Noise Reduction Rating Research Articles

Attenuation performance of double hearing protection devices.

High performance jet aircraft are capable of producing 130-150 dB of noise. Large populations work long hours in these high level noise environments and are at risk of hearing loss and related hearing disabilities. An array of hearing protection devices (HPD), both single hearing protection (SHP) and double hearing protection (DHP), are issued to protect aircrew and ground crew members from the nearly constant assault of hazardous noise. American National Standard Institute (ANSI) Methods for Measuring the Real-Ear Attenuation (REAT) of Hearing Protectors S12.6 were used to measure attenuation performance of nearly 20 SHP and DHP configurations. The term DHP can be slightly deceiving. DHP means two devices are worn in combination, not the sum of the attenuation from each device worn singly. The passive attenuation performance of inserts (earplugs) and helmets worn singly and in combination were reported using REAT octave band data, EPA Noise Reduction Ratings (NRR), and ANSI Methods of Estimating Effective A-Weighted Sound Pressure Levels when Hearing Protectors are Worn: the Noise Level Reduction Statistics for use with A-Weighting (NRSA), and the Noise Level Reduction Statistic, Graphical (NRSG).

A Modified Structure for Feed Forward Active Noise Control Systems With Improved Performa

Several approaches have been introduced in literature for active noise control (ANC) systems. Since FxLMS algorithm appears to be the best choice as a controller filter, researchers tend to improve performance of ANC systems by enhancing and modifying this algorithm. In this paper, the existing FxLMS algorithm is modified which provides a new structure for improving the noise reduction and convergence rate. Here the proposed method uses two variable step sizes, one for control filter and another for modelling filter. The control filter step size is varied based on the secondary path threshold signal λ δ ˆ . The modelling filter step size is varied based on error signal ) (n f . It is shown that in the proposed method ANC system noise reduction rate and convergence rate are improved dynamically than the FxLMS variable step size methods. The computer simulations results indicate effectiveness of the proposed method.

Analysis of Frequency Mismatch in Narrowband Active Noise Control

This paper analyzes the effects of frequency mismatch between the actual frequency of the primary noise and the frequency of the synthesized reference signal in narrowband active noise control (ANC) systems using the filtered-X least-mean-square (FXLMS) algorithm. Two stability conditions are derived in terms of the frequency mismatch and the phase error of the estimated secondary-path model. Conditions to guarantee that ANC systems can achieve noise reduction are also developed by considering the overall effect of frequency mismatch and phase error. The optimum step-size to reach the minimum excess mean-square error is obtained and analyzed. Theoretical analysis shows that the noise reduction and convergence rate are functions of frequency mismatch, step size, and phase error in the secondary-path model for the FXLMS algorithm. Computer simulations are conducted to verify the analysis results presented in the paper.

Angle dependent effects for impulse noise reduction for hearing protectors.

The proposed U.S. Environmental Protection Agency regulation for labeling hearing protection devices (HPDs) includes an impulsive noise reduction rating . In 2009, the American National Standards Institute Subcommittee for noise approved a revised standard for measuring the impulsive insertion loss of HPDs, ANSI/ASA S12.42-2009. The exposure at the ear in response to a forward-propagating wave depends strongly on the orientation of the head with respect to the direction of propagation. Furthermore, the insertion loss varies with the peak sound pressure level. This paper reports the results of tests performed using an acoustic shock tube to produce peak impulses of approximately 160-dB peak sound pressure level. Two manikins were evaluated: the GRAS KEMAR manikin equipped with 1/2 and 1/4 in. microphone in a GRAS 711 IEC coupler and the Institute de Saint Louis manikin equipped with a Bruel & Kjaer IEC 711 coupler equipped with a 1/4 in. microphone. The manikin heads were rotated through ±90 deg relative to the direction of the oncoming wavefront and impulsive peak insertion loss was measured according to S12.42-2009. [Portions of the research were supported by U.S. EPA Interagency Agreement No. 75921973-01-0.]

