Hearing protection devices and methods used for their evaluation: A military perspective
Publication: Journal of Military, Veteran and Family Health
Volume 5, Number 1
Abstract
Abstract
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.
Résumé
Introduction : Les soldats sont régulièrement exposés à des bruits potentiellement dangereux, tels que ceux des moteurs des véhicules de transport et le son impulsionnel des armes. Le son impulsionnel peut être particulièrement dangereux, surtout le son intense de l’artillerie ou celui des projectiles tirés depuis l’épaule, par exemple les lance-roquettes. Suite à des contrôles administratifs et d’ingénierie, les systèmes de protection auditive sont une pierre d’assise des programmes de sauvegarde de l’ouïe. Le choix de la protection selon la mission doit être effectué avec soin. Les systèmes varient des simples bouchons au options les plus technologiquement avancées. Méthodologie : Les façons de caractériser l’atténuation offerte par les systèmes de protection contre le son impulsionnel sont complexes et en évolution. Résultats : Pour le soldat, le besoin de balancer l’atténuation sonore contre l’interférence avec le reste de sas capacités d’audition, tel que le besoin d’entendre les sons plus doux, de comprendre les commandements, ou de localiser des sons, est un dilemme constant. Discussion : Cette revue souligne quelques-uns des défis d’évaluation et de choix des systèmes de protection protégeant nos soldats en vue d’aider les nouveaux venus dans la discussion à comprendre cet important sujet.
INTRODUCTION
Hearing loss is costly in terms of loss of both human health and human resources. To understand how to best use hearing protection devices (HPDs) to protect against hearing loss, the characteristics that make noise injurious need to be appreciated. Much research has been devoted to deciphering the association between exposure to high levels of noise and hearing loss. Noise sound pressure level (SPL), spectral frequency, temporal spacing of impulses, peakedness of impulses, duration of exposure, and genetics are known to influence hearing loss.1 In this article, we focus on HPDs and protection from noise, especially impulse noise, from a military perspective.
A single unprotected gunshot at close range can cause temporary or permanent hearing loss and hinder performance on the battlefield and afterward. Depending on the severity of a temporary hearing loss (also known as a temporary threshold shift), it may take minutes to months of restorative quiet (assuming a normal hearing threshold to start)2 for hearing to return to normal. A permanent hearing loss is defined as an irreversible threshold shift.3 Both temporary and permanent hearing loss may be accompanied by tinnitus, loss of sensitivity to certain frequencies, or subtle deficits of hearing in noise, all of which are preventable when HPDs are worn consistently and properly when indicated. HPDs have been developed along two different avenues: active and passive.
Passive hearing protection devices
Passive HPDs do not use embedded electronics and can be further subdivided into noise-level-dependent and noise-level-independent attenuation devices.4
Passive level-dependent HPDs’ noise attenuation is affected by noise intensity, whereas level-independent HPDs’ noise attenuation is not. Both types of HPD are based on physical barriers that aim to prevent the propagation of noise into the ear canal, but with level-independent HPDs, constant energy reduction is achieved for different noise SPLs.5
Unlike level-independent HPDs, level-dependent HPDs provide less attenuation at lower intensity SPLs (e.g., speech or low-SPL environmental noise). Level-dependent HPDs typically contain a narrow inner channel throughout the length of the earplug. The acoustic impedance of this narrow channel increases non-linearly with the exterior sound level. At low SPLs, the channel dimensions allow sound waves to move through the earplug without interruption, but when the SPL increases, the pressure acts on the plug, causing the air at the plug’s entrance to move circularly, thus preventing the passage of sound waves.4 Preventing the passage of sound waves causes an energetic reduction (sound attenuation) in the ear canal. Such HPDs are often called non-linear earplugs or selective-to-shooting earplugs because of their selective ability to reduce high-intensity weapon noise. Selective-to-shooting earplugs allow better hearing of speech and environmental sounds yet protect against sudden impulse noise, a basis for their popularity in the military.
