ASME B133.8-2011 [Revisi on of ASME B1 33.8-1 977 (R2001 )] REAFFIRMED 201 Gas Turbine Installation Sound Emissions A N A M E R I C A N N A T I O N A L S TA N D A R D ASME B133.8-2011 [Revision of ASME B1 33.8-1 977 (R2001 )] Gas Turbine Installation Sound Emissions AN AM ERI CAN N AT I O N A L S TA N D A R D Three Park Avenue • New York, NY • 001 USA Date of Issuance: March 5, 201 The next edition of this Standard is scheduled for publication in 201 ASME issues written replies to in quiries cern in g in terpretations of tech nical aspects of th is Stan dard I n terpretations are publish ed on th e ASME Web site un der th e Com m ittee Pages at http://cstools.asme.org/ as they are issued Errata to codes and standards may be posted on the ASME Web site under the Committee Pages to provide corrections to incorrectly published items, or to correct typographical or grammatical errors in codes and standards Such errata shall be used on the date posted The Committee Pages can be found at 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of the publisher The American Society of Mechanical Engineers Three Park Avenue, New York, NY 001 6-5990 Copyright © 201 by THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS All rights reserved Printed in U.S.A CONTENTS Foreword Committee Roster Correspondence With the PTC Committee Introduction iv v vi vii Section Object and Scope 1 Section Sound Emissions Specification 2 2 Section Field Sound Measurement Guidelines 6 6 6 7 10 10 11 Section References 13 13 13 Estimated Sound Pressure Level at Far-Field Measurement Positions Near-Field Contours Gas Turbine Sound Level Measurement Locations Gas Turbines in a Combined-Cycle Installation Operating in Simple-Cycle Mode 10 Corrections for Sound-Reflecting Surfaces 11 1-1 1-2 2-1 2-2 2-3 2-4 3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 4-1 4-2 Figures 2-2-1 3-6-1 3-6-2 3-6-3 Table 3-9.4-1 Forms 2-2.1-1 2-2.2-1 Object Scope Introduction Environmental Sound Emissions Specification Procedures Environmental Sound Emissions Specification Format Near-Field Sound Emissions Specification Introduction Qualifications Gas Turbine Operation Acoustic Environment Sound Measurement Instruments Microphone Locations Sound Measurements Data Reporting Average and Maximum Sound Level Calculation Comparison of Measured and Specified Sound Level References Bibliography Procedure Turbine Procedure (120 m) A: Specified Sound Levels at 400 ft (120 m) for Total Gas Installation at Contract Conditions B: Specified Octave Band Sound Pressure Levels at 400 ft for Total Gas Turbine Installation at Contract Conditions Nonmandatory Appendices A B Guide to Determining Acceptable A-Weighted Sound Level Guide to Determining Specified C-Weighted Sound Level iii 5 15 22 FOREWORD The purpose of this Standard is to provide format and criteria for the preparation of gas turbine procurement acoustical specifications for industrial, pipeline, and utility applications This Standard will also be useful for response to such specifications Field sound measurement guidelines to determine specified sound emissions compliance and to report field data are also presented This Standard provides essential information for the procurement of gas turbine power plants involving acoustical requirements This Standard applies to simple-cycle gas turbines and combined-cycle gas turbines operating in simple-cycle mode with simple-cycle bypass capability, and conventional or advanced low-emissions combustion systems for industrial, marine, and electric power applications Auxiliaries needed for proper operation are included Gas turbines applied to earth-moving machines, agricultural and industrial-type tractors, automobiles, trucks, buses, and aeropropulsion units are not included For gas turbines using unconventional or special heat sources (such as chemical processes, nuclear reactors, or furnaces for supercharged boilers), this Standard may also be useful; however, appropriate modifications may be necessary The intent of this Standard is to cover the normal requirements of the majority of applications as determined by the consensus of the B133 Committee, recognizing that economic tradeoffs and reliability implications may differ in some applications The user may desire to add, delete, or modify the requirements in this Standard to meet specific needs, and may so in the procurement specification In the 1990s, the B133 Committee decided not to update the B133 standards, but instead to work with the ISO TC 192 Committee to prepare a series of gas turbine standards These standards would essentially replace the B133 series of standards As the ISO 3977 series of standards was released, the related B133 standards were withdrawn In the first decade of this century, the