1. Peak Reductions of Nonlinear Hearing Protection Devices
William J. Murphy, Ph.D.
National Institute for Occupational Safety and Health
Hearing Loss Prevention Section

2. New Technology Nonlinear Orifice Sound Restoration
Circular Orifice (Institute de Saint Louis)
EAR Combat Arms & Bilsom 655 ISL
Slit Orifice: EAR Ultra 9000
Sound Restoration
Level-limiting: Peltor, Bilsom
Compression Circuits: Howard Leight Pro-Ears
Active Noise Reduction
Bose, Telex, David Clark

3. New Standards? International Standards Organization
ISO 4869 parts 4-7
Primarily Electronic
Proposed European Standards
EN352 parts 5-7
Electronic Devices
American National Standards Institute
Not yet proposed

4. Measurement of Attenuation
Real Ear Attenuation at Threshold (REAT)
Low levels = Low attenuations
Microphone in Real Ear (MIRE)
High levels = Increased risk to subjects
Microphone placement for earplugs
Impractical (disrupts seal with ear canal)
Insufficient acoustic isolation

5. Measurement of Attenuation
Acoustic Test Fixture
Not real ear = Not representative
Bone-conduction
Flanking pathways
Masking
Speech Intelligibility in noise
Signal detection

6. Research Questions
1. Do nonlinear hearing protection devices provide sufficient attenuation for impulsive noise?
2. What is the typical attenuation performance for an electroacoustic hearing protection device?

7. Methods Generate impulsive signals with gunshots
Measure the protected and unprotected signals
¼” B&K 4136 microphone outside HPD
ISL mannequin with 4165 B&K ½” under HPD
Record signals with Tascam DA-P1 digital audio tape (DAT) recorder.
48000 samples/second.

8. ISL Mannequin Measurements
Bruel & Kjaer 4136
¼” microphone
This photograph details the arrangement of the microphone next to the mannequin. The height of the mannequin was adjusted so that its ear canal was at the same height as the shooter. The mannequin was mounted on a tripod and was positioned approximately 1 meter to the left of the shooter. A second tripod was positioned to give the shooter a hand rest from which each shot was fired. The mannequin was facing down range.
4165 ½” B&K microphone
4157 B&K ear simulator
Head Acoustics pinna and ear canal
Isolated housing for 4157 assembly

9. Methods Measured ten weapons Five shots per condition and weapon
0.357 Smith & Wesson 586 and 686 pistols
0.450 Colt 1991A1, Para-ordinance pistols
0.400 Glock 22 and 27 pistols
9mm Pocket 9 and Sig Sauer P228 pistols
12 gauge Remington 870 & shotguns
Five shots per condition and weapon

10. Methods Measured Passive Earplugs Two passive nonlinear earplugs
Bilsom 655 with ISL cartridge
EAR Combat Arms with ISL cartridge
Six passive linear earplugs
Bilsom 555, 655 NST
EAR Combat Arms, HiFi, UltraTech, Classic
One passive mechanical earplug
North Sonic Earvalvs

11. Methods Measured Passive Earmuffs One nonlinear earmuff
EAR Ultra 9000
One passive linear earmuff
David Clark Model 27

12. Methods Measured Active Earplugs and Earmuffs
Six electroacoustic earmuffs
Bilsom Targo and 707 Impact II
Peltor Tactical 6s and 7s
Howard Leight Leightning and Thunder with ProEars
One electroacoustic earplug
Electronic Shooters Protection Elite
Hearing aid with compression circuit

13. Gunshot Impulse Analysis
42.67 msec analysis window (2048 samples)
Maximum Impulse Levels,
Unprotected & Protected
Peak Level Reduction
Unprotected – Protected
Third-octave Analysis of Attenuation

14. 167 dB
147 dB

15. 167 dB
An example that was typical of an impulse measured underneath an earmuff is shown in this figure. The predominant feature is the slow ringing of the impulse. The sharp feature of the previous figure is filtered by the poor low-frequency performance that typifies most earmuffs. The peak level of this impulse is 137 dB, about 0.15 kPa.
136 dB

