Attenuation Of Hearing Protection Devices Research Articles

A Tester to Evaluate the Correct Placement of Earplugs.

The use of hearing protection devices is one possible way of reducing the negative impact of noise on hearing. However, it is important to keep in mind that only properly used hearing protection devices provide adequate hearing protection. The aim of this article is to describe a newly developed tester to verify the correct placement of earplugs in the ear canal. This tester was developed using easily accessible and low-cost components. It implements the real-ear attenuation at threshold (REAT) method by which the sound attenuation of hearing protection devices is determined. The headphones with a greater low-frequency attenuation value were selected for use in the tester. The results of the sound attenuation measurement performed with the use of the tester did not differ by more than 5 dB compared to the measurements performed with the use of the Norsonic NOR838 system dedicated to this purpose. The developed tester is considered to be a device that will obtain reliable sound attenuation values. Thus, it can also be used as a device with which the correct placement of earplugs in the ear canal can be assessed.

Detection thresholds with active hearing protection devices in quiet and noise

The Real-Ear Attenuation at Threshold (REAT) method is a standardized (ANSI/ASA S12.6) method of assessing the passive attenuation of hearing protection devices (HPD), including electronic HPDs with the electronics turned off. Characterizing HPDs with electronics turned on is performed with the Microphone in Real Ear (MIRE) method, or with an acoustic test fixture, as described in ANSI/ASA S12.42. It has been proposed that the REAT method may be useful for characterizing the effect of HPDs on detection of ambient sounds for HPDs with hear-through microphones, including the effects of electronics noise from the HPD. In this study, detection thresholds were measured with three electronic earmuff-type HPDs using a method similar to the REAT method. Detection thresholds were obtained for warble tones presented from a loudspeaker placed in front of the subject in a sound treated room. Thresholds were measured with unoccluded ears and with three HPDs with hear-through capability in quiet and in noise. In addition, thresholds were measured with one of the HPDs with foam earplugs inserted into both ears to simulate a conductive hearing loss. The results point to the interaction between device noise, ambient noise, and human absolute thresholds as limitations on the detectability of ambient sounds.

Individual Fit Testing of Hearing-Protection Devices Based on Microphones in Real Ears among Workers in Industries with High-Noise-Level Manufacturing.

Hearing-protection devices (HPDs) are particularly important in protecting the hearing of workers. The aim of this study was to prevent hearing damage in workplaces in Taiwan. It was conducted to determine the actual sound attenuation of the personal attenuation rating (PAR) values when wearing HPDs via measurements from field microphones in workers’ real ears (F-MIRE). Across 105 measurement trials for the Classic™ roll-down foam earplug HPDs worn by the workers, there were 23 cases of ineffective protection (including caution and fail); the proportion was 20% (including the first measurement and re-wear of HPDs after education and training). In addition, re-education and training in how to wear the HPDs was provided, improving wearing skills. A total of 29 testees wearing the Classic™ roll-down foam earplug HPDs failed to meet the pass standard for the first PAR test, and 6 of them improved and subsequently passed the PAR test. The improvement rate was 20%. These 23 testees switched to another HPD, namely Kneading-Free Push-Ins™ earplugs. From this group, 16 effective sound attenuation values were obtained, with an improvement rate of 70%. However, seven testees failed to pass the PAR test, and after education, training, and replacement of HPDs with different types, they still could not pass the PAR test. At that time, even if the UltraFit™ pre-molded earplugs were adopted again for wear and replacement, they were still unable to pass the PAR test. This HPD was eventually replaced with the PELTOR X4A Earmuff HPD and then tested again, with these HPDs finally passing the PAR test. In Taiwan, the use of fit testing has been increasing but it is not a common practice, and few studies on hearing-protection fit testing have been conducted in this country. The goal of this study was to gain more insight into the current hearing protection situation, including field attenuation of HPDs obtained by workers, the effects of training on improving the attenuation of HPDs after F-MIRE measurements, and the awareness of hearing health and motivation on the use of HPDs in a high-noise-level environment.

Observations on the correlation of subjective and objective attenuation measurements on earplugs.