B133 Committee was disbanded and all but one of the B133 standards were withdrawn The B133.8 Standard, Gas Turbine Installation Sound Emissions, was considered of sufficient continuing interest in U.S industry to merit its retention Subsequently, it was felt that it should be updated PTC Committee 36 on Measurement of Industrial Sound was considered to be the closest in subject matter among the ASME Standards and Certification Committees to undertake such a revision In 2008, several former members of the B133 Committee responsible for the B133.8 Standard agreed to serve on the PTC 36 Committee The 2011 edition of B133.8 improves on the 1977 edition in the following aspects: (a) addition of a near-field measurement specification along a prescribed near-field source envelope contour (b) clarification of the gas turbine acoustical specification in the case of combined-cycle applications for both near field and far field (c) clarification of the definition of measurement positions and distance corrections (d) additional guidance regarding measurement methodology (e) clarification of the treatment and evaluation of reflective surface effects (f) additional and updated references, including Nonmandatory Appendix B on low-frequency airborne sound Suggestions for improvement of this Standard are welcome They should be stated as specifically as possible, and sent to the Secretary, PTC 36 Committee, The American Society of Mechanical Engineers, Three Park Avenue, New York, NY 10016 This edition of the B1 33.8 Standard was approved by the PTC Standards Committee on April 18, 2011, and was approved as an American National Standard by the American National Standards Institute on August 17, 2011 iv ASME PTC COMMITTEE Performance Test Codes (The following is the roster of the Committee at the time of approval of this Standard.) STANDARDS COMMITTEE OFFICERS J R Friedman, Chair J W Milton, Vice Chair J H Karian, Secretary STANDARDS COMMITTEE PERSONNEL D R Keyser, Survice Engineering S J Korellis, Electric Power Research Institute M P McHale, McHale & Associates, Inc P M McHale, McHale & Associates, Inc T K Kirpatrick, Alternate, McHale & Associates, Inc J W Milton, RRI Energy, Inc S P Nuspl, Consultant R R Priestley, Consultant J A Silvaggio, Siemans Demag Delaval, Inc W G Steele, Mississippi State University T L Toburen, T2E3, Inc G E Weber, Midwest Generation EME LLC C Wood, Duke Energy, Inc P G Albert, General Electric Co R P Allen, Consultant J M Burns, Burns Engineering, Inc W C Campbell, Southern Company Services, Inc M J Dooley, Alstom Power, Inc J R Friedman, Siemens Energy, Inc G J Gerber, Consultant P M Gerhart, University of Evansville T C Heil, Consultant S A Scavuzzo, Alternate, Babcock & Wilcox Co R A Henry, Sargent & Lundy, Inc J H Karian, The American Society of Mechanical Engineers HONORARY MEMBERS G H Mittendorf, Consultant J W Siegmund, Consultant R E Sommerlad, Consultant R L Bannister, Consultant W O Hays, Consultant R Jorgensen, Consultant F H Light, Consultant PTC 36 COMMITTEE — MEASUREMENT OF INDUSTRIAL SOUND R A Putnam, Chair, Siemens Energy, Inc B Brooks, Vice Chair, Brooks Acoustics Corp G Osolsobe, Secretary, The American Society of Mechanical Engineers S A Hambric, State College, Pennsylvania G F Hessler, Hessler Associates, Inc R S Johnson, Johnson Acoustical Consulting D Ozgur, Consultant D J Parzych, Power Acoustics, Inc R J Peppin, Scantek, Inc H A Scarton, Rensselaer Polytechnic Institute v CORRESPONDENCE WITH THE PTC COMMITTEE General ASME Codes are developed and maintained with the intent to represent the consensus of concerned interests As such, users of this Code may interact with the Committee by requesting interpretations, proposing revisions, and attending Committee meetings Correspondence should be addressed to Secretary, PTC Standards Committee The American Society of Mechanical Engineers Three Park Avenue New York, NY 10016-5990 Proposing Revisions Revisions are made periodically to the Code to incorporate changes that appear necessary or desirable, as demonstrated by the experience gained from the application of the Code Approved revisions will be published periodically The Committee welcomes proposals for revisions to this Code Such proposals should be as specific as possible, citing the paragraph number(s), the proposed wording, and a detailed description of the reasons for the proposal, including any pertinent documentation Proposing a Case Cases may be issued for the purpose of providing alternative rules when justified, to permit early implementation of an approved revision when the need is urgent, or to provide rules not covered by existing provisions Cases are effective immediately upon ASME approval and shall be posted on the ASME Committee Web page