Uncertainty calculation for hearing protector noise attenuation measurements by REAT method

The objective of this paper is to quantify the uncertainty of hearing protector attenuation measurements and present the metrology study necessary for the accreditation of the Industrial Noise Laboratory (LARI) at The Federal University of Santa Catarina and Individual Protection Equipment laboratory (LAEPI) of NR consultancy Ltda. - Brazil for hearing protector noise attenuation procedures using the Real Ear Attenuation at Threshold - REAT method of the Brazilian National Institute of Metrology Standardization and Industrial Quality - INMETRO. A model for the calculation of uncertainty was developed based on the document: Guide to expression of uncertainty in measurement published by the International Organization for Standardization, first edition, corrected and reprinted in 1995, Geneva, Switzerland. The uncertainty of each source of error was estimated and the overall uncertainty of the hearing protector noise attenuation was calculated for each 1/1 octave band frequency and the results used in the single number (NRRsf - Noise Reduction Rating for subject fit) uncertainty calculation. It was concluded that the largest uncertainty is due to the determination of the subject hearing thresholds.

Hearing protection devices: The regulatory journey to change.

The regulation for the labeling of hearing protection devices (40 CFR Part 211, Subpart B) was promulgated in 1979 and has been the directive for 30 years. During this time, there have been significant changes in the industry with respect to newly developed products, which has resulted in the demand for various types of hearing protectors and an increased awareness of hearing loss in both occupational and non-occupational environments. The foundation of the newly proposed regulation of August 5, 2009 is based on these observations and those that were presented by industry, hearing conservation professionals, and the general public during the EPA Workshop on Hearing Protection Devices that was held in March 2003. While these observations will serve as the backdrop of the presentation, the scenery will be the key elements of the proposed regulation, such as the testing methodology, noise reduction rating range, label format and content, recurrent testing, and reporting requirements of testing data. The audience will journey through these scenic elements but will be delivered to its final destination, which are regulatory change and the prospects of withstanding another 30 years.

Angle dependent effects for impulse noise reduction for hearing protectors.

The proposed U.S. Environmental Protection Agency regulation for labeling hearing protection devices (HPDs) includes an impulsive noise reduction rating. In 2009, the American National Standards Institute Subcommittee for noise approved a revised standard for measuring the impulsive insertion loss of HPDs, ANSI/ASA S12.42-2009. The exposure at the ear in response to a forward-propagating wave depends strongly on the orientation of the head with respect to the direction of propagation. Furthermore, the insertion loss varies with the peak sound pressure level. This paper reports the results of tests performed using an acoustic shock tube to produce peak impulses of approximately 160-dB peak sound pressure level. Two manikins were evaluated: the GRAS KEMAR manikin equipped with 1/2- and 1/4-in. microphone in a GRAS 711 IEC coupler and the Institute de Saint Louis manikin equipped with a Bruel & Kjaer IEC 711 coupler equipped with a 1/4 in. microphone. The manikin heads were rotated through ±90 deg relative to the direction of the oncoming wavefront and impulsive peak insertion loss was measured according to S12.42-2009. [Portions of the research were supported by U.S. EPA Interagency Agreement No. 75921973-01-0.]

Impulse noise reduction for hearing protectors.

In 2009, the United States Environmental Protection Agency will propose a revision to the federal regulation for the labeling of hearing protection devices, 40 CFR 211 Subpart B. One of the new features of the proposed rule was the measurement of an impulse noise reduction rating for hearing protection devices. Measurement of impulsive sounds is challenging technically from an acoustics perspective. This paper will report on the performance of an acoustic shock tube used to generate impulses between 140 and 170 dB peak sound pressure level. The calibration methods for the microphones will be discussed and the measurements for a variety of hearing protectors will be presented. Typical earmuffs are capable of impulsive noise reduction ratings of between 20 and 35 dB. Earplugs provide similar range of performance. Combinations of earmuff and earplug have yielded impulse peak reductions of more than 50 dB. The reduction of the impulse peak level should provide a means to predict exposure at the ear when a hearing protector is worn in an impulsive noise environment.

