ASA S12 68-2007 American National Standard Methods of Estimating Effective A-Weighted Sound Pressure Levels When Hearing Protectors are Worn (Includes Access to Additional Content).pdf

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1、 Access to Additional Content for ANSI/ASA S12.68 R2012 (Click here to view the publication) This Page is not part of the original publication This page has been added by IHS as a convenience to the user in order to provide access to additional content as authorized by the Copyright holder of this d

2、ocument Click the link(s) below to access the content and use normal procedures for downloading or opening the files. Files associated with ANSI/ASA S12.68 Information contained in the above is the property of the Copyright holder and all Notice of Disclaimer acquisition of the data themselves is de

3、scribed in other ANSI standards. Though number ratings have been heretofore specified by regulation in the U.S., this is the first American National Standard method that specifies how to compute such numbers. This standard compares to ISO 4869-2, Acoustics Hearing Protectors Part 2: Estimation of Ef

4、fective A-Weighted Sound Pressure Levels When Hearing Protectors are Worn. Like that standard it includes three different rating methods of increasing accuracy, but unlike that document it includes a number that is suitable for direct application to A-weighted sound pressure level measurements (NRSA

5、), whereas the ISO documents simplest approach still requires the measurement of C-weighted sound pressure level measurements. Both standards include a multi-number rating that requires use of both A- and C-weighted sound pressure level measurementsin the ANSI document an NRSGand in the ISO document

6、 an HML. The NRSGis similar in application to the HML as specified in ISO 4869-2:1994; however, the two methods differ in how they are calculated. The NRSGmore accurately achieves the targeted protection rate while providing a simplified graphical presentation for ease of use. Both standards include

7、 as their most accurate descriptor an octave-band computational method identical in all aspects except that the ANSI document excludes 63 Hz from the computations since such attenuation data are not normally available and even when they are, they usually have negligible impact on the overall A-weigh

8、ted noise reduction. The standard was editorially corrected and republished in July 2009 after discovery of a typographical error in Table 2 which carried through to Equation (14). This edition contains the corrected table and equation. The MicrosoftExcelworkbook provided with this American National

9、 Standard is entirely informative and provided for the convenience of the user. Use of the provided workbook is not required for conformance with the Standard. ASA makes no representation or warranty or condition of any kind, whether express or implied (either in fact or by operation of law) with re

10、spect to any part of this product, including, without limitation, with respect to the sufficiency, accuracy or utilization of, or any information or opinion contained or reflected in, any of the product. ASA expressly disclaims all warranties or conditions of merchantability or fitness for a particu

11、lar purpose. No officer, director, employee, member, agent, representative or publisher of the copyright holder is authorized to make any modification, extension, or addition to this limited warranty. Acoustical Society of America 2007 All rights reserved iv At the time this Standard was submitted t

12、o Accredited Standards Committee S12, Noise for approval, the membership was as follows: R.D. Hellweg, Chair W.J. Murphy, Vice-Chair S.B. Blaeser, Secretary Acoustical Society of America.R.D. Hellweg D. Lubman (Alt.) Aearo TechnologiesE.H. Berger Air-Conditioning and Refrigeration Institute . S. Lin

13、d . D. Brown (Alt.) Alcoa Inc. . W.D. Gallagher American Academy of Otolaryngology R.A. Dobie L.A. Michael (Alt.) American Industrial Hygiene Association. D. Driscoll .S.N. Hacker (Alt.) American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) R.J. Peppin American Speech-He

14、aring-Language Association .L.A. Wilber . V. Gladstone (Alt.) Bruel FAX: 631-390-0217; E-mail: asastdsaip.org Acoustical Society of America 2007 All rights reserved viiIntroduction Though there exists today, and has for many years, an American National Standard for the measurement of real-ear attenu

15、ation at threshold (ANSI S12.6), there has never been a U. S. national standard for the estimation of effective A-weighted sound pressure levels when hearing protection devices (HPDs) are worn. This standard addresses that need by specifying procedures for estimation, values suitable for labeling of

16、 HPDs, and guidelines on the accuracy that can be expected. Many issues are involved in estimating the protection that users achieve while wearing HPDs. These include obtaining valid estimates of the hearing protectors attenuation as influenced by user training and motivation, the proportion of expo