Comprehensive examination of the measured attenuation of a wide variety of level-dependent HPDs over a range of sound levels from threshold to 190 decibels commenced during the past decade.6 Measurements (including real ear attenuation at threshold [REAT] and artificial test fixture [ATF], which are discussed later in this article) have revealed that level-dependent HPDs begin to provide amplitude sensitivity when SPLs equal or exceed 110 decibels. Level-dependent HPDs are therefore only suited for protection against impulse noises. They provide limited protection for constant noise below 110 decibels, which is insufficient attenuation for hearing protection.6
Active hearing protection devices
The active approach incorporates active noise reduction (ANR) algorithms in electrical devices that actively cancel noise, thereby enhancing the signal-to-noise ratio (SNR), where the signal is the desired sound or speech. Active HPDs include a passive component and an active electrical component that consists of a pre-amplifier and a tiny microphone, which serve as another independent physical barrier.5 Another option is to position the electrical mechanism outside the ear canal, including other electrical components that are essential to its activity, such as a reference microphone, an electrical circuit, and hardware to process the signal, as well as operating switches and a volume control.
The electrical system reduces noise at different levels and at different frequencies according to the user’s demands. It can distinguish between frequencies and intensities that typically characterize human voices versus environmental noises and sounds, such as gunfire or explosions. These devices have been found to be effective mainly at low frequencies because they do not allow the passage of sound waves lower than 1000 hertz through the ear canal. However, they have been found to be ineffective in attenuation of SPLs higher than 150 decibels.4
Active HPDs with directional microphones facing forward in relation to the user enable users to adjust the sound level in a way that allows them to hear human voices in front while minimizing the peripheral noise.4 In the context of military operations, these HPDs enable soldiers to adjust the sound level they can hear and understand commands and environmental noises while reducing background noise. Such devices contain military communication components (e.g., radio transmission devices) and are called tactical communication and hearing protection systems (TCAPS).4 The obvious advantage of these devices is their ability to enhance speech intelligibility in noisy surroundings, thus preventing misunderstanding of vocal or radio-transmitted inputs. These devices are typically designed to be able to be used in harsh conditions and environments for military applications. The drawback is that with only forward-angled microphones, some models can interfere with localization of sound sources from behind.7 In these conditions, acoustical situational awareness will be decreased if threats are coming from the back and sides. Testing protocols and models are under development to evaluate TCAPS’ capability to allow good localization of sound sources.8
METRICS FOR MEASURING ATTENUATION OF HEARING PROTECTION DEVICES AGAINST CONTINUOUS NOISE
Several measurement techniques of noise attenuation are relevant to the process of choosing an HPD. With noise reduction, two microphones, one fixed outside the ear canal (open field equivalent) for reference and another inside the canal (closed ear; influenced by the canal’s shape and tissues), can be used to measure the difference in SPLs on either side of the HPD at the time of a noise.4
Common laboratory metrics for continuous noise include the noise reduction rating (NRR), in which the HPD is fit by the experimenter. With the single number rating (SNR)4 and noise reduction rating (subject fit; NRR [SF]), however, the HPD is fit by the subject. Proper insertion of an earplug can yield 30–40 decibels of frequency attenuation, whereas a poor and unsatisfying insertion can yield only 15–30 decibels.9 Subject fit metrics, such as the SNR and NRR (SF) are more real world. These indexes describe the efficiency of an HPD using a single numeric value (in decibels) and are based on attenuation values across the frequency domain. The index is usually found on the package of an HPD or obtained from the manufacturer and should be displayed along with the attenuation values at each frequency. The SNR can be used to grade or classify the HPD to the noise so as to provide adequate sound attenuation while avoiding overprotection.