Wavelet Based Noise Reduction in CT-Images Using Correlation Analysis

The projection data measured in computed tomography (CT) and, consequently, the slices reconstructed from these data are noisy. We present a new wavelet based structure-preserving method for noise reduction in CT-images that can be used in combination with different reconstruction methods. The approach is based on the assumption that data can be decomposed into information and temporally uncorrelated noise. In CT two spatially identical images can be generated by reconstructions from disjoint subsets of projections: using the latest generation dual source CT-scanners one image can be reconstructed from the projections acquired at the first, the other image from the projections acquired at the second detector. For standard CT-scanners the two images can be generated by splitting up the set of projections into even and odd numbered projections. The resulting images show the same information but differ with respect to image noise. The analysis of correlations between the wavelet representations of the input images allows separating information from noise down to a certain signal-to-noise level. Wavelet coefficients with small correlation are suppressed, while those with high correlations are assumed to represent structures and are preserved. The final noise-suppressed image is reconstructed from the averaged and weighted wavelet coefficients of the input images. The proposed method is robust, of low complexity and adapts itself to the noise in the images. The quantitative and qualitative evaluation based on phantom as well as real clinical data showed, that high noise reduction rates of around 40% can be achieved without noticeable loss of image resolution.

Earplugs and the NRR

Earplugs are sold in most pharmacies. Because I teach at a college of pharmacy, I want my students to understand a particular feature of earplugs called the Noise Reduction Rating, or NRR. The NRR is a rating of the effectiveness of earplugs at decreasing the amount of sound (or noise) entering the ear. Typical values range from 12 dB to 33 dB; the ones I demonstrate in my class have a value of 29 dB.

Variability of Real-World Hearing Protector Attenuation Measurements

The attenuation provided by a hearing protection device (HPD) in the field is usually estimated by applying a derating factor to the laboratory-determined noise reduction rating (NRR) of the HPD. However, attenuation is highly dependent on individual-specific HPD fit. Prediction of an individual's attenuation depends on the accuracy of the measurement system and the variability of attenuation over time (e.g. after HPD refitting). Variability in attenuation and attenuation test systems has not been adequately characterized to allow for such an assessment. This study compared attenuation measurements made with two systems, Real-Ear-at-Threshold (REAT) and Microphone-in-Real-Ear (MIRE), on 20 workers using two earplugs (foam and custom-molded). Workers' perceptions of the earplugs were also evaluated. Individuals' attenuation results were summarized as personal attenuation ratings (PARs, similar to NRRs). Variability in PARs from between-subject, within-subject and within-day (i.e. repeated tests on a subject without earplug refitting) differences was assessed and used to present the lower confidence limit, or uncertainty factor (UF), of an average individual's attenuation. The custom-molded earplug PARs achieved a higher mean percentage of labeled attenuation than did the foam earplug with both test systems. The custom-molded earplugs also had higher overall acceptance among workers. MIRE PAR levels were lower than REAT levels for both earplugs, but the relationship between the two test systems was highly variable. The MIRE system had lower within-day variability than the REAT system. One individual's MIRE results were highly influential; removal of these results greatly reduced the UF for the custom-molded earplug/MIRE combination. UFs ranged from 8.8 to 13.5 dB. These findings highlight the importance of evaluating variability in individual-specific protection results for personal protective equipment like HPDs, rather than relying on single measurements.

Improvement of “SAFIA” source separation method under reverberant conditions

A sound source separation method with robustness in real environments is needed to achieve clear speech communications and improve noise immunity in a noisy environment. One of the problems in source separation using multiple microphones is to improve performance under reverberant conditions. This paper applies two improvements to SAFIA, a conventional source separation method, in order to improve its performance under reverberant conditions. One of the improvements is smoothing of the interchannel level difference in the time direction in order to reduce SAFIA's decision error. In this way the noise reduction rate (NRR) can be improved by 7.0 to 8.0 dB while the distortion of the target speech is reduced. The other is the addition of a mechanism that separates signals overlapping on the same frequency to the first stage of SAFIA. The mechanism calculates the ratio of the transfer functions of the source and the microphones from the ratio of the received signals of the channels, so that the inverse transfer function between the sources and the microphones can be estimated. These techniques improved the NRR of the proposed method (enhanced SAFIA) by 7.3 dB and its distortion by 12.6 dB on average compared with the conventional SAFIA, which is a great improvement in performance. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 89(3): 22–37, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ ecjc.20160

