Level-dependent Hearing Protectors Research Articles

Development and Human Factors Evaluation of a Portable Auditory Localization Training System.

To design and develop a Portable Auditory Localization Acclimation Training (PALAT) system capable of producing psychoacoustically accurate localization cues; evaluate the training effect against a proven full-scale, laboratory-grade system under three listening conditions; and determine if the PALAT system is sensitive to differences among electronic level-dependent hearing protection devices (HPDs). In-laboratory auditory localization training has demonstrated the ability to improve localization performance with the open (natural) ear, that is, unoccluded, and while wearing HPDs. The military requires a portable system capable of imparting similar training benefits as those demonstrated in laboratory experiments. In a full-factorial repeated measures design experiment, 12 audiometrically normal participants completed localization training and testing using an identical, optimized training protocol on two training systems under three listening conditions (open ear, TEP-100, and ComTac™ III). Statistical tests were performed on mean absolute accuracy score and front-back reversal errors. No statistical difference existed between the PALAT and laboratory-grade DRILCOM systems on two dependent localization accuracy measurements at all stages of training. In addition, the PALAT system detected the same localization performance differences among the three listening conditions. The PALAT system imparted similar training benefits as the DRILCOM system and was sensitive to HPD localization performance differences. The user-operable PALAT system and optimized training protocol can be employed by the military, law enforcement, and various industries, to improve auditory localization performance in conditions where auditory situation awareness is critical to safety.

Selection of Level-Dependent Hearing Protectors for Use in An Indoor Shooting Range.

The high sound pressure level generated by impulse noise produced in an indoor shooting range makes it necessary to protect the hearing of the people it affects. Due to the need for verbal communication during training at a shooting range, level-dependent hearing protectors are useful. The objective of this study was to answer the question of whether it is possible to properly protect the hearing of a shooting instructor using level-dependent hearing protectors. The noise parameters were measured in the places where the instructor was present at the shooting range. The division of a specific group of trained shooters into subgroups consisting of three or six simultaneously shooting individuals did not significantly affect the exposure of the shooting instructor to the noise. An assessment of noise reduction was carried out for eight models of earmuffs and two variants of earplugs, using computational methods for the selection of hearing protectors. Among the noise parameters, both the A-weighted equivalent sound pressure level and the C-weighted peak sound pressure level were taken into account. Depending on the assessment criterion adopted, a sufficient reduction in impulse noise was provided by either four or six out of the 10 hearing protectors included in the study.

Localization of Vehicle Back-Up Alarms by Users of Level-Dependent Hearing Protectors under Industrial Noise Conditions Generated at a Forge.

The use of hearing protectors in various noisy workplaces is often necessary. For safety reasons, auditory information may be required to correctly localize the direction of an auditory danger signal. The purpose of this study was to verify if the selection of a specific level-dependent hearing protector may be important for the ability to localize a vehicle back-up alarm signal. The laboratory conditions reflected industrial conditions, under which an impulse noise was emitted against a background of continuous noise. A passive mode and a level-dependent mode (maximum and incomplete amplification) were considered. Four different models of level-dependent earmuffs and one model of level-dependent earplugs were included in the tests. The tests enabled differentiation between the individual hearing protectors. The use of earplugs in level-dependent mode did not significantly affect the ability to correctly localize the back-up alarm signal. For the earmuffs, the global assessment of the impact of a mode change revealed that, depending on the model of the earmuffs, the impact may be insignificant, but may also result in considerable impairment of the ability to localize the back-up alarm signal.

Free-field sound localization on the horizontal plane as a function of stimulus level with an electronic, level-dependent hearing protection device

Electronic, level-dependent hearing protection devices (HPD) provide different levels of attenuation as a function of the input sound pressure level, so that loud sounds are attenuated for protection, but soft and moderate sound levels may be presented with little or no attenuation, or even with a small positive gain. While previous experiments have investigated sound localization with HPDs and moderate stimulus levels, it is important to understand the impact of level-dependent HPDs on the localization of both low- and high-level sounds. In this experiment, horizontal-plane sound localization judgments were obtained from human listeners with and without an earplug-type electronic, level-dependent HPD at several stimulus levels from 20 to 80 dB SPL. The data were analyzed in terms of the proportion of front/back reversals, and the mean absolute lateral error after correcting for front/back reversals. There were very few front/back reversals in the open-ear conditions at any level, and the mean absolute la...

