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REVISION FOR REVIEW 21 May 2010 Engineering Technical Practice Engineering Group Practice GP 15-01 Noise Control
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Page 1: GP 15-01 21 May 2010 - Guidance on Practice for Noise Control

REVISION FOR REVIEW 21 May 2010 Engineering Technical Practice Engineering

Group Practice

GP 15-01

Noise Control

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Noise Control

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Table of Contents

Page

Foreword............................................................................................................................................................. 3 1. Scope ........................................................................................................................................................ 4 2. Normative references ............................................................................................................................... 4 3. Terms and definitions ............................................................................................................................... 5 4. Symbols and abbreviations ....................................................................................................................... 7 5. General noise limits .................................................................................................................................. 8

5.1. Local regulations .......................................................................................................................... 8 5.2. Hearing conservation work area noise limit ................................................................................ 8 5.3. Speech and work interference .................................................................................................... 10 5.4. Accommodation (offshore) ........................................................................................................ 10 5.5. Exceptional or emergency operating conditions ........................................................................ 11 5.6. Additional restrictions for tonal or impulsive noise ................................................................... 11

6. Equipment noise limits ........................................................................................................................... 11 7. Noise reduction techniques .................................................................................................................... 13 8. Key responsibilities ................................................................................................................................ 14 Annex A (Informative) Noise calculations ....................................................................................................... 15 Bibliography ..................................................................................................................................................... 16

List of Tables

Table 1 - Maximum allowable background sound levels in work spaces ........................................................ 10 Table 2 - Maximum allowable background sound levels in accommodations ................................................. 11 Table 3 - Guidance for determining existence of a pure tone (from ANSI S12.9 Part 4, Annex C, with

permission) ............................................................................................................................................. 11 Table A.1 - Additive effect of noise ................................................................................................................. 15

Copyright © 2010 BP International Ltd. All rights reserved. This document and any data or information generated from its use are classified, as a minimum, BP Internal. Distribution is intended for BP authorized recipients only. The information contained in this document is subject to the terms and conditions of the agreement or contract under which this document was supplied to the recipient's organization. None of the information contained in this document shall be disclosed outside the recipient's own organization, unless the terms of such agreement or contract expressly allow, or unless disclosure is required by law.

In the event of a conflict between this document and a relevant law or regulation, the relevant law or regulation shall be followed. If the document creates a higher obligation, it shall be followed as long as this also achieves full compliance with the law or regulation.

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Foreword

This is a revised issue of Engineering Technical Practice (ETP) GP 15-01. This Group Practice (GP) is based on parts of heritage documents from the merged BP companies as follows:

Amoco (ACES) A EN-NC-00-E Environmental-Noise Control-Engineering Specification. A EN-NC-00-G Environmental-Noise Control-Guide.

ARCO (APCES) Std 600 Noise Control Standard for Refinery Operating Units. Std 601 Equipment Noise Control.

British Petroleum (RPSE) RP 14-1 Guide to Noise Control.

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1. Scope

This GP provides Group practice for general noise control requirements for complete plant and individual equipment items. Unless otherwise stated, all requirements apply to normal operating conditions.

BP Group Safety and Operations recognises noise as one of BP Group level risks that if not properly managed can cause harm to people. Following the hierarchy of controls logic, engineering solutions should be the first option for consideration when considering noise control solutions.

Noise control at the procurement or design stage of a project is critical as industry studies show it costs 10 to 13 times more to retrofit as compared to the cost of including engineering controls on the front end of a project. Retrofit solutions often make operating and maintenance of equipment more cumbersome and introduce other unintended issues. Reliance upon hearing protection cannot be depended upon as a primary means of ensuring employees are protected against hazardous noise levels.

Consequently, eliminating or even reducing the need for required employee participation in a Hearing Conservation Program (HCP), by implementing effective engineering controls, will make for a safer and healthier workplace. It will also be a significant long term cost savings to the company.

2. Normative references

The following referenced documents may, to the extent specified in subsequent clauses and normative annexes, be required for full compliance with this GP:

• For dated references, only the edition cited applies.

• For undated references, the latest edition (including any amendments) applies.

BP GIS 15-011 Noise Control. GN 15-01 Guidelines on Noise Control.

American National Standards Institute (ANSI) ANSI S12.9 Part 4 Quantities and Procedures for Description and Measurement of

Environmental Sound - Part 4: Noise Assessment and Prediction of Long-Term Community Response.

