Microphone In Real Ear Research Articles

A comparison of methods to measure the sound attenuation of active noise reduction hearing protectors

Hearing protectors equipped with active noise reduction (ANR) functions have become popular and widespread in recent years. It is important to assess the sound attenuation performance of the ANR hearing protectors properly, which remains a problem. This work studies several typical measurement methods, including the real-ear attenuation at threshold (REAT), microphone in real-ear (MIRE), and the acoustic test fixture (ATF). In order to reduce the masking effect from the noise generated from the ANR electronic system when ANR is turned on during the REAT test, a modified tone sound instead of one-third octave band noise is used as the test signal. The results obtained from the different measurement methods on several ANR headphones and earphones are compared and the differences are discussed. It is found that the measurement differences among those methods are huge. The values of active insertion loss measured by the REAT method are much lower than those assessed by the MIRE procedure. This work contributes to the development of the test methods to assess the sound attenuation of the ANR hearing protectors.

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.

High fidelity hearing protection devices: Attenuation in human ears and manikin devices

Flatness of attenuation was measured for high-fidelity hearing protection devices (HPDs) using microphone-in-real-ear (MIRE) and behavioral real-ear-attenuation-at-threshold (REAT) protocols. In addition, participants completed the words-in-noise (WIN) test with and without HPDs, and provided subjective ratings of perceived sound quality via surveys administered after listening to music with and without HPDs. Attenuation was also measured using a manikin device, with both the manikin-based microphone and the MIRE probe tube used to make measurements inside the artificial ear coupler. Among human participants, statistically significant correlations revealed decreased WIN performance and poorer music quality ratings as achieved HPD attenuation increased. There was also a statistically significant correlation in which shallower slope (flatter attenuation) was correlated with higher perceived music quality ratings. Achieved attenuation decreased in the coupler when the probe tube was inserted into the manikin, suggesting HPD seals were compromised by the probe tube. Consistent with the manikin data, attenuation in human ears was greater when measured using REAT, suggesting similar compromise of the HPD seal during MIRE tests. To assure high fidelity HPDs are well fit, REAT testing is recommended to verify both overall attenuation and flatness of attenuation in individual listeners.

A study of the real-world noise attenuation of the current hearing protection devices in typical workplaces using Field Microphone in Real Ear method.

Actual noise reduction of the earmuffs is considered as one of the main challenges for the evaluation of the effectiveness of a hearing conservation program. The current study aimed to determine the real world noise attenuation of current hearing protection devices in typical workplaces using a field microphone in real ear (FMIRE) method. In this cross-sectional study, five common earmuffs were investigated among 50 workers in two industrial factories with different noise characteristics. Noise reduction data was measured with the use of earmuffs based on the ISO 11904 standard, field microphone in real ear method, using noise dosimeter (SVANTEK, model SV 102) equipped with a microphone SV 25 model. The actual insertion losses (IL) of the tested earmuffs in octave band were lower than the labeled insertion loss data (p < 0.05). The frequency nature of noise to which workers are exposed has noticeable effects on the actual noise reduction of earmuffs (p < 0.05). The results suggest that the proportion of time using earmuffs has a considerable impact on the effective noise reduction during the workday. Data about the ambient noise characteristics is a key criterion when evaluating the acoustic performance of hearing protectors in any workplaces. Comfort aspects should be considered as one of the most important criteria for long-term use and effective wearing of hearing protection devices. FMIRE could facilitate rapid and simple measurement of the actual performance of the current earmuffs employed by workers during different work activities.

Evaluation of a calculation method of noise exposure from communication headsets

Specialized standardized methods for the measurement of noise exposure from communication headsets or sound sources close to the ear include the Microphone in Real Ear and manikin techniques, as specified in ISO 11904-1/2. The 2013 version of Canadian standard Z107.56 introduced a simpler calculation method to increase accessibility to communication headset exposure assessments for the widest range of stakeholders in hearing loss prevention. The calculation method only requires general and widely accessible sound measurement equipment and basic computational steps that account for the main determinants of exposure such as the background noise around the user, the sound attenuation of the communication headset, and the expected communication duration and effective listening signal-to-noise ratio. This paper reviews recent research on the effects of the spectral and temporal characteristics of the background noise and the headset configuration on the speech listening level. Results indicate that the listening level is largely insensitive to spectral and temporal variations in the background noise and that A-weighted noise level is a good predictor of listening level once headset attenuation is taken into account. It is also found that one-sided headsets increase exposure by 6-7 dB compared to two-sided headsets due to binaural summation.

