Hearing Protector Attenuation Research Articles

Influence of reference transducer location on hearing protector attenuation metrics for impulse noise

The ANSI/ASA S12.42-2010 standard can be used to estimate the attenuation for impulsive noises between 130 and 170 decibels peak sound pressure level (dB pSPL). The ANSI/ASA S12.42 method uses a complex transfer function between the signal received in the ear of an acoustic test fixture (ATF) and a reference transducer. The reference transducer is at the same radial distance from the impulse source and at less than 30 degrees from the ATF angle. The reference transducer is specified as a pencil-type pressure (> 40.6 cm long). The maximum angle and the minimum pencil probe dimensions could be incompatible near the source, or may introduce artifacts or hazards. In this study, several hearing protectors were evaluated with two rifles (A-Bolt .300 Winchester Magnum and Colt AR-15 5.56 caliber), two ATFs (GRAS 45CB), and five transducers (four ¼” microphones and one surface mount microphone). The ATFs were on opposite sides of the gun at distances for field levels between 180- and 140-dB pSPL. For each location and rifle, S12.42 metrics were calculated for all 20 combinations of ATF ear and reference transducer. Results indicated that S12.42 metrics were minimally affected by the locations of the reference transducers.

Measuring hearing protector attenuation of impulse noise on acoustic test fixtures using maximum A-weighted energy reduction

Hearing protection devices (HPDs) and firearm suppressors can help mitigate risk of hearing loss from small caliber firearms. ANSI S12.42 is the standard for measuring HPD impulse peak insertion loss (IPIL). No consensus standard exists for measuring firearm suppressor noise reduction, though the industry is working toward a standard using maximum accumulated A-weighted energy. A common attenuation metric for different impulse noise reduction technologies would inform the individual and combined impact of these technologies. The maximum A-weighted energy reduction for several HPDs was assessed using firearm impulses from two rifles (0.300 Winchester Magnum and 5.56x45) with two ATFs and five field microphones positioned on opposite sides of the gun at four locations (field levels between 180- and 140-dB pSPL). No adjustments were made to account for tissue/bone conduction. Maximum A-weighted energy reduction values for single protection ranged between 17 dB (earmuff with safety glasses) and 43 dB (preformed earplug). Similar to IPIL, maximum A-weighted energy reduction increased with increased impulse levels. Advantages, disadvantages, and technical issues associated with the procedure will be discussed.

Design and evaluation of a noise dosimeter integrated into an ear-muff hearing protector

In impulse-noise environments, such as during weapons training, it can be difficult to characterize an individual’s true cumulative exposure, and consequently their risk of hearing loss. Direct measurements of sound pressure in the ear canal underneath hearing protection are ideal for assessing the protected noise exposure because the effects of distance, orientation, protection, etc., are automatically included in the measurement. However, these types of measurements are not commonly collected outside the laboratory. In this research, we develop and evaluate a portable a noise dosimeter that is integrated into a commonly used muff-based hearing protector (3M™ PELTOR™ ComTac™ V). The design of the device aims to simultaneously measure both the external and the under-the-muff sound pressures in a portable, wearable form-factor. First, we demonstrate that the modifications do not affect the hearing protector attenuation using an acoustic test fixture and a fit-check system across 10 refits (p = 0.14). Next, we show through testing with a shock-tube and acoustic test fixture that it is possible to obtain valid pressure measurements through this system. Finally, we report on initial field testing during weapons training. Future application of this technology can be used to develop damage risk criteria based on in-ear measurements that will provide for more accurate individualized risk assessment.

A new method for measurement of impulsive peak level attenuation of hearing protectors

ASA/ANSI S12.42-2010 contains the first standardized test method for measuring the performance of hearing protection devices (HPDs) with high-level impulsive noise. This standard defines the impulsive peak insertion loss (IPIL) as a time-domain metric that quantifies the reduction in peak sound pressure level provided by an HPD for impulsive noises. However, IPIL as defined in S12.42-2010 is dependent on the spectrum of the impulsive noise source used for measurements of HPDs. Recent studies of HPDs with impulsive noise have led to the investigation of frequency-domain metrics and calculation methods. Using frequency-domain calculations allows the impulsive peak level attenuation to be computed for a wide range of impulsive noises, not only those used for the measurement. Consequently, significant revisions to S12.42 are under consideration and a new standard, ISO 4869-7, is being developed in parallel with the ANSI revision. This presentation highlights key proposals and substantive changes including: measurement and calculation of the frequency-dependent impulsive insertion loss (IIL), incorporation of frequency-domain aspects such as real-ear attenuation at threshold (REAT) limits to attenuation, and a new impulsive peak level attenuation metric (IPLA) to be calculated from the REAT-bounded IIL.