ANSI S12.16 Guidelines for the Specification of Noise of New Machinery.

British Standards (BS) BS 4142 Method for Rating Industrial Noise Affecting Mixed Residential and

Industrial Areas.

International Organization for Standardization (ISO) ISO 11690-1 Acoustics - Recommended practice for the design of low-noise

workplaces containing machinery - Part 1: Noise control strategies. ISO 15664 Acoustics - Noise control design procedures for open plant.

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3. Terms and definitions

For the purpose of this GP, the following terms and definitions apply:

Acoustics (1) The science of sound, including the generation, transmission, and effects of sound waves, both audible and inaudible. (2) The physical qualities of a room or other enclosure (such as size, shape, amount of noise) that determine the audibility and perception of speech and music within the room.

Acoustical expert Person formally trained in acoustics, noise control engineering, and noise survey techniques and data analysis. Typically, this individual holds the status of Board Certified Noise Control Engineer by the Institute of Noise Control Engineering (INCE), or an equivalent professional association.

Administrative control Changes to employee work routines that affect their daily noise exposure in a positive manner.

Attenuation The reduction of sound intensity by various means (e.g., air, humidity, porous materials, sound transmission loss materials, etc.).

A-weighted sound level A measure of sound pressure level designed to reflect the acuity of the human ear, which does not respond equally to all frequencies. Units are expressed in dBA. Specific response is defined by ANSI/ISO standards.

Background noise (ambient noise) Total of all noise in a room or area, independent of the presence of the intended equipment or machines for potential procurement.

Calibrator (acoustical) Device which produces a known sound pressure on the microphone of a sound level measurement system and is used to adjust the system to standard specifications.

dBA Unit of sound level. Decibels measured using the A frequency weighting filter on the sound level metre.

Decibel A unit of sound pressure level, abbreviated dB. Decibels are a logarithmic scale.

Equivalent continuous A-weighted sound level (LAeq,T, also written LpAeq,T) The constant sound level that, in a given time period, would convey the same sound energy as the actual time varying A-weighted sound level.

Frequency Number of times per second the sine wave of sound repeats itself, or the sine wave of a vibrating object repeats itself. Now expressed in hertz (Hz), but was formerly expressed in cycles per second (cps).

Governing authorities Local, regional, national, or any other authorities that specify and enforce criteria for environmental and occupational noise.

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Industrial hygienist Professional qualified by education, training, and experience to anticipate, recognise, evaluate, and develop controls for occupational health hazards and environmental issues.

Intermittent sound (fluctuating or cyclical sound) Sounds that fluctuate in amplitude with time by more than 3 dB.

Line source Source that radiates sound as if along a cylindrical line, such as a pipeline, traffic on a highway, trains moving along a railroad track, etc. Very large equipment, such as cooling towers, will radiate sound waves as a line source near the equipment. However, as the distance increases beyond multiple dimensions away from the centre of the source, the equipment will appear to radiate sound as a point source. Typically, point source radiation tends to exist at distances 3 to 4 times the largest dimension of very large equipment.

Management of change (MOC) Recommendations that apply to any “change” to plant, process, or people whether permanent, temporary, or emergency, with the potential to introduce health, safety, security, environmental, or operational hazards.

Maximum A-weighted sound pressure level (LAmax) Greatest A-weighted sound pressure level, within a stated time interval. Time interval for “slow” response is 1 s and for “fast” response is 125 ms.

Noise level For airborne sound, unless specified to the contrary, it is the A-weighted sound pressure level.

Noise limit Noise levels not to be exceeded and given as a sound power level, sound pressure level, or noise exposure.

Point source Source that radiates sound as if from a single point. Most equipment, such as pumps, motors, compressors, etc., may be assumed to be point sources, unless they fit the description of a line source.

Project specification Document defining the scope of the project.

Restricted area Those areas in the plant where is it not reasonably practicable to reduce the noise level below the work area limit and/or any routinely unoccupied space (e.g., compressor room, blower room, etc.).

Sound (1) Oscillation in pressure, stress, particle displacement, particle velocity, etc., in an elastic or partially elastic medium, or the superposition of such propagated alterations. (2) Auditory sensation evoked by the oscillation described above. Not all sound waves can evoke an auditory sensation (e.g., ultrasound).

