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

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

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

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

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

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?

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.

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

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 & 11-87 shotguns Five shots per condition and weapon

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

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

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

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

167 dB 147 dB

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

Peak Reduction Growth for Nonlinear HPDs

Peak Reduction Growth for Nonlinear HPDs

Peak Reduction Growth for Nonlinear HPDs

Peak Reduction Growth for Passive HPDs

Bilsom Linear Protectors

EAR Linear Protectors

Nonlinear Protectors

Peak Reduction by Protector 5 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

Bilsom 707 Impact II Earmuffs The attenuation performance of the device was calculated from 25 to 10000 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 42.67 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 1983. 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.

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.

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.

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.

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.

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.

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.

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

AHAAH Exposure Analysis

Conclusions Simple Relationship for Impulsive Risk Verified by AHAAH model ADU = -12.9 + 0.00086 e(0.0829 * Protected Peak) Number of Shots per Day = 500 / ADU Ultra 9000 and Bilsom 655 NST protectors have not completely saturated.

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.