Hearing Protection Systems Research Articles

Method to measure the effect of acoustic leaks on active noise reduction device performance

Passive hearing protection (HP) systems are designed to reduce the level of noise at the ear. Passive HP typically excel in attenuating mid- to high-frequency noise. Active noise reduction (ANR) has the potential to provide additional attenuation of low-frequency noise. There are documented reports describing unwanted sound being generated by ANR systems if an acoustic leak is present. The objective of this effort was to develop a method to measure the performance of ANR devices with and without the introduction of an acoustic leak. A number of ANR earcups were selected for this study. Data were collected with an artificial ear and a flat base plate in a noise environment that ranged from 65–115 dB SPL. Wedges, designed in-house, created a consistent acoustic leak that ranged in size from 1–2 mm in radius. The results revealed that some ANR earcups provided high levels of protection when the earcup was well-sealed; however, when an acoustic leak was introduced, the earcups became unstable and generated noise in the earcup. This instability was not seen in all ANR devices. The outcome of these measurements solidified the need for a method to measure ANR device performance when acoustic leaks are introduced.

An explanation of the decrease in the earplug noise reduction when combined with an earmuff on an acoustic test fixture

For people working in high noise level environments, typically above 105 dBA, double hearing protection (DHP) systems (earplugs worn in combination with earmuffs) are always recommended. However, it is very difficult to predict the DHP overall sound attenuation due to the complexity of the system. A recent experimental study has shown on human subjects and an acoustic test fixture (ATF) that the noise reduction (NR) of the earplug decreases considerably when an earmuff is worn over it but the reason is still not fully understood. In this work, specially designed experiments using an ATF are proposed in order to explain this observation. The focus is put on the NR of the single earplug and that of the earplug in the DHP system. The respective effects on the DHP attenuation of various sound transmission paths involved in the system (both air-borne and structure-borne) are analyzed by modifying the system coupling conditions or controlling the sound pressure level under the earmuff. This work will help better understand the interactions between the system components and will provide the grounds for developing a prospective numerical model to predict the sound attenuation of a DHP system.

Diagnostic value of experimental parameters for measurement of localization ability

This research examines the test parameters in two methods proposed by an ANSI/ASA working group for measurement of auditory localization with tactical communications and hearing protection systems (TCAPS). The localization task consisted of responding to pink noise bursts (0.250s and 7000s). Participants responded by pointing to the perceived source with a laser pointer mounted on an instrumented chair. 12 participants completed the study—half of the trials while wearing no headset and half while wearing an active Invisio X5 headset. Method 1 used four pairs of loudspeakers, separated by 8°, 10°, or 12°, and the listener was tested in two orientations (0° and 45°). Intended for use in untreated acoustic environments, Method 1 provides only a coarse estimate of localization accuracy and proportion of front/back errors. Method 2 used 36 equally spaced loudspeakers, a sound treated environment, and a chair instrumented with a laser pointer. Performance is compared as a function of separation between loudspeaker pairs in Method 1 and test method. The analyses include measures of bias, magnitude, and whether errors are due to poor acuity or front/back confusion. The utility of the proposed test configurations to assess the effects of headgear on auditory spatial awareness will be discussed.

Assessing the performance capabilities of tactical hearing protection and communication devices

Military personnel work in unpredictable noise environments, which require flexible types of hearing protection (i.e., tactical hearing protection systems) in order to maintain mission effectiveness and situation awareness while reducing the risk of hearing loss. Acquisition decisions need to be made relative to accurate and complete measures of the total performance capabilities of tactical hearing protection systems and their effect on the user. Understanding the noise attenuation performance of tactical hearing protection systems has been a priority in order to protect the user from excessive noise exposure. However, active electronic tactical hearing protection systems have been designed to allow for enhanced communication and situation awareness, while at the same time protecting the auditory system from both impulsive and continuous noise. The Air Force Research Laboratory conducted a multifactorial assessment on currently available tactical hearing protection systems to determine the overall impact of these devices on performance and to enable users to make data-driven, informed acquisition decisions. The assessments included the following measurements: continuous noise attenuation, impulsive peak insertion loss, auditory localization, and speech intelligibility. The methods and results will be presented as well as a discussion on how the results promote an informed device selection.