Sound level (dBA) Weighted sound pressure level obtained by the use of a sound level metre while employing the A-weighted frequency network, as specified in ANSI or ISO specifications for sound level metres.

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Sound level metre Instrument comprised of a microphone, amplifier, output metre, and frequency weighting networks which is used for the measurement of noise and sound levels.

Sound power Total sound energy radiated by a source per unit time. Unit of measurement is watt.

Sound power level 10 times the logarithm, to the base 10, of the ratio of the power (in watts) of the sound measured to the reference power (wr), which is 1 x 10-12 W. In equation form, sound pressure level in units of decibels is expressed as SPL (dB) = 10 log w/wr.

Sound pressure Instantaneous difference between actual pressure produced by a sound wave and average of barometric pressure at a given point in space.

Sound pressure level (SPL) 20 times the logarithm, to the base 10, of the ratio of the pressure (p) of the sound measured to the reference pressure (pr), which is 20 x 10-6 Pa. In equation form, sound pressure level in units of decibels is expressed as SPL (dB) = 20 log p/pr.

Tonal noise Sounds that contain pure tones.

Weighting Prescribed frequency filtering provided in a sound level metre.

Work area Any location employees routinely work or visit during their normal course of duty. Work area also includes any position accessible by an employee that is at least 1 m (3.3 ft) away from equipment or piping surfaces, including any mezzanine platform, catwalk, or fixed ladder.

4. Symbols and abbreviations

For the purpose of this GP, the following symbols and abbreviations apply:

HCMP Noise and Hearing Conservation Management Program.

HPD Hearing protection device.

LAeq,8 Daily 8 hr equivalent average noise exposure.

LAeq,T Equivalent continuous A-weighted sound level for the sample time.

MOC Management of change.

SME Subject matter expert.

SPL Sound pressure level.

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5. General noise limits

5.1. Local regulations

5.1.1. In plant noise

a. Limits on plant noise are often based on worker noise exposure. Therefore, if feasible, the routine or typical work patterns leading to noise exposure shall be agreed upon by the parties involved. Noise limits may also be based on maximum SPLs for work areas.

b. BP is committed to complying with all applicable laws and regulations as they pertain to in-plant noise levels and/or personnel noise exposure. All applicable noise regulations or standards shall be investigated and adhered to.

c. In instances where local regulations, statutory requirements, or national standards are more stringent than this GP, the applicable more stringent limits shall be stated in the project specification, or any other document defining the scope of the project.

5.1.2. Environmental and community noise

a. Noise generated by the plant should be assessed through common BP processes such as Health Impact Assessment (HIA) and Environmental Impact Assessment (EIA).

b. Effects of noise shall be considered during the construction phase of the project. However, it is important to note this GP is not intended to address the effects of environmental noise.

c. The quality of the sound and community characteristics shall be considered.

1. Unusual sounds, such as discrete tones and impulsive sounds, need more attention.

2. Sounds with strong low frequency content require special attention.

3. Different noise limits may exist for different times of day or night and for workdays or weekends.

d. The most critical of the factors discussed in c shall be the basis of design, taking due account of the period of operation of the plant. Allowances for occasional higher noise levels that may be acceptable to local authorities (e.g., such as for emergencies) shall be included in the environmental noise limits.

e. In those instances where local regulations for environmental noise do not exist, this aspect of plant design shall still be considered at the project definition stage to anticipate adverse community reactions at some later date. BS 4142 may be used for guidance.

5.2. Hearing conservation work area noise limit For new facility areas, including expansion of an existing area, the following noise limits shall apply and be satisfied by the vendor:

a. Absolute noise limit

1. A-weighted SPL anywhere in a work area shall not exceed 115 dBA at 1 m (3.3 ft) from the source in any situation.

2. Absolute limit of 115 dBA shall remain valid in all areas. However, feasible noise control measures should be used to reduce the level to as low as reasonably practical, and preferably below 85 dBA.

b. Work area noise limit

1. Objective is to keep all employee Leq,8 noise exposures below 85 dBA, without regard for hearing protection.

2. Toward this objective, the work area A-weighted SPL should not exceed 85 dBA.

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3. Any work area exceeding 85 dBA shall be listed as a restricted area.

c. Restricted area noise limit

1. To assist with determining the feasibility of implementing noise control measures, employee exposures, occupancy of the space, and overall area noise levels should be considered. Obviously, the desired result is to obtain the maximum employee noise exposure reduction for each dollar invested.