Speech perception and production with one level-dependent hearing protector

Electronic level-dependent hearing protectors aim to enhance communications and situational awareness in the noisy workplace compared to passive hearing protectors. From a listener’s perspective, the fixed attenuation provided by passive hearing protection interferes with speech recognition in noise in users with hearing loss. From a talker’s perspective, they also disrupt the Lombard reflex and induce an occlusion effect, which leads to a reduced vocal output in noise and compounds the problem. This paper presents laboratory data on speech recognition and production in noise with one level-dependent earmuff device. The device was tested in the level-dependent mode at two different gain settings as well as in a powered OFF condition to simulate passive protection within the same device. Level-dependent hearing protection provided significant speech recognition benefits in noise compared to passive protection, especially for users with hearing loss, and performance often exceeded unprotected listening. Tal...

Effect of a level-dependent hearing protector on detection thresholds, perceived urgency, and localization performance of reverse alarms

Previous studies have shown that perception of warning signals in noisy workplaces can be compromised when wearing conventional hearing protectors. Level-dependent hearing protectors have been developed to increase environmental awareness and, by extension, to improve worker safety. A set of studies was conducted to assess the potential benefits of a level-dependent hearing protector operating in its passive and active modes, in quiet and in noise, for a range of listeners with normal hearing listening to reverse alarms. As expected, the detection of reverse alarms in quiet was improved in the active mode compared to the passive mode, but no significant benefit was found in noise. The active mode of the device only marginally helped to restore the sense of urgency conveyed by back-up alarms in noise, which was largely affected by passive attenuation.The localization of reverse alarms was slightly worse when wearing the device compared to unprotected listening, and was not improved in the active mode compared to the passive mode. In summary, when choosing a hearing protector to ensure adequate perception of reverse alarms, for example, different psychoacoustic tasks must be considered, also keeping in mind that level-dependent hearing protection does not necessarily restore performance to unprotected targets.

Sound localization performance with level-dependent hearing protection devices

Level-dependent hearing protection devices (HPDs) offer the promise of the ability to hear low-level sounds while providing protection against dangerous high-level sounds. These devices are useful for people who may be exposed to intermittent, unexpected loud sounds while operating in environments that are otherwise generally quiet. Often, it is important for these users to be able to localize sounds and to maintain auditory situation awareness in these environments while wearing the HPDs. Measurements were made of listeners’ abilities to localize quiet and loud sound sources while wearing different level-dependent devices, as well as with unprotected ears. Electroacoustic measurements were also made with the devices on an acoustic test fixture to characterize their input-output functions, effective bandwidths, and directional transfer functions. Sound localization performance was typically worse when wearing the HPDs than with unprotected ears, with more front/back confusions, especially for the devices ...

The interaction of hearing loss and level-dependent hearing protection on speech recognition in noise

Objectives: To determine the effects of different control settings of level-dependent hearing protectors on speech recognition performance in interaction with hearing loss. Design: Controlled laboratory experiment with two level-dependent devices (Peltor® PowerCom Plus™ and Nacre QuietPro®) in two military noises. Study sample: Word recognition scores were collected in protected and unprotected conditions for 45 participants grouped into four hearing profile categories ranging from within normal limits to moderate-to-severe hearing loss. Results: When the level-dependent mode was switched off to simulate conventional hearing protection, there were large differences across hearing profile categories regarding the effects of wearing the devices on speech recognition in noise; participants with normal hearing showed little effect while participants in the most hearing-impaired category showed large decrements in scores compared to unprotected listening. Activating the level-dependent mode of the devices produced large speech recognition benefits over the passive mode at both low and high gain pass-through settings. The category of participants with the most impaired hearing benefitted the most from the level-dependent mode. Conclusions: The findings indicate that level-dependent hearing protection circuitry can provide substantial benefits in speech recognition performance in noise, compared to conventional passive protection, for individuals covering a wide range of hearing losses.