Communication and situation awareness needs of hearing protection systems in high noise environments

The primary mission of aircraft carriers is to launch and recover military aircraft. Critical to the highly choreographed operations on the flight deck of the aircraft carrier is communications between aircraft directors, plane crews, and pilots as well as maintaining situation awareness in a highly dynamic environment. This must occur in an acoustic environment of 110 to 150 + dBA. These noise levels readily exceed the requirements for double hearing protection. A dilemma is generated in trying to provide adequate hearing protection while maintaining critical communications and situation awareness during flight operations at these noise levels. This paper will review previous studies of hearing protection effectiveness, compliance, factors affecting compliance, and impact on job performance in environments that readily exceed legacy hearing protection. To address the situation awareness shortfalls, a tri-service group (US Army, Air Force and Navy) is developing a standard for detection and localization criteria (commonly known as situation awareness) to assess operational suitability. The standard is intended to drive manufacturers to measure beyond the attenuation a hearing protector provides to address operational needs. New products and techniques are presented which demonstrate a potential solution sets to this critical issue.

Sound-localization performance with the hearing protectors

Hearing-protection system that provide level-dependent sound attenuation can protect the ear against potentially damaging sounds (such as loud impulsive noises), while at the same time allowing the perception of moderate-level signals (such as speech). Such systems come in two forms: passive (nonlinear-attenuation earplugs) and active (talk-through system). This study sought to quantify the effect of these systems on spatial hearing. To this aim, sound-localization performance was measured in twenty subjects, with and without ear protectors on. Five protectors (two passives and three actives) were tested. The results showed significant increases in the proportions of errors during the use of one of the systems tested. To clarify the origin of this effect, “protected head-related transfer functions” (PHRTFs), i.e., HRTFs obtained with the ear-protectors on, were measured in the horizontal plane for each of the systems tested. The comparisons of these measures between PHRTFs with HRTFs were found to be in agreement with the subjective tests.

Use of Hearing Protection on Military Operations

Noise Induced Hearing Loss (NIHL) continues to be a significant source of morbidity for UK service personnel. The Personal Interfaced Hearing Protection (PIHP) system was procured as an Urgent Operational...

Speech understanding in noise with integrated in-ear and muff-style hearing protection systems

Integrated hearing protection systems are designed to enhance free field and radio communications during military operations while protecting against the damaging effects of high-level noise exposure. A study was conducted to compare the effect of increasing the radio volume on the intelligibility of speech over the radios of two candidate systems, in-ear and muff-style, in 85-dBA speech babble noise presented free field. Twenty normal-hearing, English-fluent subjects, half male and half female, were tested in same gender pairs. Alternating as talker and listener, their task was to discriminate consonant-vowel-consonant syllables that contrasted either the initial or final consonant. Percent correct consonant discrimination increased with increases in the radio volume. At the highest volume, subjects achieved 79% with the in-ear device but only 69% with the muff-style device, averaged across the gender of listener/talker pairs and consonant position. Although there was no main effect of gender, female listener/talkers showed a 10% advantage for the final consonant and male listener/talkers showed a 1% advantage for the initial consonant. These results indicate that normal hearing users can achieve reasonably high radio communication scores with integrated in-ear hearing protection in moderately high-level noise that provides both energetic and informational masking. The adequacy of the range of available radio volumes for users with hearing loss has yet to be determined.

A Comparison Study of Foam versus Custom Silicone Earplugs Used as Part of an Intelligent Electronic Hearing Protector System

During the development of an intelligent hearing protection and communication system the attenuation of two different earplugs were measured. Both earplugs were measured separately and in combination with earmuffs. Foam earplugs and custom-moulded silicone earplugs were both used. The hearing protection system in question is able to measure the ear canal with respect to leaks. If a leak is detected, the system will warn the user. The measurements show that the foam plug gives higher attenuation than the silicone plug at all frequencies, but particularly at frequencies below 2 kHz. It has a steadily increasing attenuation from 30 dB – 43 dB over the frequency range 125 Hz – 8 kHz. The silicone plug attenuates around 26 dB, from 125 Hz – 1 kHz. Above this range, the attenuation increases to approximately 40 dB. With extra earmuffs added, the attenuation is 40 dB or better at all frequencies except 125 Hz, and the two plugs offer nearly identical protection. The results show that the mean and standard deviation of the attenuation for the foam earplug is as good as for an optimally fitted earplug. In the case of the silicone earplug, the mean attenuation is comparable to a typical custom earplug, but the standard deviation is better than comparable earplug. This finding is a result of the leakage control acting as a ’supervisor’ in the fitting of the earplugs.