Minimizing switching noise in a power distribution network using external coupled resistive termination

Switching noise-induced voltage fluctuation might result in a serious timing skew or false triggering in a high-speed digital system. We propose a new approach called external coupled resistive termination (ECRT) for suppressing this noise. Analyzing the resonant field patterns reveals that the bulk noise is accumulated close to the plane edges because of the open-end boundary condition. Thus, adding extra loading at the field bulk effectively reduces the quality factors and consequently minimizes the noise accumulation. A Y-shaped power distribution network was studied with a frequency controllable input-output (I/O) buffer model. Simulation results indicate that the noise level is significantly suppressed. The optimal noise reduction rate is 80% at resonance with an overall reduction rate of 50%. Scattered placement of the ECRT was also investigated and the corresponding noise reduction rate was 50%. Reasons for that will be discussed.

Uncertainty calculation for noise attenuation measurements of hearing protectors

The objective of this paper is to present a metrology study necessary for accreditation of the Industrial Noise Laboratory (LARI) at The Federal University of Santa Catarina-Brazil for hearing protector noise attenuation procedures using the Real Ear Attenuation at Threshold (REAT) method of the Brazilian National Institute of Metrology Standardization and Industrial Quality-INMETRO. A model for the calculation of measurement uncertainty was developed. The uncertainty calculation was based on the document, ‘‘Guide to expression of uncertainty in measurement,’’ by the International Organization for Standardization, first edition, corrected and reprinted in 1995, Geneva, Switzerland. The uncertainty of each source of error was estimated. The overall uncertainty of the noise attenuation measurement of hearing protectors was calculated for each 1/1 octave band frequency test and the results applied in the single number (NRRSF—noise reduction rating for subject fit) uncertainty calculation. It was concluded that the largest uncertainty is due to the determination of the subject’s hearing thresholds.

Field impact insulation testing: Inadequacy of existing normalization methods and proposal for new ratings analogous to those for airborne noise reduction

The field test method for determining the Field Impact Isolation Class (FIIC) rating of a floor/ceiling assembly, prescribed in ASTM E 1007, requires an estimation of the receiving room absorption and the normalization of the receiving room sound pressure levels based on a standard quantity of room absorption. Normalization is intended to remove the effects of receiving room acoustical characteristics, but an analysis of field impact testing indicates that this method has a limited engineering value. The normalization correction is strongly dependent on receiving room volume, does not correlate with the normalization used in airborne sound isolation testing, and can be unreasonably large under certain conditions. The current test method leads to potential errors in engineering judgment, as illustrated by example field tests. By contrast, airborne noise reduction ratings (prescribed in ASTM E 336) are either non-normalized or normalized to a standard reverberation time, which provides significant advantages over normalizing to a standard amount of absorption. New impact noise metrics, the non-normalized impact sound rating (ISR) and the normalized impact sound rating (NISR), analogous to the airborne noise reduction metrics NIC and NNIC, are proposed for incorporation into ASTM E 1007. Revised building code requirements using the proposed metrics are suggested.

In-situ personal assessment of hearing protector performance–recommendations for an updated standard

Standardized method currently used to assess Hearing Protection Devices (HPD) performance have little correspondence with protection achieved by users in practice. To overcome this problem new measurement method are being proposed. They no longer rely on statistically-based, population-based estimate, such as the Noise Reduction Rating (NRR), but on an individual assessment of the performance of the HPD. These new methods are highly beneficial for effective Hearing Conservation Programs as they provide individual attenuation values for noise-exposed workers in lieu of the statistically-based laboratory performance estimate, but their implementation will necessitate complete revision of existing standards. Among the issues to be addressed, are the aspect of the accuracy of the measurements performed by those methods and associated devices and also the replacement of any single number rating of HPD (such as the NRR) in existing standards. Solutions to address theses issues and some general guidelines for an updated standard will be presented.

