Lessons Learned At DOD Indoor Firing Ranges Chuck Jokel Noise Control Engineer, Army Hearing Program Office UNCLASSIFED National Hearing Conservation Association Annual Conference 25 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. UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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 3 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

First Example of Multi-lane Indoor Firing Range 4 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Second Example of Multi-lane Indoor Firing Range 5 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Narrower Multi-lane Indoor Firing Range 6 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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 7 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Reflections Special Challenges with Indoor Weapon Noise 8 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

The Reflected Ceiling Level is Incorrect 9 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC Primary blast and wall reflection Ceiling reflections

Reflections Clipping Special Challenges with Indoor Weapon Noise 10 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Special Challenges; a case in point 11 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Special Challenges; a case in point 12 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Special Challenges; a case in point 13 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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 Special Challenges with Indoor Weapon Noise 14 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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 15 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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 16 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Impulse Noise Hazard Depends on: Peak Level and B-duration 17 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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” 18 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Peak levels and B-durations are plugged into a formula contained in MIL-STD 1474D, yielding the Allowable Number of Rounds per Day (ANR), when single hearing protection (earmuffs or earplugs) are worn. A 165 dBP shot with a 45 ms B-duration calculates to have an ANR of 1998, for example. This, in turn, establishes the dose for a single impulse at that level and B-duration combination: –Dose, in percent = 100 times (1/ANR) –For example, if the ANR for a round =2000, each similar round contributes 100(1/2000) or.05% of the allowable exposure Noise “Dose” UNCLASSIFIED

Typical Exposure Does, In Fact, Consist of Individually Identifiable Impulses 20 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Putting it All Together 21 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC 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 WEAPON Sniper RifleM4 Rifle M4 burst45 cal9 mmM240BM249 Directly From Shooter's Own Weapon Directly From All Other Weapons From Nearest Reflections Total dose/round Resultant ANR for single hearing protection Resultant ANR for double hearing protection

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 22 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Example of a Narrow Multi-lane, Treated Range 23 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Another Example of Range Surface Treatment 24 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Treatment Comparison (Effect of Surface Treatment Extends only to Reflected Sounds) 25 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

Treatment Comparison, Effect on ANOR 26 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC 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 Weapon Sniper Rifle M4 M4 (burst mode) 45 cal9 mm M240B (burst mode) M249 (burst mode) Bare walls (Ft Campbell) Some treatment (Little Creek; 1-inch acoustic foam covering plywood) Ideal Treatment (Like Little Creek but with 2-inch acoustic foam covering plywood or Troy System)

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 27 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC

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 28 UNCLASSIFIED Chuck Jokel MCHB-IP-MHC