measured firing under a steel ceiling covered with plywood bullet traps

– Measured firing under a steel ceiling covered with plywood bullet traps having 1 inch thick acoustical foam as a surface treatment

– Predicted firing under a steel ceiling covered with an “ideal” acoustical treatment

Treatment Comparisons


Example of a Narrow Multi-lane, Treated Range


Another Example of Range Surface Treatment


•

Treatment Comparison (Effect of Surface Treatment Extends only to Reflected Sounds)


Treatment Comparison, Effect on ANOR


Allowed Number of Rounds per Day (or Bursts for Automatic Weapon Fire) when using Double Hearing Protection, for Indoor Ranges with Three Degrees of Acoustic Treatment

Degree of Treatment WeaponSniper Rifle

M4 M4 (burst mode)

45 cal 9 mm M240B (burst mode)

M249 (burst mode)

Bare walls (Ft Campbell) 689 704 94 1622 2201 48 58

Some treatment (Little Creek; 1-inch acoustic foam covering plywood)

825 997 139 3757 5308 85 225

Ideal Treatment (Like Little Creek but with 2-inch acoustic foam covering plywood or Troy System)

924 1162 164 4265 6266 97 269

• The acoustic environment of indoor firing ranges can be modeled with basic input about the noise of weapons involved, physical dimensions of the range, and ordinary descriptors about acoustical performance of room treatments (absorption coefficients)

• Surface treatments can moderate the effects of reflected sound, altering B-durations and overall energy content of received noise, thus modestly increasing ANR. Audible effects are more pronounced.

• Improvement is limited by exposure to direct sounds

• Broadband absorption is needed to provide optimum effect of the surface treatments. 2-inch thick material is better than 1-inch thick

• In a tactical range, it is necessary to treat the entire ceiling surface area. Not so with a fixed position range. Ceiling treatment should be given first priority; side and rear wall surfaces are of lesser importance

Conclusions


• Spread shooters out whenever possible, but avoid shooting from end lanes if possible

• Unlike SOCOM ranges, other indoor ranges may cause noise issues outside the range proper

Conclusions


Lessons Learned At DOD Indoor Firing Ranges Chuck Jokel Noise Control Engineer, Army Hearing Program...

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