Requests for Cases shall provide a Statement of Need and Background Information The request should identify the Code, the paragraph, figure or table number(s), and be written as a Question and Reply in the same format as existing Cases Requests for Cases should also indicate the applicable edition(s) of the Code to which the proposed Case applies Interpretations Upon request, the PTC Standards Committee will render an interpretation of any requirement of the Code Interpretations can only be rendered in response to a written request sent to the Secretary of the PTC Standards Committee The request for interpretation should be clear and unambiguous It is further recommended that the inquirer submit his/her request in the following format: Subject: Edition: Question: Cite the applicable paragraph number(s) and the topic of the inquiry Cite the applicable edition of the Code for which the interpretation is being requested Phrase the question as a request for an interpretation of a specific requirement suitable for general understanding and use, not as a request for an approval of a proprietary design or situation The inquirer may also include any plans or drawings that are necessary to explain the question; however, they should not contain proprietary names or information Requests that are not in this format will be rewritten in this format by the Committee prior to being answered, which may inadvertently change the intent of the original request ASME procedures provide for reconsideration of any interpretation when or if additional information that might affect an interpretation is available Further, persons aggrieved by an interpretation may appeal to the cognizant ASME Committee or Subcommittee ASME does not “approve,” “certify,” “rate,” or “endorse” any item, construction, proprietary device, or activity Attending Committee Meetings The PTC Standards Committee and PTC Committees hold meetings regularly, which are open to the public Persons wishing to attend any meeting should contact the Secretary of the PTC Committee vi INTRODUCTION This Standard is intended to be primarily a sound emissions procurement specification for gas turbines, as opposed to a standardized test procedure The PTC 36 Committee believes that although there are many test procedures available to users of this Standard, procurement and testing are inextricably and unavoidably connected Whether the specification calls for measured airborne sound levels at particular positions, or for source sound power levels from components, a significant degree of measurement methodology is absolutely necessary Whenever sound levels at particular positions are called out, requirements are also specified for controlling the operating conditions, the instrumentation, the definition of locations, the averaging methodology, the meteorological constraints, the measurement duration, the test tolerances permitted, the environmental corrections to be applied, and the manner and methodologies used in any such corrections Without such defined constraints, similar to test procedures, the sound emissions specification will be deficient Sound power level specifications require even more explicit details to adapt and apply any of the several possible sound power test procedures for gas turbine installations Failure to provide detailed qualifications to sufficiently control and limit the range of possible interpretations on the specification or commitments can have adverse effects These qualifications should be provided at the earliest possible stage in the specification (and by extension, in the contract documents and vendor guarantees) to avoid disagreements regarding the limitations and constraints of any specified sound emissions Sound emissions must be considered in any gas turbine installation, as they are central to declarations of provisional acceptance, final acceptance, etc Furthermore, the degree of financial exposure as a result of disagreements as to whether acoustical compliance has been achieved, or the degree to which some shortfall has occurred, is always a function of the precision of the definition of measurement methodology In turn, any such disagreements affect the cost of both retrofit corrections or liquidated damages involved While contractual considerations are beyond the scope of this Standard, an awareness of the context within which the specification will be implemented is needed This awareness influences the work of PTC 36 on this Standard, since it bears directly on the revisions contained herein The gas turbine literature often uses “gas turbine” and “combustion turbine” interchangeably PTC 36 regards the use of “gas turbine” herein to be synonymous with “combustion turbine.” vii I N TE N TI O N ALLY LE FT B LAN K viii ASME