Individual Fit Testing of Hearing Protection Devices Based on Microphone in Real Ear.

BackgroundLabeled noise reduction (NR) data presented by manufacturers are considered one of the main challenging issues for occupational experts in employing hearing protection devices (HPDs). This study aimed to determine the actual NR data of typical HPDs using the objective fit testing method with a microphone in real ear (MIRE) method.MethodsFive available commercially earmuff protectors were investigated in 30 workers exposed to reference noise source according to the standard method, ISO 11904-1. Personal attenuation rating (PAR) of the earmuffs was measured based on the MIRE method using a noise dosimeter (SVANTEK, model SV 102).ResultsThe results showed that means of PAR of the earmuffs are from 49% to 86% of the nominal NR rating. The PAR values of earmuffs when a typical eyewear was worn differed statistically (p < 0.05). It is revealed that a typical safety eyewear can reduce the mean of the PAR value by approximately 2.5 dB. The results also showed that measurements based on the MIRE method resulted in low variability. The variability in NR values between individuals, within individuals, and within earmuffs was not the statistically significant (p > 0.05).ConclusionThis study could provide local individual fit data. Ergonomic aspects of the earmuffs and different levels of users experience and awareness can be considered the main factors affecting individual fitting compared with the laboratory condition for acquiring the labeled NR data. Based on the obtained fit testing results, the field application of MIRE can be employed for complementary studies in real workstations while workers perform their regular work duties.

Attenuation of earmuffs used simultaneously with respiratory protective devices

In the work environment, apart from the noise, employees may be exposed to other harmful factors. Therefore, they wear hearing protectors and other personal protective equipment. The aim of the study was to determine whether simultaneous use of earmuffs and respiratory protective devices affects the attenuation of earmuffs. The study was conducted in laboratory conditions using the subjective REAT (Real Ear Attenuation at Threshold) and objective MIRE (Microphone in Real Ear) methods. The REAT method was used to measure sound attenuation of earmuffs, while MIRE was used to determine changes in attenuation of earmuffs due to the use of other personal protective equipment. The study showed reduction in attenuation of earmuffs due to the use of a full face mask up to 20 dB. Using a full face mask causes that attenuation of earmuffs in the low frequency range is close to zero. Reduction in attenuation due to the use of half masks for complete with particle filters (half masks) is 3-15 dB. Simultaneous use of earmuffs and filtering half masks makes small changes in attenuation not exceeding 3 dB. The study showed that full face masks give the greatest reduction in attenuation of earmuffs. On the other hand, the least reduction is observed in the case of filtering half masks. There is a significant difference between the reduction in attenuation of earmuffs worn with half masks for complete with particle filters because they may be equipped with different kind of the head strap. Med Pr 2017;68(3):349-361.

Noise reduction ratings of active noise cancellation headsets

The recent promulgation of the MIL-STD 1474 E, “Design Criteria Standard Noise Limits,” combines data from the Real Ear Attenuation at Threshold (REAT) and the Microphone in Real Ear (MIRE) measurements to estimate the noise reduction rating of active noise cancellation (ANC) hearing protection devices. This paper will examine the American National Standards Institute standards, S12.6, S12.42, and S12.68 to assess the performance of four aviation headsets measured with REAT and MIRE methods. We will compare the protection estimates for different noises with the Noise Reduction Statistic for A-weighting (NRSA), Noise Reduction Statistic for Graphical (NRSG), and Octave Band rating methods. The NRSG method provides an efficient and accurate alternative to the Octave Band rating method. In the International Standards Organization, the ISO 4869 standard part 6 is under development to measure the attenuation of ANC protectors. Comparison with the proposed ISO 4869-6 standard will be conducted.