Measuring effective detection and localization performance of hearing protection devices

Awareness of the surrounding acoustic environment is essential to the safety of persons. However, the use of hearing protection devices can degrade the ability to detect and localize sounds, particularly quiet sounds. There are ANSI/ASA standards describing methods for measuring attenuation, insertion loss, and speech intelligibility in noise for hearing protection devices, but currently there are no standard methods to measure the effects of hearing protection devices on localization and/or detection performance. A method for measuring the impact of hearing protectors on effective detection and localization performance has been developed at AFRL. This method measures the response time in an aurally aided visual search task where the presentation levels are varied. The performance with several level-dependent hearing protection devices will be presented.

Continuous and impulsive noise attenuation performance of passive level dependent earplugs

Level dependent hearing protectors, earplugs and earmuffs, have advanced in technology due to the needs of military personnel and others to reduce the risk of hearing damage from impulsive noise. These hearing protectors were developed to preserve ambient listening capabilities therefore improving situational awareness while reducing the risk of noise induced hearing loss by attenuating both continuous and impulsive noise. Four commercially available passive level dependent earplugs were assessed for both continuous noise attenuation and impulsive insertion loss performance. The continuous noise attenuation results were collected using American National Standard Institute (ANSI) S12.6-2008 Methods for Measuring the Real-Ear Attenuation of Hearing Protectors while the impulsive insertion loss results were collected using ANSI S12.42-2010 Methods for the Measurement of Insertion Loss of Hearing Protection Devices in Continuous or Impulsive Noise Using Microphone-in-Real-Ear (MIRE) or Acoustic Test Fixture Procedures. The presentation will include the passive noise attenuation performance of level dependent earplugs for both continuous and impulsive noise. The impulsive insertion loss results for these particular hearing protectors will be applied to impulsive noise damage risk criteria for an estimate of allowable impulsive noise exposure.

Speech intelligibility in military noise for normal-hearing and hearing-impaired listeners using level-dependent tactical hearing protectors

The effects of two tactical headsets with level-dependent hearing protection capabilities on face-to-face speech intelligibility in military noise were investigated. Devices were the Nacre QuietPro and the Peltor Powercom Plus. Noises recorded from light-artillery vehicles in the Canadian Forces were reproduced in a simulation room at 80-95 dBA. Over 45 subjects covering a wide range of hearing profiles were tested using sentences from the Hearing-In-Noise Test. When used as passive devices with the electronics powered off, the devices performed as expected from conventional protectors having the same amount of attenuation. In this mode, there were large performance differences among subject groups in terms of the effects of wearing the devices compared to unprotected listening. When used in active talk-through (or surround) mode, both devices showed large intelligibility benefits over the passive mode and demonstrated a level of performance often exceeding that in unprotected listening. The subject group with the most impaired hearing benefitted the most from the active mode. The findings indicate that the current technology of high-end tactical headsets could provide substantial benefits in situational awareness during noisy military operations for a wide range of hearing profiles. [Work based on Defence R&D Canada Contractor Report DRDC Toronto CR-2011-101.]