Real-ear attenuation at threshold (REAT) has been recognized as the gold-standard method to evaluate the attenuation of hearing protection devices (HPDs). The question is often posed as to whether or not REAT measurements could be replaced with objective, insertion loss (IL) measurements conducted on acoustical test fixtures for assessing the performance and consistency of HPDs. To understand how IL data correspond to REAT data for earplugs, both methods were used to assess three earplugs. The resulting data were compared, and the HPD attenuation ranked at each frequency. In most cases, after correction for bone conduction limits, occlusion effect, and physiological noise, IL data show good agreement with REAT values. Discrepancies at some frequencies were found and varied by HPD. The IL data did not identify the high standard deviation of the REAT results on one earplug. The IL data did not correspond well to the REAT values, resulting in differences as large as 12 dB at some frequencies. In conclusion, IL and REAT procedures are used for different purposes when assessing attenuation characteristics of HPDs. While close agreement was found at most frequencies, IL data may not reflect the REAT performance of HPDs equivalently in all aspects.

Hearing protection devices and methods used for their evaluation: A military perspective

Introduction: Soldiers are regularly exposed to potentially harmful noise such as the constant noise of transport vehicle engines and the impulse noise of weapons. Impulse noise can be particularly hazardous, especially, for example, the high-intensity noise of artillery or shoulder-fired projectile launchers. After administrative and engineering controls, hearing protection devices (HPDs) are a cornerstone of hearing conservation programs. Yet selecting the appropriate protection for the various mission tasks must be done with care. HPDs can range from simple earplugs to high-tech options. Methods: Methods of characterizing the attenuation of HPDs against high-level impulse noise are complex and evolving. Results: For the soldier, the need to balance the degree of measured sound attenuation against interference with other auditory abilities – such as the need to hear soft sounds, to understand commands, or to localize sound – is a common dilemma. Discussion: This article outlines some of the challenges of assessing and choosing HPDs that keep soldiers safe from noise exposure with a view to helping those new to hearing conservation understand more about this important subject.

Evaluating the effect of training along with fit testing on earmuff users in a Chinese textile factory

ABSTRACTA hearing protection fit testing was conducted on workers (n = 189) in a textile factory in southern China. The 3M E-A-Rfit Dual-Ear Validation System was used to measure the personal attenuation rating (PAR) on an over-the-head style earmuff at the work site. In addition, PARs were obtained several times throughout the same work shift to capture different fits on 39 workers. Follow-up visits were conducted in approximate 6-month or 12-month intervals to repeat the fit testing. Both the immediate and residual effects of training on the field attenuation of hearing protection devices obtained by workers, and the effects of training toward improving the attenuation and protection of earmuffs after a period of daily use, were observed. A questionnaire survey was used to assess the awareness of hearing health and motivation toward the use of earmuffs in a high temperature environment. The results showed that there were wide variations in PARs among the workers tested. The one-on-one training together with fit testing improved PAR and helped to reduce the negative impact caused by use of protective hair covers. Since earmuffs are often used for long periods of time (>1 year), repeating fit tests could be helpful in order to verify the attenuation received throughout their useful life.

Observation on the variability of fitting by two experimenters at REAT and the agreement with objective measurement on three premolded earplugs

Real ear attenuation at threshold (REAT) has been recognized as the gold standard method to evaluate the attenuation of hearing protection devices (HPDs). It has been standardized in ANSI, ISO/EN and AS/NZS documents. The fitting of HPDs is one of the key factors of REAT measurement. The experimenter fit protocol of ANSI S3.19 is considered one precise approach to obtain the “best fit” of HPDs. A study of the attenuation obtained between the three fits for two experimenters on the same ten subjects on three premolded three-flange earplugs was conducted at 3M E-A-RCAL laboratory, including two models of Chinese brands earplugs and 3MTM E-A-RTM UltraFit earplug. Good agreement was shown between the REAT results that were obtained by the two experimenters. The REAT result of one Chinese brand earplug shows much higher variability within subjects. Comparing with the insertion loss (IL) measured by G.R.A.S 45CB, which has been corrected for bone conduction, occlusion effects and physiological noise, produced IL for two of the tested earplugs that did not showed good agreement with REAT results at most, but not frequencies. The discrepancies will be discussed.

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.

