Melissa A Theis

Noise exposure criteria for continuous noise: A case for reducing the 8 hour open ear exposure level

Personnel working in hazardous noise environments can be protected from noise induced hearing loss (NIHL) or other hearing related disabilities when their exposures are limited. Limiting noise exposures can be accomplished by engineering controls, administrative controls, and/or with the use of personal protective equipment. Damage risk criteria (DRC) were developed as a guide to limit personnel noise exposure in order to reduce the risk of NIHL. With the exception of the US Occupational Safety and Health Administration (OSHA), the US and many European countries accepted the DRC of 85 dB for 8 hours with a 3 dB per doubling exchange rate. Recent studies of the auditory response to noise dose, conducted at the Air Force Research Laboratory, found that A-weighted open ear exposures may be more hazardous (i.e., have a higher effective noise dose) than equal A-weighted level protected ear exposures. A potential conclusion of these studies is that hearing protectors are more effective than previously thought and reducing the open ear exposure criteria may be needed in order to minimize NIHL. This presentation will describe the pros and cons of reducing the 85 dBA DRC.

Operational challenges associated with the use of fit test systems in the U.S. Air Force

Hearing protection fit test systems provide immediate and direct feedback on the goodness-of-fit for a specific individual based on the personal attenuation rating (PAR) obtained from the measurement. Fit test systems may provide improved retention of hearing protection training, particularly when used in combination with face-to-face training. One challenge for the U.S. Air Force is determining when face-to-face training should occur, and how detailed this training should be to maximize the benefits for end users. A second challenge is determining if various hearing loss patterns render PAR data inaccurate, because many Air Force personnel have hearing loss. The ability of individuals with hearing loss to achieve a desired PAR for a workplace may be influenced by the dynamic range of the system, particularly when measuring the protected condition. This presentation will discuss how the two challenges mentioned above must be investigated before implementing fit test systems as a hearing preservation tool ...

How long are inexperienced-subjects “naive” for ANSI S12.6?

ANSI S12.6-2008 describes the methods for measuring the real-ear attenuation of hearing protectors. Method A, trained-subject fit, was intended to describe the capabilities of the devices fitted by thoroughly trained users while Method B, inexperienced-subject fit, was intended to approximate the protection that can be attained by groups of informed users in workplace hearing conservation programs. Inexperienced subjects are no longer considered “naive” according to ANSI S12.6 after 12 or more sessions measuring the attenuation of earplugs or semi-insert devices. However, an inexperienced subject that has received high quality video instructions may no longer be considered “naive” or “inexperienced” even after just one session. AFRL conducted an ANSI S12.6-2008 Method B study to determine what effect, if any, high quality instructions had on the performance of naive or inexperienced subjects and the number of trials where the subject could still be considered naive or inexperienced. This experiment used t...

Effective attenuation performance of passive hearing protectors: A temporary threshold shift study

Passive hearing protectors have been used for decades to reduce the risk of noise induced hearing loss. Hearing protectors (earmuffs, earplugs, helmets) have traditionally been the first line of defense for personnel working in hazardous noise environments. According to ANSI S12.68-2007, the “gold standard” method of estimating effective A-weighted sound pressure levels when hearing protectors are worn is the classical octave band method. The octave band method subtracts the hearing protector noise attenuation from the ambient noise level for each relevant octave band to estimate the noise exposure at the ear, under the hearing protector. ANSI S12.6-2008 Methods for Measuring the Real-Ear Attenuation of Hearing Protectors was used to measure the attenuation of the hearing protectors. The purpose of this study was to measure the effective attenuation of a hearing protector in terms of temporary threshold shift (TTS) response for individual human subjects with and without hearing protection. This presentati...

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 P...

Hearing Protection With Integrated In-Ear Dosimetry: A Noise Dose Study

Military personnel working in high noise environments can be exposed to continuous noise levels up to 150 dB. United States (US) Department of Defense (DoD) Hearing Conservation Programs (HCPs) [1–3] set safe noise exposure limits to reduce the risk for noise induced hearing loss. These daily noise exposure limits were based on ambient noise levels and the duration of time spent in that noise environment. Current dosimeters, worn on the lapel of personnel and at least one system worn under a hearing protector, were designed to measure noise levels and calculate noise dose, but do not provide a validated measure of noise dose external to or under a hearing protector. Noise dose under hearing protectors can be estimated by subtracting the real ear attenuation (REAT) data, collected in accordance with the American National Standards Institute (ANSI) S12.6 [4], at each octave band from the ambient octave band noise. This procedure gives accurate results for group data, but does not account for individual variations in effective attenuation. To address this issue, the US Naval Air Systems Command (NAVAIR) led the development of ship suitable in-ear dosimetry integrated into a hearing protector, and co-sponsored an effort executed by the Air Force Research Laboratory (AFRL) to calibrate in-ear noise dose readings. This was accomplished by conducting human noise exposure experiments, with and without hearing protection, which calculated noise dose from temporary threshold shifts (TTS) in hearing. Ten subjects participated in the study. Noise levels were 91, 94, and 97 dB for up to 2 hrs, 1 hr, and 30 minutes respectively. These exposure levels were well within US DoD safe noise exposure guidelines (DoD HCP) [1–3]. Data will be presented describing the open and occluded (protected) ear TTS response to noise dose achieved by subjects in the experiment. Preliminary findings indicate that human subject data is extremely important in developing and validating calibration factors for any type of noise dosimeter but is especially important for in-ear dosimetry. Results from this study demonstrated that the REAT noise dose estimations and the in-ear dosimetry earplugs consistently overestimated the effective noise dose received by subjects. However, more than 10 subjects are required to improve the confidence level of the estimated calibration factor.Copyright

Attenuation performance of active noise reduction headsets using American National Standard s12.42 and the proposed U.S. Environmental Protection Agency Hearing Protector Labeling Regulation

The attenuation performance and noise reduction rating (NRR) of six commercially available active noise reduction (ANR) headsets was assessed using the proposed environmental protection agency (EPA) regulation. The passive attenuation results were collected using American National Standard Institute (ANSI) S12.6 method for measuring real-ear attenuation at threshold (REAT) of hearing protectors while the active attenuations results were collected using ANSI S12.42 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. ANSI/ASA S12.68 methods of estimating effective A-weighted sound pressure levels when hearing protectors are worn was used to compute noise reduction metrics including the noise reduction statistic A-weighted (NRSA) and the graphical noise reduction statistic (NRSG). The proposed NRR labels for the ANR headsets were computer per the guidance in the draft U.S. EPA reg...

Bone and/or tissue conducted noise: Implications for advanced hearing protection

Military personnel working in various high noise environments may be exposed to continuous noise levels reaching 150 dB. At those levels, bone and tissue conduction pathways become the dominate pathway for sound transmission to the cochlea. Accordingly, hearing protection devices designed to attenuate noise transmitted via air conduction pathways may not be sufficient for meeting hearing conservation requirements. The goal of adequately protecting personnel in these types of environments requires a better understanding of the bone and/or tissue conduction flanking pathways, the susceptibility of the cochlea to bone and tissue conducted energy, and the accompanying mitigation strategies. This experiment investigated the linearity of air conducted noise to the transmission of bone, tissue, and bone/tissue conducted noise. Specially designed bone and tissue conduction drivers, which primarily isolate and excite the respective desired pathways, were used to conduct a loudness matching study. Preliminary findi...