Study of Noise Reduction Rate of Hearing Protection Devices Based on Microphone in Real Ear (MIRE) Under Laboratory Conditions

Study of Noise Reduction Rate of Hearing Protection Devices Based on Microphone in Real Ear (MIRE) Under Laboratory Conditions

Measurement of hearing-protector attenuation using auditory steady state responses

There is a need to assess hearing protection device (HPD) attenuation to ensure that individuals receive adequate protection from noise. Present methods of attenuation measurement have limitations. Objective measurements such as field microphone in real ear (F-MIRE) 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 response. We explored using auditory steady state responses (ASSR) as a technique that may overcome these limitations. ASSRs were recorded in ten normal hearing adults, using both “normal” and “occluded” conditions. Stimuli included both narrow band noises and pure tones (500 and 1000 Hz), amplitude modulated at 40 Hz. Stimuli were presented through either loudspeakers or headphones, at 45, 55, and 65 dB SPL. “Physiological attenuation” was calculated as the difference between ASSR values (both amplitude and phase) for normal and occluded conditions. Physiological attenuation estimates were compared to in-ear (objective) and psychophysical (subjective) measurements. Grand mean ASSR data complied well with in-ear and subjective measurements. Further work is needed to provide accurate assessment of ASSR-based physiological attenuation for individual subjects.

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.

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 regulation. The presentation will include the baseline passive, active, and total attenuation, the NRSA and the Graphical NRSG, and the proposed EPA labels for passive attenuation and total attenuation while in an active mode.

MIRE- IL methodology applied to measuring the noise attenuation of earmuff hearing protectors

This article describes an objective methodology for measuring the noise attenuation of earmuff hearing protectors using as a reference the method known as microphone-in-real-ear (MIRE). The methodology implements the insertion loss ( IL) paradigm, in which IL is measured using miniature microphones, specially designed to comply with ANSI and ISO standards for the MIRE technique. The results for four different hearing protectors are compared with the subjective method known as real-ear-attenuation-at-threshold ( REAT). Correction factors are included in the methodology to account for external effects such as physiological noise and bone conduction. The objective method predicted well the real protection of the earmuffs and the proposed methodology showed lower standard deviation values than the REAT method.

Earmuff noise attenuation by microphone in real ear technique.

When using a hearing protector at the workplace, it is necessary to quantify its noise attenuation by laboratory measurements. This paper aims at developing an objective methodology for measuring hearing protector noise attenuation of earmuff type using “microphone in real ear” (MIRE) method. The methodology uses the “insertion loss” (IL) as parameter, which is the difference between the sound pressure level with and without the hearing protector, using MIRE, and then calculating the hearing protector IL. The results for four different hearing protectors are compared with the subjective method “real-ear attenuation at threshold” (REAT). Factors such as physiological noise and bone conduction are quantified using the relation between MIRE and REAT method. Results are compared with values obtained from the literature.

Digital Filters for Accurately Verifying the Performance of Hearing Protectors in Use

The performance of hearing protectors can be assessed in situ by measuring the sound pressure inside the ear canal following the Microphone In Real Ear (MIRE) protocol. Thus a custom-made earplug has been designed with an inner bore, allowing the insertion of the MIRE measurement microphone. However, the actual exposure level can only be accurately predicted if the relationship, henceforth called transfer function, between the sound level at the microphone and at the eardrum is known. Previous research has revealed that the transfer function can be precisely approximated with an individualized FDTD model, but a simplified method is needed for practical implementation due to the time-consuming nature of this numerical technique. In this matter, a one-dimensional analytical model appears inadequate, hence an approximation to the detailed FDTD model based on digital filter design is proposed instead. Two different approaches have been applied to estimate the individualized filter coefficients: multiple linear regression and Multivariate Orthonormal Vector Fitting (MOVE). In general, both methods can predict an individual's transfer function quite accurately if the length of the earplug's inner bore and the length of the residual part of the ear canal behind the protector are known. However, MOVE seems more reliable for ears with a longer residual part.

Verifying the attenuation of earplugs in situ: Method validation on human subjects including individualized numerical simulations

The microphone in real ear (MIRE) protocol allows the assessment of hearing protector's (HPD) attenuation in situ by measuring the difference between the sound pressure outside and inside the ear canal behind the HPD. Custom-made earplugs have been designed with an inner bore to insert the MIRE probe containing two microphones, the reference microphone measuring the sound pressure outside and the measurement microphone registering the sound pressure behind the HPD. Previous research on a head and torso simulator reveals a distinct difference, henceforth called transfer function, between the sound pressure at the MIRE measurement microphone and the sound pressure of interest at the eardrum. In the current study, similar measurements are carried out on humans with an extra microphone to measure the sound pressure at the eardrum. The resulting transfer functions confirm the global frequency dependency found earlier, but also show substantial variability between the ears with respect to the exact frequency and amplitude of the transfer functions' extrema. In addition, finite-difference time-domain numerical models of an ear canal with earplug are developed for each individual ear by including its specific geometrical parameters. This approach leads to a good resemblance between the simulations and their corresponding measurements.