A comparison of methods to measure the sound attenuation of active noise reduction hearing protectors

Hearing protectors equipped with active noise reduction (ANR) functions have become popular and widespread in recent years. It is important to assess the sound attenuation performance of the ANR hearing protectors properly, which remains a problem. This work studies several typical measurement methods, including the real-ear attenuation at threshold (REAT), microphone in real-ear (MIRE), and the acoustic test fixture (ATF). In order to reduce the masking effect from the noise generated from the ANR electronic system when ANR is turned on during the REAT test, a modified tone sound instead of one-third octave band noise is used as the test signal. The results obtained from the different measurement methods on several ANR headphones and earphones are compared and the differences are discussed. It is found that the measurement differences among those methods are huge. The values of active insertion loss measured by the REAT method are much lower than those assessed by the MIRE procedure. This work contributes to the development of the test methods to assess the sound attenuation of the ANR hearing protectors.

A finite element model to predict the double hearing protector effect on an in-house acoustic test fixture.

The sound attenuation of double hearing protectors (DHPs), earplugs combined with earmuffs, generally falls short of the sum of each single protector's attenuation when used independently. This phenomenon, referred to as the DHP effect, is found to be related to structure-borne sound transmission involving the outer ear and can also be observed on acoustic test fixtures (ATFs). At present, it still remains not fully understood, and no available model can help demonstrate the associated sound transmission mechanisms. In this work, a finite element model is proposed to study the DHP effect on an ATF between 100 Hz and 5 kHz. Power balances are calculated with selected configurations of the ATF in order to (i) quantify the contribution of each sound path, and study the effects of (ii) the artificial skin and (iii) acoustic excitation on the ATF exterior boundaries. The DHP effect is shown to originate from the structure-borne sound power injected from the ATF boundaries and/or earmuff cushion. The important influence of earcanal wall vibration is highlighted when the skin is accounted for. The simulation results allow for gaining more insight into the sound transmission through a DHP/ATF system.

A deep neural-network classifier for photograph-based estimation of hearing protection attenuation and fit

Occupational and recreational acoustic noise exposure is known to cause permanent hearing damage and reduced quality of life, which indicates the importance of noise controls including hearing protection devices (HPDs) in situations where high noise levels exist. While HPDs can provide adequate protection for many noise exposures, it is often a challenge to properly train HPD users and maintain compliance with usage guidelines. HPD fit-testing systems are commercially available to ensure proper attenuation is achieved, but they often require specific facilities designed for hearing testing (e.g., a quiet room or an audiometric booth) or special equipment (e.g., modified HPDs designed specifically for fit testing). In this study, we explored using visual information from a photograph of an HPD inserted into the ear to estimate hearing protector attenuation. Our dataset consists of 960 unique photographs from four types of hearing protectors across 160 individuals. We achieved 73% classification accuracy in predicting if the fit was greater or less than the median measured attenuation (29 dB at 1 kHz) using a deep neural network. Ultimately, the fit-test technique developed in this research could be used for training as well as for automated compliance monitoring in noisy environments to prevent hearing loss.

Elliott Berger's contributions to military hearing conservation and standards

Military organizations world-wide experience the great majority of the extreme occupational noise exposures. The noise sources range from tanks and armored personnel carriers to fighter jets and shoulder launched missiles. It is a difficult task to protect the men and women who serve their countries and experience these extreme environments where the continuous noise levels can reach 150 dBA and the peak noise levels from firing weapons can reach 195 dBpeak. Stable methods to measure hearing protector attenuation benefit the development and optimization of hearing protection. National and international standards measuring hearing protection attenuation have been the focus of much of Elliott's work during his career and now in semi-retirement. These standards have been the catalysts that have enabled the development of improved hearing protectors that help conserve the hearing of service members in military noise environments. Data will be presented showing ranges of military continuous and impulsive noise exposures, the amount of hearing protection attenuation needed to meet US DoD noise exposure criteria, and the ANSI standards for the measurement of hearing protector attenuation.