Double hearing protection device

A double hearing protection system (120) comprising an earplug portion (122) of the system contains a tether attachment mechanism (123) that may be selectively attached to the interior of a circumaural hearing protector (125) designed with the mating attachment (154). The tether (123) retracts into the earcup (125) when both the earplug (122) and circumaural hearing protector (125) are worn, ensuring that the earseal (126) against the head is not broken by the retractable tether (123). The attachment point (154) inside the earcup (125) mates to the end of the tether (123) in a way that prevents foreign object damage in dangerous work environments, and limits the vibratory path from the earcup (125) to the earplug (122) through a flexible attachment mechanism (154).

Army Moves to Higher Level of Hearing Protection

Army Moves to Higher Level of Hearing Protection

Impulse and continuous noise reduction of tactical hearing protection systems

Tactical hearing protectors are devices designed to protect the wearer from high levels of impulse noise while providing some ambient listening and communication capability. Many of these devices also provide some attenuation of continuous noise. The efficacy of these types of devices depends on the amount of impulse noise protection and continuous noise protection, the quality of the ambient listening capability, and the intelligibility of the speech communication capability. This paper will describe the peak noise reduction of approximately 1 ms duration impulses with peak levels at 165 dB and 195 dB, for several earplugs and earmuffs. The frequency dependent reduction of impulse noise measured using an acoustic test fixture (the French German Research Institute at Saint Louis Head). Generally the data show higher levels of impulse noise reduction with earplugs than with earmuffs. Additionally, continuous noise attenuation was measured using human subjects performing the ANSI S12.6 Real Ear At Threshold (REAT) test. High speed (20k frames/sec) video will show the dynamic motion of typical tactical hearing protection earplugs and earmuffs when stimulated with a 195 dB impulse.

Free‐field sound localization with nonlinear hearing protection devices

Hearing enhancement and protection devices are being developed for both civilian and military markets that permit the wearer to maintain an awareness of the ambient sound environment and permit face‐to‐face communications while still protecting the user from high‐level impulse noises or blast waves from, for example, handguns, rifles, and shotguns. The impulse noise protection for these devices can range from less than 9 dB peak pressure reduction to reductions of more than 40 dB peak. There has been little research, however, on the ability to use binaural cues to localize sound with such devices. An experiment to evaluate the ability of normal‐hearing listeners to localize broadband noise bursts while wearing various nonlinear hearing enhancement and protection systems was conducted in an anechoic environment. Subjects were asked to localize a 60‐dB SPL, 200‐ms broadband noise burst from a speaker in 1 of 12 different azimuth locations (0 to 330 deg in 30‐deg steps) and five different elevations (0, ±15, ±30 deg). The azimuth and elevation errors were measured for each device using circular statistical techniques. Front/back reversals were corrected. The results show a 15‐ to 20‐deg degradation in the wearer’s ability to localize sound when using these hearing protectors.

Compact test method for the evaluation of acoustical transmission loss and insertion loss of new helmet material samples

There is a need to establish a simple and accurate measurement technique for determining the transmission loss of sample materials for helmets over a frequency range of 300–3 kHz. Standard methods, e.g., ASTM E 90-02, for measuring transmission loss of building materials and structures, based on adjacent reverberation chambers, are too costly and impractical. A 1.22-m-long double-wall tube, packed with Owens Corning R13 insulation, has been fabricated using QUIK-TUBETM cardboard concrete forms of 200 and 300 mm diameters. A circular sample of material, also 300 mm in diameter, is placed on the end of the tube and subjected to an external sound field. Transmission loss is established by external and internal microphones. This paper describes the measurement and analysis procedures and examines the associated variables and error terms. Results are presented for 16 material samples with surface weights covering a range from 0.3 to 14.7 kg/m2 and compared with analytical predictions including mass law models. The acoustical characteristics of commercial helmet materials and liners are evaluated in the context of hearing protection systems. The transmission loss measurement procedure has the potential for meeting standardization objectives.