Human subject investigation of MIRE microphone location during insertion loss testing of Active Noise Reduction hearing protectors in active and passive modes

The Microphone in Real Ear or MIRE protocol, standardized as ANSI S12.42-1995, does not specify a standardized location for measurement microphone placement in the human ear. Prior research is mixed as to the potential benefits and/or shortcomings of placing the measurement microphone outside versus inside the ear canal. This study captured and compared spectral attenuation data at three different microphone locations: in the concha of the outer ear, in the ear canal-shallow, and in the ear canal-deep (with a probe tube microphone positioned near the tympanic membrane), using human test participants across five active noise reduction (ANR) headphones of differing design. ANR headphones were used because this paper is an excerpt of a larger research effort designed to assess attenuation measurement methods and instrumentation. Research goals were to determine whether spectral sound pressure levels (SPL) differed significantly at the three microphone locations tested and whether any changes in frequency significantly affected SPL measured at each location; the results showed small, but statistically significant differences between microphone locations, but with frequency-specificity. The results provide assistance to practitioners in determining where to place measurement microphones to best suit their particular needs when using MIRE for hearing protector attenuation testing.

Verifying the attenuation of earplugs in situ: Method validation using artificial head and numerical simulations

The use of in situ measurements of hearing protectors' (HPD's) attenuation following the microphone in real ear (MIRE) protocol is increasing. The attenuation is hereby calculated from the difference in sound levels outside the ear and inside the ear canal behind the HPD. Custom-made earplugs have been designed with an inner bore that allows inserting a miniature microphone. A thorough understanding of the difference, henceforth called transfer function, between the sound pressure of interest at the eardrum and the one measured at the inner bore of the HPD is indispensable for optimizing the MIRE technique and extending its field of application. This issue was addressed by measurements on a head-and-torso-simulator and finite difference time domain numerical simulations of the outer ear canal occluded by an earplug. Both approaches are in good agreement and reveal a clear distinction between the sound pressure at the MIRE microphone and at eardrum, but the measured transfer functions appear to be stable and reproducible. Moreover, the most striking features of the transfer functions can be traced down to the geometrical and morphological characteristics of the earplug and ear canal.

Verifying the attenuation of earplugs in situ: variability of transfer functions among human subjects

The use of in situ measurements of hearing protectors' (HPD's) attenuation following the MIRE‐protocol (Microphone In Real Ear) is increasing. The attenuation is hereby calculated from the difference in sound levels outside the ear and inside the ear canal behind the HPD. Custom‐made earplugs have been designed with an inner bore that allows inserting a miniature microphone. A thorough understanding of the difference, henceforth called 'transfer function', between the sound pressure of interest at the eardrum and the one measured at the inner bore of the HPD is indispensable for optimizing this technique and extending its field of application. Of particular interest is the variation of these transfer functions among humans. This was checked experimentally on 19 subjects. Differences in sound pressure were measured at the HPD's inner bore, by the MIRE‐microphone, and at the eardrum by inserting an extra tube microphone in the ear canal. All transfer functions showed a comparable shape, however variability was substantial for the exact frequency and amplitude of the resonance peaks. The link between this variability and the morphology of the individual's HPD and ear canal was addressed using FDTD‐simulations (Finite‐Difference Time‐Domain) of the outer ear canal occluded by an earplug with inner bore.

Effectiveness of hearing protector devices in impulse noise verified with transiently evoked and distortion product otoacoustic emissions

This study aimed to evaluate the effect of impulse noise on otoacoustic emissions (OAEs) while a passive non-linear earplug or an active level-dependent earmuff was worn.Since none of the standardized attenuation measurement techniques, REAT (real ear at threshold), ATF (acoustic test fixture), or MIRE (microphone in real ear), is designed to test both types in real-wearing condition, OAEs, suitable for detecting subtle changes in the functional integrity of the cochlear outer hair cells, are used.First, DPOAEs (distortion product) and TEOAEs (transiently evoked) of 24 subjects were compared before, immediately after gunfire practice, and after one hour of non-exposure. Secondly, both types of OAEs were evaluated in 31 subjects before and after exposure during a five-day military practice.Significant differences existed between the ears in most cases; the emissions from the right ear had a tendency to be more robust. There were no significant changes in OAEs either before and after exposure, or in the second experiment over multiple days. These findings suggest that the HPDs are able to prevent cochlear damage.