17、sure time during which users actually wear the devices, and accurate measurements of the noise exposure in question. Perhaps of greatest concern is the issue of individual variability in the fit and performance users achieve. Even with precise computational schemes such as an octave-band analysis of

18、 the noise, the issue of variability remains critical. Once predictions are made, one can estimate the percentage of users in various noises that achieve the targeted protection values, called the protection performance, and use this metric to evaluate the accuracy of various rating systems. For exa

19、mple, if the goal is to protect 84% of the population to a “safe” exposure level, it is desirable to know how closely the protection performance approaches the desired value. Numerous rating systems have been proposed in the past 30 years. Those materials were used as the basis for an extensive rese

20、arch project that was reviewed and approved by ANSI Accredited Standards Committee S12/WG11 (Gauger and Berger, 2004). That project expanded upon the prior published literature by introducing new concepts and new data. Methods of varying complexity were examined, from an octave-band approach to ones

21、 involving ratings used with C-weighted sound levels or exposures, from those that work with A-weighted measurements to those that are simple class or grading schemes. It became apparent that the straightforwardness of what are called A A ratings is appealing. Such ratings predict, by simple subtrac

22、tion from the A-weighted ambient noise levels, the effective A-weighted levels (LA) when an HPD is worn. A A ratings, which by their very nature are easier to use and less prone to computational errors, are of sufficient precision for most applications considering the many sources of variability inh

23、erent in predicting protection. An important collateral issue to the development of a rating procedure is the underlying attenuation data from which the rating is to be computed. Gauger and Berger examined various techniques, especially Methods A and B as specified in the ANSI S12.6 standard. Subseq

24、uent to publication of their report an interlaboratory study was completed (Murphy et al., 2006) and the results demonstrated differences in the repeatability and reproducibility of the two methods. Since both methods have merits and applications as discussed in ANSI S12.6 and the selection of one f

25、or labeling purposes is primarily a matter of public policy, the decision was made to incorporate both methods as options in this standard. Various sets of representative noise data have been published since the 1950s to provide a picture of the occupational noise scene. They originated from around

26、the globe and included industrial, military, and specialized environments. Most of the prior hearing protection analytical studies have based their work on the “NIOSH 100” (Kroes et al., 1975). Gauger and Berger assessed a variety of additional data sets to make sure that the 60-year old data from w

27、hich the NIOSH 100 were selected were indeed still representative. They were, and thus those values are utilized for the most simplified of the proposed ratings. For the more complex graphical approach, data were included from specialized Air Force and aviation spectra in order to assure the suitabi

28、lity of the recommendations for a broader range of noises. Acoustical Society of America 2007 All rights reserved viii The basis for the research in support of this standard is the pioneering work of Dick Waugh (1976, 1984). Building upon his analytical methods allowed examination of the ratings tha

29、t best met the goals of simplicity, consistency, and accuracy. What emerged were two ratings, the Noise Level Reduction Statistic for use with A-weighting (NRSA), and the somewhat more complex and more accurate Noise Level Reduction Statistic, Graphical (NRSG). Finally as the “gold standard” for com

30、parison purposes and for cases in which the maximum accuracy is warranted, a classical octave-band noise-reduction computational scheme is described as a third method. A substantial divergence in this standard from prior publications and other standards (CSA Z94.2; ISO 4869-2; SA/SNZ 1270) is the re

31、commendation that the simplified ratings be presented as pairs of numbers in order to provide additional information about the precision of the ratings, and to supply better user guidance for labeling purposes. This pair of values describes the range of performance at the 80thand 20thpercentile leve

32、l; the specific meaning depends upon whether Method-A or Method-B data are utilized, as defined below: Method A, NRSA80(80thpercentile value) - the protection that is possible for most individually trained users to achieve or exceed. Method B, NRSA80(80thpercentile value) - the protection that is po

33、ssible for most users to achieve or exceed. Method A or Method B, NRSA20(20thpercentile value) - the protection that is possible for a few motivated proficient users to achieve or exceed. The 20thpercentile number has the same meaning regardless of the procedure, Method-A or Method-B, since the resu