Speech Intelligibility and Passive, Level-Dependent Earplugs

Noise-induced hearing loss is one of the most common occupational diseases. Military personnel are at especially high risk due to the broad range of military noise hazards and the frequency of exposure. Hearing protectors are vital for this particular workforce, yet they can impede the ability to understand necessary communication in the field. Level-dependent hearing protectors are designed to protect the auditory system from the hazards of impulse noise, while preserving the ability to hear speech and other important auditory signals. The aim of this study was to evaluate the effect of two different passive, level-dependent earplugs (Combat Arms Earplugs; Sonic II Ear valves) on speech understanding of normal-hearing listeners in the presence of low-level background noise. The Combat Arms Earplug, developed specifically for use by military personnel, represented devices that attenuate impulse noise using small orifices and the Sonic II ear valve represented devices using an internal diaphragm. This study used a repeated-measures experimental design. Four scrambled lists of each of the four Northwestern University No. 6 50-word lists were presented in random order at 65 dB SPL in the presence of quiet and two different types of background noise: multitalker and military vehicle noise; using three ear conditions: NP (open ear), CA (Combat Arms Earplugs), and SO (Sonic II earplugs); and three signal-to-noise ratios (SNRs): -10, 0, and +10 dB. Word recognition scores (WRSs) of 18 native English-speaking adults with normal hearing sensitivity were measured in all test conditions. The percentage of words correctly repeated was used to determine differences between the two different level-dependent devices, types of background noise, and SNRs. Results showed a statistically significant increase in WRS as SNR increased from -10 to +10 dB. A repeated-measures analysis of variance for ear condition × noise × SNR indicated a significant main effect for SNR but not for type of noise or ear condition. A slight but significant interaction was found for SNR and ear condition. SNR had great impact on the ability of listeners to understand speech in the presence of background noise; however, the type of noise and the type of level-dependent device used did not. The results of the study support the notion that individuals potentially subjected to high-level impulse noise should be able to use level-dependent earplugs in low-level continuous noise without compromising speech understanding. More specifically, the passive, level-dependent earplugs currently used by military personnel do not appear to be detrimental to speech communication for listeners with normal hearing when the speech is at an average conversational level and the listener is actively attending to the signal.

Evaluation by industrial workers of passive and level-dependent hearing protection devices

Level-dependent hearing protection devices (HPDs) provide protection from intense sound, while offering amplification for speech and other signals in lower levels of noise. These HPDs have been developed in response to the communication and operational needs of noise-exposed persons in industry and the military. This study was conducted to examine industrial workers' perceptions of the performance of two level-dependent HPDs (one with integrated radio communication capability and one without it) and their customary passive HPDs. This research took place at a plastic film manufacturing plant in Rhode Island, USA, following a mixed-measures design. Fifteen maintenance technicians at the plant evaluated the two level-dependent HPDs, plus their customary passive HPDs, in three separate trial periods. Data were collected via a questionnaire designed for this purpose. Mixed-model analyses of variance were performed on all dependent measures. Linear and quadratic effect sizes were assessed with eta. Results revealed that the two level-dependent HPDs offered better perceived communication and situational awareness than the workers' customary passive HPDs. However, the level-dependent HPDs were rated lower than the passive HPDs in terms of usability and comfort. To increase workers' acceptance of level-dependent HPDs, usability issues must be addressed by the HPD manufacturers.

Sound localization with an army helmet worn in combination with an in-ear advanced communications system

Conventional hearing protection devices result in decrements mainly in the ability to distinguish front from rearward sound sources. The aim of this study was to investigate the effect of wearing an earplug with advanced communications capability, in combination with an army helmet, on horizontal plane speaker identification. Ten normal-hearing male subjects were tested in a semi-reverberant sound proof booth under eight conditions defined by combinations of two levels of ear occlusion (unoccluded and occluded by the earplug) and four levels of the helmet (head bare and fitted with the helmet modified to give no, partial and full ear coverage). Percent correct speaker identification was assessed using a horizontal array of eight loudspeakers surrounding the subject at one meter. These were positioned close to the midline and interaural axes of the head, at ear level. The stimulus was a 75-dB SPL, 300-ms broadband white noise. Both degree of ear coverage and ear occlusion significantly determined outcome. Overall percent correct ranged from 93.6% (bareheaded) to 79.7% (full ear coverage) with the ears unoccluded, and from 83.4%-77.5% with ear occlusion. Both variables affected the prevalence of mirror image confusions for positions 30 degrees apart in front and back of the interaural axis. With ear occlusion, front given back errors were more likely than back given front errors, increasing with degree of ear coverage to 49% and 25.4%, respectively. These errors also increased with ear coverage with the ears unoccluded, but were similar. Both degree of ear coverage and ear occlusion significantly impacted horizontal plane speaker identification, particularly for sources close to the interaural axis. However, overall percent correct was higher than observed in a previous study with conventional and level-dependent hearing protection devices, using the same array.