Comparison of hearing protector ratings for statistically significant differences

During the development of the American National Standards Institute S12.68 standard for estimating effective A-weighted sound pressure levels when hearing protector device (HPD) are worn, the need to provide a mechanism to compare HPD ratings was identified. The current Noise Reduction Rating (NRR) is a single number that describes the potential of a product to reduce noise at the user’s ears. The Noise Reduction Statistic for A-weighting (NRSA) describes a range of protection performance and estimates the uncertainty associated with the rating. The uncertainty for NRSA is estimated using a bootstrap procedure. This method can be applied to current ratings and comparisons between rating evaluations from different laboratories. This paper will discuss NRR and NRSA error bars ratings and present a modification of the bootstrap method to estimate directly the statistical significance of differences. Results from two different studies of hearing protector attenuation measurements and training protocols will be examined. The studies’ results will also be compared against the manufacturers’ published attenuation values.

Attenuation characteristics of an extended-wear hearing aid: Impulse and continuous noise.

The extended-wear hearing aid (EWHA) is a hearing assistive device that combines a low-power analog amplification circuit with a soft biocompatible foam plug that allows it to remain in the ear canal for several months at a time without replacement. EWHAs fit snugly in the ear canal and are not vented and so produce insertion losses comparable to a passive earplug when inserted into the ear canal with the active circuitry turned off. However, EWHAs are not marketed as hearing protection devices, and other than a general warning to users that the device will have impaired auditory awareness when the device is inserted in the "off" mode, relatively little has been reported about the attenuation characteristics of EWHAs. In this study, commercially-available EWHAs were evaluated using the ANSI standard procedures for measuring hearing protector attenuation in impulse noise [ANSI (2010). S1242-2010, Methods for the Measurement of Insertion Loss of Hearing Protective Devices in Continuous or Impulsive Noise Using Microphone-In-Real-Ear or Acoustic Text Fixture Procedures (American National Standards Institute, New York)] and in continuous noise [ANSI (2006). S12.6, Methods for Measuring the Real-Ear Attenuation of Hearing Protectors (American National Standards Institute, New York)]. Attenuation values were also measured in double and triple protection conditions that combined EWHAs with traditional earplugs and earmuffs. The results show that properly-fit EWHAs can provide passive attenuation comparable to conventional passive earplugs, which may make it possible to use them to provide persistent protection from intermittent noise sources.

Hearing Protector Attenuation and Noise Exposure Among Metal Manufacturing Workers.

This study utilized personal noise measurements and fit-testing to evaluate the association between noise exposures and personal attenuation rating (PAR) values among participating workers, and second, to compare the attenuated exposure levels received by the workers and the British Standards Institute's recommended noise exposure range of 70 to 80 dBA. We measured hearing protection device (HPD) attenuation among a sample of 91 workers at 2 US metal manufacturing facilities, through performance of personal noise dosimetry measurements and HPD fit-testing over multiple work shifts. We compared this testing with participant questionnaires and annual audiometric hearing threshold results. The average 8-hr time-weighted average noise exposures for study participants was 79.8 dBA (SD = 7.0 dBA), and the average PAR from fit-testing was 20.1 dB (±6.7 dB). While differences existed between sites, 84% of the 251 PAR measurements resulted in effective protection levels below the recommended 70 dBA (indicating overprotection), while workers were underprotected (i.e., effective exposures >80 dBA) during <1% of monitored shifts. Our results also demonstrated a significant positive relationship between measured noise exposure and PAR among non-custom-molded plug users (p = 0.04). Non-custom-molded plug wearers also showed a significant increase in PAR by sequential fit-test interaction (p = 0.01), where on average, subsequent fit-testing resulted in increasingly higher HPD attenuation. Workers at site 1 showed higher PARs. PARs were significantly related to race, even when adjusting for site location. While age, hearing threshold level, task, and self-reported tinnitus showed no significant effect on individual PAR in an unadjusted model, site, race, and sand- or water-blasting activities were significant predictors in adjusted models. Within-worker variability in time-weighted averages and PARs across repeated measurements was substantially lower than variability between workers. Careful selection of HPDs is necessary to minimize instances of overprotection to workers in low and moderate occupational noise environments. The use of fit-testing in hearing conservation programs to evaluate PAR is recommended to avoid overprotection from noise exposure while also minimizing instances of under-attenuation.