METRICS FOR MEASURING THE ATTENUATION OF HEARING PROTECTION DEVICES AGAINST IMPULSE NOISE
For impulse noise, the energetic gap between two separate noise exposures at the eardrum without an HPD (open ear) and with an HPD (closed ear) is called the insertion loss (IL). This value is unique to each HPD, with a given noise stimulus at a given SPL4 It is an expression of the pressure attenuation as a function of the frequency.10
The SPL measurement is obtained by a microphone or a pressure gauge and is demonstrated graphically with a one-third octave diagram in the frequency domain. This is the accepted way of determining the frequency-dependent energy that is transferred to the cochlea.11 Traditionally, IL was obtained in a quiet environment. This was altered once the non-linear effects of HPDs were noticed, because the amount of energy reduction obtained is not necessarily linearly associated with the impulse noise level. Today, the usual practice, at least in the Israel Defense Forces, is to obtain the IL in a noisy environment and with different levels of noise.9
Objective and subjective methods have been developed to evaluate HPD performance. An accepted subjective method is the REAT. This method reflects a statistical sample (20 subjects for earplugs and 10 subjects for earmuffs) and is therefore considered reliable, according to the American National Standards Institute.11 REAT is considered the most accurate measurement system and has been standardized around the world. It was the first system to be codified in national and international guidelines and has the fewest measurement artefacts.12 One drawback of REAT is that it measures the subject’s threshold of hearing response to narrow-band tone stimuli in a quiet environment, whereas the response to these stimuli in a noisy environment is more important.9
The two main objective methods to measure the efficiency of HPDs are the ATF and the microphone-in-real-ear (MIRE) methods. In the MIRE method, a small microphone is inserted into the subject’s ear canal to measure the SPL.10 MIRE measurements account only for sound travelling through the ear canal; they do not calculate all pathways of noise transmission (bone and other tissues) as does REAT. MIRE measurements are performed with and without an HPD, similar to the subjective REAT method, but with no need for the subject to acknowledge that he or she has heard the signal. Measuring SPL with and without the HPD yields the IL value. Measurements can be performed in laboratory settings or in field conditions, allowing the testing of a variety of HPDs under various conditions. However, MIRE measurements without HPDs are only possible if the sound does not present a risk to the subjects, which is why MIRE is not commonly used to evaluate HPDs for impulse noise.
The other objective method is the ATF method. ATFs can be adjusted to fit all people by changing the head width and ear size, although these may not be critical for all attenuation measurements.13 This method uses a head and torso mannequin, mimicking the human head and ear with a microphone built right into the ear canal, which, in addition to air conduction, picks up the acoustic energy absorbed from the bones and soft tissues. Bone conduction is the term used to describe the mechanism of noise propagation into the inner ear, not air conduction. The characteristics of the simulated tissue of the ATF are temperature dependent, and later ATF models are heated to body temperature. Acoustic energy is also dispersed differently according to the shape of the ear canal because the sound waves fracture and return in unique angles.9 The ATF method is typically used to measure high levels of sound, thus preventing the risk to human volunteers of hearing loss due to noise exposures, as well as allowing for a relatively reliable measurement of noise levels.
Even if an impulse noise source is tuned to give the same signal in different repeated trials (open ear in comparison with closed ear), small changes are expected to occur and must be considered. For that reason, another microphone, defined as the reference microphone, is placed outside the ATF, measuring the sound level in the air. The energetic uniqueness of a specific ATF without an HPD and the reference microphone in the field is called the transfer function of the open ear (TFOE) and is a fixed value for each ATF.4 The energetic change in a noise peak measurement between two situations – with HPD and without HPD, after taking into consideration the TFOE – is called the impulse peak insertion loss (IPIL).4 The calculation of IPIL requires two sequential impulses, and given that no two weapon impulses are exactly the same, error is introduced into the measurements. A disadvantage of the ATF method is that in studies statistical analysis is not inherent in the method.
Recently, a new approach, termed impulse spectrum insertion loss (ISIL), was introduced. ISIL aims to look beyond the peak value of the IPIL in the frequency domain. ISIL represents the third-octave band and adds further description of the HPD’s performance by providing knowledge of the frequency at which IPIL is achieved relative to the noise source impulse.14,15 Moreover, ISIL can incorporate bone conduction exceedances. For example, ISIL analysis of the use of an E-A-R Classic earplug when exposed to grenade launcher and a 12.7-mm machine gun noise reveals that bone conduction limits were exceeded beyond 1000 hertz and 2000 hertz, respectively.15 While using IPIL, one can overestimate the amount of protection because bone conduction corrections are not part of the calculations. ISIL is gradually being practiced by military policy-makers in the field of hearing protection, such as in the Israel Defense Forces. The ISIL can help to determine the allowable number of exposures (ANEs) to weapons and guide the choice of HPD to be used.