Derivation of an analytic expression for the error associated with the noise reduction rating

Hearing protection devices are assessed using the Real Ear Attenuation at Threshold (REAT) measurement procedure for the purpose of estimating the amount of noise reduction provided when worn by a subject. The rating number provided on the protector label is a function of the mean and standard deviation of the REAT results achieved by the test subjects. If a group of subjects have a large variance, then it follows that the certainty of the rating should be correspondingly lower. No estimate of the error of a protector’s rating is given by existing standards or regulations. Propagation of errors was applied to the Noise Reduction Rating to develop an analytic expression for the hearing protector rating error term. Comparison of the analytic expression for the error to the standard deviation estimated from Monte Carlo simulation of subject attenuations yielded a linear relationship across several protector types and assumptions for the variance of the attenuations.

A new hearing protector rating: The Noise Reduction Statistic for use with A weighting (NRSA).

An important question to ask in regard to hearing protection devices (HPDs) is how much hearing protection they can provide. With respect to the law, at least, this question was answered in 1979 when the U. S. Environmental Protection Agency (EPA) promulgated a labeling regulation specifying a Noise Reduction Rating (NRR) measured in decibels (dB). In the intervening 25 years many concerns have arisen over this regulation. Currently the EPA is considering proposing a revised rule. This report examines the relevant issues in order to provide recommendations for new ratings and a new method of obtaining the test data. The conclusion is that a Noise Reduction Statistic for use with A weighting (NRSA), an A–A’ rating computed in a manner that considers both intersubject and interspectrum variation in protection, yields sufficient precision. Two such statistics ought to be specified on the primary package label—the smaller one to indicate the protection that is possible for most users to exceed, and a larger one such that the range between the two numbers conveys to the user the uncertainty in protection provided. Guidance on how to employ these numbers, and a suggestion for an additional, more precise, graphically oriented rating to be provided on a secondary label, are also included.

Estimating the precision error in hearing protector ratings

The variance associated with hearing protector rating methods is derived for the Noise Reduction Rating, Noise Reduction Rating Subject Fit, and Single Number Rating methods. Real-ear attenuation at threshold (REAT) data have an error that is a function of stimulus frequency, protector fit, protector type, and the subject performance during occluded and unoccluded threshold estimation. In addition to the error associated with conducting the REAT tests, there is an additional error that results from applying the rating to a noise spectrum different from the spectrum used to derive the rating. For this paper, a distinction is made between the accuracy of the rating and the precision of the rating. The precision is an inherent property of the subject panel, protector type and fit, and the frequencies tested. The accuracy will be attributed to the application of the rating to an arbitrary noise. [Portions of work supported by the U.S. EPA IA 75090527.]

Blind Source Separation Combining Independent Component Analysis and Beamforming

We describe a new method of blind source separation (BSS) on a microphone array combining subband independent component analysis (ICA) and beamforming. The proposed array system consists of the following three sections: (1) subband ICA-based BSS section with estimation of the direction of arrival (DOA) of the sound source, (2) null beamforming section based on the estimated DOA, and (3) integration of (1) and (2) based on the algorithm diversity. Using this technique, we can resolve the low-convergence problem through optimization in ICA. To evaluate its effectiveness, signal-separation and speech-recognition experiments are performed under various reverberant conditions. The results of the signal-separation experiments reveal that the noise reduction rate (NRR) of about 18 dB is obtained under the nonreverberant condition, and NRRs of 8 dB and 6 dB are obtained in the case that the reverberation times are 150 milliseconds and 300 milliseconds. These performances are superior to those of both simple ICA-based BSS and simple beamforming method. Also, from the speech-recognition experiments, it is evident that the performance of the proposed method in terms of the word recognition rates is superior to those of the conventional ICA-based BSS method under all reverberant conditions.