Fit-testing systems for hearing protectors: ANSI standards to the rescue

To standardize the performance of fit-testing systems for hearing protection devices (HPDs), an ANSI/ASA standard has been in development since 2008. As of this writing, BSR/ASA S12.71 “Performance criteria for systems that estimate the attenuation of passive hearing protectors for individual users” has been submitted for public comment and ballot. This standard refers to fit-testing systems as field attenuation estimation systems (FAESs). In this talk, we introduce important FAES concepts, as handled in the draft S12.71. There are two types of FAESs: physical systems that take an objective measurement and psychophysical systems that rely on subjective responses from the user being tested. FAESs may produce a quantitative output as a personal attenuation rating (PAR) or simply indicate pass/fail for a given HPD fit. For all types of systems, S12.71 specifies methods to assess the quality of the attenuation estimates, by comparing the FAES results to a laboratory real-ear attenuation at threshold (REAT) measurement. Other important considerations covered in S12.71 include the maximum levels of ambient noise in which FAESs may be operated, the incorporation of uncertainties due to HPD fitting and spectral variability in users’ noise exposures, calibration intervals, and information that must be provided to users.

A field microphone in real ear hearing protector attenuation measurement system

A Field-Microphone in Real Ear (F-MIRE) hearing protector Field Attenuation Estimation System (FAES) was developed for individual or group testing of any type or style of hearing protector device (HPD). Both occluded and unoccluded ear canal stimulus level measurements are made with a tiny Micro Electro-Mechanical System (MEMS) microphone mounted on a 2-mil thick flat flexible cable. The microphone is mounted inside of a conventional foam ear dam to protect the microphone and the ear canal and to facilitate insertion into the ear canal. The presence of the cable in the ear canal was demonstrated to have a negligible effect on attenuation measurements for both muff and insert-type HPDs. The system measures HPD attenuation in 1/1 or 1/3 octave bands and it is capable of simultaneously measuring HPD attenuation on up to eight human test subjects. Stimuli can be presented via headphones or loudspeakers. Human subject testing using both the laboratory based ANSI S12.6-2016 method and the F-MIRE system was performed using muff- and insert- type HPDs. Regression equations were generated from these data. These equations are incorporated in the F-MIRE software to allow valid comparisons between F-MIRE data and labeled HPD attenuation data.

Improved hearing protector attenuation through the use of a lubricant

ABSTRACTA study of 40 novice hearing protection users was conducted to determine if the use of a lubricant with hearing protective plugs would enhance their attenuation. Three types of hearing protection devices (i.e., earplugs) were evaluated: roll-down PVC foam, mushroom-style polyurethane foam, and elastomeric flanged. For each earplug type, the right and left ear (dry and lubricated, respectively) personal attenuation rating was determined using a commercially available microphone in real-ear apparatus.Earplug efficacy was observed to increase in the presence of a lubricant for all 3 plug types, with average mean elevations by earplug type ranging from 1.5–5.1 dB. Two of three earplug types saw statistically significant improvements when used with lubrication, demonstrating heightened mean personal attenuation ratings of 2.8 dB and 5.1 dB.The issues of handedness and gender differences (vis à vis ear canal size) are explored, and the importance of proper fitting is discussed in detail. Several specific questions yet to be answered are posed, and directions for further research indicated.

Measurement of Earmuffs Attenuation at High Audible Frequencies

Abstract Standardised measurements of sound attenuation of hearing protectors are performed in the frequency range from 125 Hz to 8 kHz. However, noise present at many workplaces contains significant components at higher audible frequency. Therefore, the knowledge about noise attenuation with earmuffs in the audible frequency range above 8 kHz is also necessary for proper hearing protection. The aim of this study was to obtain values of the noise attenuation with 27 commonly-used earmuffs models in the 1/3 octave bands of 10, 12.5 and 16 kHz. The measurements were conducted with a real ear at threshold (REAT) method with participation of subjects. The study showed that attenuation of earmuffs ranged from 24.7 to 42.8 dB, depending on model of earmuffs and frequency band. Furthermore, the measurements were performed with the use of acoustic test fixture which is designed especially for testing hearing protectors. Results obtained with the use of acoustic test fixture indicated that this measurement method can lead to values close to attenuation measured with participation of subjects. On the other hand, values obtained with the use of acoustic test fixture may differ average up to 14 dB from REAT method.

Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation.

Introduction:Present methods of measuring the attenuation of hearing protection devices (HPDs) have limitations. Objective measurements such as field microphone in real-ear do not assess bone-conducted sound. Psychophysical measurements such as real-ear attenuation at threshold (REAT) are biased due to the low frequency masking effects from test subjects’ physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is explored as a technique which might overcome these limitations.Subjects and Methods:Pure tone stimuli (500 and 1000 Hz), amplitude modulated at 40 Hz, are presented to 10 normal-hearing adults through headphones at three levels in 10 dB steps. Two conditions were assessed: unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level are linearized using least-square regressions. The “physiological attenuation” is then calculated as the average difference between the two measurements. The technical feasibility of measuring earplug attenuation is demonstrated for the group average attenuation across subjects.Results:No significant statistical difference is found between the average REAT attenuation and the average ASSR-based attenuation.Conclusion:Feasibility is not yet demonstrated for individual subjects since differences between the estimates occurred for some subjects.