B133.8-2011 (c) the measured average or maximum, weighted or octave band sound levels (whichever have been specified) are equal to or less than the specified levels at locations agreed to by the user and manufacturer This paragraph may apply to sites where governmental regulations or other specific limits have been contractually agreed to (d) the measured average or maximum, weighted or octave band sound levels (whichever have been specified) are equal to or less than the value specified except for the additive corrections given in Forms 2-2.1-1 and 2-2.2-1 [These corrections only apply when there are major buildings, fences, walls, or other large structures (see para 3-4.2) within a distance of ? ( ? is the wavelength of sound at the lowest frequency of interest) of the turbine installation or the microphone location In this context an additive correction consists of subtracting the applicable values of Forms 2-2.1-1 and 2-2.2-1 from the appropriate measured value If the specification is in the form of an A-weighted sound level, the lowest frequency of interest shall be 100 Hz (? p 55 ft) If the C-weighted sound levels are specified, the lowest frequency of interest shall be 31.5 Hz (? p 55/0.31 p 175 ft).] 12 ASME B133.8-2011 Section References 4-1 REFERENCES Adequate Margin o f S afety,” U S E nvironme ntal Protection Agency, 550/9-74-004, March 1974 [13] K M Eldred, “Assessment of Community Noise,” Noise Control Engineering, Vol 3, No 2, September– October 1974 [1 4] ANSI S1 3, Measurement of Sound Pressure Levels in Air [1 ] R M Hoover, “ Beware Low-Frequency Gas-Turbine Noise,” Power, May 1973 [16] G F Hessler, “Proposed criteria in residential communities for low frequency noise emissions from industrial sources,” Noise Control Engineering Journal, Vol 52, No 4, July–August 2004 [17] N Broner, “A simple criterion for low-frequency noise emission assessment,” Journal of Low Frequency Noise, Vibration and Active Control, Vol 129, No I, Multi-Science Publishing, 2010 [1] ISO 9613, Acoustics — Attenuation of sound during propagation outdoors — Part 1: Calculation of the absorption of sound by the atmosphere [2] ANSI S1.4, Specification for Sound Level Meters [3] ASME PTC 36-2004, Measurement of Industrial Sound [4] ISO 10494, Gas turbines and gas turbine sets — Measurement of emitted airborne noise — Engineering/ survey method [5] DIN 45635, Part 47, Measurement of noise emitted by machines; airborne noise emissions; enveloping surface method [6] ANSI S1 8, Procedures for Outdoor Measurement of Sound Pressure Level [7] ANSI S1 43, Sp ecifications for Integrating– Averaging Sound Level Meters [8] ANSI S1 1, Specification for Octave-Band and Fractional-Octave-Band Analog and Digital Filters [9] ANSI S6.1, Qualifying a Sound Data Acquisition System [1 0] ANSI S1 9, Quantities and Procedures for Descrip tion and Measurement of Environmental Sound — Part 4: Noise Assessment and Prediction of Long-term Community Response [1 ] “ Community Noise,” U S Environmental Protection Agency Report No NTID 300.3, December 1971 [12] ”Information on Levels of Environmental Noise Requisite to Protect Health and Welfare with an 4-2 BIBLIOGRAPHY ISO 9613, Acoustics — Attenuation of sound during propagation outdoors — Part 2: General method of calculation R B Tatge, “Effect of community population on the applicability of noise rating procedures,” Noise Control Engineering, Vol 4, No 1, January–February 1975 L L Beranek, Noise and Vibration Control, McGrawHill, New York, 1971 Noise Control Engineering, Vol 4, No 1, January– February 1975 13 I N TE N TI O N ALLY LE FT B LAN K 14 ASME B133.8-2011 NONMANDATORY APPENDIX A GUIDE TO DETERMINING ACCEPTABLE A-WEIGHTED SOUND LEVEL This No nmandatory Ap p e nd ix is no t a p a rt of ASME B133.8 but is included for information purposes only Users interested in estimating acceptable sound emissions should also refer to reference [10] Where local or state regulatory criteria exist they will supersede this Nonmandatory Appendix This Nonmandatory Appendix suggests a procedure that may be used to select acceptable A-weighted gas turbine sound emissions for an installation site where there is a nearby community It may be used to develop procurement sound level specifications, or alternatively, if previously established sound level limits such as property line noise regulations are to be used as a design goal, to predict the expected community subjective response to the noise Available information regarding community response to noise has been used (see references [11], [12] and [13]) However, community response criteria relates to average group response, and in some cases, individual judgments may vary, as shown by Fig A-1 Therefore, some degree of uncertainty is inherent in this