NOISE EXPOSURE LIMITS
The exposure limits to noise vary around the world and may be different for civilian and military applications. In many countries, exposure limits are based on the A-weighted equivalent sound level, or LAeq, with the same energy content as the varying acoustical signal measured on the basis of the A-weighted sound level. Often this is expressed as the allowable equivalent sound exposure over 8 hours, or LAeq8, the typical shift length in industry.
US MIL-STD-1474E provides the option of applying the equal energy model to impulse noise in the form of the LIAeq and the LIAeq100ms metrics, for impulses of less than 2.5 ms and 2.5 ms or more, respectively. With the latter, a waveform such as with an artillery round is truncated after the first 100-ms interval.16 These metrics allow the energy in impulse noise to be added to the dose of continuous noise, the LAeq. The usual approach in the US standard is the use of the Military Auditory Hazards Assessment Algorithm for Humans (AHAAH), a biomechanical model, which predicts the ear’s response to noise and can be used to evaluate the ANE by analysing the recorded noise. The algorithm can be applied to situations in which a person uses, or does not use, an HPD, and it considers the middle ear reflex as well as the fatigue of the cochlea’s organ of Corti.5,9 However, although the middle ear reflex is common in those with good hearing, it is felt to not be pervasive enough (i.e., sufficiently prevalent to provide 95% confidence that it has at least 95% prevalence) to be used in damage risk criteria and health hazard assessments for impulse noise17
A 2003 North Atlantic Treaty Organisation review of impulse noise recommended use of the quantitative value of sound exposure level, or SEL, defined as the constant sound level that has the same amount of energy in 1 second as the original noise event.9,18 Noise exposure limits were established using this metric on the basis of the available temporary threshold shift data from impulse noise exposure experiments done in the past. This metric is, in fact, a simple and easy-to-use mathematical equation, but without measuring the noise with a protected ATF, one cannot use the SEL to calculate the ANE.
The organ of hearing within the inner ear responds to the amplitude and frequency of the acoustic stimuli. The ear is more sensitive to certain frequencies, most commonly 1–4 kilohertz, the natural resonate frequencies of the human ear. Exposure to these noise frequencies could potentially damage hearing to different degrees, and HPDs are chosen to attenuate the noise in a given frequency spectrum. ANE may be determined by means of the ATF or the REAT methods but, as we describe later, the ATF method may be more conservative. REAT data are obtained using a clinical audiometer that measures hearing thresholds in the 125- to 8000-hertz range, whereas the ATF measures SPL in the 20- to 20000-hertz range. The difference between these two methods yields a two-tailed frequency region, a low-frequency tail of 20–125 hertz and a high-frequency tail of 8000–20000 hertz. Although these frequencies pose less threat to the organ of Corti, especially the low frequencies,19 they are embedded in the calculation of the ANE and may result in overprotection by lowering the ANE to rounds in military training. Others think that for earplugs used with high-level impulse noise, REAT should not be used as IL.20
Another aspect in determining the effect of noise on hearing is the ear’s sensitivity to the signal’s frequency range. The concept of auditory weighting, such as A-weighting, used in industry, for example, reflects the sensitivity of the human ear to common industrial noise frequencies, and it is used to determine the intensity of noise inside the ear. Another approach, C-weighting, is felt to follow the frequency sensitivity of the human ear at high SPLs according to the equal loudness contours.21 By contrast, zero-weighting reflects the absolute amount of noise at all frequencies without the influence of the ear structure and shape. A newly proposed F-weighting, based on a blending of A-weighting and C-weighting with two new parameters, kurtosis (a measure of peakedness) and oscillation coefficients (energy density distribution), has been proposed to evaluate noise hazard and has been validated against animal exposure data for complex noise, the combination of high-intensity impulse and constant noise.22 These researchers, however, do state how to integrate this newly proposed F-weighting scale with HPD attenuation calculations.