A finite element model to improve noise reduction based attenuation measurement of earmuffs in a directional sound field

The real attenuation of hearing protection devices (HPD) can be assessed in the field using a method based on continuous field microphone-in-real-ear (F-MIRE) measurements. The two-microphone method provides an indicator called the measured noise reduction (NR∗), defined as the difference between the measured exterior (outside the protector) and interior (under the protector) sound pressure levels (SPL). The HPD’s attenuation expressed in terms of the more common insertion loss (IL) can then be obtained from NR∗ using compensation factors. For earmuffs, NR∗ has been shown to vary of up to 20dB depending on the angle of incidence of the sound source. Therefore, there is a need to use sound incidence dependent compensation factors to relate NR∗ and IL. To evaluate these factors and more generally to improve the continuous F-MIRE method, a finite-element (FE) model of an earmuff on an ATF (acoustic test fixture) exposed to a directional sound field has been developed and its predictions compared with lab measurements for several incidence angles. Regarding the external microphone SPL and the NR∗, in one-third of octave bands, the model correlates very well with measurements for frequencies below 1250Hz whatever the sound incidence. Above 1250Hz, the FE model captures the trends, as a function of the incidence angle, but the agreement generally decreases with increasing frequency. A better correlation between the FE model and the experimental data is achieved for the variation of NR∗ (ΔNR∗) as a function of the sound incidence. Actions, such as (i) accounting for the headband in the model, (ii) refining the modeling of the sound source, (iii) improving the cushion modeling and (iv) better describing the backplate/cushion coupling conditions, are suggested to improve the model accuracy. To illustrate the potential of the modeling to improve the continuous F-MIRE measurement method, the FE model is used to determine an optimal position of the external microphone and to obtain estimates of exposure levels using the left and right ear exterior microphones.

Field fit testing with E-A-Rfit Dual-Ear validation system in China

This presentation introduces hearing protection fit testing on over 1000 workers in 6 different factories, each with different hearing conservation practices in China. The 3MTM E-A-RfitTM Dual-Ear validation system was used to measure the Personal Attenuation Rating (PAR), on three different types of Hearing Protection Devices (HPDs), including a foam earplug, a premolded earplug, and a headband earmuff at the work site. A follow up visit to repeat fit-testing was conducted approximately 6 months after the initial visit. Field attenuation of HPDs obtained by workers’ and the effects of training toward improving the attenuation were observed. In addition, PARs for the Peltor™X4A earmuff users were obtained at several points throughout the work shift to capture different fits. Result shows that there is wide variability in PARs and poor agreement between PAR and labelled noise attenuation ratings obtained from laboratory measurement. Hearing protection fit testing is essential to verify the sufficiency of a...

Sound field uncertainty budget for real-ear attenuation at threshold measurement per ANSI S12.6 standards

In many national and international standards, the attenuation of Hearing Protection Devices is rated according to a psychophysical method called Real-Ear Attenuation at Threshold (REAT), which averages on a group of test-subjects the difference between the open and occluded auditory thresholds. In ANSI S12.6 standard, these REAT tests are conducted in a diffuse sound field in which sound uniformity and directionality are assessed by two objective microphone measurements. While the ANSI S12.6 standard defines these two criteria, it does not link the microphone measurements to the actual variation of sound pressure level at the eardrum that may originate from natural head movements during testing. This presentation examines this issue with detailed measurements conducted in an ANSI S12.6-compliant audiometric booth using an Artificial Test Fixture (ATF). The sound pressure level variations were recorded for movements of the ATF along the three main spatial axes and two rotation planes. From these measured variations and different head movements hypothetical scenarios, various sound field uncertainty budgets were computed. These findings will be discussed in order to eventually include them for uncertainty budget in a revised version of the ANSI S12.6 standard.

Measurements of bone-conducted impulse noise from weapons using a head simulator

High-intensity impulse sounds are generally considered to be more damaging than continuous sounds, so understanding the attenuation performance of hearing protection devices against impulse noise is key to providing adequate protection for exposed persons. The maximum attenuation of hearing protection devices is limited by bone-conducted sound. Weapon fire noise in the form of short duration impulses can reach peak levels of 170 dB SPL at the shooter’s ear, a sound level for which maximum hearing protection is recommended and for which bone-conducted sound will be a significant factor. However, current acoustic test fixtures do not capture the bone-conducted sound paths. In this study, an anatomically correct head simulator built specifically to measure bone-conducted sound was used to evaluate the effects of impulse noise generated by hand guns and rifles at several peak sound pressure levels ranging between 120 dB SPL and 170 dB SPL. Time histories of the acceleration of the temporal bones and the sound pressure transmitted into the cranial cavity were recorded. Results investigating the linearity of the bone-conducted response to impulse noise at high peak levels and the effects of hearing protection on the sound level and vibrations inside the head are presented.