34、lts of the Murphy et al. (2006) study demonstrated that the high-performing users achieved approximately the same protection regardless of the test procedure. This was not the case for the low-performing subjects. The rationale for the two-number approach is: It indicates that a range of performance

35、 is to be anticipated. It represents, via the range between the high and low numbers, products that offer more or less inter-subject variability. It diverts the attention of the buyer from a single value and the associated tendency to focus on the seeming “accuracy” of that value. It supports the ra

36、ting of the product with a conservative number that may appear low to some observers, while still indicating a much higher level of protection that is potentially attainable when a hearing protector is fit in an exemplary manner. It draws attention, via the higher number, to the possibility of overp

37、rotection. It may also encourage more careful fitting of hearing protection, especially among consumers who are buying products for their own use, by explicitly demonstrating what exacting application of the product can achieve. The Gauger and Berger report summarizes the rationale for the choices m

38、ade in this standard and provides comprehensive recommendations on how to implement them. It included presentation of the data in a primary label much like the existing primary label required by the U.S. Environmental Protection Agency (1979) that incorporated a pair of NRSAvalues and new explanator

39、y wording along with supporting information. AMERICAN NATIONAL STANDARD ANSI/ASA S12.68-2007 Acoustical Society of America 2007 All rights reserved 1American National Standard Methods of Estimating Effective A-Weighted Sound Pressure Levels When Hearing Protectors are Worn 1 Scope and Applications 1

40、.1 Scope This standard specifies a choice of three methods for use with hearing protector attenuation data to estimate the effective A-weighted sound pressure levels when a hearing protector is worn. The three methods, the Noise Level Reduction Statistic for use with A-weighting (NRSA), the Noise Le

41、vel Reduction Statistic, Graphical (NRSG), and the octave-band method are presented in order of increasing complexity of use and potential accuracy. Furthermore, the standard specifies in the case of the NRSAand the NRSGthat values will be presented for both the 80thand 20thpercentiles, indicated as

42、 NRSA80and NRSA20, and as NRSG80 and NRSG20, to reflect the range of attenuation that can be anticipated. The NRSAspecifies an attenuation value, the Noise Level Reduction Statistic for use with A-weighting, determined from the octave-band attenuation data of a hearing protector in an ensemble of 10

43、0 representative noises, which may be directly subtracted from an A-weighted noise assessment to estimate LA, the effective A-weighted sound pressure level when the hearing protector is worn. The NRSGspecifies an estimated noise level reduction value deduced from a graph (or, alternatively, an arith

44、metic interpolation done by a spreadsheet) that relates the protection in a given A-weighted exposure to the difference between the C- and A-weighted sound pressure levels of the noise. It requires two noise measures (A- and C-weighted), instead of the single measure (A-weighted) necessary for use w

45、ith the NRSA. The NRSGis determined by applying the octave-band attenuation data for a hearing protector to an ensemble of 67 noises that span a broader range of spectral types than used for the NRSAcomputation. The octave-band method specifies a procedure for directly applying the octave-band atten

46、uation data of a hearing protector to a set of octave-band measurements of the noise. The computation includes a correction that is a multiple of the standard deviation in order to adjust the prediction for the desired protection performance. 1.2 Applications The methods of this standard are applica

47、ble to estimating either the sound pressure level or the equivalent continuous or time-weighted average sound pressure levels that are effective when hearing protectors are worn.1Although primarily intended for steady noise exposures, the methods are also 1The estimated value represents the “effecti

48、ve” sound level when the hearing protector is worn, i.e., the A-weighted sound level at the head center with the listener absent (commonly estimated by the on-the-shoulder measurement with a dosimeter), minus the attenuation of the HPD. This is not the same as the sound level in the earcanal. The ea

49、rcanal sound level differs from that in the sound field by the transfer function of the open ear. It cannot simply be estimated by subtracting the HPDs attenuation from the A-weighted level. However, it is the “effective” values that are required to assess noise hazard, as it is those values that are normally compared to the classical damage-risk curves and permissible exposure limits. ANSI/ASA S12.68-2007 Acoustical Society of America 2007 All rights reserved 2 applicable to noises containing impulsive components. These methods may