2. Initial and on-going costs of the noise control measures should be weighed against the amount of noise reduction that can be achieved.

3. Keeping in mind that noise exposures are a function of magnitude and duration, the following feasibility considerations may be useful in helping to develop a plan of action:

a) Will employee LAeq,8 noise exposures routinely be above 85 dBA?

b) How many employees will typically be in the area(s) of concern?

c) How much time per day will employees routinely be exposed to high noise source(s)?

d) Will administrative controls reduce noise exposures to less than 85 dBA?

e) Will engineering controls reduce noise exposures to less than 85 dBA?

f) What are the initial costs? What are the on going costs?

g) Does the technology exist to control the noise source(s)?

h) Are the control measures reasonably practical from an employee operations and/or production viewpoint? Is there an increased maintenance impact?

4. BP responsible engineer should weigh the answers to these questions in their totality if attempting to determine the feasibility, reasonableness, and practicality of each noise control measure.

5. Noise sources should be prioritised for implementation of engineering controls based on the highest rank order of their contribution to actual worker noise exposures.

6. LAeq,8 noise exposures shall be kept below 85 dBA without regard for hearing protection for personnel who access restricted areas.

7. Vendor, working in concert with BP, shall determine employee work patterns or routines in restricted areas and estimate the LAeq,8 noise exposure for affected personnel. LAeq,8 (also written LEX,8h) is calculated as follows:

+=

o

eTAeqAeq T

TLL log10,8,

Where:

LAeq,8 = Daily 8 hr equivalent average noise exposure (dBA).

LAeq,T = Equivalent continuous A-weighted SPL (dBA).

Te = Effective duration of the working day (hr).

To = Reference duration (To = 8 hr).

Note 1: The term above is also called the “daily noise exposure” or simply “noise exposure” in some references and standards. For this GP, the expression “noise exposure” is used.

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Note 2: In those instances where the effective duration of the workday is equal to 8 hr, then LAeq,T equals LAeq,8.

Note 3: Attenuation provided by hearing protection does not apply toward these measurements or exposure calculations.

8. BP shall ensure appropriate hearing protection devices are used by employees at all times in restricted areas. Appropriate hearing protection means the “protected” or LAeq,8 under the device, as per HCMP Section 2.3.2 or the relevant clause in GN 15-01, is below 85 dBA, and preferably between 75 dBA to 80 dBA. For areas above 105 dBA, double hearing protection shall be required.

9. Vendor shall identify potential restricted areas to BP. Written permission shall be obtained from BP to designate an area as a restricted area.

10. Areas of 85 dBA or more shall be posted as “hearing protection required” areas at the entries and boundaries of restricted areas.

11. Equipment or areas of 105 dBA or more shall be posted as “dual hearing protection required” areas.

5.3. Speech and work interference To ensure acceptable background sound levels exist in nonrestricted work areas, other than equipment or machinery spaces, the limits in Table 1 should apply.

Table 1 - Maximum allowable background sound levels in work spaces

5.4. Accommodation (offshore) For offshore accommodation, the maximum background SPLs provided in Table 2 should apply.

Work space description Maximum allowable sound pressure level (dBA)

Workshops and machinery buildings where communication is required.

70

Workshops for light maintenance. Workshop offices. 60 Control rooms, not continuously manned. Computer rooms. Wash places, changing rooms, and toilets. Production area supervisory offices. 55 Guard stations. Shipping and receiving rooms. Control rooms, continuously manned. 50 Open plan offices. Social rooms. Enclosed offices, conference rooms, and training areas.

45

Note: Sounds produced by users of the space are not to be considered when evaluating the background sound level in an area.

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Table 2 - Maximum allowable background sound levels in accommodations

5.5. Exceptional or emergency operating conditions a. Allowance for occasional higher noise levels that may be acceptable to governing

authorities (such as emergencies, startup, shutdown, and maintenance activities) shall be included in the project specification.

b. Noise limits applicable during construction or deconstruction (dismantling) of the facility shall be stated separately in the project specification.

5.6. Additional restrictions for tonal or impulsive noise a. Since tonal noise or impulsive noise can be annoying to the workforce and/or community,

these events should be taken into account when specifying equipment noise limits.

b. To determine whether a pure tone exists, refer to Table 3.