Messungen zum Lärmschutz bei der funktionellen MR-Tomographie

Functional magnetic resonance imaging (fMRI) can detect changes in oxygen saturation of the brain. Fast changing high gradient fields are necessary which produce high levels of noise. In studies of the auditory cortex, auditory stimuli have to be perceived and discriminated against the noise level of the activated tomograph. The generated frequency bands and their intensities during fMRI with a Siemens Magnetom Vision, 1.5 T, EPI sequence were measured in the outer ear canal of a dummy head. Noise attenuation was evaluated with four different noise muffs (simple/inexpensive products, quality product, specialized fMRI muffs). Without protection, peak noise levels reached up to 111 dB(A) near 1000 Hz in the dummy ear canal. Major noise attenuation was only found at higher frequencies (4000 Hz by about 25 dB; 8000 Hz by about 35 dB) with the quality product and the specialized fMRI muffs. Only quality noise products can sufficiently protect patients from high sound pressure levels of tomograph noise. If in the future higher gradient fields are applied at faster slew rates, acoustic stimuli can safely be applied only in combination with increased hearing protection systems in order to minimize the risk of noise trauma.

Skull simulator for design of hearing protection systems for bone-conducted sound

A skull simulator was developed for an investigation of bone-conducted sound transmission, which can be a significant source of sound in extreme noise environments. The simulator was built from a human skull, with silicon gel used to model internal organs and silicon and latex used to model the skin. Accelerometers attached to the skull bones were used to measure the skull vibration response. Mechanical point impedance measurements on the simulator were compared to results reported in the literature for humans, cadavers, and skulls. Reasonable agreement with these results served to validate the simulation. The link between skull vibration measured on the skull simulator and bone-conducted sound at the cochlea of a human subject was determined by comparing the response at the mastoid measured on the skull with similar measurements on a human subject. The response of the skull simulator in a sound field was then measured. The frequency response peaked from 1 to 3 kHz, which is where the bone-conduction limit to hearing protection attenuation is a minimum. The simulator was used to determine the effectiveness of various hearing protection components in reducing skull vibration. The results are compared to attenuation measurements of these components on human subjects.

Real-world performance of hearing protection systems in Army aviation

Methods to measure the performance of hearing protective devices using naive subjects were developed to better estimate the hearing protection that can be reasonably achieved in operational environments. A ‘‘preference’’ for subject-fit methods over experimenter-fit or experimenter-supervised-fit methods has been growing among many hearing conservation professionals but has not received universal acceptance. The real-world performance of hearing protection used in Army aviation was measured. Army aviators who had completed 38 weeks of rotary-wing aircraft training were used in personal hearing protector evaluations using ANSI S12.6 real-ear attenuation at threshold procedures. Using their own flight helmets (group 1) or helmets and earplugs (group 2), the aviators were instructed to fit their helmet or helmets with earplugs as they do for flight operations (i.e., an informed-user fit). Following this evaluation, an experimenter-supervised fit evaluation was performed, which included helmet and earplug fit by a technician trained and experienced in helmet fit procedures. For both groups, the real-ear attenuation at threshold results for the informed-user and experimenter-supervised fitting procedures were virtually identical. Thus, the experimenter-supervised fit procedure (ANSI S12.6-1997 Method A) is appropriate for evaluating the performance of personal hearing protective headgear in the Army aviation environments.

High-level impulsive noise source

To assist in the development of hearing protection and communication systems for use by crews of artillery weapons, a source of high-level impulsive noise has been produced which has operating characteristics allowing its use within the laboratory. Generating peak sound pressure levels in excess of 180 dB and amenable to automatic operation, the source produces no fumes or high temperatures. Its basic principle of operation is the sudden release of compressed air to the atmosphere from a pressure chamber. The physical characteristics and acoustical performance of the source are described.