procedure Use of this Nonmandatory Appendix requires anticipation of the gas turbine installation operation cycle, and a familiarity with the proposed site and its surrounding neighborhood Special consideration may be required for sites with unusual topographic or demographic features The procedure described should be followed for each proposed gas turbine installation location For those installations where noise may have an environmental impact or there are unusual topographic or demographic features, professional advice may be needed Two community sound emissions limits are calculated: one for turbines that not emit prominent discrete tones, and one for turbines that emit prominent discrete tones The user may want to reference both values in the bid specifications so that a prospective manufacturer can identify which specified noise level is appropriate to the offering The manufacturer should state whether or not the sound emissions from his plant include a prominent discrete tone Care should be taken to ensure proper definition of “prominent discrete tone” if the provision is included in the specification, since all such determinations are dependent on such variables as ambient background sound levels, measurement sampling durations, time of day, distance and direction from the gas turbine installation, and the influence of other plant sources Sound specifications apply to the sound emissions from an entire installation, and not to the individual turbines in a multiple array Form A-1 is a step-by-step worksheet for selecting an acceptable A-weighted community sound level for a complete gas turbine installation The A-weighted sound level may be calculated by completing the following steps and entering the results in the appropriate boxes on Form A-l: Step 1: An expected community subj ective sound acceptance response category is selected on Fig A-l (ordinate) and the corresponding normalized outdoor day/night sound level, L dn, is determined Table A-1, showing typical residential area A-weighted sound levels, may assist in this selection Step 2: Corrections are then obtained from Table A-2 for the season of operation and the ambient sound characteristics of the nearby neighborhood, and from Table A-3 and Fig A-2 for the expected daily operational cycle of the gas turbine installation The corrections are then summed Step 3: The sum of the corrections from Step is then added to the normalized Ldn criteria to yield the gas turbine A-weighted sound level in decibels (dB) at the selected community location, usually the closest neighbor Step 4: By reference to Fig A-3, determine the correction in decibels for the distance between the nearest residential area and the standard sound specification distance of 400 ft (120 m) Step 5: Add the distance correction factor from Step to the sound level in dB(A) of Step to obtain the sound level in dB(A) at 400 ft (120 m) Step 6: Add a correction of –5 dB(A) to the level calculated at 400 ft (120 m) in Step if the turbine sound emissions contain one or more prominent discrete tones For this procedure, a prominent discrete tone is taken to be as defined in Annex A of reference [14], with the quantity X equal to 10 dB Actually, X may range from dB, for more-critical installations, to 15 dB, for less-critical installations 15 ASME B133.8-2011 Step 7: (This procedure should be repeated for other noise-sensitive locations that surround the proposed site, and the most stringent noise emissions requirement would be used in Step 7.) Enter the calculated A-weighted sound level in dB(A) from Step or as the A-weighted specification sound level for Procedure A as shown in Form 2-2.1-1 Fig A-1 Community Reaction to Noises of Many Types as a Function of the Normalized Outdoor Day/Night Sound Level of the Intruding Noise Com m u n i ty Reacti on Vi g orou s acti on Several th reats of l eg al acti on or stron g appeal s to l ocal 90% l i m i ts offi ci al s to stop n oi se Wi despread com pl n ts or si n g l e th reat of l eg al acti on Sporadi c com pl n ts N o reacti on al th ou g h n oi se i s g en eral l y n oti ceabl e 40 50 60 70 80 90 N orm al i zed Ou tdoor Day/N i g h t Sou n d Level of I n tru di n g N oi se, Ldn, dB [ N ote (1 )] NOTE: (1 ) Ldn designates the day/night soun d level that is the equivalen t A-weighted sound level during a 24-hr time period with a 0-dB weightin g applied to the equivalen t sound level during the hours of p.m to a.m 16 ASME B133.8-2011 Form A-1 Suggested Procedure to Develop A-Weighted Sound Level Specification at Standard Distance of 400 ft (1 20 m) Steps Value Choose appropriate value of normalized outdoor day/night sound level at nearest residence (see Fig A-1 and Table A-1 ) Enter corrections dB (a) seasonal (see Table A-3) dB (b) background sound (see Table A-2) dB (c) operational cycle (see Table A-3 and Fig A-2) [Note (1 )] dB Sum of corrections dB Line value + sum of corrections in Line = recommended sound level at nearest residence dB(A) Enter distance correction (see Fig A-3 or para 3-9.4) dB(A) Line value + Line value = specification level at 400 ft (1 20 m) for installations without prominent discrete tones dB(A) Line value – dB = specification level at 400 ft (1 20 m) for installations with prominent discrete tones dB(A) Specify the appropriate value from Line or for Procedure A as shown in Form 2-2.1 -1 dB(A) NOTE: (1 ) Sum = D + N 17 ASME B133.8-2011 Table A-1 Residential Area Sound Levels Daytime Sound Level Exceeded 90% of the Time, dB re 20 ?Pa [Note (1)] Description Typical Range Very quiet rural area Quiet suburban residential Normal suburban residen tial Urban residential Noisy urban residen tial Very noisy urban residen tial GENERAL NOTE: 31 36 41 46 51 56 to to to to to to 35 40 45 50 55 60 Average in clusive in clusive in clusive in clusive in clusive in clusive 33 38 43 48 53 58 See referen ce [1 ] NOTE: (1 ) Add dB to estimate median sound level Table A-2 Corrections for Background Sound Type of Correction Description Amount Correction, dB Seasonal Sum mer for year-round operation Winter only for windows always closed Background soun d [Note (1 )] Quiet suburban or rural com munity (remote from large cities an d from industrial activity an d trucking) –1 Normal suburban commun ity (not located near industrial activity) –5 Urban residential com munity (n ot im mediately adjacen t to heavily traveled roads an d industrial areas) Noisy urban residen tial community (n ear relatively busy roads or in dustrial areas) Very noisy urban residen tial com munity +5 +5 +1 NOTE: (1 ) These corrections are based on reported typical residual n oise levels as shown in Table A-2 I f measured data at the site under in vestigation differs sign ifican tly from the Table, different corrections may be warranted The residual sound level is that sound level exceeded 90% of the time 18 ASME B133.8-2011 Table A-3 Operational Cycle Correction Typical Number of Hours of Operation Between a.m and 10 p.m Outdoor Day Sound Level Correction, D 0 0.04 0.08 0.1 0.1 0.21 0.25 0.29 0.33 0.38 10 0.42 11 0.46 12 0.50 13 0.54 14 0.58 15 0.63 Typical Number of Hours of Operation Between 10 p.m and a.m Outdoor Night Sound Level Correction, GENERAL NOTE: N 0 0.42 0.83 3 2.1 2.5 2.9 3.3 3.8 Sum p D + N (see 19 Fig A-2) ASME B133.8-2011 Fig A-2 Operational Cycle Correction 14 12 Operati on al Cycl e Correcti on 10 −2 −4 −6 −8 01 Su m , GENERAL NOTES: (a) Choose n earest whole n um ber correction (b) See Table A-3 20 D+ N 10 ASME B133.8-2011 Fig A-3 Distance Correction for Weighted Sound Levels 25 A-weighted sound level Distance Correction, dB 20 15 C-weighted sound level 10 -5 200 (60) 300 (90) 500 (1 50) ,000 (300) 2,000 (600) Distance From Gas Turbine Installation to Nearest Residence, ft (m) GENERAL NOTE: Choose nearest whole number 21 5,000 (1 500) ASME B133.8-2011 NONMANDATORY APPENDIX B GUIDE TO DETERMINING SPECIFIED C-WEIGHTED SOUND LEVEL The sound level sp ecified for Procedure A (see para 2-2.1) should also include a C-weighted level for simple-cycle gas turbine installations located near sensitive receivers (such as an occupied frame structure) to avoid complaints of building/window vibration caused by low-frequency airborne sound (see references [10], [15], and [16]) Users interested in estimating acceptable C-weighted sound emissions should refer to references [10] and [17] Where local or state regulatory criteria exist, they will supersede this Nonmandatory Appendix The first step in this procedure is to select a maximum value for the C-weighted sound level outside the nearest sensitive receptor The upper limit for this C-weighted level at the nearest frame structure should not exceed 75 dB(C) to 80 dB(C) The range of values is given because there is some uncertainty as to the sound level required to induce structural vibration in a frame structure See references [1 6] and [1 7] for recommended C-weighted limits for residential receptors After selecting an appropriate C-weighted sound level, the second step is to find a distance correction from Fig A-2 that is added to the selected level to obtain the C-weighted sound level at the specification distance at 400 ft (120 m) As an example, for a selected sound level of 75 dB(C) at a residence at 1,000 ft (300 m) from the gas turbine installation, the specified level at 400 ft (120 m) should not exceed 83 dB(C) This calculated C-weighted level at 400 ft (1 20 m) is entered into Form 2-2.1-1 for the specification in Procedure 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