Table 3 - Guidance for determining existence of a pure tone (from ANSI S12.9 Part 4, Annex C, with permission)

6. Equipment noise limits

For the purchase of a specific machine or equipment item, the following noise limits per qualifying condition shall apply and be satisfied by the vendor:

a. Noise limit for equipment emitting steady state noise (continuous sound levels)

1. Employee LAeq,8 noise exposures shall be kept below 85 dBA without regard for hearing protection.

2. Steady state broadband A-weighted SPL propagated by equipment should not exceed 85 dBA beyond a 1 m (3.3 ft) perimeter from all equipment surfaces.

Accommodation area Maximum allowable sound pressure level (dBA)

Washroom, toilets 60 Dining rooms 55 Recreation areas 50 Cinemas, theatres, meeting rooms 45 Sleeping areas, cabins 45 Medical rooms 45 Note:

Sounds produced by users of the space are not to be considered when evaluating the background sound level in an area.

Range of octave band centre frequencies (Hz)

Difference between arithmetic average of SPLs in two adjacent bands (dB)

25 to 125 15 160 to 400 8 500 to 10 000 5 Note:

Obtain the arithmetic average of the SPLs in the 1/3 octave bands immediately above and below the frequency of concern. Subtract this average value from the SPL in the 1/3 octave band containing the suspected pure tone. If the difference equals or exceeds the value indicated for the respective frequency range listed in the table, a discrete or pure tone may be assumed to exist.

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b. Noise limit for equipment emitting intermittent noise

1. If equipment emits an intermittent, fluctuating, or cyclical noise (e.g., depressurising, boiler blowdown, sump pump, etc.), LAeq,T, as measured over sufficient machine cycles to accurately document a representative average level, should not exceed 85 dBA beyond a 1 m (3.3 ft) perimeter from all equipment surfaces. The following equation may be used to approximate the average sound level from a series of individual SPLs:

≅ ∑

=

n

i

L

iTAeq

pi

tT

L1

10, 101log10

Where:

LAeq,T = Equivalent continuous A-weighted sound level (dBA).

T = Time period (hr).

ti = Duration of the ith increment.

Lpi = SPL for each increment.

2. Maximum A-weighted SPL (LAmax) should not be more than 90 dBA.

c. Noise limit for equipment used less than a full work shift

1. If the equipment is of the type that will not be used consistently throughout a work shift, written permission from BP, in the form of an MOC, may be given for emitting a higher LAeq,T (above 85 dBA) provided LAeq,8 at 1 m (3.3 ft) does not exceed 85 dBA.

2. For the noise exposure assessment, the maximum duration of potential equipment operation per work shift shall be used.

d. Equipment located outside the work area

1. Maximum allowable SPL at 1 m (3.3 ft) from a machine or equipment located outside the work area may be higher than the limits given in a, b, and c.

2. At positions that are inaccessible by personnel, such as may be the case for vent stacks and certain control valves, the allowable increase shall be one of the following:

a) 20 log(x) dBA for a point source (e.g., vent stack, compressor, etc.).

b) 10 log(x) dBA for a line source (e.g., piping).

3. In the two expressions in 2, “x” is the shortest distance from the equipment under consideration to the nearest work area, expressed in metres. For valves, the distance should be taken from directly connected piping to the nearest work area.

e. Additional restrictions for tonal or impulsive noise

1. Further restrictions shall apply if the noise of an equipment item contains tonal or impulsive components.

2. Equipment noise limit shall be reduced by 5 dBA for such equipment.

f. Packages

1. Specified noise limits apply to the complete equipment package being supplied by the vendor. Examples of equipment supplied as a package are as follows:

a) Agitators, gear boxes, and drivers.

b) Pumps and drivers.

c) Burners and forced draft fans.

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d) Compressors, gear boxes, and drivers.

2. If different packages are added together, located adjacent to one another, or placed in an area with other noise sources, the decibel manipulation procedures presented in Annex A shall be used to determine the added effect on the overall noise level in the area of concern.

g. Piping

1. Noise propagated by piping is of primary concern in plant noise control and shall be subject to the same equipment noise limits as presented above for general machines, equipment, and/or packages.

2. Noise shall be controlled by selection of low noise equipment by design, often available from the original equipment vendor. Alternatively, when it is not reasonably practical to affect the sound levels by design, inline silencers and/or acoustical insulation may be used to attenuate piping noise.