DUAL PROTECTION WITH HEARING PROTECTION DEVICES
ANR HPDs can be added to passive devices to achieve maximal noise reduction.23 One passive device that could be added to achieve the desired attenuation is the earmuff. On the basis of prior experiments, combining an ANR with a passive device can yield up to a 47-decibel attenuation.23 This reduction does not account for energy conducted through bone and other tissue. To yield a higher attenuation, the issue of energy absorbed through bone conduction must be considered.23
SPEECH INTELLIGIBILITY AND SPEECH QUALITY
Considering the soldier’s need for acoustic communication while using HPDs, we should emphasize that reducing or increasing sound intensity does not correlate with or ensure its clarity. The draft version of the international standard ISO 9921 defines speech intelligibility (SI) as “rating of the proportion of speech that is understood”24(p.2) SI should not be confused with speech quality. SI refers to the number of speech items identified correctly by a listener, whereas speech quality refers to the quality of a recovered sound signal in relation to the amount of distortion made by the mathematically engineered processing. Sound quality is an important component in evaluating the performance of an ANR when reducing the background noise. One should aspire to achieve a situation in which the recovered sound, after activating the ANR, would be as close as possible to the original signal without the background noise. A known side effect when activating an ANR algorithm (or any other method to process signals) is alteration of the sound signal. Different objective methods, which are based on physical indexes of communication, do not predict sound clarity but rather set the physical parameters used by other models to do so. Subjective tests for both SI and sound quality also exist and are based on using different words and sentences in a quiet or a noisy background.24
HEARING PROTECTION DEVICE SELECTION SUMMARY
Although the effects of HPDs are widely known, the data comparing devices, scales, and measurements are limited. Accurate matching of an HPD in the military environment is not a simple matter of choosing a consumer metric to match the noise SPL as it is with continuous industrial noise. It is a complex task, taking into consideration the user’s environment, compatibility with other personal protective equipment, the user’s need for situational awareness and communication needs, and the characteristics of the noise and that of the HPD.
In addition, methods such as AHAAH and SEL are used to assess hearing risk in relation to a measured noise and its characteristics, which enables the determination, directly or indirectly, of the allowed number and duration of exposures without risking permanent hearing loss.
Although there is no ideal HPD, in this article we provide insight into the various types of HPD and how they must be integrated with soldiers’ military ensemble so as not to interfere with their varied military tasks. The goal is to allow them to perform at peak levels yet protect them from disabling hearing loss.
Being aware of the basic concepts of noise attenuation, the different ways to achieve it, and the objective methods of measuring efficacy of the various HPDs contributes significantly to the understanding of how to optimize HPD selection today and the ability to assess the HPDs of the future.
ACKNOWLEDGEMENTS
We thank Professor David S. Gertz and Mr. Pierre Lamontagne for their comments and proofreading of this work. Their effort is highly appreciated. We thank the two anonymous reviewers whose comments and suggestions helped to improve and clarify this article.
REFERENCES
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Published In
Journal of Military, Veteran and Family Health
Volume 5 • Number 1 • 2019
Pages: 141 - 147
Copyright
© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence. For permission to share, copy, or redistribute, contact [email protected].
History
Published in print: 2019
Published online: 29 April 2019
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Authors
Contributors
Nir Fink and Hagar Zvia Pikkel contributed equally to this work. Nir Fink, Hagar Zvia Pikkel, and Gregory A. Banta designed the study, conducted the literature search, and acquired and analyzed data. Arik Eisenkraft helped select the research questions and data set. All authors revised the article for important intellectual content. All authors drafted and approved the final version submitted for publication.
Competing Interests
Nir Fink and Arik Eisenkraft had a past patent pending on the development of a novel passive ear protection device.
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None declared.
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Journal of Military, Veteran and Family Health 2019 5:1, 141-147
Journal of Military, Veteran and Family Health 2019 5:1, 141-147