Development of an advanced hearing protection evaluation system

Acoustic Test Fixtures (ATFs) are practical and often necessary tools for testing Hearing Protection Devices (HPDs) especially with extremely loud impulsive and/or continuous noise, for which the use of live subjects might not be advisable. Although there have been various standardized and laboratory ATFs from past research, there still exists large uncertainty in the correlation between the attenuation results obtained from ATFs and those obtained from actual human subject tests, particularly for intraaural HPDs. It is suspected that one of the main factors contributing to the discrepancy may be insufficient fidelity in the circumaural/intraaural flesh and bone. Human subject testing was performed to obtain median parameters of ear canal geometry and eardrum reflectance, which are considered to be critical parameters for circumaural/intraaural HPD attenuation performance. This presentation discusses the research methodologies and design implementation of these important subsystems in this advanced Hearing Protection Evaluation System (HPES).

Development of an advanced hearing protection evaluation system.

Acoustic test fixtures (ATFs) are practical and often necessary tools for testing hearing protection devices (HPDs) especially with extremely loud impulsive and/or continuous noise, for which the use of live subjects might not be advisable. Although there have been various standardized and laboratory ATFs from past research, there still exists large uncertainty in the correlation between the attenuation results obtained from ATFs and those obtained from actual human subject tests, particularly for intraaural HPDs. It is suspected that one of the main factors contributing to the discrepancy may be insufficient fidelity in the circumaural/intraaural flesh system, whose underlying dynamics is not yet clear. Therefore the first goal of this research is to better understand this biomechanical system and implement it into a new ATF prototype which emulates circumaural/intraaural HPD attenuation performance. This prototype ATF will be validated against human subjects. This presentation discusses the research methodologies and design strategies for developing this advanced hearing protection evaluation system.

Hearing Protectors: Topicality and Research Needs

Occupational noise specialists do not generally recommend hearing protection devices (HPDs) as a preferred solution to noise risk prevention. Nevertheless, these devices are widely used and are in fact often necessary. Selection of an HPD should take into account comfort and the capacity for perceiving external signals, when they are worn. Current European regulations require that HPD attenuation be considered, when comparing noise exposure to limit values. However, HPD attenuation is effectively unknown under real-world conditions. Some methods are designed to give approximate attenuation values and these provide results within a wide statistical range. Field measurement methods and current standards have been developed to deal with this situation. The specific characteristic of impulse noise requires establishment of dedicated criteria and tools for HPD selection and testing. This paper introduces a number of avenues for research, which could be of assistance in improving HPD selection, qualification and design.

"Real world" noise exposure beneath hearing protectors : a scattered international practise

Assessing occupational noise exposure "beneath" Hearing Protection Devices (HPD) is a topical subject. Standardized methods allow to calculate the exposure by using the HPD attenuation. The HPD attenuations declared by manufacturers are much higher from the "real‐world" ones. To reduce this difference, some "compensation" methods are proposed. The methods and their rules vary a lot from one country to another. The "derating" rmethod reduces the declared values from a certain amount, according to the HPD kind. "Double‐labelling" method uses attenuation values measured in laboratory with untrained subjects. "Statistical range enlargement" widens the statistical confidence of the laboratory‐test results. All these methods propose a global solution: the reasons due to human behaviour (mainly bad HPD wearing) , products quality, and the difference between laboratory and industrial situations are mixed. This "blind" approach could be considered as endorsing the lack of workers' training, and could impede the progress in HPD developments. But a "short‐term" answer to the problem is needed. It should allow to build a more relevant medium‐term answer, which could be worked out possibly on an international scale.