7. Noise reduction techniques

a. There are many variable factors that can affect the degree of protection each individual receives while wearing their HPD. Given the variability of hearing protection, noise reduction techniques shall be considered if sound pressure levels are above the 85 dBA recommendation.

b. Listed in c are the techniques to be followed in order of importance, starting with the most important. In all cases, use of HPDs will be considered as the last option of choice.

c. The following basic techniques should be used for noise reduction:

1. Eliminating noise at the source is the most effective long term control method. GN 15-01 contains information on noise control and implementing this technique. These controls should be incorporated at the design stage of the new plant/modification.

2. Partial or total enclosure of the noise source(s) to physically separate a source of noise from those people who may be harmed or inconvenienced by that noise.

3. Silencing or damping of the source of noise by means of a specially designed acoustic silencer or damping material.

4. Isolation by distance.

5. Regular monitoring, maintenance, and repair of equipment.

6. Administrative controls of work scheduling and employee rotation.

d. Items to check during the design stage of any new plant/modification should include the following items:

1. Insulation.

2. Relief valve discharges.

3. Control valves.

4. Fans.

5. Silencers and acoustic enclosures.

6. Pipes.

7. Flares.

e. A noise control study should include the following:

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1. A list of potentially noisy equipment, including those sources having characteristic high or low frequency noise emissions and those sources subject to vibration effects.

2. A record of the proposed location and position of equipment and assessment to determine if these are arranged to minimise overall noise level.

3. Details on how the BP noise specifications will be achieved for both internal site limits and those in the neighbourhood surrounding the facility.

4. A list of noise that will occur during construction, startup, and shutdown of the plant or equipment.

5. A list of noise control systems to be used.

6. Estimated noise exposure for all job classifications that will work in the areas. This includes both work areas and restricted areas.

Note: Refer to GN 15-02 which contains a spreadsheet tool for making the noise exposure estimation.

8. Key responsibilities

a. The division of responsibilities for noise control between BP and vendor should be as defined in GIS 15-011.

b. Nominated BP SME shall be responsible for the achievement of the noise specification.

c. A qualified industrial hygienist should be contacted for assistance in identifying noise control expertise.

d. In the event that the noise specification cannot be met, the BP SME shall nominate a specialist review team to further investigate the particular circumstances and to recommend an appropriate action plan. Review team shall use the BP MOC process where applicable.

e. Review team should be comprised of the project lead engineer, industrial hygienist, and vendor as a minimum.

f. Any action plan shall ensure the most appropriate corrective action is undertaken such that the resultant noise levels are economically controlled as near to the original specification as possible.

g. Vendor data should be checked to determine if the quoted noise levels in the noise control study are truly representative and not quoted artificially high as this might lead to the specification of unnecessary abatement equipment.

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Annex A (Informative)

Noise calculations

a. When reviewing equipment vendor data or product literature, it is normally useful to undertake some simple, basic noise calculations to determine the effect of the proposed equipment.

b. Detailed calculations of noise can involve some complex mathematics and the associated measurements necessitate specialist sound level/noise metres (as required by EEMUA PUB NO 140).

c. If a less technical approach is warranted or desired, the log nature of the decibel scale associated with noise measurement makes it possible to derive some simple “rule of thumb” guidelines, namely:

1) A doubling of the sound level results in an increase of 3 dBA (e.g., if there are two items of equipment both emitting 85 dBA, the combined noise level will be 88 dBA).

2) Where two or more sources of noise are involved, the resultant sound level can be calculated by use of Table A.1.

Table A.1 - Additive effect of noise

d. Pressure waves associated with sound energy obey the “inverse square law”. In far field conditions, this means that their intensity decreases in proportion to the square of the distance from the noise source (i.e., SPL decreases by 6 dB for each doubling of distance). Therefore, if the distance between the source and receptor is increased by a factor of 10, the sound intensity decreases by a factor of 100. When the sound intensity has decreased by a factor of 100, this is equivalent to a noise reduction of 20 dB.

Difference in noise levels between two sources (dBA)

Add the following to the higher value to get the resultant

(combined) noise level (dBA) 0 to 1 3 2 to 3 2 4 to 9 1 10 or more 0

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Bibliography

[1] GN 15-02, Noise Exposure Estimation.

[2] EEMUA PUB NO 140, Noise Procedure Specification.