Review of the 2016 Edition of ANSI S12.6 and its place in the panoply of standards on the measurement of real-ear attenuation at threshold since 1957

The American standard specifying the procedure for the measurement of real-ear attenuation at threshold (REAT), often termed the gold standard in measuring hearing protector attenuation, was recently approved as an updated version, ANSI 12.6-2016. REAT was first standardized worldwide in the late 1950s in an American standard ANSI Z24.22-1957 and the method has evolved with time. Changes have affected the electroacoustic requirements for the sound field, instrumentation, audiometric method, and permissible background noise, but more importantly have also improved the specification of how the experimenter works with and fits the test subjects. So too, estimates of uncertainty are now included, and in the 2016 version they have been clarified and brought into harmony with ISO 4869-1. The author, who has been the chair since 1985 of the ANSI working group responsible for S12.6, will compare and contrast the various methods and present representative data as well as a discussion of the expanded uncertainties that are specified. Those values, for the 1/3-octave band test bands from 125 Hz to 8000 Hz, vary from approximately 1.5-2 dB for earmuffs and 2-3 dB for earplugs for within-laboratory testing, to 4-6 dB for earmuffs and 6.5-8 dB for earplugs for between laboratory measurements.

Attenuation of hearing protectors: A systematic comparison of subjective and objective measurement methods

A key component when selecting a hearing protector is the noise attenuation offered by the device. The subjective Real-Ear Attenuation at Threshold (REAT) test method is the most commonly used procedure to measure attenuation. On the other hand, with the increase popularity of individual fit testing and miniaturization of electronic components, the Microphone-In-Real-Ear approach (MIRE), and its field counterpart F-MIRE, are becoming more appealing and well suited for estimating attenuation in laboratory or in “real world” occupational conditions. In this approach, two miniature microphones are used to measure sound pressure levels in the ear canal under the protector and outside of the protector. This study presents a systematic evaluation of the various factors relating the subjective and objective attenuation values. Experiments on human subjects were carried out where the subjects were instrumented on both ears with microphones outside and underneath their protector. They were then asked to go through...

Speech recognition in noise under hearing protection: A computational study of the combined effects of hearing loss and hearing protector attenuation

Objective: To investigate the effects of hearing protection on speech recognition in noise. Design: Computational study using a speech recognition model that was previously empirically validated. Study sample: Recognition scores were calculated in unprotected and protected conditions for four sets of hearing protector attenuation functions in two different noises, for three simulated hearing profiles illustrative of those anticipated in the noisy workplace. Results: For a normal-hearing profile, recognition scores were not sensitive to the slope of the attenuation function and the overall amount of noise reduction, but protected conditions provided a small but consistent 7–12% benefit compared to unprotected listening. For profiles simulating hearing loss, recognition scores were much more sensitive to the attenuation function. Substantial drops of 30% or more were found compared to unprotected listening in some conditions of steep attenuation slopes and large noise reductions. Attenuation functions modelled from real hearing protectors with nearly-flat attenuation yielded a benefit compared to unprotected listening for all hearing profiles studied. These findings were true in both noises. Conclusions: Limiting the slope of the hearing protector attenuation function and/or the overall amount of noise reduction is useful and warranted for workers with hearing loss to prevent adverse effects on speech recognition.

Calculating total daily noise exposure using aircrew flight equipment noise attenuation and fight segment noise data

The workplace of military aircrew is louder and more varied than most industrial environments. In the recent revision of MIL-STD-1474E, Noise Limits, octave band level design limits for helicopters have been replaced by a broader requirement based on noise exposure at the ear. This change brings 1474E into alignment with DoD hearing conservation policies which require that total daily noise exposure does not exceed an Leq of 85 dBA during an 8 hour period. Although these polices also require use of hearing protection whenever levels exceed 85 dBA, there is currently little written guidance for how to use hearing protector attenuation data to estimate exposure at the ear over the course of a typical mission profile. This paper will explain how attenuation data for flight helmets and communication headsets has been used by the Naval Air Systems Command to estimate noise exposure for crew and passengers in various types of aircraft.