Variability of hearing protection devices attenuation as a function of source location

It is of common knowledge, and well documented, that laboratory-measured noise attenuation values of most hearing protection devices (HPD) exceed significantly the attenuation values obtained in real-world workplace environments. Various reasons may explain such discrepancies (lack of training, wearing time, lack of comfort, bad fitting, noise environments, etc.) but very few of them have been studied in details due to the complexity of the problem. This study focuses on the variability of the attenuation of HPDs as a function of the location of the noise source. Laboratory measurements were performed where subjects, wearing a HPD and facing a loudspeaker, were asked to rotate slowly on a rotating chair to simulate different angular positions of the head relative to the source. The protected and unprotected sound pressure signals for both ears were recorded as time signals using miniature microphones placed respectively inside and outside the HPD (F-MIRE technique). The microphones signals were processed to obtain attenuation values for the different angular positions. Results for different type of HPD (ear-muffs and ear-plugs) are presented and discussed.

Empirical evaluation using impulse noise of the level-dependency of various passive earplug designs

An objective in the development of hearing protection devices (HPDs) has been the design of a passive earplug that provides modest or no attenuation at low sound levels, with greater protection at high sound levels. This raises the issue of not only how to construct such a device, but also how to evaluate it. There is the related question of whether conventional HPDs are actually level independent. Passive level dependency is typically accomplished via an orifice that causes sound transmission to decrease as input level increases. We utilized an impulsive noise source (explosives) with peak levels from 110 to 190 dB SPL to measure the insertion loss of a variety of commercially available and developmental earplugs. The tests were conducted at frontal incidence in a reflection-free outdoor environment using the Institute of Saint-Louis acoustical test fixture specifically constructed for HPD attenuation measurements. Conventional foam and premolded earplugs exhibited attenuation that was essentially constant with level, whereas the best of the level-dependent designs provided attenuation that increased by about 25 dB over the 80-dB range of test impulse levels. This latter design has been successfully utilized since 2000 in the Combat Arms™ Plug widely fielded in the U. S. Military.

The Effectiveness of Hearing Protection Among Construction Workers

Effective hearing conservation programs in the construction industry are rare. Where programs are present, they often rely on workers' use of hearing protection devices (HPDs) rather than on exposure controls to reduce noise exposure levels. Dependence on HPDs for protection from high noise is problematic, as the protection provided by the HPD depends on both the HPD's attenuation level and the time the HPD is used. This article presents an analysis of data on noise exposure and hearing protection among construction workers drawn from several large datasets covering nine construction trades. A unique combination of 1-min dosimetry noise exposure levels and simultaneous self-reported use of HPDs was evaluated, as were occupational and nonoccupational HPD use data collected by questionnaire as part of a longitudinal study of noise exposure and hearing loss among apprentices. Direct measurements of HPD attenuation were also made on workers at their work site. The workers assessed in this study were found to use hearing protection less than one-quarter of the time that they were exposed above 85 dBA. Workers who reported “always” using HPDs in high noise on questionnaires were found to wear them only one-third of the time their exposures exceeded 85 dBA. Workers' self-reported use of HPDs during most noisy nonoccupational activities was also found to be low. Direct attenuation measurements found that workers were able to achieve more than 50% of the rated attenuation of their HPD on average, but that the variability in achieved attenuation was large. When the measured HPD attenuation levels and use time data were combined, the effective protection afforded by HPDs was less than 3 dB, a negligible amount given the high exposure levels associated with construction work. However, there was substantial variation in effective protection among the different trades assessed. These results demonstrate the need for better hearing conservation programs and expanded noise control efforts in the construction industry.

An objective method for measuring the attenuation of hearing protection devices using otoacoustic emissions

An objective human subject test technique for measuring the attenuation performance of hearing protection devices is presented. The technique is similar to the real-ear attenuation at threshold (REAT) technique, except distortion product otoacoustic emission (DPOAE) measurements in the ear canal replace the standard audiometric threshold measurements. The primary advantages of the new technique are that the measurement is objective, relatively fast, and can be applied at supra-threshold sound levels. Microphone in real ear (MIRE) data is also available from the DPOAE probe in the ear canal. Disadvantages are the restricted frequency range in which DPOAE signals may be reliably recorded (2–5 kHz) and the need for an in-ear probe. Attenuation data for a set of earmuffs was measured on a pool of seven subjects using DPOAE, MIRE, and threshold methods. The DPOAE and threshold measurements were in agreement, validating the DPOAE method. The MIRE data were significantly higher than the DPOAE or threshold data. The difference in the attenuation levels represents sound reaching the cochlea that is bypassing the ear canal. This difference was smaller for measurements of the attenuation performance of a helmet, which provides increased shielding from bypass transmission paths through the head.