16. Peak Reduction Growth for Nonlinear HPDs

17. Peak Reduction Growth for Nonlinear HPDs

18. Peak Reduction Growth for Nonlinear HPDs

19. Peak Reduction Growth for Passive HPDs

20. Bilsom Linear Protectors

21. EAR Linear Protectors

22. Nonlinear Protectors

23. Peak Reduction by Protector5 10 15 20 25 30 35
Bilsom 707 Impact II
Bilsom Targo Electronic
Howard Leight Leightning with Pro-Ears
Howard Leight Thunder with Pro-Ears
Peltor Tactical 6-S
Peltor Tactical 7
Electronic Shooter Protection Elite
Max
Unity
Off

24. Bilsom 707 Impact II Earmuffs
The attenuation performance of the device was calculated from 25 to Hz using a library of octave band analysis routines available from Mathworks for Matlab. A set of digital filters estimate the one-third octave band levels. Below 1000 Hz, the routines perform a decimation of the signal and resample the signal. The msec sample window resulted in increased error below 100 Hz. Although the data are shown, little consideration should be given to the data below 100 Hz. The Cyan colored curve represents the Bone-conduction limit for humans estimated by Berger While the ISL mannequin exceeds the bone-conduction limit, attenuation above the bone-conduction limit should be examined with caution. The Bilsom 707 earmuffs show almost no difference as a function of volume setting below 4000 Hz. Above 4000 Hz, the difference between the passive and active settings is about 10 dB or more. Above 300 Hz, the 707 yields 30 dB or more attenuation.

25. Bilsom Targo Electronic Earmuffs
The Bilsom Targo Electronic earmuffs exhibited excellent agreement for the three conditions. Above 300 Hz, the attenuations were better than 30 dB.

26. Peltor Tactical 6-S Earmuffs
The Peltor Tactical 6S earmuff exhibited the greatest dependence of attenuation on volume setting out of all the electroacoustic earmuffs. The maximum volume setting yielded attenuations that were 5 dB reduced over a range from 200 to 1600 Hz. In addition to being reduced, the earmuff overall had less attenuation than the other earmuffs. In part, the reduced attenuation might be attributed to the smaller cup volume, lower mass and less attenuating material inside the cup.

27. Peltor Tactical 7 Earmuffs
The Peltor Tactical 7 earmuffs yielded attenuations that were typical of the other large volume earmuffs. Little difference as a function of volume setting was observed.

28. Howard Leight Leightning Earmuffs
The Howard Leight Leightning Earmuffs with Pro-Ears exhibited the most peak reduction of the earmuffs tested and had the most attenuation. Again little difference was observed as a function of volume setting.

29. Howard Leight Thunder Earmuffs
The Howard Leight Thunder earmuffs with Pro-Ears also exhibited a consistent performance as a function of the volume setting. Above 400 Hz, the attenuation was approximately 40 dB or more.

30. ESP Elite Protectors
The ESP Elite was not originally planned to be included with this paper. However, it represents a class of protectors or potential protectors for use with gunshots. The passive performance exceeded the bone-conduction limits between 2000 and 8000 Hz. The passive performance exhibited better than 25 dB attenuation at all frequencies. Below 100 Hz, the attenuation was dependent on the volume setting. The ESP device was fit for the mannequin and may not have the same performance on a real ear due to different terminating impedance characteristics of the B&K 4157 ear simulator and the occluded volume.

31. Auditory Hazard Assessment Model
Proposed Model to determine exposure criteria for U.S. Army
Mathematical model of the auditory periphery
Estimates the basilar membrane velocity
Estimates impulsive stress on the BM
Yields Auditory Damage Units
Verified with an animal model

32. AHAAH Exposure Analysis

33. Conclusions Simple Relationship for Impulsive Risk
Verified by AHAAH model
ADU = e( * Protected Peak)
Number of Shots per Day = 500 / ADU
Ultra 9000 and Bilsom 655 NST protectors have not completely saturated.

34. Conclusions
Electroacoustic/Sound restoration HPDs yield consistent peak reduction with respect to volume setting.
Recommend dual protection for gunshot exposure
Recommend pairing Electroacoustic muff with passive earplug to get below the 130 dB peak level of single protector.